Structs with subscript() setter don't actually set

gistfile1.txt

//
//  Matrices.swift
//  Game
//
//  Created by Eryn Wells on 2015-10-24.
//  Copyright © 2015 Eryn Wells. All rights reserved.
//

/** A square matrix. */
public protocol Matrix {
    /** The identity matrix. */
    static var identity: Self { get }

    /** Number of elements in the underlying container. */
    static var count: Int { get }

    /**
     * Number of elements in a single row or column of the matrix. Since
     * matrices are always square, this applies to row and column.
     */
    static var dimension: Int { get }

    /** Initialize a Matrix filled with zeroes. */
    init()

    /** 
     * Initialize a Matrix with a given array of values. The number of values
     * must match the count of the array; otherwise, a MatrixError.InvalidSize
     * error is thrown.
     */
    init(values: [Float]) throws

    /** The underlying data array. */
    var data: [Float] { get }

    /** 
     * Element access
     * @{
     */
    subscript(idx: Int) -> Float { get set }
    subscript(row: Int, col: Int) -> Float { get set }
    /** @} */
}

extension Matrix {
    public static var count: Int {
        return dimension * dimension
    }
}

public enum MatrixError: ErrorType {
    case InvalidSize(given: Int, expected: Int)
}

// MARK: - Matrices

public struct Matrix4: Matrix {
    public static var identity: Matrix4 = {
        var matrix = Matrix4()
        for i in 0..<Matrix4.dimension {
            matrix[i,i] = 1.0
        }
        return matrix
    }()

    public private(set) var data: [Float]

    public init() {
        data = [Float](count: Matrix4.count, repeatedValue: Float(0))
    }

    public init(values: [Float]) throws {
        guard values.count == Matrix4.count else {
            throw MatrixError.InvalidSize(given: values.count, expected: Matrix4.count)
        }
        data = values
    }


    init(translationWithX x: Float, y: Float, z: Float) {
        // NOTE: Call will never fail.
        try! self.init(values: [1, 0, 0, x,
                                0, 1, 0, y,
                                0, 0, 1, z,
                                0, 0, 0, 1])
    }

    init(scaleWithX x: Float, y: Float, z: Float) {
        // NOTE: Call will never fail.
        try! self.init(values: [x, 0, 0, 0,
                                0, y, 0, 0,
                                0, 0, z, 0,
                                0, 0, 0, 1])
    }

    init(rotationAboutX x: Float) {
        let c = cos(x)
        let s = sin(x)
        try! self.init(values: [1,  0, 0, 0,
                                0,  c, s, 0,
                                0, -s, c, 0,
                                0,  0, 0, 1])
    }

    init(rotationAboutY y: Float) {
        let c = cos(y)
        let s = sin(y)
        try! self.init(values: [c, 0, -s, 0,
                                0, 1,  0, 0,
                                s, 0,  c, 0,
                                0, 0,  0, 1])
    }

    init(rotationAboutZ z: Float) {
        let c = cos(z)
        let s = sin(z)
        try! self.init(values: [ c, s, 0, 0,
                                -s, c, 0, 0,
                                 0, 0, 1, 0,
                                 0, 0, 0, 1])
    }

    init(rotationAboutX x: Float, y: Float, z: Float) {
        let r = Matrix4(rotationAboutX: x) * Matrix4(rotationAboutY: y) * Matrix4(rotationAboutZ: z)
        try! self.init(values: r.data)
    }

    public static var dimension: Int {
        return 4
    }

    public subscript(idx: Int) -> Float {
        get {
            print("Matrix4.subscript(\(idx)) -> \(data[idx])")
            return data[idx]
        }
        mutating set(value) {
            data[idx] = value
            print("Matrix4.subscript(\(idx)) = \(value) -> \(data[idx])")
        }
    }

    public subscript(row: Int, col: Int) -> Float {
        get {
            return data[indexFromCoordinates(row, col)]
        }
        mutating set(value) {
            data[indexFromCoordinates(row, col)] = value
        }
    }

    /** Convert a (row, col) pair into an index into the data array. */
    private func indexFromCoordinates(row: Int, _ col: Int) -> Int {
        return row * Matrix4.dimension + col
    }
}