//
//  Qwen3CoreML.swift
//  Qwen3 CoreML Example
//
//  Swift Example for Qwen3 CoreML Integration
//  Qwen3-0.6B CoreML integration with Stateful KV-Cache and Int4 quantization
//
//  Requirements:
//  - iOS 18.0+ / macOS 15.0+ (Apple Neural Engine support)
//  - 400-500MB RAM for both models
//  - swift-transformers package
//
//  Usage:
//  let qwen3 = Qwen3CoreML()
//  await qwen3.loadModels()
//  let response = await qwen3.generate("Hello, world!")
//

import Foundation
import CoreML
import Tokenizers

/// Qwen3-0.6B CoreML model wrapper with Stateful KV-Cache
@MainActor
public final class Qwen3CoreML {

    // MARK: - Configuration

    public struct Config {
        public static let maxContextLength = 1024
        public static let maxTokens = 512
        public static let temperature: Float = 0.7
        public static let topK = 40
        public static let topP: Float = 0.9

        // Model paths (relative to app bundle or absolute)
        public static let prefillModelName = "Qwen3-0.6B-Prefill-Int4"
        public static let decodeModelName = "Qwen3-0.6B-Decode-Int4"
        public static let tokenizerModelId = "Qwen/Qwen3-0.6B"
    }

    // MARK: - State

    private var prefillModel: MLModel?
    private var decodeModel: MLModel?
    private var tokenizer: Tokenizer?
    private var decodeState: MLState?

    private(set) var isModelsLoaded = false
    private(set) var isGenerating = false

    // Qwen3 special tokens
    private let eosTokenIds: Set<Int> = [151643, 151645] // <|endoftext|>, <|im_end|>
    private let bosTokenId = 151643
    private let chatTemplate = "<|im_start|>system\nYou are a helpful assistant.<|im_end|>\n<|im_start|>user\n%@<|im_end|>\n<|im_start|>assistant\n"

    // Performance tracking
    private(set) var tokensPerSecond: Double = 0
    private(set) var currentPosition = 0

    // MARK: - Initialization

    public init() {
        print("🤖 Qwen3CoreML initialized")
    }

    // MARK: - Model Loading

    /// Load both Prefill and Decode CoreML models and tokenizer
    public func loadModels() async throws {
        guard !isModelsLoaded else {
            print("🤖 Qwen3: Models already loaded")
            return
        }

        print("🤖 Qwen3: Loading CoreML models and tokenizer...")

        do {
            // Load Prefill model
            try await loadModel(named: Config.prefillModelName, into: &prefillModel)
            print("✅ Prefill model loaded")

            // Load Decode model with state
            try await loadModel(named: Config.decodeModelName, into: &decodeModel, withState: true)
            print("✅ Decode model loaded")

            // Load tokenizer via Tokenizers framework
            tokenizer = try await AutoTokenizer.from(pretrained: Config.tokenizerModelId)
            print("✅ Tokenizer loaded")

            isModelsLoaded = true
            print("🎉 Qwen3 models loaded successfully")

        } catch {
            print("❌ Failed to load Qwen3 models: \(error.localizedDescription)")
            throw Qwen3Error.modelLoadingFailed(error.localizedDescription)
        }
    }

    /// Load a single CoreML model
    private func loadModel(named modelName: String, into model: inout MLModel?, withState: Bool = false) async throws {
        let config = MLModelConfiguration()
        config.computeUnits = .cpuAndNeuralEngine  // Use ANE when available

        // Try Bundle first, then local paths
        var modelURL: URL?

        // Check main bundle
        if let url = Bundle.main.url(forResource: modelName, withExtension: "mlpackage") {
            modelURL = url
        }
        // Check app support directory
        else if let appSupport = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first {
            let appDir = appSupport.appendingPathComponent("Qwen3CoreML")
            let modelsDir = appDir.appendingPathComponent("Models")
            let modelPath = modelsDir.appendingPathComponent("\(modelName).mlpackage")
            if FileManager.default.fileExists(atPath: modelPath.path) {
                modelURL = modelPath
            }
        }

        guard let modelURL = modelURL else {
            throw Qwen3Error.modelNotFound(modelName)
        }

        // Compile and load model
        let compiledURL = try await MLModel.compileModel(at: modelURL)
        model = try MLModel(contentsOf: compiledURL, configuration: config)

        // Create state for decode model only
        if withState {
            decodeState = model?.makeState()
        }
    }

    // MARK: - Text Generation

    /// Generate text response for user message (streaming)
    public func generate(
        userMessage: String,
        systemPrompt: String = "You are a helpful assistant.",
        maxTokens: Int = Config.maxTokens,
        temperature: Float = Config.temperature,
        enableThinking: Bool = false
    ) -> AsyncStream<String> {
        AsyncStream { continuation in
            Task {
                await generateInternal(
                    userMessage: userMessage,
                    systemPrompt: systemPrompt,
                    maxTokens: maxTokens,
                    temperature: temperature,
                    enableThinking: enableThinking,
                    continuation: continuation
                )
            }
        }
    }

    /// Generate text response for user message (non-streaming)
    public func generateSync(
        userMessage: String,
        systemPrompt: String = "You are a helpful assistant.",
        maxTokens: Int = Config.maxTokens,
        temperature: Float = Config.temperature,
        enableThinking: Bool = false
    ) async throws -> String {
        guard isModelsLoaded, let tokenizer = tokenizer else {
            throw Qwen3Error.modelNotLoaded
        }

        var result = ""

        for await chunk in generate(
            userMessage: userMessage,
            systemPrompt: systemPrompt,
            maxTokens: maxTokens,
            temperature: temperature,
            enableThinking: enableThinking
        ) {
            result += chunk
        }

        return result
    }

    /// Reset conversation and KV-Cache state
    public func resetConversation() {
        decodeState = decodeModel?.makeState()
        currentPosition = 0
        print("🔄 Qwen3 conversation reset")
    }

    // MARK: - Private Generation

    private func generateInternal(
        userMessage: String,
        systemPrompt: String,
        maxTokens: Int,
        temperature: Float,
        enableThinking: Bool,
        continuation: AsyncStream<String>.Continuation
    ) async {
        guard isModelsLoaded,
              let prefillModel = prefillModel,
              let decodeModel = decodeModel,
              let tokenizer = tokenizer,
              var decodeState = decodeState else {
            continuation.finish()
            return
        }

        isGenerating = true
        let startTime = Date()

        defer {
            isGenerating = false
            continuation.finish()
        }

        do {
            // Format chat prompt
            let chatPrompt = formatChatPrompt(
                userMessage: userMessage,
                systemPrompt: systemPrompt,
                enableThinking: enableThinking
            )

            // Tokenize prompt
            let inputTokens = tokenizer.encode(text: chatPrompt)

            // Check context length
            guard inputTokens.count + maxTokens <= Config.maxContextLength else {
                print("⚠️ Prompt too long, truncating...")
                // Truncate if needed
                let truncatedTokens = Array(inputTokens.suffix(Config.maxContextLength - maxTokens))
                // Add BOS token if missing
                let tokensToProcess = truncatedTokens.first == bosTokenId ? truncatedTokens : [bosTokenId] + truncatedTokens
                try await processTokens(tokensToProcess, model: prefillModel)
            }

            // Process initial tokens with Prefill model
            try await processTokens(inputTokens, model: prefillModel)

            // Generate new tokens with Decode model
            var generatedTokens: [Int] = []
            var isInThinkingBlock = false

            for _ in 0..<maxTokens {
                let nextToken = try await generateNextToken(
                    temperature: temperature,
                    decodeModel: decodeModel,
                    decodeState: &decodeState
                )

                // Check for end of generation
                if eosTokenIds.contains(nextToken) {
                    break
                }

                generatedTokens.append(nextToken)

                // Handle thinking blocks (thinking mode)
                if nextToken == 151667 { // <think>
                    isInThinkingBlock = true
                } else if nextToken == 151668 { // </think>
                    isInThinkingBlock = false
                    if !enableThinking {
                        continue
                    }
                }

                // Decode token to text
                let tokenText = tokenizer.decode(tokens: [nextToken])

                // Stream token if not in thinking block or thinking enabled
                if !isInThinkingBlock || enableThinking {
                    continuation.yield(tokenText)
                }
            }

            // Calculate performance
            let elapsed = Date().timeIntervalSince(startTime)
            tokensPerSecond = Double(generatedTokens.count) / elapsed
            print("📊 Generation: \(generatedTokens.count) tokens in \(String(format: "%.2f", elapsed))s (\(String(format: "%.1f", tokensPerSecond)) tok/s)")

        } catch {
            print("❌ Generation failed: \(error.localizedDescription)")
            // Note: We don't throw here since continuation is already finished
        }
    }

    /// Process initial tokens using Prefill model
    private func processTokens(_ tokens: [Int], model: MLModel) async throws {
        let seqLen = tokens.count

        // Create causal mask for all tokens
        let causalMask = createCausalMask(seqLen: seqLen, totalLen: seqLen)
        let inputIdsTensor = MLTensor(
            shape: [1, seqLen],
            scalars: tokens.map { Int32($0) },
            scalarType: Int32.self
        )

        let inputs = try MLDictionaryFeatureProvider(dictionary: [
            "inputIds": MLFeatureValue(tensor: inputIdsTensor),
            "causalMask": MLFeatureValue(tensor: causalMask)
        ])

        // Run prefill inference
        _ = try await model.prediction(from: inputs)
        currentPosition = seqLen
    }

    /// Generate next token using Decode model
    private func generateNextToken(
        temperature: Float,
        decodeModel: MLModel,
        decodeState: inout MLState
    ) async throws -> Int {
        // Current position as input
        let positionIds = [Int32(currentPosition)]

        let positionTensor = MLTensor(
            shape: [1, 1],
            scalars: positionIds,
            scalarType: Int32.self
        )

        // We need a dummy input ID, actual logit generation uses past KV cache
        let dummyInputTensor = MLTensor(
            shape: [1, 1],
            scalars: [Int32(0)], // Will be ignored in decode model
            scalarType: Int32.self
        )

        let inputs = try MLDictionaryFeatureProvider(dictionary: [
            "inputIds": MLFeatureValue(tensor: dummyInputTensor),
            "positionIds": MLFeatureValue(tensor: positionTensor),
        ])

        let output = try await decodeModel.prediction(from: inputs, using: decodeState)

        guard let logitsTensor = output.featureValue(for: "logits")?.tensorValue(of: Float16.self) else {
            throw Qwen3Error.inferenceError("No logits in model output")
        }

        // Sample from logits
        let nextToken = sampleToken(from: logitsTensor, temperature: temperature)

        // Update position for next step
        currentPosition += 1

        return nextToken
    }

    /// Sample next token from logits
    private func sampleToken(from logitsTensor: MLTensor, temperature: Float) -> Int {
        // Extract logits for the last token [1, 1, vocab_size] -> [vocab_size]
        let vocabSize = logitsTensor.shape[2]

        var logitsArray = [Float](repeating: 0, count: vocabSize)
        logitsTensor.withUnsafeBufferPointer(of: Float16.self) { buffer in
            for i in 0..<vocabSize {
                logitsArray[i] = Float(buffer[vocabSize + i]) // Last token position
            }
        }

        if temperature <= 0 {
            // Greedy sampling
            return logitsArray.enumerated().max(by: { $0.element < $1.element })?.offset ?? 0
        }

        // Apply temperature and sample
        let scaledLogits = logitsArray.map { $0 / temperature }
        let maxLogit = scaledLogits.max() ?? 0
        let expLogits = scaledLogits.map { exp($0 - maxLogit) }
        let sumExp = expLogits.reduce(0, +)
        let probs = expLogits.map { $0 / sumExp }

        // Sample from distribution
        let random = Float.random(in: 0..<1)
        var cumulative: Float = 0

        for (index, prob) in probs.enumerated() {
            cumulative += prob
            if random < cumulative {
                return index
            }
        }

        return vocabSize - 1
    }

    /// Create causal attention mask
    private func createCausalMask(seqLen: Int, totalLen: Int) -> MLTensor {
        var maskData = [Float16](repeating: Float16(-Float.infinity), count: seqLen * totalLen)

        for i in 0..<seqLen {
            for j in 0..<(totalLen - seqLen + i + 1) {
                maskData[i * totalLen + j] = Float16(0)
            }
        }

        return MLTensor(
            shape: [1, 1, seqLen, totalLen],
            scalars: maskData,
            scalarType: Float16.self
        )
    }

    /// Format chat prompt using Qwen3 chat template
    private func formatChatPrompt(userMessage: String, systemPrompt: String, enableThinking: Bool) -> String {
        let chatTemplate = "<|im_start|>system\n\(systemPrompt)<|im_end|>\n<|im_start|>user\n\(userMessage)<|im_end|>\n<|im_start|>assistant\n"

        if enableThinking {
            return chatTemplate
        } else {
            return chatTemplate + "/no_think\n"
        }
    }
}

// MARK: - Errors

public enum Qwen3Error: LocalizedError {
    case modelNotFound(String)
    case modelNotLoaded
    case modelLoadingFailed(String)
    case inferenceError(String)
    case tokenizationError

    public var errorDescription: String? {
        switch self {
        case .modelNotFound(let modelName):
            return "Model '\(modelName)' not found. Place it in app bundle or ~/Library/Application Support/Qwen3CoreML/Models/"
        case .modelNotLoaded:
            return "Models are not loaded. Call loadModels() first."
        case .inferenceError(let message):
            return "Inference error: \(message)"
        case .tokenizationError:
            return "Tokenization error"
        }
    }
}

// MARK: - Helper Methods

/// Extension with utility methods for text processing
extension Qwen3CoreML {

    /// Correct text using Qwen3 (compatible with LLMRunner.correct())
    public func correct(text: String) async throws -> String {
        return try await generateSync(
            userMessage: """
            Please correct the following text by fixing punctuation, capitalization, and grammatical errors.
            Keep the original language. Only output the corrected text, nothing else.

            Text: \(text)

            Corrected:
            """,
            systemPrompt: "You are a professional proofreader and text editor.",
            maxTokens: 256,
            temperature: 0.1 // Low temperature for consistent corrections
        ).trimmingCharacters(in: .whitespacesAndNewlines)
    }
}