From ea3cfa05bd81d0411bc413a3c5647a82de92c586 Mon Sep 17 00:00:00 2001
From: Bob Nystrom <robert@stuffwithstuff.com>
Date: Fri, 14 Feb 2014 20:10:41 -0800
Subject: [PATCH] Allow [0..-1] and [0...0] to work on empty lists.

---
 builtin/core.wren              |  5 +----
 src/wren_core.c                | 17 +++++++++++++----
 test/list/subscript_range.wren |  4 ++++
 3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/builtin/core.wren b/builtin/core.wren
index 0906afa0..1fd024de 100644
--- a/builtin/core.wren
+++ b/builtin/core.wren
@@ -10,10 +10,7 @@ class List {
   }
 
   + that {
-    var result = []
-    for (element in this) {
-      result.add(element)
-    }
+    var result = this[0..-1]
     for (element in that) {
       result.add(element)
     }
diff --git a/src/wren_core.c b/src/wren_core.c
index 7081e30b..6db28350 100644
--- a/src/wren_core.c
+++ b/src/wren_core.c
@@ -53,10 +53,7 @@ static const char* libSource =
 "  }\n"
 "\n"
 "  + that {\n"
-"    var result = []\n"
-"    for (element in this) {\n"
-"      result.add(element)\n"
-"    }\n"
+"    var result = this[0..-1]\n"
 "    for (element in that) {\n"
 "      result.add(element)\n"
 "    }\n"
@@ -389,6 +386,18 @@ DEF_NATIVE(list_subscript)
 
   ObjRange* range = AS_RANGE(args[1]);
 
+  // TODO: This code is pretty hairy. Is there a more elegant way?
+  // Corner case: an empty range at zero is allowed on an empty list.
+  // This way, list[0..-1] and list[0...list.count] can be used to copy a list
+  // even when empty.
+  if (list->count == 0) {
+    if ((range->from == 0 && range->to == -1 && range->isInclusive) ||
+        (range->from == 0 && range->to == 0 && !range->isInclusive))
+    {
+      RETURN_OBJ(wrenNewList(vm, 0));
+    }
+  }
+
   int from = validateIndexValue(vm, args, list->count, range->from,
                                 "Range start");
   if (from == -1) return PRIM_ERROR;
diff --git a/test/list/subscript_range.wren b/test/list/subscript_range.wren
index 6c7d1f2f..08a03a44 100644
--- a/test/list/subscript_range.wren
+++ b/test/list/subscript_range.wren
@@ -29,3 +29,7 @@ IO.print(list[1..-2]) // expect: [b, c, d]
 IO.print(list[2...-1]) // expect: [c, d]
 IO.print(list[4..-5]) // expect: [e, d, c, b, a]
 IO.print(list[3...-6]) // expect: [d, c, b, a]
+
+// An empty range at zero is allowed on an empty list.
+IO.print([][0...0]) // expect: []
+IO.print([][0..-1]) // expect: []