[PATCH] manual: Clarify the documentation of strverscmp [BZ #20524]

[fweimer at redhat dot com (Florian Weimer)]

2016-08-29  Florian Weimer  <fweimer@redhat.com>

	[BZ #20524]
	* manual/string.texi (String/Array Comparison): Clarify the
	strverscmp behavior.

diff --git a/manual/string.texi b/manual/string.texi
index bce81a7..3577017 100644
--- a/manual/string.texi
+++ b/manual/string.texi
@@ -1374,46 +1374,70 @@ The @code{strverscmp} function compares the string @var{s1} against
 @var{s2}, considering them as holding indices/version numbers.  The
 return value follows the same conventions as found in the
 @code{strcmp} function.  In fact, if @var{s1} and @var{s2} contain no
-digits, @code{strverscmp} behaves like @code{strcmp}.
+digits, @code{strverscmp} behaves like @code{strcmp}
+(in the sense that the sign of the result is the same).
 
-Basically, we compare strings normally (byte by byte), until
-we find a digit in each string - then we enter a special comparison
-mode, where each sequence of digits is taken as a whole.  If we reach the
-end of these two parts without noticing a difference, we return to the
-standard comparison mode.  There are two types of numeric parts:
-"integral" and "fractional" (those  begin with a '0').  The types
-of the numeric parts affect the way we sort them:
+The comparison algorithm which the @code{strverscmp} function implements
+differs slightly from other version comparison algorithms.  The
+implementation is based on a finite state machine, whose behavior is
+approximated below.
 
 @itemize @bullet
 @item
-integral/integral: we compare values as you would expect.
+The input strings are each split into sequences of non-digits and
+digits.  These sequences can be empty at the beginning and end of the
+string.  Digits are determined by the @code{isdigit} function and are
+thus subject to the current locale.
 
 @item
-fractional/integral: the fractional part is less than the integral one.
-Again, no surprise.
+Comparison starts with a (possibly empty) non-digit sequence.  The first
+non-equal sequences of non-digits or digits determines the outcome of
+the comparison.
 
 @item
-fractional/fractional: the things become a bit more complex.
-If the common prefix contains only leading zeroes, the longest part is less
-than the other one; else the comparison behaves normally.
+Corresponding non-digit sequences in both strings are compared
+lexicographically.  If their lengths differ, the shorter non-digit
+sequence is amended with the input string character immediately
+following it (which can be the null terminator).  This character is
+compared against the non-digit character in the other sequence, at the
+same string offset, and their difference determines the outcome of the
+comparison.
+
+@item
+For two digit sequences, the number of leading zeros is counted (which
+can be zero).  If the count differs, the string with more leading zeros
+in the digit sequence is considered smaller than the other string.
+
+@item
+If the two digit sequences have no leading zeros, they are compared as
+integers, that is, the string with the longer digit sequence is deemed
+larger, and if both sequences are of equal length, they are compared
+lexicographically.
+
+@item
+If both digit sequences have an equal, positive number of leading zeros,
+they are compared lexicographically.  If their length differs, another
+character is added to to the shorter sequence, to determine the outcome
+of the comparison, as described above for the non-digit sequence case.
 @end itemize
 
+The treatment of leading zeros and the tie-breaking characters (which in
+effect propagate across non-digit/digit sequence boundaries) differ from
+other version comparison algorithms.
+
 @smallexample
 strverscmp ("no digit", "no digit")
     @result{} 0    /* @r{same behavior as strcmp.} */
 strverscmp ("item#99", "item#100")
     @result{} <0   /* @r{same prefix, but 99 < 100.} */
 strverscmp ("alpha1", "alpha001")
-    @result{} >0   /* @r{fractional part inferior to integral one.} */
+    @result{} >0   /* @r{different number of leading zeros (0 and 2).} */
 strverscmp ("part1_f012", "part1_f01")
-    @result{} >0   /* @r{two fractional parts.} */
+    @result{} >0   /* @r{lexicographical comparison with leading zeros.} */
 strverscmp ("foo.009", "foo.0")
-    @result{} <0   /* @r{idem, but with leading zeroes only.} */
+    @result{} <0   /* @r{different number of leading zeros (2 and 1).} */
 @end smallexample
 
-This function is especially useful when dealing with filename sorting,
-because filenames frequently hold indices/version numbers.
-
 @code{strverscmp} is a GNU extension.
 @end deftypefun