improve fast returns of find and rfind (#18127)

2 files changed, 31 insertions, 1 deletions

diff --git a/lib/pure/strutils.nim b/lib/pure/strutils.nim
index 5bbd850cc..3fb67ead1 100644
--- a/lib/pure/strutils.nim
+++ b/lib/pure/strutils.nim
@@ -1890,7 +1890,7 @@ func find*(s, sub: string, start: Natural = 0, last = 0): int {.rtl,
   ## See also:
   ## * `rfind func<#rfind,string,string,Natural>`_
   ## * `replace func<#replace,string,string,string>`_
-  if sub.len > s.len: return -1
+  if sub.len > s.len - start: return -1
   if sub.len == 1: return find(s, sub[0], start, last)
 
   template useSkipTable {.dirty.} =
@@ -1967,6 +1967,8 @@ func rfind*(s, sub: string, start: Natural = 0, last = -1): int {.rtl,
   ## * `find func<#find,string,string,Natural,int>`_
   if sub.len == 0:
     return -1
+  if sub.len > s.len - start:
+    return -1
   let last = if last == -1: s.high else: last
   result = 0
   for i in countdown(last - sub.len + 1, start):
diff --git a/tests/stdlib/tstrutils.nim b/tests/stdlib/tstrutils.nim
index 771dddcaf..637661ba3 100644
--- a/tests/stdlib/tstrutils.nim
+++ b/tests/stdlib/tstrutils.nim
@@ -243,6 +243,34 @@ template main() =
     doAssert "/1/2/3".rfind('/', last=1) == 0
     doAssert "/1/2/3".rfind('0') == -1
 
+    block:
+      const haystack: string = "ABCABABABABCAB"
+      doAssert haystack.len == 14
+      doAssert haystack.rfind("ABC") == 9
+      doAssert haystack.rfind("ABC", last=13) == 9
+      doAssert haystack.rfind("ABC", last=12) == 9
+      doAssert haystack.rfind("ABC", last=11) == 9
+      doAssert haystack.rfind("ABC", last=10) == 0
+
+      doAssert haystack.rfind("ABC", start=0) == 9
+      doAssert haystack.rfind("ABC", start=1) == 9
+      doAssert haystack.rfind("ABC", start=2) == 9
+      doAssert haystack.rfind("ABC", start=9) == 9
+      doAssert haystack.rfind("ABC", start=10) == -1
+      doAssert haystack.rfind("ABC", start=11) == -1
+      doAssert haystack.rfind("ABC", start=12) == -1
+
+      doAssert haystack.rfind("ABC", start=0, last=13) == 9
+      doAssert haystack.rfind("ABC", start=0, last=12) == 9
+      doAssert haystack.rfind("ABC", start=0, last=11) == 9
+      doAssert haystack.rfind("ABC", start=0, last=10) == 0
+      doAssert haystack.rfind("ABC", start=1, last=10) == -1
+
+    doAssert "".rfind("/") == -1
+    doAssert "/".rfind("/") == 0
+    doAssert "/".rfind("//") == -1
+    doAssert "///".rfind("//", start=3) == -1
+
   block: # trimZeros
     var x = "1200"
     x.trimZeros()