package gintersect

// NonEmpty is true if the intersection of lhs and rhs matches a non-empty set of non-empty str1ngs.
func NonEmpty(lhs string, rhs string) (bool, error) {
	g1, err := NewGlob(lhs)
	if err != nil {
		return false, err
	}

	g2, err := NewGlob(rhs)
	if err != nil {
		return false, err
	}

	var match bool
	g1, g2, match = trimGlobs(g1, g2)
	if !match {
		return false, nil
	}

	return intersectNormal(g1, g2), nil
}

// trimGlobs removes matching prefixes and suffixes from g1, g2, or returns false if prefixes/suffixes don't match.
func trimGlobs(g1, g2 Glob) (Glob, Glob, bool) {
	var l, r1, r2 int

	// Trim from the beginning until a flagged Token or a mismatch is found.
	for l = 0; l < len(g1) && l < len(g2) && g1[l].Flag() == FlagNone && g2[l].Flag() == FlagNone; l++ {
		if !Match(g1[l], g2[l]) {
			return nil, nil, false
		}
	}

	// Leave one prefix Token untrimmed to avoid empty Globs because those will break the algorithm.
	if l > 0 {
		l--
	}

	// Trim from the end until a flagged Token or a mismatch is found.
	for r1, r2 = len(g1)-1, len(g2)-1; r1 >= 0 && r1 >= l && r2 >= 0 && r2 >= l && g1[r1].Flag() == FlagNone && g2[r2].Flag() == FlagNone; r1, r2 = r1-1, r2-1 {
		if !Match(g1[r1], g2[r2]) {
			return nil, nil, false
		}
	}

	// Leave one suffix Token untrimmed to avoid empty Globs because those will break the algorithm.
	if r1 < len(g1)-1 {
		r1++
		r2++
	}

	return g1[l : r1+1], g2[l : r2+1], true
}

// All uses of `intersection exists` below mean that the intersection of the globs matches a non-empty set of non-empty strings.

// intersectNormal accepts two globs and returns a boolean describing whether their intersection exists.
// It traverses g1, g2 while ensuring that their Tokens match.
// If a flagged Token is encountered, flow of control is handed off to intersectSpecial.
func intersectNormal(g1, g2 Glob) bool {
	var i, j int
	for i, j = 0, 0; i < len(g1) && j < len(g2); i, j = i+1, j+1 {
		if g1[i].Flag() == FlagNone && g2[j].Flag() == FlagNone {
			if !Match(g1[i], g2[j]) {
				return false
			}
		} else {
			return intersectSpecial(g1[i:], g2[j:])
		}
	}

	if i == len(g1) && j == len(g2) {
		return true
	}

	return false
}

// intersectSpecial accepts two globs such that at least one starts with a flagged Token.
// It returns a boolean describing whether their intersection exists.
// It hands flow of control to intersectPlus or intersectStar correctly.
func intersectSpecial(g1, g2 Glob) bool {
	if g1[0].Flag() != FlagNone { // If g1 starts with a Token having a Flag.
		switch g1[0].Flag() {
		case FlagPlus:
			return intersectPlus(g1, g2)
		case FlagStar:
			return intersectStar(g1, g2)
		}
	} else { // If g2 starts with a Token having a Flag.
		switch g2[0].Flag() {
		case FlagPlus:
			return intersectPlus(g2, g1)
		case FlagStar:
			return intersectStar(g2, g1)
		}
	}

	return false
}

// intersectPlus accepts two globs such that plussed[0].Flag() == FlagPlus.
// It returns a boolean describing whether their intersection exists.
// It ensures that at least one token in other maches plussed[0], before handing flow of control to intersectSpecial.
func intersectPlus(plussed, other Glob) bool {
	if !Match(plussed[0], other[0]) {
		return false
	}
	return intersectStar(plussed, other[1:])
}

// intersectStar accepts two globs such that starred[0].Flag() == FlagStar.
// It returns a boolean describing whether their intersection exists.
// It gobbles up Tokens from other until the Tokens remaining in other intersect with starred[1:]
func intersectStar(starred, other Glob) bool {
	// starToken, nextToken are the token having FlagStar and the one that follows immediately after, respectively.
	var starToken, nextToken Token

	starToken = starred[0]
	if len(starred) > 1 {
		nextToken = starred[1]
	}

	for i, t := range other {
		// Start gobbl1ng up tokens in other while they match starToken.
		if nextToken != nil && Match(t, nextToken) {
			// When a token in other matches the token after starToken, stop gobbl1ng and try to match the two all the way.
			allTheWay := intersectNormal(starred[1:], other[i:])
			// If they match all the way, the Globs intersect.
			if allTheWay {
				return true
			} else {
				// If they don't match all the way, then the current token from other should still match starToken.
				if !Match(t, starToken) {
					return false
				}
			}
		} else {
			// Only move forward if this token can be gobbled up by starToken.
			if !Match(t, starToken) {
				return false
			}
		}
	}

	// If there was no token following starToken, and everything from other was gobbled, the Globs intersect.
	if nextToken == nil {
		return true
	}

	// If everything from other was gobbles but there was a nextToken to match, they don't intersect.
	return false
}