//
//  Metaballs.swift
//  Metaballs
//
//  Created by Eryn Wells on 7/30/17.
//  Copyright © 2017 Eryn Wells. All rights reserved.
//

import Foundation
import MetalKit

public enum MetaballsError: Error {
    case metalError(String)
}

public struct Ball {
    let radius: CGFloat
    var position = CGPoint()
    var velocity = CGVector()

    internal var bounds: CGRect {
        let diameter = radius * 2
        return CGRect(x: position.x - radius, y: position.y - radius, width: diameter, height: diameter)
    }

    internal mutating func update() {
        position.x += velocity.dx
        position.y += velocity.dy
    }
}

public class Field {
    public var size: CGSize {
        didSet {
            if size != oldValue {
                let numberOfBallsBeforeFilter = balls.count

                // Remove balls that fall outside the new bounds.
                balls = balls.filter { bounds.contains($0.bounds) }

                // Update Metal state as needed.
                updateThreadgroupSizes(withFieldSize: size)
                sampleTexture = nil
                if numberOfBallsBeforeFilter != balls.count {
                    ballBuffer = nil
                }
            }
        }
    }

    private(set) var balls = [Ball]()

    internal var bounds: CGRect {
        return CGRect(origin: CGPoint(), size: size)
    }

    public init(size s: CGSize) {
        size = s
    }

    public func update() {
        let selfBounds = bounds
        for var ball in balls {
            // Update position of ball.
            ball.update()

            if !selfBounds.contains(ball.position) {
                // Degenerate case. If the ball finds itself outside the bounds of the field, plop it back in the center.
                ball.position = CGPoint(x: selfBounds.midX, y: selfBounds.midY)
            } else {
                // Do collision detection with walls.
                let ballBounds = ball.bounds
                if !selfBounds.contains(ballBounds) {
                    if ballBounds.minX < selfBounds.minX || ballBounds.maxX > selfBounds.maxX {
                        ball.velocity.dx *= -1
                    }
                    if ballBounds.minY < selfBounds.minY || ballBounds.maxY > selfBounds.maxY {
                        ball.velocity.dy *= -1
                    }
                }
            }
        }
    }

    public func add(ballWithRadius radius: CGFloat) {
        NSLog("Adding ball with r=\(radius); fieldSize=\(size)")
        let insetBounds = bounds.insetBy(dx: radius, dy: radius)
//        let x = CGFloat(UInt32(insetBounds.minX) + arc4random_uniform(UInt32(insetBounds.width)))
//        let y = CGFloat(UInt32(insetBounds.minY) + arc4random_uniform(UInt32(insetBounds.height)))
        let position = CGPoint(x: insetBounds.midX, y: insetBounds.midY)
        // TODO: Randomly generate velocity too.
        let ball = Ball(radius: radius, position: position, velocity: CGVector())
        balls.append(ball)
    }

    // MARK: - Metal Configuration

    private var device: MTLDevice?
    private var sampleComputeState: MTLComputePipelineState?
    private var parametersBuffer: MTLBuffer?
    private var ballBuffer: MTLBuffer?
    public private(set) var sampleTexture: MTLTexture?

    private var threadgroupCount = MTLSize()
    // TODO: It might be possible to (more dynamically) right-size this.
    private var threadgroupSize = MTLSize(width: 16, height: 16, depth: 1)

    /// Create the Metal buffer containing basic parameters of the simulation.
    private func makeParametersBufferIfNeeded(withDevice device: MTLDevice) -> MTLBuffer? {
        if parametersBuffer == nil {
            parametersBuffer = device.makeBuffer(length: MemoryLayout<Int>.size * 3, options: [])
        }
        return parametersBuffer
    }

    /// Create a Metal buffer containing the current set of metaballs.
    /// @param device The Metal device to use to create the buffer.
    /// @return A new buffer containing metaball data.
    private func makeBallBufferIfNeeded(withDevice device: MTLDevice) -> MTLBuffer? {
        if ballBuffer == nil {
            let sizeOfBall = MemoryLayout<Ball>.size
            let length = balls.count * sizeOfBall
            ballBuffer = device.makeBuffer(length: length, options: [])
        }
        return ballBuffer
    }

    /// Create a Metal texture to hold sample values created by the sampling compute shader.
    /// @param device The Metal device to use to create the texture.
    /// @return A new texture.
    private func makeSampleTextureIfNeeded(withDevice device: MTLDevice) -> MTLTexture? {
        if sampleTexture == nil {
            let desc = MTLTextureDescriptor()
            desc.pixelFormat = .r16Float
            desc.width = Int(size.width)
            desc.height = Int(size.height)
            desc.usage = [.shaderWrite, .shaderRead]
            sampleTexture = device.makeTexture(descriptor: desc)
        }
        return sampleTexture
    }

    /// Update the threadgroup divisions based on the size of the field.
    /// @param size The size of the field.
    private func updateThreadgroupSizes(withFieldSize size: CGSize) {
        let width = Int(size.width)
        let height = Int(size.height)
        threadgroupCount = MTLSize(width: width + threadgroupSize.width - 1, height: height + threadgroupSize.height - 1, depth: 1)
    }

    /// Copy metaballs data into the parameters buffer.
    private func updateParametersBuffer() {
        guard let parameters = parametersBuffer,
              let balls = ballBuffer
        else {
            return
        }

        var ptr = parameters.contents()
        
        var width = Int(size.width)
        ptr = writeValue(value: &width, to: ptr)
        var height = Int(size.height)
        ptr = writeValue(value: &height, to: ptr)

        var numberOfBalls = self.balls.count
        ptr = writeValue(value: &numberOfBalls, to: ptr)

        ptr = balls.contents()
        for ball in self.balls {
            var radius = Float(ball.radius)
            ptr = writeValue(value: &radius, to: ptr)

            var posX = Float(ball.position.x)
            ptr = writeValue(value: &posX, to: ptr)
            var posY = Float(ball.position.y)
            ptr = writeValue(value: &posY, to: ptr)

            var dx = Float(ball.velocity.dx)
            ptr = writeValue(value: &dx, to: ptr)
            var dy = Float(ball.velocity.dy)
            ptr = writeValue(value: &dy, to: ptr)
        }
    }

    private func writeValue<T>(value: inout T, to ptr: UnsafeMutableRawPointer) -> UnsafeMutableRawPointer {
        let sizeOfType = MemoryLayout<T>.size
        ptr.copyBytes(from: &value, count: sizeOfType)
        return ptr.advanced(by: sizeOfType)
    }

    public func setupMetal(withDevice device: MTLDevice) throws {
        guard self.device == nil else {
            return
        }
        self.device = device
        sampleComputeState = try computePipelineStateForSamplingKernel(withDevice: device)
    }

    public func computePipelineStateForSamplingKernel(withDevice device: MTLDevice) throws -> MTLComputePipelineState? {
        let bundle = Bundle(for: type(of: self))
        let library = try device.makeDefaultLibrary(bundle: bundle)
        guard let samplingKernel = library.makeFunction(name: "sampleFieldKernel") else {
            throw MetaballsError.metalError("Unable to create sampling kernel function")
        }
        let state = try device.makeComputePipelineState(function: samplingKernel)
        return state
    }

    public func computeEncoderForSamplingKernel(withDevice device: MTLDevice, commandBuffer buffer: MTLCommandBuffer) throws -> MTLComputeCommandEncoder {
        guard let parametersBuffer = makeParametersBufferIfNeeded(withDevice: device),
              let ballBuffer = makeBallBufferIfNeeded(withDevice: device),
              let sampleTexture = makeSampleTextureIfNeeded(withDevice: device),
              let state = sampleComputeState
        else {
            throw MetaballsError.metalError("Missing Metal buffers or compute state")
        }

        let encoder = buffer.makeComputeCommandEncoder()
        encoder.setComputePipelineState(state)
        encoder.setBuffer(parametersBuffer, offset: 0, at: 0)
        encoder.setBuffer(ballBuffer, offset: 0, at: 1)
        encoder.setTexture(sampleTexture, at: 0)
        encoder.dispatchThreadgroups(threadgroupCount, threadsPerThreadgroup: threadgroupSize)
        encoder.endEncoding()

        updateParametersBuffer()

        return encoder
    }
}