From 74fa24ce456a2af018117e61a1e9de373e71853d Mon Sep 17 00:00:00 2001
From: flywind <43030857+xflywind@users.noreply.github.com>
Date: Mon, 23 Nov 2020 18:27:09 +0800
Subject: [PATCH] fix rope index (#16100)

* fix rope index

* add testcase

(cherry picked from commit ab2c082cb3e9e411571fa0936776762bc0c8fd94)
---
 lib/pure/ropes.nim      |  2 +-
 tests/stdlib/tropes.nim | 66 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 1 deletion(-)
 create mode 100644 tests/stdlib/tropes.nim

diff --git a/lib/pure/ropes.nim b/lib/pure/ropes.nim
index 41d6211b41..0797bbe060 100644
--- a/lib/pure/ropes.nim
+++ b/lib/pure/ropes.nim
@@ -204,8 +204,8 @@ proc `[]`*(r: Rope, i: int): char {.rtl, extern: "nroCharAt".} =
       if x.left.length > j:
         x = x.left
       else:
+        dec(j, x.left.length)
         x = x.right
-        dec(j, x.len)
 
 iterator leaves*(r: Rope): string =
   ## iterates over any leaf string in the rope `r`.
diff --git a/tests/stdlib/tropes.nim b/tests/stdlib/tropes.nim
new file mode 100644
index 0000000000..9678e26c35
--- /dev/null
+++ b/tests/stdlib/tropes.nim
@@ -0,0 +1,66 @@
+import ropes
+
+
+block:
+  let r: Rope = nil
+  doAssert r[0] == '\0'
+
+block:
+  var
+    r1 = rope("Hello")
+    r2 = rope("Nim-Lang")
+
+  let r = r1 & r2
+  let s = $r
+  for i in 0 ..< r.len:
+    doAssert r[i] == s[i]
+
+  doAssert r[66] == '\0'
+
+block:
+  let r = rope("Hello, Nim-Lang")
+
+  let s = $r
+  for i in 0 ..< r.len:
+    doAssert r[i] == s[i]
+
+  doAssert r[66] == '\0'
+
+block:
+  var r: Rope
+  r.add rope("Nim ")
+  r.add rope("is ")
+  r.add rope("a ")
+  r.add rope("great ")
+  r.add rope("language")
+
+  let s = $r
+  for i in 0 ..< r.len:
+    doAssert r[i] == s[i]
+
+  doAssert r[66] == '\0'
+
+block:
+  var r: Rope
+  r.add rope("My Conquest")
+  r.add rope(" is ")
+  r.add rope("the Sea of Stars")
+
+  let s = $r
+  for i in 0 ..< r.len:
+    doAssert r[i] == s[i]
+
+  doAssert r[66] == '\0'
+
+block:
+  var r: Rope
+  r.add rope("My Conquest")
+  r.add rope(" is ")
+  r.add rope("the Sea of Stars")
+
+  var i: int
+  for item in r:
+    doAssert r[i] == item
+    inc i
+
+  doAssert r[66] == '\0'