| package sortorder |
| |
| // Natural implements sort.Interface to sort strings in natural order. This |
| // means that e.g. "abc2" < "abc12". |
| // |
| // Non-digit sequences and numbers are compared separately. The former are |
| // compared bytewise, while the latter are compared numerically (except that |
| // the number of leading zeros is used as a tie-breaker, so e.g. "2" < "02") |
| // |
| // Limitation: only ASCII digits (0-9) are considered. |
| type Natural []string |
| |
| func (n Natural) Len() int { return len(n) } |
| func (n Natural) Swap(i, j int) { n[i], n[j] = n[j], n[i] } |
| func (n Natural) Less(i, j int) bool { return NaturalLess(n[i], n[j]) } |
| |
| func isdigit(b byte) bool { return '0' <= b && b <= '9' } |
| |
| // NaturalLess compares two strings using natural ordering. This means that e.g. |
| // "abc2" < "abc12". |
| // |
| // Non-digit sequences and numbers are compared separately. The former are |
| // compared bytewise, while the latter are compared numerically (except that |
| // the number of leading zeros is used as a tie-breaker, so e.g. "2" < "02") |
| // |
| // Limitation: only ASCII digits (0-9) are considered. |
| func NaturalLess(str1, str2 string) bool { |
| idx1, idx2 := 0, 0 |
| for idx1 < len(str1) && idx2 < len(str2) { |
| c1, c2 := str1[idx1], str2[idx2] |
| dig1, dig2 := isdigit(c1), isdigit(c2) |
| switch { |
| case dig1 != dig2: // Digits before other characters. |
| return dig1 // True if LHS is a digit, false if the RHS is one. |
| case !dig1: // && !dig2, because dig1 == dig2 |
| // UTF-8 compares bytewise-lexicographically, no need to decode |
| // codepoints. |
| if c1 != c2 { |
| return c1 < c2 |
| } |
| idx1++ |
| idx2++ |
| default: // Digits |
| // Eat zeros. |
| for ; idx1 < len(str1) && str1[idx1] == '0'; idx1++ { |
| } |
| for ; idx2 < len(str2) && str2[idx2] == '0'; idx2++ { |
| } |
| // Eat all digits. |
| nonZero1, nonZero2 := idx1, idx2 |
| for ; idx1 < len(str1) && isdigit(str1[idx1]); idx1++ { |
| } |
| for ; idx2 < len(str2) && isdigit(str2[idx2]); idx2++ { |
| } |
| // If lengths of numbers with non-zero prefix differ, the shorter |
| // one is less. |
| if len1, len2 := idx1-nonZero1, idx2-nonZero2; len1 != len2 { |
| return len1 < len2 |
| } |
| // If they're equal, string comparison is correct. |
| if nr1, nr2 := str1[nonZero1:idx1], str2[nonZero2:idx2]; nr1 != nr2 { |
| return nr1 < nr2 |
| } |
| // Otherwise, the one with less zeros is less. |
| // Because everything up to the number is equal, comparing the index |
| // after the zeros is sufficient. |
| if nonZero1 != nonZero2 { |
| return nonZero1 < nonZero2 |
| } |
| } |
| // They're identical so far, so continue comparing. |
| } |
| // So far they are identical. At least one is ended. If the other continues, |
| // it sorts last. |
| return len(str1) < len(str2) |
| } |