`strutils.split/rsplit` now return src on an empty sep (#22136)

This is a rebase of an earlier rejected PR. Following the discussion
around it, this commit provides a valid output for and edge case
of an empty separator for `split` and `rsplit` routines. The empty
separator is interpreted as "split by no separators" and the initial
string is returned. This is consistent with the behaviour of the
`set[char]` version of `split`/`rsplit` routines and unifies them all.

Compared to a commit merged earlier, this one has a benefit of
not using assertions that will be removed in release builds
and thus still not preventing possible infinite loops (which was the
earlier behaviour for this edge case for separator of type `string`).

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

1 files changed, 37 insertions, 23 deletions

diff --git a/lib/pure/strutils.nim b/lib/pure/strutils.nim
index fe9f2a8c3..6c1e49564 100644
--- a/lib/pure/strutils.nim
+++ b/lib/pure/strutils.nim
@@ -366,11 +366,14 @@ func cmpIgnoreStyle*(a, b: string): int {.rtl, extern: "nsuCmpIgnoreStyle".} =
 # --------- Private templates for different split separators -----------
 
 func substrEq(s: string, pos: int, substr: string): bool =
-  var i = 0
+  # Always returns false for empty `substr`
   var length = substr.len
-  while i < length and pos+i < s.len and s[pos+i] == substr[i]:
-    inc i
-  return i == length
+  if length > 0:
+    var i = 0
+    while i < length and pos+i < s.len and s[pos+i] == substr[i]:
+      inc i
+    i == length
+  else: false
 
 template stringHasSep(s: string, index: int, seps: set[char]): bool =
   s[index] in seps
@@ -487,15 +490,15 @@ iterator split*(s: string, seps: set[char] = Whitespace,
   ##   "22"
   ##   "08"
   ##   "08.398990"
-  ## 
-  ## .. warning:: `seps` should not be empty.
+  ##
+  ##  .. note:: Empty separator set results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `rsplit iterator<#rsplit.i,string,set[char],int>`_
   ## * `splitLines iterator<#splitLines.i,string>`_
   ## * `splitWhitespace iterator<#splitWhitespace.i,string,int>`_
   ## * `split func<#split,string,set[char],int>`_
-  assert seps.card > 0, "Empty separator"
   splitCommon(s, seps, maxsplit, 1)
 
 iterator split*(s: string, sep: string, maxsplit: int = -1): string =
@@ -515,15 +518,17 @@ iterator split*(s: string, sep: string, maxsplit: int = -1): string =
   ##   "is"
   ##   "corrupted"
   ##
-  ## .. warning:: `sep` should not be empty.
+  ##  .. note:: Empty separator string results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `rsplit iterator<#rsplit.i,string,string,int,bool>`_
   ## * `splitLines iterator<#splitLines.i,string>`_
   ## * `splitWhitespace iterator<#splitWhitespace.i,string,int>`_
   ## * `split func<#split,string,string,int>`_
-  assert sep.len > 0, "Empty separator"
-  splitCommon(s, sep, maxsplit, sep.len)
+  let sepLen = if sep.len == 0: 1 # prevents infinite loop
+    else: sep.len
+  splitCommon(s, sep, maxsplit, sepLen)
 
 
 template rsplitCommon(s, sep, maxsplit, sepLen) =
@@ -591,16 +596,16 @@ iterator rsplit*(s: string, seps: set[char] = Whitespace,
   ##   "bar"
   ##   "foo"
   ##
-  ## Substrings are separated from the right by the set of chars `seps`.
-  ## 
-  ## .. warning:: `seps` should not be empty.
+  ## Substrings are separated from the right by the set of chars `seps`
+  ##
+  ##  .. note:: Empty separator set results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `split iterator<#split.i,string,set[char],int>`_
   ## * `splitLines iterator<#splitLines.i,string>`_
   ## * `splitWhitespace iterator<#splitWhitespace.i,string,int>`_
   ## * `rsplit func<#rsplit,string,set[char],int>`_
-  assert seps.card > 0, "Empty separator"
   rsplitCommon(s, seps, maxsplit, 1)
 
 iterator rsplit*(s: string, sep: string, maxsplit: int = -1,
@@ -619,17 +624,19 @@ iterator rsplit*(s: string, sep: string, maxsplit: int = -1,
   ##   "bar"
   ##   "foo"
   ##
-  ## Substrings are separated from the right by the string `sep`.
-  ## 
-  ## .. warning:: `sep` should not be empty.
+  ## Substrings are separated from the right by the string `sep`
+  ##
+  ##  .. note:: Empty separator string results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `split iterator<#split.i,string,string,int>`_
   ## * `splitLines iterator<#splitLines.i,string>`_
   ## * `splitWhitespace iterator<#splitWhitespace.i,string,int>`_
   ## * `rsplit func<#rsplit,string,string,int>`_
-  assert sep.len > 0, "Empty separator"
-  rsplitCommon(s, sep, maxsplit, sep.len)
+  let sepLen = if sep.len == 0: 1 # prevents infinite loop
+    else: sep.len
+  rsplitCommon(s, sep, maxsplit, sepLen)
 
 iterator splitLines*(s: string, keepEol = false): string =
   ## Splits the string `s` into its containing lines.
@@ -740,7 +747,8 @@ func split*(s: string, seps: set[char] = Whitespace, maxsplit: int = -1): seq[
   ## The same as the `split iterator <#split.i,string,set[char],int>`_ (see its
   ## documentation), but is a func that returns a sequence of substrings.
   ##
-  ## .. warning:: `seps` should not be empty.
+  ##  .. note:: Empty separator set results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `split iterator <#split.i,string,set[char],int>`_
@@ -750,6 +758,7 @@ func split*(s: string, seps: set[char] = Whitespace, maxsplit: int = -1): seq[
   runnableExamples:
     doAssert "a,b;c".split({',', ';'}) == @["a", "b", "c"]
     doAssert "".split({' '}) == @[""]
+    doAssert "empty seps return unsplit s".split({}) == @["empty seps return unsplit s"]
   accResult(split(s, seps, maxsplit))
 
 func split*(s: string, sep: string, maxsplit: int = -1): seq[string] {.rtl,
@@ -759,7 +768,8 @@ func split*(s: string, sep: string, maxsplit: int = -1): seq[string] {.rtl,
   ## Substrings are separated by the string `sep`. This is a wrapper around the
   ## `split iterator <#split.i,string,string,int>`_.
   ##
-  ## .. warning:: `sep` should not be empty.
+  ##  .. note:: Empty separator string results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `split iterator <#split.i,string,string,int>`_
@@ -773,6 +783,7 @@ func split*(s: string, sep: string, maxsplit: int = -1): seq[string] {.rtl,
     doAssert "a  largely    spaced sentence".split(" ") == @["a", "", "largely",
         "", "", "", "spaced", "sentence"]
     doAssert "a  largely    spaced sentence".split(" ", maxsplit = 1) == @["a", " largely    spaced sentence"]
+    doAssert "empty sep returns unsplit s".split("") == @["empty sep returns unsplit s"]
   accResult(split(s, sep, maxsplit))
 
 func rsplit*(s: string, sep: char, maxsplit: int = -1): seq[string] {.rtl,
@@ -822,7 +833,8 @@ func rsplit*(s: string, seps: set[char] = Whitespace,
   ## .. code-block:: nim
   ##   @["Root#Object#Method", "Index"]
   ##
-  ## .. warning:: `seps` should not be empty.
+  ##  .. note:: Empty separator set results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `rsplit iterator <#rsplit.i,string,set[char],int>`_
@@ -851,7 +863,8 @@ func rsplit*(s: string, sep: string, maxsplit: int = -1): seq[string] {.rtl,
   ## .. code-block:: nim
   ##   @["Root#Object#Method", "Index"]
   ##
-  ## .. warning:: `sep` should not be empty.
+  ##  .. note:: Empty separator string results in returning an original string,
+  ##   following the interpretation "split by no element".
   ##
   ## See also:
   ## * `rsplit iterator <#rsplit.i,string,string,int,bool>`_
@@ -867,6 +880,7 @@ func rsplit*(s: string, sep: string, maxsplit: int = -1): seq[string] {.rtl,
     doAssert "".rsplit("Elon Musk") == @[""]
     doAssert "a  largely    spaced sentence".rsplit(" ") == @["a", "",
         "largely", "", "", "", "spaced", "sentence"]
+    doAssert "empty sep returns unsplit s".rsplit("") == @["empty sep returns unsplit s"]
   accResult(rsplit(s, sep, maxsplit))
   result.reverse()