regex/matching.go

package main

// a matchIndex represents a match. It contains the start index and end index of the match
type matchIndex struct {
	startIdx int
	endIdx   int
}

// takeZeroState takes the 0-state (if such a transition exists) for all states in the
// given slice. It returns the resulting states. If any of the resulting states is a 0-state,
// the second parameter is true.
func takeZeroState(states []*State) (rtv []*State, isZero bool) {
	for _, state := range states {
		if len(state.transitions[EPSILON]) > 0 {
			rtv = append(rtv, state.transitions[EPSILON]...)
		}
	}
	for _, state := range rtv {
		if len(state.transitions[EPSILON]) > 0 {
			return rtv, true
		}
	}
	return rtv, false
}

// findAllMatches tries to findAllMatches the regex represented by given start-state, with
// the given string
func findAllMatches(start *State, str string) (indices []matchIndex) {
	return findAllMatchesHelper(start, str, make([]matchIndex, 0), 0)
}
func findAllMatchesHelper(start *State, str string, indices []matchIndex, offset int) []matchIndex {
	// 'Base case' - exit if string is empty.
	if len(str) == 0 {
		return indices
	}

	foundPath := false
	startIdx := 0
	endIdx := 0
	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
	startingFrom := i               // Store starting index
	// Increment until we hit a character matching the start state (assuming not 0-state)
	if start.isEmpty == false {
		for i < len(str) && int(str[i]) != start.content {
			i++
		}
		startIdx = i
		startingFrom = 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)

	// Main loop
	for i < len(str) {
		foundPath = false

		zeroStates := make([]*State, 0)
		// Keep taking zero-states, until there are no more left to take
		// Objective: If any of our current states have transitions to 0-states, replace them with the 0-state. Do this until there are no more transitions to 0-states
		zeroStates, isZero := takeZeroState(currentStates)
		tempStates = append(tempStates, zeroStates...)
		for isZero == true {
			zeroStates, isZero = takeZeroState(tempStates)
			tempStates = append(tempStates, zeroStates...)
		}

		currentStates = append(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])]...)
				foundPath = true
			}
		}

		if foundPath == false {
			// This enables the 'greedy' behavior - last-state status is only checked if we didn't find a path forward
			for _, state := range currentStates {
				if state.isLast {
					endIdx = i
					indices = append(indices, matchIndex{startIdx + offset, endIdx + offset})
				}
			}
			// Recursion - match with rest of string if we have nowhere to go. If we haven't moved in the string, increment the counter by 1 to ensure we don't keep trying the same string over and over
			if i == startingFrom {
				i++
			}
			return findAllMatchesHelper(start, str[i:], indices, offset+i)
		}
		currentStates = make([]*State, len(tempStates))
		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]...)
		}
	}
	currentStates = append(currentStates, tempStates...)
	tempStates = nil

	for _, state := range currentStates {
		// Only add the match if we the start index is in bounds
		if state.isLast && startIdx+offset < len(str)+offset {
			endIdx = i
			indices = append(indices, matchIndex{startIdx + offset, endIdx + offset})
		}
	}

	// Default
	return indices
}