import Foundation

public struct ByteBuffer {
    
    /// Storage is a container that would hold the memory pointer to solve the issue of
    /// deallocating the memory that was held by (memory: UnsafeMutableRawPointer)
    @usableFromInline final class Storage {
        // This storage doesn't own the memory, therefore, we won't deallocate on deinit.
        private let unowned: Bool
        /// pointer to the start of the buffer object in memory
        var memory: UnsafeMutableRawPointer
        /// Capacity of UInt8 the buffer can hold
        var capacity: Int
        
        init(count: Int, alignment: Int) {
            memory = UnsafeMutableRawPointer.allocate(byteCount: count, alignment: alignment)
            capacity = count
            unowned = false
        }

        init(memory: UnsafeMutableRawPointer, capacity: Int, unowned: Bool) {
            self.memory = memory
            self.capacity = capacity
            self.unowned = unowned
        }
        
        deinit {
            if !unowned {
              memory.deallocate()
            }
        }
        
        func copy(from ptr: UnsafeRawPointer, count: Int) {
            assert(!unowned, "copy should NOT be called on a buffer that is built by assumingMemoryBound")
            memory.copyMemory(from: ptr, byteCount: count)
        }
        
        func initialize(for size: Int) {
            assert(!unowned, "initalize should NOT be called on a buffer that is built by assumingMemoryBound")
            memset(memory, 0, size)
        }
        
        /// Reallocates the buffer incase the object to be written doesnt fit in the current buffer
        /// - Parameter size: Size of the current object
        @usableFromInline internal func reallocate(_ size: Int, writerSize: Int, alignment: Int) {
            let currentWritingIndex = capacity &- writerSize
            while capacity <= writerSize &+ size {
                capacity = capacity << 1
            }
            
            /// solution take from Apple-NIO
            capacity = capacity.convertToPowerofTwo
            
            let newData = UnsafeMutableRawPointer.allocate(byteCount: capacity, alignment: alignment)
            memset(newData, 0, capacity &- writerSize)
            memcpy(newData.advanced(by: capacity &- writerSize), memory.advanced(by: currentWritingIndex), writerSize)
            memory.deallocate()
            memory = newData
        }
    }
    
    @usableFromInline var _storage: Storage
    
    /// The size of the elements written to the buffer + their paddings
    private var _writerSize: Int = 0
    /// Aliginment of the current  memory being written to the buffer
    internal var alignment = 1
    /// Current Index which is being used to write to the buffer, it is written from the end to the start of the buffer
    internal var writerIndex: Int { return _storage.capacity &- _writerSize }

    /// Reader is the position of the current Writer Index (capacity - size)
    public var reader: Int { return writerIndex }
    /// Current size of the buffer
    public var size: UOffset { return UOffset(_writerSize) }
    /// Public Pointer to the buffer object in memory. This should NOT be modified for any reason
    public var memory: UnsafeMutableRawPointer { return _storage.memory }
    /// Current capacity for the buffer
    public var capacity: Int { return _storage.capacity }

    /// Constructor that creates a Flatbuffer object from a UInt8
    /// - Parameter bytes: Array of UInt8
    public init(bytes: [UInt8]) {
        var b = bytes
        _storage = Storage(count: bytes.count, alignment: alignment)
        _writerSize = _storage.capacity
        b.withUnsafeMutableBytes { bufferPointer in
            self._storage.copy(from: bufferPointer.baseAddress!, count: bytes.count)
        }
    }

    /// Constructor that creates a Flatbuffer from the Swift Data type object
    /// - Parameter data: Swift data Object
    public init(data: Data) {
        var b = data
        _storage = Storage(count: data.count, alignment: alignment)
        _writerSize = _storage.capacity
        b.withUnsafeMutableBytes { bufferPointer in
            self._storage.copy(from: bufferPointer.baseAddress!, count: data.count)
        }
    }

    /// Constructor that creates a Flatbuffer instance with a size
    /// - Parameter size: Length of the buffer
    init(initialSize size: Int) {
        let size = size.convertToPowerofTwo
        _storage = Storage(count: size, alignment: alignment)
        _storage.initialize(for: size)
    }
  
    #if swift(>=5.0)
    /// Constructor that creates a Flatbuffer object from a ContiguousBytes
    /// - Parameters:
    ///   - contiguousBytes: Binary stripe to use as the buffer
    ///   - count: amount of readable bytes
    public init<Bytes: ContiguousBytes>(
        contiguousBytes: Bytes,
        count: Int
    ) {
        _storage = Storage(count: count, alignment: alignment)
        _writerSize = _storage.capacity
        contiguousBytes.withUnsafeBytes { buf in
            _storage.copy(from: buf.baseAddress!, count: buf.count)
        }
    }
    #endif
    
    /// Constructor that creates a Flatbuffer from unsafe memory region without copying
    /// - Parameter assumingMemoryBound: The unsafe memory region
    /// - Parameter capacity: The size of the given memory region
    public init(assumingMemoryBound memory: UnsafeMutableRawPointer, capacity: Int) {
        _storage = Storage(memory: memory, capacity: capacity, unowned: true)
        _writerSize = capacity
    }

    /// Creates a copy of the buffer that's being built by calling sizedBuffer
    /// - Parameters:
    ///   - memory: Current memory of the buffer
    ///   - count: count of bytes
    internal init(memory: UnsafeMutableRawPointer, count: Int) {
        _storage = Storage(count: count, alignment: alignment)
        _storage.copy(from: memory, count: count)
        _writerSize = _storage.capacity
    }

    /// Creates a copy of the existing flatbuffer, by copying it to a different memory.
    /// - Parameters:
    ///   - memory: Current memory of the buffer
    ///   - count: count of bytes
    ///   - removeBytes: Removes a number of bytes from the current size
    internal init(memory: UnsafeMutableRawPointer, count: Int, removing removeBytes: Int) {
        _storage = Storage(count: count, alignment: alignment)
        _storage.copy(from: memory, count: count)
        _writerSize = removeBytes
    }

    /// Fills the buffer with padding by adding to the writersize
    /// - Parameter padding: Amount of padding between two to be serialized objects
    @usableFromInline mutating func fill(padding: Int) {
        assert(padding >= 0, "Fill should be larger than or equal to zero")
        ensureSpace(size: padding)
        _writerSize = _writerSize &+ (MemoryLayout<UInt8>.size &* padding)
    }
    
    ///Adds an array of type Scalar to the buffer memory
    /// - Parameter elements: An array of Scalars
    @usableFromInline mutating func push<T: Scalar>(elements: [T]) {
        let size = elements.count &* MemoryLayout<T>.size
        ensureSpace(size: size)
        elements.reversed().forEach { (s) in
            push(value: s, len: MemoryLayout.size(ofValue: s))
        }
    }

    /// A custom type of structs that are padded according to the flatbuffer padding,
    /// - Parameters:
    ///   - value: Pointer to the object in memory
    ///   - size: Size of Value being written to the buffer
    @available(*, deprecated, message: "0.9.0 will be removing the following method. Regenerate the code")
    @usableFromInline mutating func push(struct value: UnsafeMutableRawPointer, size: Int) {
        ensureSpace(size: size)
        memcpy(_storage.memory.advanced(by: writerIndex &- size), value, size)
        defer { value.deallocate() }
        _writerSize = _writerSize &+ size
    }
    
    /// Prepares the buffer to receive a struct of certian size.
    /// The alignment of the memory is already handled since we already called preAlign
    /// - Parameter size: size of the struct
    @usableFromInline mutating func prepareBufferToReceiveStruct(of size: Int) {
        ensureSpace(size: size)
        _writerSize = _writerSize &+ size
    }
    
    /// Reverse the input direction to the buffer, since `FlatBuffers` uses a back to front, following method will take current `writerIndex`
    /// and writes front to back into the buffer, respecting the padding & the alignment
    /// - Parameters:
    ///   - value: value of type Scalar
    ///   - position: position relative to the `writerIndex`
    ///   - len: length of the value in terms of bytes
    @usableFromInline mutating func reversePush<T: Scalar>(value: T, position: Int, len: Int) {
        var v = value
        memcpy(_storage.memory.advanced(by: writerIndex &+ position), &v, len)
    }

    /// Adds an object of type Scalar into the buffer
    /// - Parameters:
    ///   - value: Object  that will be written to the buffer
    ///   - len: Offset to subtract from the WriterIndex
    @usableFromInline mutating func push<T: Scalar>(value: T, len: Int) {
        ensureSpace(size: len)
        var v = value
        memcpy(_storage.memory.advanced(by: writerIndex &- len), &v, len)
        _writerSize = _writerSize &+ len
    }

    /// Adds a string to the buffer using swift.utf8 object
    /// - Parameter str: String that will be added to the buffer
    /// - Parameter len: length of the string
    @usableFromInline mutating func push(string str: String, len: Int) {
        ensureSpace(size: len)
        if str.utf8.withContiguousStorageIfAvailable({ self.push(bytes: $0, len: len) }) != nil {
        } else {
            let utf8View = str.utf8
            for c in utf8View.reversed() {
                push(value: c, len: 1)
            }
        }
    }

    /// Writes a string to Bytebuffer using UTF8View
    /// - Parameters:
    ///   - bytes: Pointer to the view
    ///   - len: Size of string
    @usableFromInline mutating internal func push(bytes: UnsafeBufferPointer<String.UTF8View.Element>, len: Int) -> Bool {
        memcpy(_storage.memory.advanced(by: writerIndex &- len), UnsafeRawPointer(bytes.baseAddress!), len)
        _writerSize = _writerSize &+ len
        return true
    }

    /// Write stores an object into the buffer directly or indirectly.
    ///
    /// Direct: ignores the capacity of buffer which would mean we are referring to the direct point in memory
    /// indirect: takes into respect the current capacity of the buffer (capacity - index), writing to the buffer from the end
    /// - Parameters:
    ///   - value: Value that needs to be written to the buffer
    ///   - index: index to write to
    ///   - direct: Should take into consideration the capacity of the buffer
    func write<T>(value: T, index: Int, direct: Bool = false) {
        var index = index
        if !direct {
            index = _storage.capacity &- index
        }
        assert(index < _storage.capacity, "Write index is out of writing bound")
        assert(index >= 0, "Writer index should be above zero")
        _storage.memory.storeBytes(of: value, toByteOffset: index, as: T.self)
    }

    /// Makes sure that buffer has enouch space for each of the objects that will be written into it
    /// - Parameter size: size of object
    @discardableResult
    @usableFromInline mutating func ensureSpace(size: Int) -> Int {
        if size &+ _writerSize > _storage.capacity {
            _storage.reallocate(size, writerSize: _writerSize, alignment: alignment)
        }
        assert(size < FlatBufferMaxSize, "Buffer can't grow beyond 2 Gigabytes")
        return size
    }
    
    /// pops the written VTable if it's already written into the buffer
    /// - Parameter size: size of the `VTable`
    @usableFromInline mutating internal func pop(_ size: Int) {
        assert((_writerSize &- size) > 0, "New size should NOT be a negative number")
        memset(_storage.memory.advanced(by: writerIndex), 0, _writerSize &- size)
        _writerSize = size
    }
    
    /// Clears the current size of the buffer
    mutating public func clearSize() {
        _writerSize = 0
    }

    /// Clears the current instance of the buffer, replacing it with new memory
    mutating public func clear() {
        _writerSize = 0
        alignment = 1
        _storage.initialize(for: _storage.capacity)
    }
    
    /// Reads an object from the buffer
    /// - Parameters:
    ///   - def: Type of the object
    ///   - position: the index of the object in the buffer
    public func read<T>(def: T.Type, position: Int) -> T {
        assert(position + MemoryLayout<T>.size <= _storage.capacity, "Reading out of bounds is illegal")
        return _storage.memory.advanced(by: position).load(as: T.self)
    }

    /// Reads a slice from the memory assuming a type of T
    /// - Parameters:
    ///   - index: index of the object to be read from the buffer
    ///   - count: count of bytes in memory
    public func readSlice<T>(index: Int32,
                             count: Int32) -> [T] {
        let _index = Int(index)
        let _count = Int(count)
        assert(_index + _count <= _storage.capacity, "Reading out of bounds is illegal")
        let start = _storage.memory.advanced(by: _index).assumingMemoryBound(to: T.self)
        let array = UnsafeBufferPointer(start: start, count: _count)
        return Array(array)
    }

    /// Reads a string from the buffer and encodes it to a swift string
    /// - Parameters:
    ///   - index: index of the string in the buffer
    ///   - count: length of the string
    ///   - type: Encoding of the string
    public func readString(at index: Int32,
                           count: Int32,
                           type: String.Encoding = .utf8) -> String? {
        let _index = Int(index)
        let _count = Int(count)
        assert(_index + _count <= _storage.capacity, "Reading out of bounds is illegal")
        let start = _storage.memory.advanced(by: _index).assumingMemoryBound(to: UInt8.self)
        let bufprt = UnsafeBufferPointer(start: start, count: _count)
        return String(bytes: Array(bufprt), encoding: type)
    }

    /// Creates a new Flatbuffer object that's duplicated from the current one
    /// - Parameter removeBytes: the amount of bytes to remove from the current Size
    public func duplicate(removing removeBytes: Int = 0) -> ByteBuffer {
        assert(removeBytes > 0, "Can NOT remove negative bytes")
        assert(removeBytes < _storage.capacity, "Can NOT remove more bytes than the ones allocated")
        return ByteBuffer(memory: _storage.memory, count: _storage.capacity, removing: _writerSize &- removeBytes)
    }
}

extension ByteBuffer: CustomDebugStringConvertible {

    public var debugDescription: String {
        """
        buffer located at: \(_storage.memory), with capacity of \(_storage.capacity)
        { writerSize: \(_writerSize), readerSize: \(reader), writerIndex: \(writerIndex) }
        """
    }
}