import Foundation

public struct ArabicPronunciationProcessor: Sendable {
    public init() {}

    public func process(
        lyrics: String,
        settings: ArabicPronunciationSettings
    ) -> ArabicPronunciationProcessingResult {
        guard settings.isEnabled else {
            return ArabicPronunciationProcessingResult(
                text: lyrics,
                notes: [.processingDisabled]
            )
        }

        let protection = protectPreservedSpellings(
            in: lyrics,
            preservedSpellings: settings.preservedSpellings
        )
        var processedText = protection.text.precomposedStringWithCanonicalMapping
        var notes = processingNotes(for: processedText, settings: settings)

        if settings.tanweenPolicy == .removeWhenUnwanted {
            processedText = removeTanween(from: processedText)
        }

        if settings.tanweenPolicy == .addWhenPronunciationRequires,
           containsArabicLetter(in: processedText),
           !containsTanween(in: processedText) {
            notes.append(.tanweenAdditionNeedsReview)
        }

        processedText = restorePreservedSpellings(
            in: processedText,
            replacements: protection.replacements
        )

        if protection.matchCount > 0 {
            notes.append(.preservedSpellingsProtected(count: protection.matchCount))
        }

        return ArabicPronunciationProcessingResult(
            text: processedText,
            notes: notes
        )
    }

    private func processingNotes(
        for text: String,
        settings: ArabicPronunciationSettings
    ) -> [ArabicPronunciationProcessingNote] {
        guard containsArabicLetter(in: text) else {
            return []
        }

        switch settings.diacritizationPolicy {
        case .unspecified:
            return [.diacritizationPolicyUnspecified]
        case .pronunciationTargeted where !containsDiacritics(in: text):
            return [.pronunciationTargetedDiacriticsNeedReview]
        case .fullTashkeel where !containsDiacritics(in: text):
            return [.fullTashkeelNeedsReview]
        default:
            return []
        }
    }

    private func protectPreservedSpellings(
        in text: String,
        preservedSpellings: [String]
    ) -> PreservedSpellingProtection {
        let spellings = Set(
            preservedSpellings.map {
                $0.trimmingCharacters(in: .whitespacesAndNewlines)
            }
        )
        .filter { !$0.isEmpty }
        .sorted {
            if $0.count == $1.count {
                return $0 < $1
            }
            return $0.count > $1.count
        }

        var protectedText = text
        var replacements: [PreservedSpellingReplacement] = []
        var matchCount = 0

        for (index, spelling) in spellings.enumerated() {
            let occurrenceCount = protectedText.components(separatedBy: spelling).count - 1
            guard occurrenceCount > 0 else {
                continue
            }

            let placeholder = uniquePlaceholder(
                for: index,
                in: protectedText
            )
            protectedText = protectedText.replacingOccurrences(
                of: spelling,
                with: placeholder
            )
            replacements.append(
                PreservedSpellingReplacement(
                    placeholder: placeholder,
                    spelling: spelling
                )
            )
            matchCount += occurrenceCount
        }

        return PreservedSpellingProtection(
            text: protectedText,
            replacements: replacements,
            matchCount: matchCount
        )
    }

    private func uniquePlaceholder(for index: Int, in text: String) -> String {
        var collisionIndex = 0
        var placeholder = "[[music-assistant-preserved-\(index)]]"

        while text.contains(placeholder) {
            collisionIndex += 1
            placeholder = "[[music-assistant-preserved-\(index)-\(collisionIndex)]]"
        }

        return placeholder
    }

    private func restorePreservedSpellings(
        in text: String,
        replacements: [PreservedSpellingReplacement]
    ) -> String {
        replacements.reduce(text) { result, replacement in
            result.replacingOccurrences(
                of: replacement.placeholder,
                with: replacement.spelling
            )
        }
    }

    private func removeTanween(from text: String) -> String {
        String(text.unicodeScalars.filter { scalar in
            !isTanween(scalar)
        })
    }

    private func containsArabicLetter(in text: String) -> Bool {
        text.unicodeScalars.contains { scalar in
            let isArabicBlock = (0x0600...0x06FF).contains(scalar.value)
                || (0x0750...0x077F).contains(scalar.value)
                || (0x08A0...0x08FF).contains(scalar.value)
            return isArabicBlock && CharacterSet.letters.contains(scalar)
        }
    }

    private func containsDiacritics(in text: String) -> Bool {
        text.unicodeScalars.contains(where: isArabicDiacritic)
    }

    private func containsTanween(in text: String) -> Bool {
        text.unicodeScalars.contains(where: isTanween)
    }

    private func isArabicDiacritic(_ scalar: Unicode.Scalar) -> Bool {
        (0x064B...0x065F).contains(scalar.value) || scalar.value == 0x0670
    }

    private func isTanween(_ scalar: Unicode.Scalar) -> Bool {
        (0x064B...0x064D).contains(scalar.value)
    }
}

private struct PreservedSpellingProtection {
    let text: String
    let replacements: [PreservedSpellingReplacement]
    let matchCount: Int
}

private struct PreservedSpellingReplacement {
    let placeholder: String
    let spelling: String
}

public struct ArabicPronunciationProcessingResult: Equatable, Sendable {
    public var text: String
    public var notes: [ArabicPronunciationProcessingNote]

    public init(text: String, notes: [ArabicPronunciationProcessingNote] = []) {
        self.text = text
        self.notes = notes
    }
}

public enum ArabicPronunciationProcessingNote: Equatable, Sendable {
    case processingDisabled
    case diacritizationPolicyUnspecified
    case pronunciationTargetedDiacriticsNeedReview
    case fullTashkeelNeedsReview
    case tanweenAdditionNeedsReview
    case preservedSpellingsProtected(count: Int)
}