natsort.go

package irelate

// This is taken from vbom.ml/util/sortorder/natsort.go and put here due to
// Installation issues on go 1.5. Here is the License from that repo:
/*
The MIT License (MIT)
Copyright (c) 2015 Frits van Bommel
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

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 not 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)
}