@preconcurrency import Foundation
@preconcurrency import Metal

public enum E4BError: Error, LocalizedError {
    case noMetalDevice
    case libraryNotFound
    case sourceCompilationFailed(String)
    case pipelineCreationFailed(String)
    case pipelineNotFound(String)
    case bufferCreationFailed
    case commandBufferCreationFailed
    case commandEncoderCreationFailed

    public var errorDescription: String? {
        switch self {
        case .noMetalDevice: return "No Metal-capable GPU found"
        case .libraryNotFound: return "Metal library not found"
        case .sourceCompilationFailed(let detail): return "Metal source compilation failed: \(detail)"
        case .pipelineCreationFailed(let detail): return "Pipeline creation failed: \(detail)"
        case .pipelineNotFound(let name): return "Kernel '\(name)' not found in library"
        case .bufferCreationFailed: return "Failed to allocate Metal buffer"
        case .commandBufferCreationFailed: return "Failed to create command buffer"
        case .commandEncoderCreationFailed: return "Failed to create command encoder"
        }
    }
}

/// Pure-Swift Metal engine with pipeline cache, proper threadgroup sizing,
/// buffer pool for reuse, and support for both runtime source compilation and pre-compiled metallib.
public final class MarkBaseEngine: @unchecked Sendable {
    public let device: MTLDevice
    public let commandQueue: MTLCommandQueue
    public let bufferPool: BufferPool
    public private(set) var library: MTLLibrary?
    private var pipelineCache: [String: MTLComputePipelineState] = [:]

    public init() throws {
        guard let device = MTLCreateSystemDefaultDevice() else {
            throw E4BError.noMetalDevice
        }
        self.device = device
        guard let queue = device.makeCommandQueue() else {
            throw E4BError.bufferCreationFailed
        }
        self.commandQueue = queue
        self.bufferPool = BufferPool(device: device)
    }
    
    /// Initialize with Metal kernels auto-compiled from source.
    /// Loads original, optimized, and fusion kernels.
    public convenience init(autoCompile: Bool = true) throws {
        try self.init()
        if autoCompile {
            try compileSource(MetalKernels.fullOptimizedSourceWithFusion)
        }
    }

    // ── Library ──────────────────────────────────────

    /// Compile Metal source at runtime.
    public func compileSource(_ source: String) throws {
        pipelineCache.removeAll()
        library = try device.makeLibrary(source: source, options: nil)
    }

    /// Load a pre-compiled .metallib from a file path.
    public func loadMetallib(path: String) throws {
        let data = try Data(contentsOf: URL(fileURLWithPath: path))
        try loadMetallib(data: data)
    }

    /// Load a pre-compiled .metallib from Data.
    public func loadMetallib(data: Data) throws {
        pipelineCache.removeAll()
        library = try data.withUnsafeBytes { rawPtr in
            try device.makeLibrary(data: DispatchData(bytes: rawPtr))
        }
    }

    // ── Pipeline (with cache) ─────────────────────────

    /// Return a cached or newly-created compute pipeline state.
    public func pipeline(named name: String) throws -> MTLComputePipelineState {
        if let cached = pipelineCache[name] { return cached }
        guard let lib = library else { throw E4BError.libraryNotFound }
        guard let fn = lib.makeFunction(name: name) else {
            throw E4BError.pipelineNotFound(name)
        }
        let pso = try device.makeComputePipelineState(function: fn)
        pipelineCache[name] = pso
        return pso
    }

    /// Clear the pipeline cache (e.g. after recompiling the library).
    public func clearPipelineCache() { pipelineCache.removeAll() }

    // ── Buffers ───────────────────────────────────────

    /// Acquire a buffer from the pool (or create new if none available)
    public func acquireBuffer(length: Int) -> MTLBuffer {
        return bufferPool.acquire(length: length)
    }
    
    /// Release a buffer back to the pool for reuse
    public func releaseBuffer(_ buffer: MTLBuffer) {
        bufferPool.release(buffer)
    }

    public func makeBuffer<T>(_ values: [T]) throws -> MTLBuffer {
        let count = values.count * MemoryLayout<T>.stride
        guard let buf = values.withUnsafeBytes({ rawPtr in
            device.makeBuffer(bytes: rawPtr.baseAddress!, length: count,
                              options: .storageModeShared)
        }) else { throw E4BError.bufferCreationFailed }
        return buf
    }

    public func makeBuffer(length: Int) throws -> MTLBuffer {
        guard let buf = device.makeBuffer(length: length, options: .storageModeShared) else {
            throw E4BError.bufferCreationFailed
        }
        return buf
    }

    // ── Threadgroup sizing ────────────────────────────

    /// Optimal 1D threadgroup size for a given pipeline.
    public func threadgroupSize1D(_ pipeline: MTLComputePipelineState,
                                  count: Int) -> MTLSize {
        let w = min(pipeline.maxTotalThreadsPerThreadgroup, count)
        return MTLSize(width: w, height: 1, depth: 1)
    }

    /// Optimal 2D threadgroup size for a given pipeline.
    public func threadgroupSize2D(_ pipeline: MTLComputePipelineState,
                                  grid: (width: Int, height: Int)) -> MTLSize {
        let w = pipeline.threadExecutionWidth
        let h = pipeline.maxTotalThreadsPerThreadgroup / w
        return MTLSize(
            width: min(w, grid.width),
            height: min(h, grid.height),
            depth: 1
        )
    }

    // ── Synchronous dispatch ──────────────────────────

    /// Synchronous 1D dispatch with proper threadgroup sizing.
    @discardableResult
    public func dispatch1D(
        _ pipeline: MTLComputePipelineState,
        buffers: [MTLBuffer],
        count: Int
    ) throws -> MTLCommandBuffer {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        let tg = threadgroupSize1D(pipeline, count: count)
        enc.setComputePipelineState(pipeline)
        for (i, buf) in buffers.enumerated() { enc.setBuffer(buf, offset: 0, index: i) }
        enc.dispatchThreads(MTLSize(width: count, height: 1, depth: 1),
                            threadsPerThreadgroup: tg)
        enc.endEncoding()
        cmdBuf.commit()
        cmdBuf.waitUntilCompleted()
        return cmdBuf
    }

    /// Synchronous 2D dispatch with proper threadgroup sizing.
    @discardableResult
    public func dispatch2D(
        _ pipeline: MTLComputePipelineState,
        buffers: [MTLBuffer],
        grid: (width: Int, height: Int)
    ) throws -> MTLCommandBuffer {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        let tg = threadgroupSize2D(pipeline, grid: grid)
        enc.setComputePipelineState(pipeline)
        for (i, buf) in buffers.enumerated() { enc.setBuffer(buf, offset: 0, index: i) }
        enc.dispatchThreads(MTLSize(width: grid.width, height: grid.height, depth: 1),
                            threadsPerThreadgroup: tg)
        enc.endEncoding()
        cmdBuf.commit()
        cmdBuf.waitUntilCompleted()
        return cmdBuf
    }

    // ── Async dispatch ────────────────────────────────

    /// Batch dispatch: execute multiple kernel operations in a single command buffer
    /// Returns after all GPU work completes
    public func batchDispatch(_ operations: [(MTLComputePipelineState, [MTLBuffer], MTLSize)]) throws {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        
        for (pso, buffers, gridSize) in operations {
            enc.setComputePipelineState(pso)
            for (i, buf) in buffers.enumerated() {
                enc.setBuffer(buf, offset: 0, index: i)
            }
            enc.dispatchThreads(gridSize, threadsPerThreadgroup: threadgroupSize1D(pso, count: Int(gridSize.width)))
        }
        
        enc.endEncoding()
        cmdBuf.commit()
        cmdBuf.waitUntilCompleted()
    }

    /// Create a batch encoder for manual use
    /// Caller is responsible for calling endEncoding() and commit/wait on returned command buffer
    public func makeBatchEncoder() throws -> (MTLCommandBuffer, MTLComputeCommandEncoder) {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        return (cmdBuf, enc)
    }

    /// Asynchronous 1D dispatch with proper threadgroup sizing.
    /// Returns when GPU work completes; buffer contents are safe to read after.
    public func dispatch1DAsync(
        _ pipeline: MTLComputePipelineState,
        buffers: [MTLBuffer],
        count: Int
    ) async throws {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        let tg = threadgroupSize1D(pipeline, count: count)
        enc.setComputePipelineState(pipeline)
        for (i, buf) in buffers.enumerated() { enc.setBuffer(buf, offset: 0, index: i) }
        enc.dispatchThreads(MTLSize(width: count, height: 1, depth: 1),
                            threadsPerThreadgroup: tg)
        enc.endEncoding()
        return await withCheckedContinuation { (continuation: CheckedContinuation<Void, Never>) in
            cmdBuf.addCompletedHandler { _ in continuation.resume() }
            cmdBuf.commit()
        }
    }

    /// Asynchronous 2D dispatch with proper threadgroup sizing.
    /// Returns when GPU work completes; buffer contents are safe to read after.
    public func dispatch2DAsync(
        _ pipeline: MTLComputePipelineState,
        buffers: [MTLBuffer],
        grid: (width: Int, height: Int)
    ) async throws {
        guard let cmdBuf = commandQueue.makeCommandBuffer() else {
            throw E4BError.commandBufferCreationFailed
        }
        guard let enc = cmdBuf.makeComputeCommandEncoder() else {
            throw E4BError.commandEncoderCreationFailed
        }
        let tg = threadgroupSize2D(pipeline, grid: grid)
        enc.setComputePipelineState(pipeline)
        for (i, buf) in buffers.enumerated() { enc.setBuffer(buf, offset: 0, index: i) }
        enc.dispatchThreads(MTLSize(width: grid.width, height: grid.height, depth: 1),
                            threadsPerThreadgroup: tg)
        enc.endEncoding()
        return await withCheckedContinuation { (continuation: CheckedContinuation<Void, Never>) in
            cmdBuf.addCompletedHandler { _ in continuation.resume() }
            cmdBuf.commit()
        }
    }

    // ── Read-back ─────────────────────────────────────

    public func readFloats(from buffer: MTLBuffer, offset: Int = 0, count: Int) -> [Float] {
        let ptr = buffer.contents().assumingMemoryBound(to: Float.self)
        return Array(UnsafeBufferPointer(start: ptr + offset, count: count))
    }

    public func writeFloats(to buffer: MTLBuffer, values: [Float], offset: Int = 0) {
        let ptr = buffer.contents().assumingMemoryBound(to: Float.self)
        for i in 0..<values.count {
            ptr[i + offset] = values[i]
        }
    }
}