Add `matchcap` combinator to pegs.

This allows running rules on captures of pegs. This is flexible, but
loses some guarantees about rules only operating on the source string.
This means that line and column information will be incorrect inside the
matchcap combinator.

src/core/peg.c

@@ -596,6 +596,67 @@ tail:
             return text + width;
         }
 
+        case RULE_MATCHCAP: {
+            const uint32_t *rule_a = s->bytecode + rule[1];
+            const uint32_t *rule_b = s->bytecode + rule[2];
+
+            int oldmode = s->mode;
+            CapState cs = cap_save(s);
+            s->mode = PEG_MODE_NORMAL;
+            int32_t old_cap = s->captures->count;
+            down1(s);
+            const uint8_t *b_result = peg_rule(s, rule_b, text);
+            up1(s);
+            s->mode = oldmode;
+            if (!b_result) return NULL;
+            int32_t new_cap = s->captures->count;
+
+            /* Check for bad captures */
+            for (int32_t i = old_cap; i < new_cap; i++) {
+                Janet capture = s->captures->data[i];
+                if (!janet_checktype(capture, JANET_STRING)) {
+                    return NULL;
+                }
+            }
+
+            /* Save captures to temporary buffer */
+            Janet *temp_mem = janet_smalloc(sizeof(Janet) * (new_cap - old_cap));
+            for (int32_t i = old_cap; i < new_cap; i++) {
+                temp_mem[i - old_cap] = s->captures->data[i];
+            }
+            cap_load(s, cs);
+
+            for (int32_t i = old_cap; i < new_cap; i++) {
+                Janet capture = temp_mem[i - old_cap];
+                const uint8_t *str = janet_unwrap_string(capture);
+                PegState subs;
+                subs.mode = PEG_MODE_NORMAL;
+                subs.text_start = str;
+                subs.text_end = str + janet_string_length(str);
+                subs.depth = s->depth - 1;
+                subs.captures = s->captures;
+                subs.tagged_captures = s->tagged_captures;
+                subs.scratch = janet_buffer(10);
+                subs.tags = s->tags;
+                subs.constants = s->constants;
+                subs.bytecode = s->bytecode;
+                subs.linemap = NULL;
+                subs.linemaplen = -1;
+                subs.has_backref = s->has_backref;
+                subs.extrac = s->extrac;
+                subs.extrav = s->extrav;
+
+                const uint8_t *a_result = peg_rule(&subs, rule_a, str);
+                if (NULL == a_result) {
+                    janet_sfree(temp_mem);
+                    return NULL;
+                }
+            }
+
+            janet_sfree(temp_mem);
+            return b_result;
+        }
+
     }
 }
 
@@ -844,6 +905,9 @@ static void spec_ifnot(Builder *b, int32_t argc, const Janet *argv) {
 static void spec_lenprefix(Builder *b, int32_t argc, const Janet *argv) {
     spec_branch(b, argc, argv, RULE_LENPREFIX);
 }
+static void spec_matchcap(Builder *b, int32_t argc, const Janet *argv) {
+    spec_branch(b, argc, argv, RULE_MATCHCAP);
+}
 
 static void spec_between(Builder *b, int32_t argc, const Janet *argv) {
     peg_fixarity(b, argc, 3);
@@ -1090,6 +1154,7 @@ static const SpecialPair peg_specials[] = {
     {"lenprefix", spec_lenprefix},
     {"line", spec_line},
     {"look", spec_look},
+    {"matchcap", spec_matchcap},
     {"not", spec_not},
     {"opt", spec_opt},
     {"position", spec_position},
@@ -1362,6 +1427,7 @@ static void *peg_unmarshal(JanetMarshalContext *ctx) {
             case RULE_IF:
             case RULE_IFNOT:
             case RULE_LENPREFIX:
+            case RULE_MATCHCAP:
                 /* [rule_a, rule_b (b if not a)] */
                 if (rule[1] >= blen) goto bad;
                 if (rule[2] >= blen) goto bad;

src/include/janet.h

@@ -1841,7 +1841,8 @@ typedef enum {
     RULE_LENPREFIX,    /* [rule_a, rule_b (repeat rule_b rule_a times)] */
     RULE_READINT,      /* [(signedness << 4) | (endianess << 5) | bytewidth, tag] */
     RULE_LINE,         /* [tag] */
-    RULE_COLUMN        /* [tag] */
+    RULE_COLUMN,       /* [tag] */
+    RULE_MATCHCAP      /* [rule_a, rule_b] */
 } JanetPegOpcod;
 
 typedef struct {

test/suite0003.janet

@@ -481,4 +481,13 @@
 (check-deep '(to "b") "aaaa" nil)
 (check-deep '(thru "b") "aaaa" nil)
 
+# matchcap
+(def matchcap-test
+  ~{:span (* "{" '(to "}") "}")
+    :parse-span (* '3 3)
+    :main (matchcap :parse-span (any :span ))})
+
+(check-deep matchcap-test "{bigrig}{fatman}{catdog}" @["big" "fat" "cat"])
+(check-deep matchcap-test "{bigrig}{fatman}{catdoggy}" @["big" "fat" "cat"])
+
 (end-suite)