strings: document Split{,N,After,AfterN} edge cases

Apparently people get confused by the fact that

  Split("", ",")

returns []{""} instead of []{}.

This is actually just a consequence of the fact that if the separator
sep (2nd argument) is not found the string s (1st argument), then the
Split* functions return a length 1 slice with the string s in it.

Document the general case: if sep is not in s, what you get is a len 1
slice with s in it; unless both s and sep are "", in that case you get
an empty slice of length 0.

Fixes #19726

Change-Id: I64c8220b91acd1e5aa1cc1829199e0cd8c47c404
Reviewed-on: https://go-review.googlesource.com/44950Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/strings/strings.go

@@ -257,33 +257,51 @@ func genSplit(s, sep string, sepSave, n int) []string {
 
 // SplitN slices s into substrings separated by sep and returns a slice of
 // the substrings between those separators.
-// If sep is empty, SplitN splits after each UTF-8 sequence.
+//
 // The count determines the number of substrings to return:
 //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
 //   n == 0: the result is nil (zero substrings)
 //   n < 0: all substrings
+//
+// Edge cases for s and sep (for example, empty strings) are handled
+// as described in the documentation for Split.
 func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
 
 // SplitAfterN slices s into substrings after each instance of sep and
 // returns a slice of those substrings.
-// If sep is empty, SplitAfterN splits after each UTF-8 sequence.
+//
 // The count determines the number of substrings to return:
 //   n > 0: at most n substrings; the last substring will be the unsplit remainder.
 //   n == 0: the result is nil (zero substrings)
 //   n < 0: all substrings
+//
+// Edge cases for s and sep (for example, empty strings) are handled
+// as described in the documentation for SplitAfter.
 func SplitAfterN(s, sep string, n int) []string {
 	return genSplit(s, sep, len(sep), n)
 }
 
 // Split slices s into all substrings separated by sep and returns a slice of
 // the substrings between those separators.
-// If sep is empty, Split splits after each UTF-8 sequence.
+//
+// If s does not contain sep and sep is not empty, Split returns a
+// slice of length 1 whose only element is s.
+//
+// If sep is empty, Split splits after each UTF-8 sequence. If both s
+// and sep are empty, Split returns an empty slice.
+//
 // It is equivalent to SplitN with a count of -1.
 func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }
 
 // SplitAfter slices s into all substrings after each instance of sep and
 // returns a slice of those substrings.
-// If sep is empty, SplitAfter splits after each UTF-8 sequence.
+//
+// If s does not contain sep and sep is not empty, SplitAfter returns
+// a slice of length 1 whose only element is s.
+//
+// If sep is empty, SplitAfter splits after each UTF-8 sequence. If
+// both s and sep are empty, SplitAfter returns an empty slice.
+//
 // It is equivalent to SplitAfterN with a count of -1.
 func SplitAfter(s, sep string) []string {
 	return genSplit(s, sep, len(sep), -1)