From 1d728731170e6f378dc27699870d6d285d9247bc Mon Sep 17 00:00:00 2001
From: Justin Ethier <justin.ethier@gmail.com>
Date: Thu, 25 Jun 2015 22:04:20 -0400
Subject: [PATCH] Added error handling to substring

---
 runtime.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/runtime.c b/runtime.c
index d435cb72..eecbb169 100644
--- a/runtime.c
+++ b/runtime.c
@@ -857,13 +857,26 @@ integer_type Cyc_string_length(object str) {
   return len;
 }
 
-TODO: need error checking below, this is much too dangerous without!
 string_type Cyc_substring(object str, object start, object end) {
-  int s = integer_value(start),
-      e = integer_value(end);
   const char *raw = string_str(str);
-  make_stringn(sub, raw + s, e - s);
-  return sub;
+  int s = integer_value(start),
+      e = integer_value(end),
+      len = strlen(raw);
+
+  if (s > e) {
+    Cyc_rt_raise2("substring - start cannot be greater than end", start);
+  }
+  if (s > len) {
+    Cyc_rt_raise2("substring - start cannot be greater than string length", start);
+  }
+  if (e > len) {
+    e = len;
+  }
+
+  {
+    make_stringn(sub, raw + s, e - s);
+    return sub;
+  }
 }
 
 integer_type Cyc_system(object cmd) {