regex/matching.go

package main

// match tries to match the regex represented by given start-state, with
// the given string
func match(start *State, str string) (startIdx int, endIdx int, matched bool) {
	currentStates := make([]*State, 0)
	tempStates := make([]*State, 0) // Used to store states that should be used in next loop iteration
	i := 0                          // Index in string
	// Increment until we hit a character matching the start state
	if start.isEmpty == false {
		for int(str[i]) != start.content {
			i++
		}
	}
	currentStates = append(currentStates, start)
	startIdx = i
	i++ // Advance to next character so that we can check for transitions
	// Main loop
	for i < len(str) {
		// If there are any 0-transitions, take those
		// TODO: Maybe I need to keep taking 0-transitions until I don't have anymore. Needs to be tested
		for _, state := range currentStates {
			if len(state.transitions[EPSILON]) > 0 {
				tempStates = append(tempStates, state.transitions[EPSILON]...)
			}
		}
		copy(currentStates, tempStates)
		tempStates = nil

		// Take any transitions corresponding to current character
		for _, state := range currentStates {
			if len(state.transitions[int(str[i])]) > 0 {
				tempStates = append(tempStates, state.transitions[int(str[i])]...)
			}
		}
		copy(currentStates, tempStates)
		tempStates = nil

		// If any of the current-states is a last state, return true.
		for _, state := range currentStates {
			if state.isLast {
				endIdx = i
				return startIdx, endIdx, true
			}
		}
		i++
	}

	// We don't seem to have reached a last-state. Return fals
	return -1, -1, false

}