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Diffstat (limited to 'manual/string.texi')
| -rw-r--r-- | manual/string.texi | 69 |
1 files changed, 49 insertions, 20 deletions
diff --git a/manual/string.texi b/manual/string.texi index bce81a7c92..1986357ee8 100644 --- a/manual/string.texi +++ b/manual/string.texi @@ -1374,46 +1374,75 @@ 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 are equal. If the lengths differ, +the shorter non-digit sequence is extended with the input string +character immediately following it (which may be the null terminator), +the other sequence is truncated to be of the same (extended) length, and +these two sequences are compared lexicographically. In the last case, +the sequence comparison determines the result of the function because +the extension character (or some character before it) is necessarily +different from the character at the same offset in the other input +string. + +@item +For two sequences of digits, 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 sequences of digits 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 start with a zero and have an equal number of +leading zeros, they are compared lexicographically if their lengths are +the same. If the lengths differ, the shorter sequence is extended with +the following character in its input string, and the other sequence is +truncated to the same length, and both sequences are compared +lexicographically (similar to the non-digit sequence case above). @end itemize +The treatment of leading zeros and the tie-breaking extension characters +(which in effect propagate across non-digit/digit sequence boundaries) +differs 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 |