FWEB version 1.53 (September 23, 1995)
Based on version 0.5 of S. Levy's CWEB [copyright (C) 1987 Princeton University]
@x—————————————————————————–
@* IMPLEMENTING the PRODUCTIONS. For the actual grammar implemented by , see ṗrod.web. This was split off in order to shorten the length of .
More specifically, a scrap is a structure consisting of a category |cat| and a |text_pointer| |trans|, which points to the translation in |tok_start|. When text is to be processed with the grammar above, we form an array |scrap_info| containing the initial scraps. Our production rules have the nice property that the right-hand side is never longer than the left-hand side. Therefore it is convenient to use sequential allocation for the current sequence of scraps. Five pointers are used to manage the parsing:
|pp| is a pointer into |scrap_info|. We will try to match the category codes |pp->cat| |(pp+1)->cat|,...to the left-hand sides of productions.
|scrap_base|, |lo_ptr|, |hi_ptr|, and |scrap_ptr| are such that the current sequence of scraps appears in positions |scrap_base| through |lo_ptr| and |hi_ptr| through |scrap_ptr|, inclusive, in the |cat| and |trans| arrays. Scraps located between |scrap_base| and |lo_ptr| have been examined, while those in positions ≥ |hi_ptr| have not yet been looked at by the parsing process.
Initially |scrap_ptr| is set to the position of the final scrap to be parsed, and it doesn't change its value. The parsing process makes sure that |lo_ptr>=pp+3|, since productions have as many as four terms, by moving scraps from |hi_ptr| to |lo_ptr|. If there are fewer than |pp+3| scraps left, the positions up to |pp+3| are filled with blanks that will not match in any productions. Parsing stops when |pp=lo_ptr+1| and |hi_ptr=scrap_ptr+1|.
Since the |scrap| structure will later be used for other purposes, we declare its second element as unions.
@<Type...@>=
typedef struct eight_bits cat; /* Category. It would be nice to |enum| this, but that would turn it into an |int|, which could be as much as four times bigger. */ eight_bits mathness; union text_pointer Trans; @<Rest of |trans_plus| union@>@; trans_plus; scrap;
typedef scrap HUGE *scrap_pointer;
@ @d trans trans_plus.Trans /* translation texts of scraps */ @d no_math 2 @d yes_math 1 @d maybe_math 0
@<Global...@>=
EXTERN long max_scraps; /* Length of the next array. */ EXTERN scrap HUGE *scrp_info; /* Dynamic memory array for scraps */ EXTERN scrap_pointer scrp_end; /* end of |scrap_info|. */
EXTERN scrap_pointer pp; /* current position for reducing productions */ EXTERN scrap_pointer scrp_base; /* beginning of the current scrap sequence */ EXTERN scrap_pointer scrp_ptr; /* ending of the current scrap sequence */ EXTERN scrap_pointer lo_ptr; /* last scrap that has been examined */ EXTERN scrap_pointer hi_ptr; /* first scrap that has not been examined */
EXTERN scrap_pointer mx_scr_ptr; /* largest value assumed by |scrap_ptr| */
@ Token lists in |@_tok_mem| are composed of the following kinds of items for TEX output.
/* In the following, note that the casts are absolutely essential on machines with sixteen-bit integers.) */ @d id_flag ID_FLAG // Signifies an identifier. @d res_flag 2*id_flag // Signifies a reserved word. @d mod_flag ((sixteen_bits)(3*(sixteen_bits)id_flag)) // Signifies a module name. @d tok_flag ((sixteen_bits)(4*(sixteen_bits)id_flag)) // signifies a token list. @d inner_tok_flag ((sixteen_bits)(5*(sixteen_bits)id_flag)) // Signifies a token list in `'.
@ The `|freeze_text|' macro is used to give official status to a token list. Before saying |freeze_text|, items are appended to the current token list, and we know that the eventual number of this token list will be the current value of |text_ptr|. But no list of that number really exists as yet, because no ending point for the current list has been stored in the |tok_start| array. After saying |freeze_text|, the old current token list becomes legitimate, and its number is the current value of |text_ptr-1| since |text_ptr| has been increased. The new current token list is empty and ready to be appended to. Note that |freeze_text| does not check to see that |text_ptr| hasn't gotten too large, since it is assumed that this test was done beforehand.
@d freeze_text *(++text_ptr)=tok_ptr
@ If ẆEAVE is being run in debugging mode, the production numbers and current stack categories will be printed out when |tracing| is set to 2; a sequence of two or more irreducible scraps will be printed out when |tracing| is set to 1.
@<Global...@>=
#ifndef COMMON_FCNS_ IN_COMMON int tracing; // Can be used to show parsing details. #endif