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 i < len(str) && int(str[i]) != start.content {
			i++
		}
		i++ // Advance to next character (if we aren't at a 0-state, which doesn't match anything), so that we can check for transitions. If we advance at a 0-state, we will never get a chance to match the first character
	}
	currentStates = append(currentStates, start)
	startIdx = i

	// 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])]...)
			} else {
				// This enables the 'greedy' behavior - last-state status is only checked if we can't match anything else
				if state.isLast {
					endIdx = i
					return startIdx, endIdx, true
				}
			}
		}
		copy(currentStates, tempStates)
		tempStates = nil

		i++
	}

	// End-of-string reached. Go to any 0-states. Then check if any of our states are in the end position.
	for _, state := range currentStates {
		if len(state.transitions[EPSILON]) > 0 {
			tempStates = append(tempStates, state.transitions[EPSILON]...)
		}
	}
	copy(currentStates, tempStates)
	tempStates = nil

	for _, state := range currentStates {
		if state.isLast {
			endIdx = i
			return startIdx, endIdx, true
		} else {
			return -1, -1, false
		}
	}

	// Default
	return -1, -1, false
}