add a new (split) PEG special

This works similarly to string/split, but the separator is a PEG.

src/core/peg.c

@@ -39,7 +39,7 @@
 typedef struct {
     const uint8_t *text_start;
     const uint8_t *text_end;
-    /* text_end will be restricted in a (sub) rule, but
+    /* text_end can be restricted by some rules, but
        outer_text_end will always contain the real end of
        input, which we need to generate a line mapping */
     const uint8_t *outer_text_end;
@@ -510,6 +510,44 @@ static const uint8_t *peg_rule(
             return window_end;
         }
 
+        case RULE_SPLIT: {
+            const uint8_t *saved_end = s->text_end;
+            const uint32_t *rule_separator = s->bytecode + rule[1];
+            const uint32_t *rule_subpattern = s->bytecode + rule[2];
+
+            const uint8_t *separator_end = NULL;
+            do {
+                const uint8_t *text_start = text;
+                CapState cs = cap_save(s);
+                down1(s);
+                while (text <= s->text_end) {
+                    separator_end = peg_rule(s, rule_separator, text);
+                    cap_load(s, cs);
+                    if (separator_end) {
+                        break;
+                    }
+                    text++;
+                }
+                up1(s);
+
+                if (separator_end) {
+                    s->text_end = text;
+                    text = separator_end;
+                }
+
+                down1(s);
+                const uint8_t *subpattern_end = peg_rule(s, rule_subpattern, text_start);
+                up1(s);
+                s->text_end = saved_end;
+
+                if (!subpattern_end) {
+                    return NULL;
+                }
+            } while (separator_end);
+
+            return s->text_end;
+        }
+
         case RULE_REPLACE:
         case RULE_MATCHTIME: {
             uint32_t tag = rule[3];
@@ -1143,6 +1181,14 @@ static void spec_sub(Builder *b, int32_t argc, const Janet *argv) {
     emit_2(r, RULE_SUB, subrule1, subrule2);
 }
 
+static void spec_split(Builder *b, int32_t argc, const Janet *argv) {
+    peg_fixarity(b, argc, 2);
+    Reserve r = reserve(b, 3);
+    uint32_t subrule1 = peg_compile1(b, argv[0]);
+    uint32_t subrule2 = peg_compile1(b, argv[1]);
+    emit_2(r, RULE_SPLIT, subrule1, subrule2);
+}
+
 #ifdef JANET_INT_TYPES
 #define JANET_MAX_READINT_WIDTH 8
 #else
@@ -1226,6 +1272,7 @@ static const SpecialPair peg_specials[] = {
     {"sequence", spec_sequence},
     {"set", spec_set},
     {"some", spec_some},
+    {"split", spec_split},
     {"sub", spec_sub},
     {"thru", spec_thru},
     {"to", spec_to},
@@ -1562,6 +1609,7 @@ static void *peg_unmarshal(JanetMarshalContext *ctx) {
                 i += 4;
                 break;
             case RULE_SUB:
+            case RULE_SPLIT:
                 /* [rule, rule] */
                 if (rule[1] >= blen) goto bad;
                 if (rule[2] >= blen) goto bad;

src/include/janet.h

@@ -2141,7 +2141,8 @@ typedef enum {
     RULE_COLUMN,       /* [tag] */
     RULE_UNREF,        /* [rule, tag] */
     RULE_CAPTURE_NUM,  /* [rule, tag] */
-    RULE_SUB           /* [rule, rule] */
+    RULE_SUB,          /* [rule, rule] */
+    RULE_SPLIT         /* [rule, rule] */
 } JanetPegOpcod;
 
 typedef struct {

test/suite-peg.janet

@@ -265,6 +265,7 @@
 (marshpeg '(group "abc"))
 (marshpeg '(sub "abcdf" "abc"))
 (marshpeg '(* (sub 1 1)))
+(marshpeg '(split "," (+ "a" "b" "c")))
 
 # Peg swallowing errors
 # 159651117
@@ -710,5 +711,50 @@
   "abcdef"
   @[])
 
+(test "split: basic functionality"
+  ~(split "," '1)
+  "a,b,c"
+  @["a" "b" "c"])
+
+(test "split: drops captures from separator pattern"
+  ~(split '"," '1)
+  "a,b,c"
+  @["a" "b" "c"])
+
+(test "split: can match empty subpatterns"
+  ~(split "," ':w*)
+  ",a,,bar,,,c,,"
+  @["" "a" "" "bar" "" "" "c" "" ""])
+
+(test "split: subpattern is limited to only text before the separator"
+  ~(split "," '(to -1))
+  "a,,bar,c"
+  @["a" "" "bar" "c"])
+
+(test "split: fails if any subpattern fails"
+  ~(split "," '"a")
+  "a,a,b"
+  nil)
+
+(test "split: separator does not have to match anything"
+  ~(split "x" '(to -1))
+  "a,a,b"
+  @["a,a,b"])
+
+(test "split: always consumes entire input"
+  ~(split 1 '"")
+  "abc"
+  @["" "" "" ""])
+
+(test "split: separator can be an arbitrary PEG"
+  ~(split :s+ '(to -1))
+  "a   b      c"
+  @["a" "b" "c"])
+
+(test "split: does not advance past the end of the input"
+  ~(* (split "," ':w+) 0)
+  "a,b,c"
+  @["a" "b" "c"])
+
 (end-suite)