Sources/FlatBuffers/FlatBufferBuilder.swift

import Foundation

public final class FlatBufferBuilder {

    /// Vtables used in the buffer are stored in here, so they would be written later in EndTable
    private var _vtable: [UInt32] = []
    /// Reference Vtables that were already written to the buffer
    private var _vtables: [UOffset] = []
    /// Flatbuffer data will be written into
    private var _bb: ByteBuffer
    /// A check if the buffer is being written into by a different table
    private var isNested = false
    /// Dictonary that stores a map of all the strings that were written to the buffer
    private var stringOffsetMap: [String: Offset<String>] = [:]
    /// A check to see if finish(::) was ever called to retreive data object
    private var finished = false
    /// A check to see if the buffer should serialize Default values
    private var serializeDefaults: Bool

    /// Current alignment for the buffer
    var _minAlignment: Int = 0 {
        didSet {
            _bb.alignment = _minAlignment
        }
    }

    /// Gives a read access to the buffer's size
    public var size: UOffset { return _bb.size }
    /// Data representation of the buffer
    public var data: Data {
        assert(finished, "Data shouldn't be called before finish()")
        return Data(bytes: _bb.memory.advanced(by: _bb.writerIndex),
                    count: _bb.capacity - _bb.writerIndex)
    }
    /// Get's the fully sized buffer stored in memory
    public var fullSizedByteArray: [UInt8] {
        let ptr = UnsafeBufferPointer(start: _bb.memory.assumingMemoryBound(to: UInt8.self),
                                      count: _bb.capacity)
        return Array(ptr)
    }
    /// Returns the written size of the buffer
    public var sizedByteArray: [UInt8] {
        let cp = _bb.capacity - _bb.writerIndex
        let start = _bb.memory.advanced(by: _bb.writerIndex)
            .bindMemory(to: UInt8.self, capacity: cp)

        let ptr = UnsafeBufferPointer(start: start, count: cp)
        return Array(ptr)
    }
    /// Returns the buffer
    public var buffer: ByteBuffer { return _bb }

    /// Returns A sized Buffer from the readable bytes
    public var sizedBuffer: ByteBuffer {
        assert(finished, "Data shouldn't be called before finish()")
        return ByteBuffer(memory: _bb.memory.advanced(by: _bb.reader), count: _bb.reader)
    }

    // MARK: - Init

    /// initialize the buffer with a size
    /// - Parameters:
    ///   - initialSize: Initial size for the buffer
    ///   - force: Allows default to be serialized into the buffer
    public init(initialSize: Int32 = 1024, serializeDefaults force: Bool = false) {
        assert(initialSize > 0, "Size should be greater than zero!")
        guard isLitteEndian else {
            fatalError("Reading/Writing a buffer in big endian machine is not supported on swift")
        }
        serializeDefaults = force
        _bb = ByteBuffer(initialSize: Int(initialSize))
    }

    /// Clears the buffer and the builder from it's data
    public func clear() {
        _minAlignment = 0
        isNested = false
        _bb.clear()
        stringOffsetMap = [:]
    }

    /// Removes all the offsets from the VTable
    public func clearOffsets() {
        _vtable = []
    }

    // MARK: - Create Tables

    /// Checks if the required fields were serialized into the buffer
    /// - Parameters:
    ///   - table: offset for the table
    ///   - fields: Array of all the important fields to be serialized
    public func require(table: Offset<UOffset>, fields: [Int32]) {
        for field in fields {
            let start = _bb.capacity - Int(table.o)
            let startTable = start - Int(_bb.read(def: Int32.self, position: start))
            let isOkay = _bb.read(def: VOffset.self, position: startTable + Int(field)) != 0
            assert(isOkay, "Flatbuffers requires the following field")
        }
    }

    /// Finished the buffer by adding the file id and then calling finish
    /// - Parameters:
    ///   - offset: Offset of the table
    ///   - fileId: Takes the fileId
    ///   - prefix: if false it wont add the size of the buffer
    public func finish<T>(offset: Offset<T>, fileId: String, addPrefix prefix: Bool = false) {
        let size = MemoryLayout<UOffset>.size
        preAlign(len: size + (prefix ? size : 0) + FileIdLength, alignment: _minAlignment)
        assert(fileId.count == FileIdLength, "Flatbuffers requires file id to be 4")
        _bb.push(string: fileId, len: 4)
        finish(offset: offset, addPrefix: prefix)
    }

    /// Finished the buffer by adding the file id, offset, and prefix to it.
    /// - Parameters:
    ///   - offset: Offset of the table
    ///   - prefix: if false it wont add the size of the buffer
    public func finish<T>(offset: Offset<T>, addPrefix prefix: Bool = false) {
        notNested()
        let size = MemoryLayout<UOffset>.size
        preAlign(len: size + (prefix ? size : 0), alignment: _minAlignment)
        push(element: refer(to: offset.o))
        if prefix { push(element: _bb.size) }
        clearOffsets()
        finished = true
    }

    /// starttable will let the builder know, that a new object is being serialized.
    ///
    /// The function will fatalerror if called while there is another object being serialized
    /// - Parameter numOfFields: Number of elements to be written to the buffer
    public func startTable(with numOfFields: Int) -> UOffset {
        notNested()
        isNested = true
        _vtable = [UInt32](repeating: 0, count: numOfFields)
        return _bb.size
    }


    /// Endtable will let the builder know that the object that's written to it is completed
    ///
    /// This would be called after all the elements are serialized, it will add the vtable into the buffer.
    /// it will fatalError in case the object is called without starttable, or the object has exceeded  the limit of
    ///  2GB,
    /// - Parameter startOffset:Start point of the object written
    /// - returns: The root of the table
    public func endTable(at startOffset: UOffset)  -> UOffset {
        assert(isNested, "Calling endtable without calling starttable")
        let sizeofVoffset = MemoryLayout<VOffset>.size
        let vTableOffset = push(element: SOffset(0))

        let tableObjectSize = vTableOffset - startOffset
        assert(tableObjectSize < 0x10000, "Buffer can't grow beyond 2 Gigabytes")

        var writeIndex = 0
        for (index,j) in _vtable.lazy.reversed().enumerated() {
            if j != 0 {
                writeIndex = _vtable.count - index
                break
            }
        }

        for i in stride(from: writeIndex - 1, to: -1, by: -1) {
            let off = _vtable[i] == 0 ? 0 : vTableOffset - _vtable[i]
            _bb.push(value: VOffset(off), len: sizeofVoffset)
        }

        _bb.push(value: VOffset(tableObjectSize), len: sizeofVoffset)
        _bb.push(value: (UInt16(writeIndex + 2) * UInt16(sizeofVoffset)), len: sizeofVoffset)

        clearOffsets()
        let vt_use = _bb.size

        var isAlreadyAdded: Int?

        let vt2 = _bb.memory.advanced(by: _bb.writerIndex)
        let len2 = vt2.load(fromByteOffset: 0, as: Int16.self)

        for table in _vtables {
            let position = _bb.capacity - Int(table)
            let vt1 = _bb.memory.advanced(by: position)
            let len1 = _bb.read(def: Int16.self, position: position)
            if (len2 != len1 || 0 != memcmp(vt1, vt2, Int(len2))) { continue }

            isAlreadyAdded = Int(table)
            break
        }

        if let offset = isAlreadyAdded {
            let vTableOff = Int(vTableOffset)
            let space = _bb.capacity - vTableOff
            _bb.write(value: Int32(offset - vTableOff), index: space, direct: true)
            _bb.resize(_bb.capacity - space)
        } else {
            _bb.write(value: Int32(vt_use) - Int32(vTableOffset), index: Int(vTableOffset))
            _vtables.append(_bb.size)
        }
        isNested = false
        return vTableOffset
    }

    // MARK: - Builds Buffer

    /// asserts to see if the object is not nested
    fileprivate func notNested()  {
        assert(!isNested, "Object serialization must not be nested")
    }

    /// Changes the minimuim alignment of the buffer
    /// - Parameter size: size of the current alignment
    fileprivate func minAlignment(size: Int) {
        if size > _minAlignment {
            _minAlignment = size
        }
    }

    /// Gets the padding for the current element
    /// - Parameters:
    ///   - bufSize: Current size of the buffer + the offset of the object to be written
    ///   - elementSize: Element size
    fileprivate func padding(bufSize: UInt32, elementSize: UInt32) -> UInt32 {
        ((~bufSize) &+ 1) & (elementSize - 1)
    }

    /// Prealigns the buffer before writting a new object into the buffer
    /// - Parameters:
    ///   - len:Length of the object
    ///   - alignment: Alignment type
    fileprivate func preAlign(len: Int, alignment: Int) {
        minAlignment(size: alignment)
        _bb.fill(padding: padding(bufSize: _bb.size + UOffset(len), elementSize: UOffset(alignment)))
    }

    /// Prealigns the buffer before writting a new object into the buffer
    /// - Parameters:
    ///   - len: Length of the object
    ///   - type: Type of the object to be written
    fileprivate func preAlign<T: Scalar>(len: Int, type: T.Type) {
        preAlign(len: len, alignment: MemoryLayout<T>.size)
    }

    /// Refers to an object that's written in the buffer
    /// - Parameter off: the objects index value
    fileprivate func refer(to off: UOffset) -> UOffset {
        let size = MemoryLayout<UOffset>.size
        preAlign(len: size, alignment: size)
        return _bb.size - off + UInt32(size)
    }

    /// Tracks the elements written into the buffer
    /// - Parameters:
    ///   - offset: The offset of the element witten
    ///   - position: The position of the element
    fileprivate func track(offset: UOffset, at position: VOffset) {
        _vtable[Int(position)] = offset
    }

    // MARK: - Vectors

    /// Starts a vector of length and Element size
    public func startVector(_ len: Int, elementSize: Int) {
        notNested()
        isNested = true
        preAlign(len: len * elementSize, type: UOffset.self)
        preAlign(len: len * elementSize, alignment: elementSize)
    }

    /// Ends the vector of at length
    ///
    /// The current function will fatalError if startVector is called before serializing the vector
    /// - Parameter len: Length of the buffer
    public func endVector(len: Int) -> UOffset {
        assert(isNested, "Calling endVector without calling startVector")
        isNested = false
        return push(element: Int32(len))
    }

    /// Creates a vector of type Scalar in the buffer
    /// - Parameter elements: elements to be written into the buffer
    /// - returns: Offset of the vector
    public func createVector<T: Scalar>(_ elements: [T]) -> Offset<UOffset> {
        return createVector(elements, size: elements.count)
    }

    ///  Creates a vector of type Scalar in the buffer
    /// - Parameter elements: Elements to be written into the buffer
    /// - Parameter size: Count of elements
    /// - returns: Offset of the vector
    public func createVector<T: Scalar>(_ elements: [T], size: Int) -> Offset<UOffset> {
        let size = size
        startVector(size, elementSize: MemoryLayout<T>.size)
        _bb.push(elements: elements)
        return Offset(offset: endVector(len: size))
    }

    /// Creates a vector of type Enums in the buffer
    /// - Parameter elements: elements to be written into the buffer
    /// - returns: Offset of the vector
    public func createVector<T: Enum>(_ elements: [T]) -> Offset<UOffset> {
        return createVector(elements, size: elements.count)
    }

    ///  Creates a vector of type Enums in the buffer
    /// - Parameter elements: Elements to be written into the buffer
    /// - Parameter size: Count of elements
    /// - returns: Offset of the vector
    public func createVector<T: Enum>(_ elements: [T], size: Int) -> Offset<UOffset> {
        let size = size
        startVector(size, elementSize: T.byteSize)
        for e in elements.lazy.reversed() {
            _bb.push(value: e.value, len: T.byteSize)
        }
        return Offset(offset: endVector(len: size))
    }

    /// Creates a vector of type Offsets  in the buffer
    /// - Parameter offsets:Array of offsets of type T
    /// - returns: Offset of the vector
    public func createVector<T>(ofOffsets offsets: [Offset<T>]) -> Offset<UOffset> {
        createVector(ofOffsets: offsets, len: offsets.count)
    }

    ///  Creates a vector of type Offsets  in the buffer
    /// - Parameter elements: Array of offsets of type T
    /// - Parameter size: Count of elements
    /// - returns: Offset of the vector
    public func createVector<T>(ofOffsets offsets: [Offset<T>], len: Int) -> Offset<UOffset> {
        startVector(len, elementSize: MemoryLayout<Offset<T>>.size)
        for o in offsets.lazy.reversed() {
            push(element: o)
        }
        return Offset(offset: endVector(len: len))
    }

    /// Creates a vector of Strings
    /// - Parameter str: a vector of strings that will be written into the buffer
    /// - returns: Offset of the vector
    public func createVector(ofStrings str: [String]) -> Offset<UOffset> {
        var offsets: [Offset<String>] = []
        for s in str {
            offsets.append(create(string: s))
        }
        return createVector(ofOffsets: offsets)
    }

    /// Creates a vector of Flatbuffer structs.
    ///
    /// The function takes a Type to know what size it is, and alignment
    /// - Parameters:
    ///   - structs: An array of UnsafeMutableRawPointer
    ///   - type: Type of the struct being written
    /// - returns: Offset of the vector
    public func createVector<T: Readable>(structs: [UnsafeMutableRawPointer],
                                          type: T.Type) -> Offset<UOffset> {
        startVector(structs.count * T.size, elementSize: T.alignment)
        for i in structs.lazy.reversed() {
            create(struct: i, type: T.self)
        }
        return Offset(offset: endVector(len: structs.count))
    }

    // MARK: - Inserting Structs

    /// Writes a Flatbuffer struct into the buffer
    /// - Parameters:
    ///   - s: Flatbuffer struct
    ///   - type: Type of the element to be serialized
    /// - returns: Offset of the Object
    @discardableResult
    public func create<T: Readable>(struct s: UnsafeMutableRawPointer,
                                    type: T.Type) -> Offset<UOffset> {
        let size = T.size
        preAlign(len: size, alignment: T.alignment)
        _bb.push(struct: s, size: size)
        return Offset(offset: _bb.size)
    }

    /// Adds the offset of a struct into the vTable
    ///
    /// The function fatalErrors if we pass an offset that is out of range
    /// - Parameter o: offset
    public func add(structOffset o: UOffset) {
        guard Int(o) < _vtable.count else { fatalError("Out of the table range") }
        _vtable[Int(o)] = _bb.size
    }

    // MARK: - Inserting Strings

    /// Insets a string into the buffer using UTF8
    /// - Parameter str: String to be serialized
    /// - returns: The strings offset in the buffer
    public func create(string str: String) -> Offset<String> {
        let len = str.count
        notNested()
        preAlign(len: len + 1, type: UOffset.self)
        _bb.fill(padding: 1)
        _bb.push(string: str, len: len)
        push(element: UOffset(len))
        return Offset(offset: _bb.size)
    }

    /// Inserts a shared string to the buffer
    ///
    /// The function checks the stringOffsetmap if it's seen a similar string before
    /// - Parameter str: String to be serialized
    /// - returns: The strings offset in the buffer
    public func createShared(string str: String) -> Offset<String> {
        if let offset = stringOffsetMap[str] {
            return offset
        }
        let offset = create(string: str)
        stringOffsetMap[str] = offset
        return offset
    }

    // MARK: - Inseting offsets

    /// Adds the offset of an object into the buffer
    /// - Parameters:
    ///   - offset: Offset of another object to be written
    ///   - position: The  predefined position of the object
    public func add<T>(offset: Offset<T>, at position: VOffset) {
        if offset.isEmpty {
            track(offset: 0, at: position)
            return
        }
        add(element: refer(to: offset.o), def: 0, at: position)
    }

    /// Pushes a value of type offset into the buffer
    /// - Parameter o: Offset
    /// - returns: Position of the offset
    @discardableResult
    public func push<T>(element o: Offset<T>) -> UOffset {
        push(element: refer(to: o.o))
    }

    // MARK: - Inserting Scalars to Buffer

    /// Adds a value into the buffer of type Scalar
    ///
    /// - Parameters:
    ///   - element: Element to insert
    ///   - def: Default value for that element
    ///   - position: The predefined position of the element
    public func add<T: Scalar>(element: T, def: T, at position: VOffset) {
        if (element == def && !serializeDefaults) {
            track(offset: 0, at: position)
            return
        }
        let off = push(element: element)
        track(offset: off, at: position)
    }

    /// Adds Boolean values into the buffer
    /// - Parameters:
    ///   - condition: Condition to insert
    ///   - def: Default condition
    ///   - position: The predefined position of the element
    public func add(condition: Bool, def: Bool, at position: VOffset) {
        if (condition == def && !serializeDefaults) {
            track(offset: 0, at: position)
            return
        }
        let off = push(element: Byte(condition ? 1 : 0))
        track(offset: off, at: position)
    }

    /// Pushes the values into the buffer
    /// - Parameter element: Element to insert
    /// - returns: Postion of the Element
    @discardableResult
    public func push<T: Scalar>(element: T) -> UOffset {
        preAlign(len: MemoryLayout<T>.size,
                 alignment: MemoryLayout<T>.size)
        _bb.push(value: element, len: MemoryLayout<T>.size)
        return _bb.size
    }
}

extension FlatBufferBuilder: CustomDebugStringConvertible {

    public var debugDescription: String {
        """
        buffer debug:
        \(_bb)
        builder debug:
        { finished: \(finished), serializeDefaults: \(serializeDefaults), isNested: \(isNested) }
        """
    }
}