import Foundation

/// SafeTensors file reader. Handles single-file and sharded (index) formats,
/// BF16→Float32 conversion, and quantized tensor grouping.
public final class SafeTensorsReader {
    public let fileURL: URL
    private let headerSize: Int
    private let rawHeader: [String: Any]
    private let fileHandle: FileHandle  // kept open for fast repeated reads
    private let lock = NSLock()  // thread-safe access to fileHandle

    // ── Init ──────────────────────────────────────────

    /// Open a single .safetensors file and parse its header.
    public init(path: String) throws {
        self.fileURL = URL(fileURLWithPath: path)
        let handle = try FileHandle(forReadingFrom: fileURL)

        let lenData = handle.readData(ofLength: 8)
        headerSize = Int(UInt64(littleEndian: lenData.withUnsafeBytes { $0.load(as: UInt64.self) }))

        let jsonData = handle.readData(ofLength: headerSize)
        guard let json = try JSONSerialization.jsonObject(with: jsonData) as? [String: Any] else {
            try? handle.close()
            throw WeightError.invalidHeader("Top-level JSON is not a dictionary")
        }
        self.rawHeader = json
        self.fileHandle = handle
    }

    deinit {
        try? fileHandle.close()
    }

    // ── Tensor listing ────────────────────────────────

    /// All tensor descriptors in this file.
    public var allTensors: [TensorDescriptor] {
        rawHeader.compactMap { name, value in
            guard let info = value as? [String: Any],
                  let dtypeStr = info["dtype"] as? String,
                  let dtype = TensorDType.from(dtype: dtypeStr),
                  let shape = info["shape"] as? [Int],
                  let offsets = info["data_offsets"] as? [Int],
                  offsets.count == 2
            else { return nil }
            return TensorDescriptor(
                name: name, dtype: dtype, shape: shape,
                dataOffset: headerSize + 8 + offsets[0],
                dataSize: offsets[1] - offsets[0]
            )
        }
    }

    /// All tensor descriptors (convenience).
    public func allDescriptors() -> [TensorDescriptor] { allTensors }

    /// Look up a specific tensor by name.
    public func tensor(named name: String) -> TensorDescriptor? {
        allTensors.first { $0.name == name }
    }

    // ── Reading raw data ──────────────────────────────

    /// Read raw bytes for a tensor.
    public func read(tensor: TensorDescriptor) throws -> Data {
        lock.lock()
        defer { lock.unlock() }
        try fileHandle.seek(toOffset: UInt64(tensor.dataOffset))
        return fileHandle.readData(ofLength: tensor.dataSize)
    }

    /// Read a specific tensor by name.
    public func read(named name: String) throws -> Data {
        guard let desc = tensor(named: name) else {
            throw WeightError.tensorNotFound(name)
        }
        return try read(tensor: desc)
    }
    
    /// Read raw bytes for a tensor as uint32 array
    public func readUint32(named name: String) throws -> [UInt32] {
        guard let desc = tensor(named: name) else {
            throw WeightError.tensorNotFound(name)
        }
        let data = try read(tensor: desc)
        return data.withUnsafeBytes { ptr in
            let uint32Ptr = ptr.bindMemory(to: UInt32.self)
            return Array(uint32Ptr)
        }
    }

    // ── BF16 → Float32 conversion ─────────────────────

    /// Convert BF16 binary data to Float32 array.
    public static func bf16ToFloat32(_ data: Data) -> [Float] {
        data.withUnsafeBytes { ptr in
            let bf16 = ptr.assumingMemoryBound(to: UInt16.self)
            return (0..<data.count / 2).map { i in
                Float(bitPattern: UInt32(bf16[i]) << 16)
            }
        }
    }
}

// ── Errors ────────────────────────────────────────────

public enum WeightError: Error, LocalizedError {
    case invalidHeader(String)
    case tensorNotFound(String)
    case unsupportedDtype(String)
    case fileNotFound(String)
    case readFailed(String)
    case bufferCreationFailed(String)

    public var errorDescription: String? {
        switch self {
        case .invalidHeader(let detail): return "Invalid SafeTensors header: \(detail)"
        case .tensorNotFound(let name): return "Tensor '\(name)' not found"
        case .unsupportedDtype(let dtype): return "Unsupported dtype: \(dtype)"
        case .fileNotFound(let path): return "File not found: \(path)"
        case .readFailed(let detail): return "Read failed: \(detail)"
        case .bufferCreationFailed(let name): return "Failed to create Metal buffer: \(name)"
        }
    }
}