lalr: clarify the API

[Akim Demaille]

commit e062b9f70daa59f193d3cf7e59b77e8ae8bea656
Author: Akim Demaille <address@hidden>
Date:   Wed Feb 27 18:33:47 2019 +0100

    lalr: clarify the API
    
    * src/state.h, src/state.c (state_reduction_find): Clarify.
    Die on errors.
    * src/lalr.c (goto_list_new): New.
    Use it.

diff --git a/src/lalr.c b/src/lalr.c
index 6a3ee044..035bf5c0 100644
--- a/src/lalr.c
+++ b/src/lalr.c
@@ -54,8 +54,16 @@ typedef struct goto_list
   goto_number value;
 } goto_list;
 
+static goto_list *
+goto_list_new (goto_number value, struct goto_list *next)
+{
+  goto_list *res = xmalloc (sizeof *res);
+  res->next = next;
+  res->value = value;
+  return res;
+}
 
-/* LA is an NLA by NTOKENS matrix of bits.  LA[l, i] is 1 if the rule
+/* LA is an nLA by NTOKENS matrix of bits.  LA[l, i] is 1 if the rule
    LArule[l] is applicable in the appropriate state when the next
    token is symbol i.  If LA[l, i] and LA[l, j] are both 1 for i != j,
    it is a conflict.  */
@@ -232,10 +240,8 @@ static void
 add_lookback_edge (state *s, rule const *r, goto_number gotono)
 {
   int ri = state_reduction_find (s, r);
-  goto_list *sp = xmalloc (sizeof *sp);
-  sp->next = lookback[(s->reductions->lookahead_tokens - LA) + ri];
-  sp->value = gotono;
-  lookback[(s->reductions->lookahead_tokens - LA) + ri] = sp;
+  int idx = (s->reductions->lookahead_tokens - LA) + ri;
+  lookback[idx] = goto_list_new (gotono, lookback[idx]);
 }
 
 
diff --git a/src/state.c b/src/state.c
index 781e49c3..b8b8e2ff 100644
--- a/src/state.c
+++ b/src/state.c
@@ -218,7 +218,7 @@ state_reduction_find (state *s, rule const *r)
   for (int i = 0; i < reds->num; ++i)
     if (reds->rules[i] == r)
       return i;
-  return -1;
+  abort ();
 }
 
 
diff --git a/src/state.h b/src/state.h
index 16066049..498323fe 100644
--- a/src/state.h
+++ b/src/state.h
@@ -239,6 +239,8 @@ void state_transitions_set (state *s, int num, state **dst);
 /* Set the reductions of STATE.  */
 void state_reductions_set (state *s, int num, rule **reds);
 
+/* The index of the reduction of state S that corresponds to rule R.
+   Aborts if there is no reduction of R in S.  */
 int state_reduction_find (state *s, rule const *r);
 
 /* Set the errs of STATE.  */