scanner.mdc: lexical scanner for Ocean.

[ocean] / csrc / scanner.mdc

# Lexical Scanner #

## The Task at Hand ##

The main task of the lexical scanner is to convert a stream of
characters into a stream of tokens.  The tokens are then typically
used by a parser to extract the syntactic structure.

The stream of characters are assumed to be in memory identified by a
linked list of blocks, such as provided by the "[mdcode][]" literate
program extractor.  A single token may never cross a block boundary.

[mdcode]: mdcode.html

###### includes
        #include "mdcode.h"

The text is assumed to be UTF-8 though some matching assumes the
ASCII subset.  If the text provided does not conform to UTF-8 an error
will be reported and some number of bytes will be skipped.

###### includes
        #include <wchar.h>
        #include <wctype.h>
        #include <unicode/uchar.h>

Tokens are returned by successive calls to the main interface
function: `token_next()` which has a `state` structure to keep track
of where it is up to.  Each token carries not just a numeric
identifier but also the code block, the line and character within that
block, and the actual start and length using the `struct text` from
"mdcode".

###### public types
        struct token {
                int               num;
                struct code_node *node;
                struct text       txt;
                int               line, col;
        };
        struct token_state;

###### private types
        struct token_state {
                ## state fields
        };

###### exported functions
        struct token token_next(struct token_state *state);

###### main functions
        struct token token_next(struct token_state *state)
        {
                ## token_next init
                while (1) {
                        wint_t ch;
                        struct token tk;

                        ## one token
                }
        }

The `line` and `col` offsets are useful for reporting errors.
The `txt` provides the content when that is important.

### Token types and configuration ##

The scanner is not completely general, yet not completely specified.
There are a fixed set of token types, though particular tokens within
those types can be distinguish via configuration.

Most token types may be explicitly ignored, as typically comments
would be.  The exact consequence of ignoring each token type varies
from token to token.

###### public types
        struct token_config {
                int ignored;    // bit set of ignored tokens.
                ## token config parameters
        };

###### state fields
        struct token_config *conf;

###### token_next init
        int ignored = state->conf->ignored;


The different tokens are numbers, words, marks, strings, comments,
newlines, EOF, and indents, each of which is examined in detail below.

There are various cases where no token can be found in part of the
input.  All of these will be reported as an `TK_error` token.

It is possible to declare a number of strings which form distinct
tokens (rather than being grouped as e.g. 'word').  These are given
token numbers from `TK_reserved` upwards.

###### public types
        enum token_num {
                TK_error,
                ## token types
                TK_reserved
        };

### Numbers

Numbers are the messiest tokens to parse, primarily because they can
contain characters that also have meaning outside of number and,
particularly, immediately after numbers.

The obvious example is the '`-`' sign.  It can come inside a number for
a negative exponent, or after a number as a subtraction operator.  To
be sure we have parsed as best as possible we need to only allow the
'`-`' inside a number if it is after an exponent character.  This can be
`e` or `p` (for hex exponents), but `e` can also be a hexadecimal
digit, so we don't allow '`-`' after just any `e`.

To make matters worse, our language designer has decided to experiment
with allowing commas to be used as the decimal indicator, and spaces
to be used to separate groups of digits in large numbers.  Both of
these can reasonably be restricted to appear between two digits, so we
have to add that condition to our tests.

So we cannot just treat numbers as starting with a digit and being
followed by some set of characters.  We need more structure than that.

So:

- Numbers must start with a digit.
- If the first digit is zero, the next character must be a base
  signifier (one of `xob`) or a decimal marker (`.` or `,`).
  In the first case the first `p` or `P` may be followed by a sign.
- If the number doesn't start with `0` followed by one of `xob`, the
  first `e` may be followed by a sign.
- Any digit or hex digit may be followed by a space or underscore
  providing that the subsequence character is also a (hex) digit.
  This rule will require an extra level of 'unget' to be
  supported when handling characters.
- Otherwise any digits or ASCII letters are allowed.  We do not at
  this point check that the digits given are permitted by the base.
  That will happen when the token is converted to a number.

To allow easy configuration, the various non alphanumeric characters
are only permitted if they are listed in a configuration parameter.

###### token config parameters
        char *number_chars;

Note that numbers may not start with a period, so `.75` is not a
number.  This is not the norm, but is not unheard of.  Excluding these
numbers simplifies the rule at very little cost.

###### token types
        TK_number,

If TK_number is ignored, digits will result in an error unless they
are declared to be a start character for words.

###### includes

        #include <string.h>

###### parse number

        if (iswdigit(ch) && !(ignored & (1<<TK_number))) {
                int prev_special = 0;
                int expect_p = 0;
                int decimal_mark = 0;
                if (ch == '0') {
                        wchar_t ch2 = get_char(state);
                        if (strchr("xobXOB", ch2) != NULL)
                                expect_p = 1;
                        unget_char(state);
                }
                while (1) {
                        int sign_ok = 0;
                        switch(expect_p) {
                        case 0:
                                if (ch == 'e')
                                        sign_ok = 1;
                                break;
                        case 1:
                                if (ch == 'p')
                                        sign_ok = 1;
                                break;
                        }
                        save_unget_state(state);
                        ch = get_char(state);
                        if (iswalnum(ch)) {
                                prev_special = 0;
                                continue;
                        }
                        if (ch == '+' || ch == '-') {
                                if (!sign_ok)
                                        break;
                                expect_p = -1;
                        }
                        if (ch == '.' || ch == ',') {
                                if (decimal_mark)
                                        break;
                                decimal_mark = 1;
                        }
                        if (prev_special) {
                                /* Don't allow that special char,
                                 * need two 'ungets'
                                 */
                                restore_unget_state(state);
                                break;
                        }
                        if (strchr(state->conf->number_chars, ch)) {
                                prev_special = 1;
                                continue;
                        }
                        /* non-number char */
                        break;
                }
                /* We seem to have a "number" token */
                unget_char(state);
                close_token(state, &tk);
                tk.num = TK_number;
                return tk;
        }

### Words
Words start with a "start" character followed by the longest
sequence of "continue" characters.  The Unicode ID_START and
ID_CONTINUE sets are always permitted, but other ASCII characters
can be added to these sets.

###### token config parameters
        char *word_start;
        char *word_cont;

###### internal functions
        static int is_word_start(wchar_t ch, struct token_config *conf)
        {
                return iswalpha(ch) ||
                       strchr(conf->word_start, ch) != NULL ||
                       u_hasBinaryProperty(ch, UCHAR_ID_START);
        }

        static int is_word_continue(wchar_t ch, struct token_config *conf)
        {
                return iswalnum(ch) ||
                       strchr(conf->word_cont, ch) != NULL ||
                       u_hasBinaryProperty(ch, UCHAR_ID_CONTINUE);
        }

Words can be either known or unknown.  Known words are referred to as
"reserved words" and get a unique token number.  Unknown words are
"identifiers" and are syntactically a single token.

###### token types
        TK_ident,

A list of known words must be provided.  This list is shared with the
"marks" which are described next.  The list must be lexically sorted
and the length of the list must be given (`known_count`).
Tokens matching these known words are reported as the index of the
list added to `TK_reserved`.

###### token config parameters
        char **words_marks;
        int known_count;

###### parse word

        if (is_word_start(ch, state->conf)) {
                int n;
                /* A word: identifier or reserved */
                do
                        ch = get_char(state);
                while (is_word_continue(ch, state->conf));
                unget_char(state);
                close_token(state, &tk);
                tk.num = TK_ident;
                if (ignored & (1<<TK_ident))
                        tk.num = TK_error;
                n = find_known(state->conf, tk.txt);
                if (n >= 0)
                        tk.num = TK_reserved + n;
                return tk;
        }

### Marks

Marks are generally one or more punctuation marks joined together.  It
would be nice to use the term "symbol" for these, but that causes
confusion in a subsequent discussion of the grammar, which has terminal
symbols and non-terminal symbols which are conceptually quite
different.  So strings of punctuation characters will be marks.

A "mark" consists of ASCII characters that are not white space, are not
"start" characters for words, and are not digits.
These will collectively be called mark characters.

###### internal functions
        static int is_mark(wchar_t ch, struct token_config *conf)
        {
                return ch > ' ' &&
                       ch < 0x7f &&
                       !iswalnum(ch) &&
                       strchr(conf->word_start, ch) == NULL;
        }

As with words, there can be known and unknown marks, though the rules
are slightly different.

Two marks do not need to be separated by a non-mark characters.  This
is different from words which do need to be separated by at least one
non-continue character.

The scanner will normally prefer longer sequences of mark characters,
but will more strongly prefer known marks over unknown marks.  So if
it finds a known mark where adding one more character does not result
in a known mark, it will return that first known mark.

If no known mark is found we will test against strings and comments
below before giving up and assuming an unknown mark.
If `TK_mark` is ignored, then unknown marks as returned as an error.

###### token types
        TK_mark,

Known marks are included in the same list as the list of known words.

###### parse mark
        tk.num = TK_error;
        while (is_mark(ch, state->conf)) {
                int n;
                close_token(state, &tk);
                n = find_known(state->conf, tk.txt);
                if (n >= 0)
                        tk.num = TK_reserved + n;
                else if (tk.num != TK_error) {
                        /* found a longest-known-mark */
                        unget_char(state);
                        close_token(state, &tk);
                        return tk;
                }
                ch = get_char(state);
        }
        unget_char(state);
        if (tk.num != TK_error)
                return tk;

###### unknown mark
        if (tk.txt.len) {
                if (ignored & (1<<TK_mark))
                        tk.num = TK_error;
                else
                        tk.num = TK_mark;
                return tk;
        }

### Strings

Strings start with one of single quote, double quote, or back quote
and continue until a matching character on the same line.  Any of
these characters can be included in the list of known marks and then
they will not be used for identifying strings.

Immediately following the close quote one or two ASCII letters may
appear.  These are somewhat like the arbitrary letters allowed in
"Numbers" above.  They can be used by the language in various ways.

If 3 identical quote characters appear in a row and are
followed by a newline, then this forms a multi-line string which
continues until an identical triple quote appears on a line preceded
only by whitespace and followed immediately by 0-2 ASCII letters and a newline.

Multi-line strings may not extend beyond the end of the `code_node` in
which they start.

Normal strings and multi-line strings are encoded as two different
token types.

###### token types
        TK_string,
        TK_multi_string,

###### internal functions
        static int is_quote(wchar_t ch)
        {
                return ch == '\'' || ch == '"' || ch == '`';
        }

#### Multi-line strings

The multi-line string is checked for first.  If they are being
ignored, we fall through and treat a triple quote as an empty string
followed by the start of a new string.

###### parse string
        if (tk.txt.len == 3 &&
            !(ignored & (1 << TK_multi_string)) &&
            is_quote(tk.txt.txt[0]) &&
            memcmp(tk.txt.txt, tk.txt.txt+1, 2) == 0 &&
            is_newline(tk.txt.txt[3])) {
                // triple quote
                wchar_t first = tk.txt.txt[0];
                int qseen = 0;
                int at_sol = 1;
                while (!at_eon(state) && qseen < 3) {
                        ch = get_char(state);
                        if (is_newline(ch)) {
                                at_sol = 1;
                                qseen = 0;
                        } else if (at_sol && ch == first) {
                                qseen += 1;
                        } else if (ch != ' ' && ch != '\t') {
                                at_sol = 0;
                                qseen = 0;
                        }
                }
                if (qseen != 3) {
                        /* Hit end of node - error.
                         * unget so the newline is seen,
                         * but return rest of string as an error.
                         */
                        unget_char(state);
                        close_token(state, &tk);
                        tk.num = TK_error;
                        return tk;
                }
                /* 2 letters are allowed */
                ch = get_char(state);
                if (iswalpha(ch))
                        ch = get_char(state);
                if (iswalpha(ch))
                        ch = get_char(state);
                /* Now we must have a newline, but we don't return it
                 * whatever it is.*/
                unget_char(state);
                close_token(state, &tk);
                tk.num = TK_multi_string;
                if (!is_newline(ch))
                        tk.num = TK_error;
                return tk;
        }

#### Single-line strings

The sequence of marks collected may be more than a single-line
string, so we reset to the start and collect characters until
we find a close quote or a newline.

If `TK_string` is ignored, then quote characters will appear as `TK_mark`s.

###### parse string
        if (tk.txt.len && is_quote(tk.txt.txt[0]) &&
            !(ignored & (1<<TK_string))) {
                wchar_t first = tk.txt.txt[0];
                reset_token(state, &tk);
                get_char(state);
                do
                        ch = get_char(state);
                while (ch != first && !is_newline(ch));
                tk.num = TK_string;
                if (is_newline(ch)) {
                        unget_char(state);
                        tk.num = TK_error;
                }
                close_token(state, &tk);
                return tk;
        }

### Comments

Single line comments may start with '`//`' or '`#`' providing that these
are not known marks.  They continue to the end of the line.

Block comments start with '`/*`' if this is not a known mark.  They
continue to the first occurrence of '`*/`' and may not contain any
occurrence of '`/*`'.

Block comments can be wholly within one line or can continue over
multiple lines.  The multi-line version should be followed immediately
by a newline.  The Linux kernel contains over 285000 multi-line
comments are only 34 are followed by characters other than white space
(which should be removed) or a backslash (only needed in macros).  So
it would not suffer from this rule.

These two comment types are reported as two separate token types, and
consequently can be ignored separately.  When ignored a comment is
parsed and discarded.

###### token types
        TK_line_comment,
        TK_block_comment,

###### internal functions
        static int is_line_comment(struct text txt)
        {
                return (txt.len >= 1 && txt.txt[0] == '#') ||
                       (txt.len >= 2 && txt.txt[0] == '/' &&
                                        txt.txt[1] == '/');
        }

        static int is_block_comment(struct text txt)
        {
                return txt.len >= 2 && txt.txt[0] == '/' &&
                       txt.txt[1] == '*';
        }

#### Single line comments

A single-line comment continues up to, but not including the newline.

###### parse comment

        if (is_line_comment(tk.txt)) {
                while (!is_newline(ch))
                        ch = get_char(state);
                unget_char(state);
                close_token(state, &tk);
                tk.num = TK_line_comment;
                if (ignored & (1 << TK_line_comment))
                        continue;
                return tk;
        }

#### Block comments

The token text collected so far could exceed the comment, so we need
to reset it first.

If we find an embedded `/*` we reset to just before the '/' and report
an error.  That way the next thing to be parsed will be the rest of
the comment.  This requires a double unget, so we need to save/restore
the unget state (explained later).

###### parse comment

        if (is_block_comment(tk.txt)) {
                wchar_t prev;
                int newlines = 0;
                reset_token(state, &tk);
                get_char(state);
                get_char(state);
                save_unget_state(state);
                ch = get_char(state);
                prev = 0;
                while (!at_eon(state) &&
                       (prev != '/' || ch != '*') &&
                       (prev != '*' || ch != '/')) {
                        if (is_newline(ch))
                                newlines = 1;
                        prev = ch;
                        save_unget_state(state);
                        ch = get_char(state);
                }
                close_token(state, &tk);
                if (at_eon(state)) {
                        tk.num = TK_error;
                        return tk;
                }
                if (prev == '/') {
                        /* embedded.  Need to unget twice! */
                        restore_unget_state(state);
                        unget_char(state);
                        tk.num = TK_error;
                        return tk;
                }
                tk.num = TK_block_comment;
                if (newlines && !(ignored & (1<<TK_newline))) {
                        /* next char must be newline */
                        ch = get_char(state);
                        unget_char(state);
                        if (!is_newline(ch))
                                tk.num = TK_error;
                }
                if (tk.num == TK_error ||
                    !(ignored & (1 << TK_block_comment)))
                        return tk;
                continue;
        }

### Indents, Newlines, and White Space.

Normally white space is ignored.  However newlines can be important as
can indents, which are either after a newline or at the start of a
node (detected by `at_son()`);

###### exported functions
        static inline int is_newline(wchar_t ch)
        {
                return ch == '\n' || ch == '\f' || ch == '\v';
        }

###### white space
        if (ch <= ' ' && !is_newline(ch)
            && ! at_son(state))
                continue;

If a line starts with more white-space than the previous non-blank
line - or if the first non-blank line in the document starts with any
white-space - then an Indent is reported at the start of the line.

Before the next non-blank line which starts with less white space, or
at the latest at the end of the document, a matching Undent token
is reported.  There will always be an exact match between Indent and
Undent tokens.

It is possible for Undent to be followed (almost) immediately by an
Indent.  This happens if, for example, the indent of three consecutive
lines are 0, 8, 4 spaces.  Before the second line we report an
Indent.  Before the third line we must report an Undent, as 4 is less
than 8, then also an Ident as 4 is greater than 0.

###### token types
        TK_indent,
        TK_undent,

For the purpose of measuring the length of white space, a tab adds at
least one space, and rounds up to a multiple of 8.

###### exported functions
        static inline int indent_tab(int indent)
        {
                return (indent|7)+1;
        }

We need to track the current levels of indent.  This requires some
sort of stack as indent levels are pushed on and popped off.  In
practice this stack is unlikely to often exceed 5 so we will used a
fixed stack of 20 indent levels.  More than this will be silently
ignored.

###### state fields
        int     indent_level;
        int     indent_sizes[20];

#### Newlines

Newlines can optionally be reported.  Newlines within a block comment
or a multi-line string are not reported separately, but each of these
must be followed immediately by a newline so these constructs cannot
hide the fact that a newline was present.

When Indents are being reported, the Newline which would normally be
reported immediately before the Indent is delayed until after the
matching undent.  This makes an indented section act like a
continuation of the previous line to some extent.

A blank line would normally be reported simply as two consecutive Newline
tokens.  However if the subsequent line is indented (and indents are being
reported) then the right thing to do is less obvious as Newlines should be
delayed - but how many Newlines?

The approach we will take is to report the extra Newlines immediately after
the Indent token, so the blank line is treated as though it were an indented
blank line.

###### token types
        TK_newline,

If we find a newline or white space at the start of a block, we keep
collecting spaces, tabs, and newlines until we find some real text.
Then depending on the indent we generate some number of tokens.  These
will be a sequence of "Newline Undent" pairs representing a decrease
in indent, then either a Newline or an Indent depending on whether the
next line is indented, then zero or more Newlines representing all the
blank lines that have been skipped.

When a Newline leads to the next block of code there is a question of
whether the various Newline and Undent/Indent tokens should appear to
pbelong to the earlier or later block.  This is addressed by processing
the tokens in two stages based on the relative indent levels of the
two blocks (each block has a base indent to which the actual indents
are added).

Any "Newline Undent" pairs needed to reduce the current indent to the
maximum of the base indents of the old and new blocks are generated
against the old block.  Then if the next block does not have an
increased indent, one more "Newline" is generated.

If further "Newline Undent" pairs are needed to get to the indent
level of the 'next' block, they are generated against that block,
though the first Newline is suppressed (it having already been
generated).

Finally the Newline or Indent for the first line of the new block is
generated, unless the Newline needs to be suppressed because it
appeared at the end of the previous block.

This means that a block may start with an Undent or an Indent, but
will only start with a Newline if it actually starts with a blank
line.

We will need to represent in the `token_state` where in this sequence
of delayed tokens we are.  As `state.col` records the target indent we
don't need to record how many undents or indents are needed.  We do
need to record the number of blank lines, and which of Newline and
Undent is needed next in the initial sequence of pairs.

For this we store one more than the number of blank lines as
`delayed_lines` and a flag for `undent_next`.

###### state fields
        int check_indent;
        int delayed_lines;
        int undent_next;

Generating these tokens involve two separate pieces of code.

Firstly we need to recognise white space and count the indents and
newlines.  These are recorded in the above state fields.

Separately we need, on each call to `token_next`, we need to check if
there are some delayed tokens and if so we need to advance the state
information and return one token.

###### white space
        if (is_newline(ch) || (at_son(state) && ch <= ' ')) {
                int newlines = 0;
                int was_son = at_son(state);
                if (ignored & (1<<TK_indent)) {
                        if (!is_newline(ch))
                                continue;
                        if (ignored & (1<<TK_newline))
                                continue;
                        tk.num = TK_newline;
                        return tk;
                }
                // Indents are needed, so check all white space.
                while (ch <= ' ' && !at_eon(state)) {
                        if (is_newline(ch))
                                newlines += 1;
                        ch = get_char(state);
                }
                if (at_eon(state)) {
                        newlines += 1;
                        if (state->node->next &&
                            state->node->next->indent > state->node->indent)
                                state->col = state->node->next->indent;
                        else
                                state->col = state->node->indent;
                } else
                        unget_char(state);
                state->delayed_lines = newlines;
                state->undent_next = was_son;
                state->check_indent = 1;
                continue;
        }


###### delayed tokens

        if (state->check_indent || state->delayed_lines) {
                if (state->col < state->indent_sizes[state->indent_level]) {
                        if (!state->undent_next &&
                            !(ignored & (1<<TK_newline))) {
                                state->undent_next = 1;
                                tk.num = TK_newline;
                                return tk;
                        }
                        state->indent_level -= 1;
                        state->undent_next = 0;
                        tk.num = TK_undent;
                        return tk;
                }
                if (state->col > state->indent_sizes[state->indent_level] &&
                    state->indent_level < sizeof(state->indent_sizes)-1) {
                        state->indent_level += 1;
                        state->indent_sizes[state->indent_level] = state->col;
                        state->delayed_lines -= 1;
                        tk.num = TK_indent;
                        return tk;
                }
                state->check_indent = 0;
                if (state->delayed_lines && !(ignored & (1<<TK_newline))) {
                        tk.num = TK_newline;
                        state->delayed_lines -= 1;
                        return tk;
                }
                state->delayed_lines = 0;
                continue;
        }

### End of File

After the last newline in the file has been processed, a special
end-of-file token will be returned.  any further attempts to get more
tokens will continue to return the same end-of-file token.

###### token types
        TK_eof,


###### white space
        if (ch == WEOF) {
                tk.num = TK_eof;
                return tk;
        }

### Unknown Marks, or errors.

We have now handled all the possible known mark-like tokens.
If the token we have is not empty and `TK_mark` is allowed,
we have an unknown mark, otherwise this must be an error.

###### unknown mark
        /* one unknown character */
        close_token(state, &tk);
        tk.num = TK_error;
        return tk;

## Tools For The Task

You may have noticed that are few gaps we left in the above -
functions used without first defining them.  Doing so above would have
broken the flow.

### Character by character

As we walk through the various `code_node`s we need to process whole
Unicode codepoints, and keep track of which line and column we are on.
We will assume for now that any printing character uses one column,
though that is not true in general.

As the text in a `code_node` may include an indent that identifies it as
being code, we need to be careful to strip that.  The `code_node` has
a flag that tells us whether or not we need to strip.

###### includes
        #include <memory.h>

###### state fields
        struct code_node *node;
        int    offset;
        int    line;
        int    col;

###### internal functions

        static void do_strip(struct token_state *state)
        {
                if (state->node->needs_strip) {
                        int n = 4;
                        while (n && state->node->code.txt[state->offset] == ' ') {
                                state->offset += 1;
                                n -= 1;
                        }
                        while (n == 4 && state->node->code.txt[0] == '\t') {
                                state->offset += 1;
                                n -= 4;
                        }
                }
        }

        static wint_t get_char(struct token_state *state)
        {
                wchar_t next;
                size_t n;
                mbstate_t mbstate;

                if (state->node == NULL)
                        return WEOF;
                if (state->node->code.len <= state->offset) {
                        do
                                state->node = state->node->next;
                        while (state->node && state->node->code.txt == NULL);
                        state->offset = 0;
                        if (state->node == NULL)
                                return WEOF;
                        do_strip(state);
                        state->line = state->node->line_no;
                        state->col = state->node->indent;
                }

                ## before get_char

                memset(&mbstate, 0, sizeof(mbstate));

                n = mbrtowc(&next, state->node->code.txt + state->offset,
                            state->node->code.len - state->offset,
                            &mbstate);
                if (n == -2 || n == 0) {
                        /* Not enough bytes - not really possible */
                        next = '\n';
                        state->offset = state->node->code.len;
                } else if (n == -1) {
                        /* error */
                        state->offset += 1;
                        next = 0x7f; // an illegal character
                } else
                        state->offset += n;

                if (next >= ' ') {
                        state->col += 1;
                } else if (is_newline(next)) {
                        state->line += 1;
                        state->col = state->node->indent;
                        do_strip(state);
                } else if (next == '\t') {
                        state->col = indent_tab(state->col);
                }
                return next;
        }

We will sometimes want to "unget" the last character as it needs to be
considered again as part of the next token.  So we need to store a
'previous' version of all metadata.

###### state fields
        int    prev_offset;
        int    prev_line;
        int    prev_col;

###### before get_char
        state->prev_offset = state->offset;
        state->prev_line   = state->line;
        state->prev_col    = state->col;

###### internal functions

        static void unget_char(struct token_state *state)
        {
                if (state->node) {
                        state->offset = state->prev_offset;
                        state->line   = state->prev_line;
                        state->col    = state->prev_col;
                }
        }

We occasionally need a double-unget, particularly for numbers and
block comments.  We don't impose this cost on all scanning, but
require those code sections that need it to call `save_unget_state`
before each `get_char`, and then `restore_unget_state` when a
double-unget is needed.

###### state fields
        int     prev_offset2;
        int     prev_line2;
        int     prev_col2;

###### internal functions
        static void save_unget_state(struct token_state *state)
        {
                state->prev_offset2 = state->prev_offset;
                state->prev_line2 = state->prev_line;
                state->prev_col2 = state->prev_col;
        }

        static void restore_unget_state(struct token_state *state)
        {
                state->prev_offset = state->prev_offset2;
                state->prev_line = state->prev_line2;
                state->prev_col = state->prev_col2;
        }

At the start of a token we don't want to be at the end of a code block
if we can help it.  To avoid this possibility, we 'get' and 'unget' a
single character.  This will move into the next non-empty code block
and leave the current pointer at the start of it.

This has to happen _after_ dealing with delayed tokens as some of them
must appear in the previous node.  When we do this, we need to reset
the data in the token.

###### delayed tokens
        if (at_eon(state)) {
                get_char(state);
                unget_char(state);
                tk.node = state->node;
                if (state->node)
                        tk.txt.txt = state->node->code.txt + state->offset;
                tk.line = state->line;
                tk.col = state->col;
                tk.txt.len = 0;
        }

### Managing tokens

The current token is initialized to line up with the first character
that we 'get' for each token.  When we have, or might have, a full
token we can call `close_token` to set the `len` of the token
appropriately.  This can safely be called multiple times.

Finally we occasionally (for single-line strings and block comments)
need to reset to the beginning of the current token as we might have
parsed too much already.  For that there is `reset_token`.

###### one token
        tk.node = state->node;
        if (state->node)
                tk.txt.txt = state->node->code.txt + state->offset;
        tk.line = state->line;
        tk.col = state->col;
        tk.txt.len = 0;

###### internal functions

        static void close_token(struct token_state *state,
                                struct token *tk)
        {
                tk->txt.len = (state->node->code.txt + state->offset)
                              - tk->txt.txt;
        }

        static void reset_token(struct token_state *state, struct token *tok)
        {
                state->prev_line = tok->line;
                state->prev_col = tok->col;
                state->prev_offset = tok->txt.txt - state->node->code.txt;
                unget_char(state);
                tok->txt.len = 0;
        }


Tokens make not cross into the next `code_node`, and some tokens can
include the newline at the and of a `code_node`, we must be able to
easily check if we have reached the end.  Equally we need to know if
we are at the start of a node, as white space is treated a little
differently there.

###### internal functions

        static int at_son(struct token_state *state)
        {
                return state->offset == 0;
        }

        static int at_eon(struct token_state *state)
        {
                // at end-of-node ??
                return state->node == NULL ||
                       state->offset >= state->node->code.len;
        }

### Find a known word

As the known-word list is sorted we can use a simple binary search.
Following the pattern established in "mdcode", we will use a `struct
text` with start and length to represent the code fragment we are
searching for.

###### internal functions
        static int find_known(struct token_config *conf, struct text txt)
        {
                int lo = 0;
                int hi = conf->known_count;

                while (lo + 1 < hi) {
                        int mid = (lo + hi) / 2;
                        int cmp = strncmp(conf->words_marks[mid],
                                          txt.txt, txt.len);
                        if (cmp == 0 && conf->words_marks[mid][txt.len])
                                cmp = 1;
                        if (cmp <= 0)
                                lo = mid;
                        else
                                hi = mid;
                }
                if (strncmp(conf->words_marks[lo],
                           txt.txt, txt.len) == 0
                    && conf->words_marks[lo][txt.len] == 0)
                        return lo;
                else
                        return -1;
        }

### Bringing it all together

Now we have all the bits there is just one section missing:  combining
all the token parsing code into one block.

The handling of delayed tokens (newlines, indents, undents) must come
first before we try getting another character.

Then we parse all the test, making sure that we check for known marks
before strings and comments, but unknown marks after strings and comments.

This block of code will either return a token, or will choose to
ignore one, in which case it will `continue` around to the top of the
loop.

###### one token
        ## delayed tokens

        ch = get_char(state);

        ## white space
        ## parse number
        ## parse word
        ## parse mark
        ## parse string
        ## parse comment
        ## unknown mark

### Start and stop

As well as getting tokens, we need to be able to create the
`token_state` to start with, and discard it later.

###### includes
        #include <malloc.h>

###### main functions
        struct token_state *token_open(struct code_node *code, struct
                                       token_config *conf)
        {
                struct token_state *state = malloc(sizeof(*state));
                memset(state, 0, sizeof(*state));
                state->node = code;
                state->line = code->line_no;
                state->conf = conf;
                return state;
        }
        void token_close(struct token_state *state)
        {
                free(state);
        }

###### exported functions
        struct token_state *token_open(struct code_node *code, struct
                                       token_config *conf);
        void token_close(struct token_state *state);

### Trace tokens

Getting tokens is the main thing but it is also useful to be able to
print out token information, particularly for tracing and testing.

Known tokens are printed verbatim.  Other tokens are printed as
`type(content)` where content is truncated to a given number of characters.

The function for printing a truncated string (`text_dump`) is also exported
so that it can be used to tracing processed strings too.

###### includes
        #include <stdio.h>

###### exported functions
        void token_trace(FILE *f, struct token tok, int max);
        void text_dump(FILE *f, struct text t, int max);

###### main functions

        void text_dump(FILE *f, struct text txt, int max)
        {
                int i;
                if (txt.len > max)
                        max -= 2;
                else
                        max = txt.len;
                for (i = 0; i < max; i++) {
                        char c = txt.txt[i];
                        if (c < ' ' || c > '~')
                                fprintf(f, "\\x%02x", c & 0xff);
                        else if (c == '\\')
                                fprintf(f, "\\\\");
                        else
                                fprintf(f, "%c", c);
                }
                if (i < txt.len)
                        fprintf(f, "..");
        }

        void token_trace(FILE *f, struct token tok, int max)
        {
                static char *types[] = {
                        [TK_ident] = "ident",
                        [TK_mark] = "mark",
                        [TK_number] = "number",
                        [TK_string] = "string",
                        [TK_multi_string] = "mstring",
                        [TK_line_comment] = "lcomment",
                        [TK_block_comment] = "bcomment",
                        [TK_indent] = "indent",
                        [TK_undent] = "undent",
                        [TK_newline] = "newline",
                        [TK_eof] = "eof",
                        [TK_error] = "ERROR",
                        };

                switch (tok.num) {
                default: /* known word or mark */
                        fprintf(f, "%.*s", tok.txt.len, tok.txt.txt);
                        break;
                case TK_indent:
                case TK_undent:
                case TK_newline:
                case TK_eof:
                        /* No token text included */
                        fprintf(f, "%s()", types[tok.num]);
                        break;
                case TK_ident:
                case TK_mark:
                case TK_number:
                case TK_string:
                case TK_multi_string:
                case TK_line_comment:
                case TK_block_comment:
                case TK_error:
                        fprintf(f, "%s(", types[tok.num]);
                        text_dump(f, tok.txt, max);
                        fprintf(f, ")");
                        break;
                }
        }

### And there we have it

We now have all the library functions defined for reading and printing
tokens.  Now we just need C files to store them, and a mk file to make them.

###### File: scanner.h
        ## public types
        ## exported functions

###### File: libscanner.c
        ## includes
        #include "scanner.h"
        ## private types
        ## internal functions
        ## main functions

###### File: scanner.mk

        CFLAGS += -Wall -g
        all ::
        scanner.mk scanner.h libscanner.c : scanner.mdc
                ./md2c scanner.mdc
        all :: libscanner.o
        libscanner.o : libscanner.c
                $(CC) $(CFLAGS) -c libscanner.c

## Processing numbers

Converting a `TK_number` token to a numerical value is a slightly
higher level task than lexical analysis, and slightly lower than
grammar parsing, so put it here - as an index if you like.

Importantly it will be used by the same testing rig that is used for
testing the token scanner.

The numeric value that we will convert all numbers into is the `mpq_t`
from the GNU high precision number library "libgmp".

###### number includes
        #include <gmp.h>
        #include "mdcode.h"

Firstly we need to be able to parse a string of digits in a given base
and possibly with a decimal marker.  We store this in an `mpz_t`
integer and report the number of digits after the decimal mark.

On error we return zero and ensure that the 'mpz_t' has been freed, or
had never been initialised.

###### number functions

        static int parse_digits(mpz_t num, struct text tok, int base,
                                int *placesp)
        {
                /* Accept digits up to 'base', ignore '_' and
                 * ' ' if they appear between two legal digits,
                 * and if `placesp` is not NULL, allow a single
                 * '.' or ',' and report the number of digits
                 * beyond there.
                 * Return number of characters processed (p),
                 * or 0 if something illegal was found.
                 */
                int p;
                int decimal = -1; // digits after marker
                enum {Digit, Space, Other} prev = Other;
                int digits = 0;

                for (p = 0; p < tok.len; p++) {
                        int dig;
                        char c = tok.txt[p];

                        if (c == '_' || c == ' ') {
                                if (prev != Digit)
                                        goto bad;
                                prev = Space;
                                continue;
                        }
                        if (c == '.' || c == ',') {
                                if (prev != Digit)
                                        goto bad;
                                if (!placesp || decimal >= 0)
                                        return p-1;
                                decimal = 0;
                                prev = Other;
                                continue;
                        }
                        if (isdigit(c))
                                dig = c - '0';
                        else if (isupper(c))
                                dig = 10 + c - 'A';
                        else if (islower(c))
                                dig = 10 + c - 'a';
                        else
                                dig = base;
                        if (dig >= base) {
                                if (prev == Space)
                                        p--;
                                break;
                        }
                        prev = Digit;
                        if (digits)
                                mpz_mul_ui(num, num, base);
                        else
                                mpz_init(num);
                        digits += 1;
                        mpz_add_ui(num, num, dig);
                        if (decimal >= 0)
                                decimal++;
                }
                if (digits == 0)
                        return 0;
                if (placesp) {
                        if (decimal >= 0)
                                *placesp = decimal;
                        else
                                *placesp = 0;
                }
                return p;
        bad:
                if (digits)
                        mpz_clear(num);
                return 0;
        }

###### number includes
        #include <ctype.h>

To parse a full number we need to consider the optional base, the
mantissa, and the optional exponent.  We will treat these one at a
time.

The base is indicated by a letter after a leading zero, which must be
followed by a base letter or a period.  The base also determines the
character which will mark an exponent.

###### number vars
        int base = 10;
        char expc = 'e';

###### parse base

        if (tok.txt[0] == '0' && tok.len > 1) {
                int skip = 0;
                switch(tok.txt[1]) {
                case 'x':
                case 'X':
                        base = 16;
                        skip = 2;
                        expc = 'p';
                        break;
                case 'o':
                case 'O':
                        base = 8;
                        skip = 2;
                        expc = 'p';
                        break;
                case 'b':
                case 'B':
                        base = 2;
                        skip = 2;
                        expc = 'p';
                        break;
                case '0':
                case '1':
                case '2':
                case '3':
                case '4':
                case '5':
                case '6':
                case '7':
                case '8':
                case '9':
                case '_':
                case ' ':
                        // another digit is not permitted
                        // after a zero.
                        return 0;
                default:
                        // must be decimal marker or trailing
                        // letter, which are OK;
                        break;
                }
                tok.txt += skip;
                tok.len -= skip;
        }

After the base is the mantissa, which may contain a decimal mark, so
we need to record the number of places.  We won't impose the number of
places until we have the exponent as well.

###### number vars
        int places =0;
        mpz_t mant;
        int d;

###### parse mantissa

        d = parse_digits(mant, tok, base, &places);
        if (d == 0)
                return 0;
        tok.txt += d;
        tok.len -= d;
        mpq_init(num);
        mpq_set_z(num, mant);
        mpz_clear(mant);

After the mantissa number may come an exponent which may be positive
or negative.  We assume at this point that we have seen the exponent
character `expc`.

###### number vars
        long lexp = 0;
        mpz_t exp;
        int esign = 1;

###### parse exponent
        if (tok.len > 1) {
                if (tok.txt[0] == '+') {
                        tok.txt++;
                        tok.len--;
                } else if (tok.txt[0] == '-') {
                        esign = -1;
                        tok.txt++;
                        tok.len--;
                }
        }
        d = parse_digits(exp, tok, 10, NULL);
        if (d == 0) {
                mpq_clear(num);
                return 0;
        }
        if (!mpz_fits_slong_p(exp)) {
                mpq_clear(num);
                mpz_clear(exp);
                return 0;
        }
        lexp = mpz_get_si(exp) * esign;
        mpz_clear(exp);
        tok.txt += d;
        tok.len -= d;


Now that we have the mantissa and the exponent we can multiply them
together, also allowing for the number of digits after the decimal
mark.

For base 10, we simply subtract the decimal places from the exponent.
For the other bases, as the exponent is alway based on 2, even for
octal and hex, we need a bit more detail.
We then recover the sign from the exponent, as division is quite
different from multiplication.

###### calc exponent
        switch (base) {
        case 10:
        case 2:
                lexp -= places;
                break;
        case 16:
                lexp -= 4*places;
                break;
        case 8:
                lexp -= 3*places;
                break;
        }
        if (lexp < 0) {
                lexp = -lexp;
                esign = -1;
        } else
                esign = 1;

Imposing the exponent on the number is also very different for base 10
than for the others.  For the binary shift `gmp` provides a simple
function.  For base 10 we use something like Russian Peasant
Multiplication.

###### calc exponent
        if (expc == 'e') {
                mpq_t tens;
                mpq_init(tens);
                mpq_set_ui(tens, 10, 1);
                while (1) {
                        if (lexp & 1) {
                                if (esign > 1)
                                        mpq_mul(num, num, tens);
                                else
                                        mpq_div(num, num, tens);
                        }
                        lexp >>= 1;
                        if (lexp == 0)
                                break;
                        mpq_mul(tens, tens, tens);
                }
                mpq_clear(tens);
        } else {
                if (esign > 0)
                        mpq_mul_2exp(num, num, lexp);
                else
                        mpq_div_2exp(num, num, lexp);
        }

Now we are ready to parse a number: the base, mantissa, and exponent.
If all goes well we check for the possible trailing letters and
return.  Return value is 1 for success and 0 for failure.


###### number functions
        int number_parse(mpq_t num, char tail[3], struct text tok)
        {
                ## number vars
                int i;

                ## parse base
                ## parse mantissa
                if (tok.len > 1 && (tok.txt[0] == expc ||
                                    tok.txt[0] == toupper(expc))) {
                        tok.txt++;
                        tok.len--;
                        ## parse exponent
                }
                ## calc exponent

                for (i = 0; i < 2; i++) {
                        if (tok.len <= i)
                                break;
                        if (!isalpha(tok.txt[i]))
                                goto err;
                        tail[i] = tok.txt[i];
                }
                tail[i] = 0;
                if (i == tok.len)
                        return 1;
        err:
                mpq_clear(num);
                return 0;
        }

Number parsing goes in `libnumber.c`

###### File: libnumber.c

        #include <unistd.h>
        #include <stdlib.h>

        ## number includes
        ## number functions

###### File: number.h
        int number_parse(mpq_t num, char tail[3], struct text tok);

###### File: scanner.mk
        all :: libnumber.o
        libnumber.o : libnumber.c
                $(CC) $(CFLAGS) -c libnumber.c

## Processing strings

Both `TK_string` and `TK_multi_string` require post-processing which
can be one of two types: literal or with escapes processed.
Even literal processing is non-trivial as the file may contain indents
which need to be stripped.

Errors can only occur when processing escapes.  Any unrecognised
character following the escape character will cause an error.

Processing escapes and striping indents can only make the string
shorter, not longer, so we allocate a buffer which is the same size as
the string and process into that.

To request escape processing, we pass the character we want to use for
quoting, usually '`\`'.  To avoid escape processing we pass a zero.

###### string main
        int string_parse(struct token *tok, char escape,
                         struct text *str, char tail[3])
        {
                ## string vars
                struct text t = tok->txt;

                str->txt = NULL;
                ## strip tail
                if (tok->num == TK_string) {
                        ## strip single
                } else {
                        ## strip multi
                }
                str->txt = malloc(t.len);
                str->len = 0;

                ## process string
                return 1;
        err:
                free(str->txt);
                str->txt = NULL;
                return 0;
        }

### strip tail

The tail of the string can be 0, 1, or 2 letters

        i = t.len;
        if (i >= 0 && isalpha(t.txt[i-1]))
                i -= 1;
        if (i >= 0 && isalpha(t.txt[i-1]))
                i -= 1;
        strncpy(tail, t.txt+i, t.len-i);
        tail[t.len-i] = 0;
        t.len = i;

###### string vars
        int i;

### strip single

Stripping the quote of a single-line string is trivial.
The only part that is at all interesting is that quote character must
be remembered.

        quote = t.txt[0];
        if (t.txt[t.len-1] != quote)
                goto err;
        t.txt += 1;
        t.len -= 2;

###### string vars
        char quote;

### strip multi

For a multi-line string we have a little more work to do.  We need to
remove 3 quotes, not 1, and need to count the indent of the close
quote as it will need to be stripped from all lines.

        quote = t.txt[0];
        if (t.len < 7 ||
            t.txt[1] != quote || t.txt[2] != quote ||
            !is_newline(t.txt[3]))
                goto err;
        t.txt += 4;
        t.len -= 4;
        i = t.len;
        if (i <= 0 || t.txt[i-1] != quote)
                goto err;
        i -= 1;
        if (i <= 0 || t.txt[i-1] != quote)
                goto err;
        i -= 1;
        if (i <= 0 || t.txt[i-1] != quote)
                goto err;
        i -= 1;
        t.len = i;
        while (i > 0 && !is_newline(t.txt[i-1]))
                i--;
        indent = 0;
        while (i < t.len) {
                if (t.txt[i] == ' ')
                        indent += 1;
                if (t.txt[i] == '\t')
                        indent = indent_tab(indent);
                i++;
        }

###### string vars
        int indent = 0;

### process string

Now we just take one byte at a time. trans-ASCII unicode won't look
like anything we are interested in so it will just be copied byte by
byte.

        cp = str->txt;
        at_sol = 1;
        for (i = 0; i < t.len; i++) {
                char c;
                if (at_sol) {
                        at_sol = 0;
                        ## strip indent
                        if (i >= t.len)
                                break;
                }
                c = t.txt[i];
                if (c != escape) {
                        *cp = c;
                        cp += 1;
                        if (is_newline(c))
                                at_sol = 1;
                } else if (i+1 >= t.len) {
                        // escape and end of string
                        goto err;
                } else {
                        i += 1;
                        c = t.txt[i];
                        ## parse escape
                }
        }
        str->len = cp - str->txt;

###### string vars
        char *cp;
        int at_sol;

### strip indent

Every time we find a start of line, we strip spaces and tabs until the
required indent is found.

        int skipped = 0;
        while (i < t.len && skipped < indent) {
                c = t.txt[i];
                if (c == ' ')
                        skipped += 1;
                else if (c == '\t')
                        skipped = indent_tab(c);
                else
                        break;
                i+= 1;
        }

### parse escape
        switch (c) {
        case 'n':
                *cp++ = '\n'; break;
        case 'r':
                *cp++ = '\r'; break;
        case 't':
                *cp++ = '\t'; break;
        case 'b':
                *cp++ = '\b'; break;
        case 'q':
                *cp++ = quote; break;
        case 'f':
                *cp++ = '\f'; break;
        case 'v':
                *cp++ = '\v'; break;
        case 'a':
                *cp++ = '\a'; break;
        case '0':
        case '1':
        case '2':
        case '3':
                // 3 digit octal number
                if (i+2 >= t.len)
                        goto err;
                if (t.txt[i+1] < '0' || t.txt[i+1] > '7' ||
                    t.txt[i+2] < '0' || t.txt[i+1] > '7')
                        goto err;
                n = (t.txt[i  ]-'0') * 64 +
                    (t.txt[i+1]-'0') *  8 +
                    (t.txt[i+2]-'0') *  1;
                *cp++ = n;
                i += 2;
                break;
        case 'x':
                // 2 hex digits
                n = take_hex(2, t.txt+i+1, t.len-i-1);
                if (n < 0)
                        goto err;
                *cp++ = n;
                i += 2;
                break;
        case 'u':
        case 'U':
                // 4 or 8 hex digits for unicode
                n = take_hex(c == 'u'?4:8, t.txt+i+1, t.len-i-1);
                if (n < 0)
                        goto err;
                memset(&pstate, 0, sizeof(pstate));
                n = wcrtomb(cp, n, &pstate);
                if (n <= 0)
                        goto err;
                cp += n;
                i += c == 'u' ? 4 : 8;
                break;
        default:
                if (c == escape)
                        *cp++ = c;
                else if (is_newline(c))
                        at_sol = 1;
                else
                        goto err;
        }

###### string vars
        long n;
        mbstate_t pstate;

For `\x` `\u` and `\U` we need to collect a specific number of
hexadecimal digits

###### string functions

        static long take_hex(int digits, char *cp, int l)
        {
                long n = 0;
                if (l < digits)
                        return -1;
                while (digits) {
                        char  c = *cp;
                        int d;
                        if (!isxdigit(c))
                                return -1;
                        if (isdigit(c))
                                d = c - '0';
                        else if (isupper(c))
                                d = 10 + c - 'A';
                        else
                                d = 10 + c - 'a';
                        n = n * 16 + d;
                        digits--;
                        cp++;
                }
                return n;
        }

#### File: libstring.c

String parsing goes in `libstring.c`

        #include <unistd.h>
        #include <stdlib.h>
        #include <stdio.h>
        #include <string.h>
        #include <ctype.h>
        #include <wchar.h>
        #include "mdcode.h"
        #include "scanner.h"
        ## string functions
        ## string main

###### File: string.h
        int string_parse(struct token *tok, char escape,
                         struct text *str, char tail[3]);

###### File: scanner.mk
        all :: libstring.o
        libstring.o : libstring.c
                $(CC) $(CFLAGS) -c libstring.c


## Testing

As "untested code is buggy code" we need a program to easily test
the scanner library.  This will simply parse a given file and report
the tokens one per line.

###### File: scanner.c

        #include <unistd.h>
        #include <stdlib.h>
        #include <fcntl.h>
        #include <errno.h>
        #include <sys/mman.h>
        #include <string.h>
        #include <stdio.h>
        #include <gmp.h>
        #include <locale.h>
        #include "mdcode.h"
        #include "scanner.h"
        #include "number.h"
        #include "string.h"

        static int errs;
        static void pr_err(char *msg)
        {
                errs++;
                fprintf(stderr, "%s\n", msg);
        }

        int main(int argc, char *argv[])
        {
                int fd;
                int len;
                char *file;
                struct token_state *state;
                char *known[] = {
                        "==",
                        "else",
                        "if",
                        "then",
                        "while",
                        "{",
                        "}",
                };
                struct token_config conf = {
                        .word_start = "_$",
                        .word_cont = "",
                        .words_marks = known,
                        .number_chars = "., _+-",
                        .known_count = sizeof(known)/sizeof(known[0]),
                        .ignored = (0 << TK_line_comment)
                                  |(0 << TK_block_comment),
                };
                struct section *table, *s, *prev;
                setlocale(LC_ALL,"");
                if (argc != 2) {
                        fprintf(stderr, "Usage: scanner file\n");
                        exit(2);
                }
                fd = open(argv[1], O_RDONLY);
                if (fd < 0) {
                        fprintf(stderr, "scanner: cannot open %s: %s\n",
                                argv[1], strerror(errno));
                        exit(1);
                }
                len = lseek(fd, 0, 2);
                file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
                table = code_extract(file, file+len, pr_err);

                for (s = table; s;
                        (code_free(s->code), prev = s, s = s->next, free(prev))) {
                        printf("Tokenizing: %.*s\n", s->section.len,
                                s->section.txt);
                        state = token_open(s->code, &conf);
                        while(1) {
                                struct token tk = token_next(state);
                                printf("%d:%d ", tk.line, tk.col);
                                token_trace(stdout, tk, 20);
                                if (tk.num == TK_number) {
                                        mpq_t num;
                                        char tail[3];
                                        if (number_parse(num, tail,tk.txt)) {
                                                printf(" %s ", tail);
                                                mpq_out_str(stdout, 10, num);
                                                mpq_clear(num);
                                        } else
                                                printf(" BAD NUMBER");
                                }
                                if (tk.num == TK_string ||
                                    tk.num == TK_multi_string) {
                                        char esc = '\\';
                                        struct text str;
                                        char tail[3];
                                        if (tk.txt.txt[0] == '`')
                                                esc = 0;
                                        if (string_parse(&tk, esc,
                                                         &str, tail)) {
                                                printf(" %s ", tail);
                                                text_dump(stdout, str, 20);
                                                free(str.txt);
                                        } else
                                                printf(" BAD STRING");
                                }
                                printf("\n");
                                if (tk.num == TK_error)
                                        errs = 1;
                                if (tk.num == TK_eof)
                                        break;
                        }
                }
                exit(!!errs);
        }
###### File: scanner.mk
        scanner.c : scanner.mdc
                ./md2c scanner.mdc
        all :: scanner
        scanner : scanner.o scanner.h libscanner.o libmdcode.o mdcode.h
                $(CC) $(CFLAGS) -o scanner scanner.o libscanner.o \
                        libmdcode.o libnumber.o libstring.o -licuuc -lgmp
        scanner.o : scanner.c
                $(CC) $(CFLAGS) -c scanner.c