From 714bd61d56efb57d536b293b01c6c828d402b274 Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Fri, 6 Mar 2020 10:01:04 -0600
Subject: [PATCH] Address #300

Check for empty capture stack in replace rule.
---
 src/core/peg.c    | 14 +++++++++-----
 test/suite8.janet |  5 +++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/core/peg.c b/src/core/peg.c
index 6e839f9e..fc05c9b7 100644
--- a/src/core/peg.c
+++ b/src/core/peg.c
@@ -369,19 +369,23 @@ tail:
             s->mode = oldmode;
             if (!result) return NULL;
 
-            Janet cap;
+            Janet cap = janet_wrap_nil();
             Janet constant = s->constants[rule[2]];
             switch (janet_type(constant)) {
                 default:
                     cap = constant;
                     break;
                 case JANET_STRUCT:
-                    cap = janet_struct_get(janet_unwrap_struct(constant),
-                                           s->captures->data[s->captures->count - 1]);
+                    if (s->captures->count) {
+                        cap = janet_struct_get(janet_unwrap_struct(constant),
+                                               s->captures->data[s->captures->count - 1]);
+                    }
                     break;
                 case JANET_TABLE:
-                    cap = janet_table_get(janet_unwrap_table(constant),
-                                          s->captures->data[s->captures->count - 1]);
+                    if (s->captures->count) {
+                        cap = janet_table_get(janet_unwrap_table(constant),
+                                              s->captures->data[s->captures->count - 1]);
+                    }
                     break;
                 case JANET_CFUNCTION:
                     cap = janet_unwrap_cfunction(constant)(s->captures->count - cs.cap,
diff --git a/test/suite8.janet b/test/suite8.janet
index f314a53e..242d80db 100644
--- a/test/suite8.janet
+++ b/test/suite8.janet
@@ -112,4 +112,9 @@
 (assert (= false (and false false)) "and 1")
 (assert (= false (or false false)) "or 1")
 
+# #300 Regression test
+
+# Just don't segfault
+(assert (peg/match '{:main (replace "S" {"S" :spade})} "S7") "regression #300")
+
 (end-suite)