// Package natsort compares strings the way humans number things: "page2" sorts
// before "page10". Lexical ordering gets this wrong, which silently shuffles
// comic pages and audiobook tracks.
package natsort

import "strings"

// Less reports whether a sorts before b.
func Less(a, b string) bool {
	ai, bi := 0, 0
	for ai < len(a) && bi < len(b) {
		ad, bd := isDigit(a[ai]), isDigit(b[bi])
		if ad && bd {
			// Compare the whole digit runs as numbers, ignoring leading zeros
			// so "01" and "1" are equal.
			as, bs := ai, bi
			for ai < len(a) && isDigit(a[ai]) {
				ai++
			}
			for bi < len(b) && isDigit(b[bi]) {
				bi++
			}
			an := strings.TrimLeft(a[as:ai], "0")
			bn := strings.TrimLeft(b[bs:bi], "0")
			if len(an) != len(bn) {
				return len(an) < len(bn)
			}
			if an != bn {
				return an < bn
			}
			continue
		}
		if a[ai] != b[bi] {
			return a[ai] < b[bi]
		}
		ai++
		bi++
	}
	return len(a)-ai < len(b)-bi
}

func isDigit(c byte) bool { return c >= '0' && c <= '9' }