Fixes for rewrite_repeat

src/libre/ast_rewrite.c

@@ -410,48 +410,33 @@ rewrite_repeat(struct ast_expr *n, enum re_flags flags)
  return 1;
  }
 
- if (h == 0 || h == 1) {
- dead = n->u.repeat.e;
-
- n->u.repeat.min = v;
- n->u.repeat.max = w;
- n->u.repeat.e = n->u.repeat.e->u.repeat.e;
-
- dead->type = AST_EXPR_EMPTY;
- ast_expr_free(dead);
-
- return 1;
- }
-
  /*
  * a{h,i}{j,k} is equivalent to a{h*j,i*k} if it's possible to combine,
- * and it's possible iff the range of the result is not more than
- * the sum of the ranges of the two inputs.
- */
- if (i != AST_COUNT_UNBOUNDED && k != AST_COUNT_UNBOUNDED) {
- /* I don't know why this is true */
- assert(w - v > i - h + k - j);
- }
-
- /*
- * a{h,}{j,}
- * a{h,i}{j,}
- * a{h,}{j,k}
+ * i.e. if for all n in [h*j,i*k], a{n} would be included in a{h,i}{j,k}.
+ * a{h*j} is trivially included, and for any a{n} that is included where
+ * n is not a multiple of i, a{n+1} is also included: if n is not a
+ * multiple of i, then one of the a{h,i} instances matched fewer than i
+ * times and can accept another a. The only case to consider is where n
+ * is a multiple of i and the rewritten expression would include a{n+1}.
+ *
+ * If j == k, this cannot be a problem. The only case where all instances
+ * of a{h,i} accept i instances of a is where n==i*j==i*k.
+ *
+ * If i == AST_COUNT_UNBOUNDED and j != 0, this cannot be a problem. It
+ * is impossible for any a{h,i} to accept i instances of a, so the problem
+ * never arises here either. Note the restriction on j != 0 though:
+ * a{2,}{0,} cannot be rewritten as a{0,}.
+ *
+ * If a{n} is included by m instances of a{i}, where j<=m<k, we know that
+ * a{n+1} is included by m+1 instances of a{h,i} if and only if
+ * h*(m+1) <= i*m+1, which simplifies to h*(j+1) <= i*j+1.
+ *
+ * Note that this last case includes all cases where h <= 1.
  */
- if ((i == AST_COUNT_UNBOUNDED || k == AST_COUNT_UNBOUNDED) && h <= 1 && j <= 1) {
- dead = n->u.repeat.e;
-
- n->u.repeat.min = v;
- n->u.repeat.max = w;
- n->u.repeat.e = n->u.repeat.e->u.repeat.e;
-
- dead->type = AST_EXPR_EMPTY;
- ast_expr_free(dead);
-
- return 1;
- }
 
- if (h > 1 && i == AST_COUNT_UNBOUNDED && j > 0) {
+ if (j == k
+ || (i == AST_COUNT_UNBOUNDED && j > 0)
+ || h * (j + 1) <= i * j + 1) {
  dead = n->u.repeat.e;
 
  n->u.repeat.min = v;

tests/pcre-repeat/in55.re

@@ -0,0 +1 @@
+^(?:a{2}){2}$

tests/pcre-repeat/out55.fsm

@@ -0,0 +1,7 @@
+0 -> 1 'a';
+1 -> 2 'a';
+2 -> 3 'a';
+3 -> 4 'a';
+
+start: 0;
+end: 4;