From 965e73197a9d46c7f629b07c4f992addde019b45 Mon Sep 17 00:00:00 2001
From: "Sven M. Hallberg" <pesco@khjk.org>
Date: Fri, 21 Feb 2020 22:51:16 +0100
Subject: [PATCH] eliminate dead rules in h_cfgrammar
fixes issue #83
---
src/cfgrammar.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 74 insertions(+)
diff --git a/src/cfgrammar.c b/src/cfgrammar.c
index 77e7ecad..aea45a45 100644
--- a/src/cfgrammar.c
+++ b/src/cfgrammar.c
@@ -42,6 +42,7 @@ void h_cfgrammar_free(HCFGrammar *g)
// helpers
static void collect_nts(HCFGrammar *grammar, HCFChoice *symbol);
static void collect_geneps(HCFGrammar *grammar);
+static void eliminate_dead_rules(HCFGrammar *g);
HCFGrammar *h_cfgrammar(HAllocator* mm__, const HParser *parser)
@@ -83,6 +84,9 @@ HCFGrammar *h_cfgrammar_(HAllocator* mm__, HCFChoice *desugared)
g->start = desugared;
}
+ // simplifications
+ eliminate_dead_rules(g);
+
// determine which nonterminals generate epsilon
collect_geneps(g);
@@ -232,6 +236,76 @@ static void collect_geneps(HCFGrammar *g)
} while(g->geneps->used != prevused);
}
+static bool mentions_symbol(HCFChoice **s, const HCFChoice *x)
+{
+ for(; *s; s++) {
+ if (*s == x)
+ return true;
+ }
+ return false;
+}
+
+static void remove_productions_with(HCFGrammar *g, const HCFChoice *x)
+{
+ HHashTableEntry *hte;
+ const HCFChoice *symbol;
+ size_t i;
+
+ for(i=0; i < g->nts->capacity; i++) {
+ for(hte = &g->nts->contents[i]; hte; hte = hte->next) {
+ if (hte->key == NULL)
+ continue;
+ symbol = hte->key;
+ assert(symbol->type == HCF_CHOICE);
+
+ HCFSequence **p, **q;
+ for(p = symbol->seq; *p != NULL; ) {
+ if (mentions_symbol((*p)->items, x)) {
+ // remove production p
+ for(q=p; *(q+1) != NULL; q++); // q = last production
+ *p = *q; // move q over p
+ *q = NULL; // delete old q
+ } else {
+ p++;
+ }
+ }
+ }
+ }
+}
+
+static void eliminate_dead_rules(HCFGrammar *g)
+{
+ HHashTableEntry *hte;
+ const HCFChoice *symbol;
+ size_t i;
+ bool found;
+
+ do {
+ found = false;
+ for(i=0; !found && i < g->nts->capacity; i++) {
+ for(hte = &g->nts->contents[i]; !found && hte; hte = hte->next) {
+ if (hte->key == NULL)
+ continue;
+ symbol = hte->key;
+ assert(symbol->type == HCF_CHOICE);
+
+ // this NT is dead if it has no productions
+ if (*symbol->seq == NULL)
+ found = true;
+ }
+ }
+ if (found) {
+ h_hashtable_del(g->nts, symbol);
+ remove_productions_with(g, symbol);
+ }
+ } while(found); // until nothing left to remove
+
+ // rebuild g->nts. there may now be symbols that no longer appear in any
+ // productions. we also might have removed g->start.
+ g->nts = h_hashset_new(g->arena, h_eq_ptr, h_hash_ptr);
+ collect_nts(g, g->start);
+}
+
HStringMap *h_stringmap_new(HArena *a)
{
--
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