From 9ca88a18897f5ebc59d6a67dfc7f9a8de8fa35ce Mon Sep 17 00:00:00 2001
From: n5m <72841454+n5m@users.noreply.github.com>
Date: Mon, 26 Sep 2022 19:39:22 +0000
Subject: [PATCH] fix #18128 rfind on empty needle returns rightmost index
 (#20430)

rfind on empty needle returns haystack len
---
 lib/pure/strutils.nim      |  3 ++-
 tests/stdlib/tstrutils.nim | 28 ++++++++++++++++------------
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/lib/pure/strutils.nim b/lib/pure/strutils.nim
index 3b315e564a..3ae953a557 100644
--- a/lib/pure/strutils.nim
+++ b/lib/pure/strutils.nim
@@ -2022,7 +2022,8 @@ func rfind*(s, sub: string, start: Natural = 0, last = -1): int {.rtl,
   ## See also:
   ## * `find func<#find,string,string,Natural,int>`_
   if sub.len == 0:
-    return -1
+    let rightIndex: Natural = if last < 0: s.len else: last
+    return max(start, rightIndex)
   if sub.len > s.len - start:
     return -1
   let last = if last == -1: s.high else: last
diff --git a/tests/stdlib/tstrutils.nim b/tests/stdlib/tstrutils.nim
index e17277ef2a..9ea4d72f2e 100644
--- a/tests/stdlib/tstrutils.nim
+++ b/tests/stdlib/tstrutils.nim
@@ -360,19 +360,23 @@ template main() =
     doAssert "///".rfind("//", start=3) == -1
 
     # searching for empty string
-    doAssert "".rfind("") == -1
-    doAssert "abc".rfind("") == -1
-    doAssert "abc".rfind("", start=1) == -1
-    doAssert "abc".rfind("", start=2) == -1
-    doAssert "abc".rfind("", start=3) == -1
-    doAssert "abc".rfind("", start=4) == -1
-    doAssert "abc".rfind("", start=400) == -1
+    doAssert "".rfind("") == 0
+    doAssert "abc".rfind("") == 3
+    doAssert "abc".rfind("", start=1) == 3
+    doAssert "abc".rfind("", start=2) == 3
+    doAssert "abc".rfind("", start=3) == 3
+    doAssert "abc".rfind("", start=4) == 4
+    doAssert "abc".rfind("", start=400) == 400
 
-    doAssert "abc".rfind("", start=1, last=3) == -1
-    doAssert "abc".rfind("", start=1, last=2) == -1
-    doAssert "abc".rfind("", start=1, last=1) == -1
-    doAssert "abc".rfind("", start=1, last=0) == -1
-    doAssert "abc".rfind("", start=1, last = -1) == -1
+    doAssert "abc".rfind("", start=1, last=3) == 3
+    doAssert "abc".rfind("", start=1, last=2) == 2
+    doAssert "abc".rfind("", start=1, last=1) == 1
+    # This returns the start index instead of the last index
+    # because start > last
+    doAssert "abc".rfind("", start=1, last=0) == 1
+    doAssert "abc".rfind("", start=1, last = -1) == 3
+    
+    doAssert "abc".rfind("", start=0, last=0) == 0
 
     # when last <= start, searching for non-empty string
     block: