/*
 * Copyright 2021 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#if !os(WASI)
import Foundation
#else
import SwiftOverlayShims
#endif

/// `Table` is a Flatbuffers object that can read,
/// mutate scalar fields within a valid flatbuffers buffer
@frozen
public struct Table {

  /// Hosting Bytebuffer
  public private(set) var bb: ByteBuffer
  /// Current position of the table within the buffer
  public private(set) var postion: Int32

  /// Initializer for the table interface to allow generated code to read
  /// data from memory
  /// - Parameters:
  ///   - bb: ByteBuffer that stores data
  ///   - position: Current table position
  /// - Note: This will `CRASH` if read on a big endian machine
  public init(bb: ByteBuffer, position: Int32 = 0) {
    guard isLitteEndian else {
      fatalError(
        "Reading/Writing a buffer in big endian machine is not supported on swift")
    }
    self.bb = bb
    postion = position
  }

  /// Gets the offset of the current field within the buffer by reading
  /// the vtable
  /// - Parameter o: current offset
  /// - Returns: offset of field within buffer
  public func offset(_ o: Int32) -> Int32 {
    let vtable = postion - bb.read(def: Int32.self, position: Int(postion))
    return o < bb
      .read(def: VOffset.self, position: Int(vtable)) ? Int32(bb.read(
        def: Int16.self,
        position: Int(vtable + o))) : 0
  }

  /// Gets the indirect offset of the current stored object
  /// (applicable only for object arrays)
  /// - Parameter o: current offset
  /// - Returns: offset of field within buffer
  public func indirect(_ o: Int32) -> Int32 {
    o + bb.read(def: Int32.self, position: Int(o))
  }

  /// String reads from the buffer with respect to position of the current table.
  /// - Parameter offset: Offset of the string
  public func string(at offset: Int32) -> String? {
    directString(at: offset + postion)
  }

  /// Direct string reads from the buffer disregarding the position of the table.
  /// It would be preferable to use string unless the current position of the table
  /// is not needed
  /// - Parameter offset: Offset of the string
  public func directString(at offset: Int32) -> String? {
    var offset = offset
    offset += bb.read(def: Int32.self, position: Int(offset))
    let count = bb.read(def: Int32.self, position: Int(offset))
    let position = Int(offset) + MemoryLayout<Int32>.size
    return bb.readString(at: position, count: Int(count))
  }

  /// Reads from the buffer with respect to the position in the table.
  /// - Parameters:
  ///   - type: Type of Element that needs to be read from the buffer
  ///   - o: Offset of the Element
  public func readBuffer<T>(of type: T.Type, at o: Int32) -> T {
    directRead(of: T.self, offset: o + postion)
  }

  /// Reads from the buffer disregarding the position of the table.
  /// It would be used when reading from an
  ///   ```
  ///   let offset = __t.offset(10)
  ///   //Only used when the we already know what is the
  ///   // position in the table since __t.vector(at:)
  ///   // returns the index with respect to the position
  ///   __t.directRead(of: Byte.self,
  ///                  offset: __t.vector(at: offset) + index * 1)
  ///   ```
  /// - Parameters:
  ///   - type: Type of Element that needs to be read from the buffer
  ///   - o: Offset of the Element
  public func directRead<T>(of type: T.Type, offset o: Int32) -> T {
    let r = bb.read(def: T.self, position: Int(o))
    return r
  }

  /// Returns that current `Union` object at a specific offset
  /// by adding offset to the current position of table
  /// - Parameter o: offset
  /// - Returns: A flatbuffers object
  public func union<T: FlatbuffersInitializable>(_ o: Int32) -> T {
    let o = o + postion
    return directUnion(o)
  }

  /// Returns a direct `Union` object at a specific offset
  /// - Parameter o: offset
  /// - Returns: A flatbuffers object
  public func directUnion<T: FlatbuffersInitializable>(_ o: Int32) -> T {
    T.init(bb, o: o + bb.read(def: Int32.self, position: Int(o)))
  }

  /// Returns a vector of type T at a specific offset
  /// This should only be used by `Scalars`
  /// - Parameter off: Readable offset
  /// - Returns: Returns a vector of type [T]
  public func getVector<T>(at off: Int32) -> [T]? {
    let o = offset(off)
    guard o != 0 else { return nil }
    return bb.readSlice(index: Int(vector(at: o)), count: Int(vector(count: o)))
  }

  /// Vector count gets the count of Elements within the array
  /// - Parameter o: start offset of the vector
  /// - returns: Count of elements
  public func vector(count o: Int32) -> Int32 {
    var o = o
    o += postion
    o += bb.read(def: Int32.self, position: Int(o))
    return bb.read(def: Int32.self, position: Int(o))
  }

  /// Vector start index in the buffer
  /// - Parameter o:start offset of the vector
  /// - returns: the start index of the vector
  public func vector(at o: Int32) -> Int32 {
    var o = o
    o += postion
    return o + bb.read(def: Int32.self, position: Int(o)) + 4
  }

  /// Reading an indirect offset of a table.
  /// - Parameters:
  ///   - o: position within the buffer
  ///   - fbb: ByteBuffer
  /// - Returns: table offset
  static public func indirect(_ o: Int32, _ fbb: ByteBuffer) -> Int32 {
    o + fbb.read(def: Int32.self, position: Int(o))
  }

  /// Gets a vtable value according to an table Offset and a field offset
  /// - Parameters:
  ///   - o: offset relative to entire buffer
  ///   - vOffset: Field offset within a vtable
  ///   - fbb: ByteBuffer
  /// - Returns: an position of a field
  static public func offset(
    _ o: Int32,
    vOffset: Int32,
    fbb: ByteBuffer) -> Int32
  {
    let vTable = Int32(fbb.capacity) - o
    return vTable + Int32(fbb.read(
      def: Int16.self,
      position: Int(vTable + vOffset - fbb.read(
        def: Int32.self,
        position: Int(vTable)))))
  }

  /// Compares two objects at offset A and offset B within a ByteBuffer
  /// - Parameters:
  ///   - off1: first offset to compare
  ///   - off2: second offset to compare
  ///   - fbb: Bytebuffer
  /// - Returns: returns the difference between
  static public func compare(
    _ off1: Int32,
    _ off2: Int32,
    fbb: ByteBuffer) -> Int32
  {
    let memorySize = Int32(MemoryLayout<Int32>.size)
    let _off1 = off1 + fbb.read(def: Int32.self, position: Int(off1))
    let _off2 = off2 + fbb.read(def: Int32.self, position: Int(off2))
    let len1 = fbb.read(def: Int32.self, position: Int(_off1))
    let len2 = fbb.read(def: Int32.self, position: Int(_off2))
    let startPos1 = _off1 + memorySize
    let startPos2 = _off2 + memorySize
    let minValue = min(len1, len2)
    for i in 0...minValue {
      let b1 = fbb.read(def: Int8.self, position: Int(i + startPos1))
      let b2 = fbb.read(def: Int8.self, position: Int(i + startPos2))
      if b1 != b2 {
        return Int32(b2 - b1)
      }
    }
    return len1 - len2
  }

  /// Compares two objects at offset A and array of `Bytes` within a ByteBuffer
  /// - Parameters:
  ///   - off1: Offset to compare to
  ///   - key: bytes array to compare to
  ///   - fbb: Bytebuffer
  /// - Returns: returns the difference between
  static public func compare(
    _ off1: Int32,
    _ key: [Byte],
    fbb: ByteBuffer) -> Int32
  {
    let memorySize = Int32(MemoryLayout<Int32>.size)
    let _off1 = off1 + fbb.read(def: Int32.self, position: Int(off1))
    let len1 = fbb.read(def: Int32.self, position: Int(_off1))
    let len2 = Int32(key.count)
    let startPos1 = _off1 + memorySize
    let minValue = min(len1, len2)
    for i in 0..<minValue {
      let b = fbb.read(def: Int8.self, position: Int(i + startPos1))
      let byte = key[Int(i)]
      if b != byte {
        return Int32(b - Int8(byte))
      }
    }
    return len1 - len2
  }
}