From dadbeb6c7039bf5185ffa18e383f94f5beae0e76 Mon Sep 17 00:00:00 2001
From: TSUYUSATO Kitsune <make.just.on@gmail.com>
Date: Sun, 31 Dec 2017 15:21:55 +0900
Subject: [PATCH] Fix String#sub with negative index

Fixed #5490
---
 spec/std/string_spec.cr | 14 ++++++++++++++
 src/string.cr           |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 8d9551914444..3b4a9136d909 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -1173,6 +1173,13 @@ describe "String" do
       string.bytesize.should eq("あいのえお".bytesize)
     end
 
+    it "subs at negative index with char" do
+      string = "abc".sub(-1, 'd')
+      string.should eq("abd")
+      string = string.sub(-2, 'n')
+      string.should eq("and")
+    end
+
     it "subs at index with string" do
       string = "hello".sub(1, "eee")
       string.should eq("heeello")
@@ -1180,6 +1187,13 @@ describe "String" do
       string.size.should eq(7)
     end
 
+    it "subs at negative index with string" do
+      string = "hello".sub(-1, "ooo")
+      string.should eq("hellooo")
+      string.bytesize.should eq(7)
+      string.size.should eq(7)
+    end
+
     it "subs at index with string, non-ascii" do
       string = "あいうえお".sub(2, "けくこ")
       string.should eq("あいけくこえお")
diff --git a/src/string.cr b/src/string.cr
index 14f9ecf4f68d..db7939a87607 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -1857,7 +1857,7 @@ class String
   end
 
   private def sub_index(index, replacement)
-    index += size + 1 if index < 0
+    index += size if index < 0
 
     byte_index = char_index_to_byte_index(index)
     raise IndexError.new unless byte_index