Slice a string into substrings, keeping the matches

split-interleaved.go

// Splits a string into parts by a given pattern, interleaving the matched
// parts with the unmatched parts.
// For example:
//  splitInterleaved(/a/, "aabbac") => []string{"a", "a", "bb", "a", "c"}
// Another way of looking at it is:
//  regexp.Split() <zip> regexp.FindAllString() <flatten>
//
func splitInterleaved(re *regexp.Regexp, str string) []string {
	splits := re.FindAllStringIndex(str, -1) // Find all the split points
	slice := make([]string, len(splits)*2+1) // Allocate enough space

	// `pos` tracks where we are in the slice
	pos := 0
	addToSlice := func(part string) {
		slice[pos] = part
		pos += 1
	}

	// `strIdx` tracks where we are in the string (which character)
	strIdx := 0
	for _, split := range splits {
		a, b := split[0], split[1]
		if a > strIdx {
			addToSlice(str[strIdx:a])
		}
		addToSlice(str[a:b])
		strIdx = b
	}

	// Pick up the end piece if it's left over
	if strIdx < len(str) {
		addToSlice(str[strIdx:len(str)])
	}

	// Re-slice to the portion we used
	return slice[:pos]
}