Floodfill Scanline pixel in iOS Swift

I am using floodFillScanline https://lodev.org/cgtutor/floodfill.html algorithm in swift but its not working as expected.

Also performance wise it looks bit slow currently I am getting

Runtime: 0.33456450200174004s

func floodFillScanlineStackV3(x: Int, y: Int,
                            color_Grid: [[Int]], newColor: Int, oldColor: Int,
                            height: Int, width: Int) -> [[Int]] {
    var colorGrid = color_Grid
    
    assert(x < width, "p.x \(x) out of range, must be < \(width)")
    assert(y < height, "p.y \(y) out of range, must be < \(height)")
    
    if oldColor == newColor { return [] }
    
    var stack : [(Int, Int)] = [(x, y)] // 0 is X, 1 is Y
    
    while let pp = stack.popLast() {
        
        var x1 = pp.0
        
        while(x1 >= 0 && colorGrid[pp.1][x1] == oldColor) {
            x1 -= 1
        }
        
        x1 += 1
        
        var spanAbove = false
        var spanBelow = false
        
        while(x1 < width && compareColor(oldColor, colorGrid[pp.1][pp.0])) {
            colorGrid[x1][pp.1] = newColor
            
            let north = (x1, pp.1-1)
            let south = (x1, pp.1+1)
            
            if spanAbove == false && pp.1 > 0 &&  compareColor(colorGrid[x1][pp.1-1], oldColor) {
                stack.append(north)
                spanAbove = true
            } else if spanAbove && pp.1 > 0 && !compareColor(colorGrid[x1][pp.1-1], oldColor) {
                spanAbove = false
            } else if spanBelow == false && pp.1 < height - 1 && compareColor(colorGrid[x1][pp.1+1], oldColor) {
                stack.append(south)
                spanBelow = true
            } else if spanBelow && pp.1 < height - 1 && compareColor(colorGrid[x1][pp.1+1], oldColor) { //!_colorIsEqual(south, pixelBuffer, rgba) {
                spanBelow = false
            }
            
            x1 += 1
        }
    }
    
    return colorGrid
}

Before flood fill

After flood fill

I think you'll have a much better chance of translating that code to Swift by keeping it closer to the original.

func floodFillScanlineStackV4(x: Int, y: Int,
                              color_Grid: [Int],
                              newColor: Int, oldColor: Int,
                              h: Int, w: Int) -> [Int] {
    
    var screenBuffer = color_Grid
    
    assert(x < w, "p.x \(x) out of range, must be < \(w)")
    assert(y < h, "p.y \(y) out of range, must be < \(h)")
    
    if oldColor == newColor { return [] }
    
    var x1: Int = 0
    
    var stack : [(Int, Int)] = [(x, y)] // 0 is X, 1 is Y
    
    while let pp = stack.popLast() {
        
        x1 = pp.0
        let y = pp.1
        
        while(x1 >= 0 && screenBuffer[y * w + x1] == oldColor) {
            x1 -= 1
        }
        x1 += 1
        
        var spanAbove = false
        var spanBelow = false
        
        while(x1 < w && screenBuffer[y * w + x1] == oldColor) {
            screenBuffer[y * w + x1] = newColor;
            
            if(!spanAbove && y > 0 && screenBuffer[(y - 1) * w + x1] == oldColor) {
                stack.append((x1, y - 1))
                spanAbove = true
            }
            else if(spanAbove && y > 0 && screenBuffer[(y - 1) * w + x1] != oldColor) {
                spanAbove = false
            }
            if(!spanBelow && y < h - 1 && screenBuffer[(y + 1) * w + x1] == oldColor) {
                stack.append((x1, y + 1))
                spanBelow = true
            }
            else if(spanBelow && y < h - 1 && screenBuffer[(y + 1) * w + x1] != oldColor) {
                spanBelow = false
            }
            x1 += 1
        }
    }
    
    return screenBuffer
}

func compareColor(_ v1: Int, _ v2: Int) -> Bool {
    return v1 == v2
}

To do that, instead of using a two-dimensional array of Int, use a one-dimensional array.

Here's some example code. Note that this is Example Code only, with minimal error checking, and is not intended to be considered "Production Ready":

class FloodVC: UIViewController {
    
    let gridWidth: Int = 12
    let gridHeight: Int = 12
    var bufLength: Int = 0
    
    let gridSpacing: CGFloat = 1
    
    let colors: [UIColor] = [
        UIColor(red: 1.00, green: 0.60, blue: 0.60, alpha: 1.0),
        UIColor(red: 0.60, green: 1.00, blue: 0.60, alpha: 1.0),
        UIColor(red: 0.20, green: 0.85, blue: 1.00, alpha: 1.0),
        UIColor(red: 1.00, green: 1.00, blue: 0.60, alpha: 1.0),
        UIColor(red: 0.60, green: 1.00, blue: 1.00, alpha: 1.0),
        UIColor(red: 1.00, green: 0.60, blue: 1.00, alpha: 1.0),
    ]
    
    var grid: [Int] = []
    
    var newColor: Int = 2
    
    lazy var gridStack: UIStackView = {
        let v = UIStackView()
        v.axis = .vertical
        v.distribution = .fillEqually
        v.spacing = gridSpacing
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    
    let colorStack: UIStackView = {
        let v = UIStackView()
        v.spacing = 8
        v.distribution = .fillEqually
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    
    let infoLabel: UILabel = {
        let v = UILabel()
        v.textAlignment = .center
        v.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        bufLength = gridWidth * gridHeight
        
        // init array to all Zeroes
        grid = Array(repeating: 0, count: bufLength)
        
        for _ in 0..<gridHeight {
            let rs = UIStackView()
            rs.distribution = .fillEqually
            rs.spacing = gridSpacing
            for _ in 0..<gridWidth {
                let v = UILabel()
                v.font = .systemFont(ofSize: 10, weight: .light)
                v.textAlignment = .center
                v.isUserInteractionEnabled = true
                let g = UITapGestureRecognizer(target: self, action: #selector(cellTap(_:)))
                v.addGestureRecognizer(g)
                rs.addArrangedSubview(v)
            }
            gridStack.addArrangedSubview(rs)
        }
        
        view.addSubview(gridStack)
        
        for i in 0..<colors.count {
            let v = UILabel()
            v.font = .systemFont(ofSize: 10, weight: .light)
            v.textAlignment = .center
            v.text = "\(i)"
            v.backgroundColor = colors[i]
            v.isUserInteractionEnabled = true
            v.heightAnchor.constraint(equalToConstant: 32).isActive = true
            v.layer.borderColor = UIColor.red.cgColor
            if i == newColor {
                v.layer.borderWidth = 1
            }
            let g = UITapGestureRecognizer(target: self, action: #selector(newColorTap(_:)))
            v.addGestureRecognizer(g)
            colorStack.addArrangedSubview(v)
        }
        
        let shapeStack: UIStackView = {
            let v = UIStackView()
            v.spacing = 8
            v.distribution = .fillEqually
            v.translatesAutoresizingMaskIntoConstraints = false
            return v
        }()
        
        ["Square", "Cross", "Triangle", "Random"].forEach { str in
            let v = UIButton()
            v.titleLabel?.font = .systemFont(ofSize: 13, weight: .light)
            v.setTitle(str, for: [])
            v.backgroundColor = .systemBlue
            v.setTitleColor(.white, for: .normal)
            v.setTitleColor(.lightGray, for: .highlighted)
            v.layer.borderColor = UIColor.blue.cgColor
            v.layer.borderWidth = 1
            v.layer.cornerRadius = 6
            v.addTarget(self, action: #selector(setupShape(_:)), for: .touchUpInside)
            shapeStack.addArrangedSubview(v)
        }
        
        let optionsStack: UIStackView = {
            let v = UIStackView()
            v.axis = .vertical
            v.spacing = 8
            v.translatesAutoresizingMaskIntoConstraints = false
            return v
        }()
        
        optionsStack.addArrangedSubview(shapeStack)
        optionsStack.addArrangedSubview(colorStack)
        
        view.addSubview(optionsStack)
        view.addSubview(infoLabel)
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            
            shapeStack.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            shapeStack.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 16),
            shapeStack.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -16),
            
            gridStack.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 8),
            gridStack.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -8),
            gridStack.heightAnchor.constraint(equalTo: gridStack.widthAnchor),
            gridStack.centerYAnchor.constraint(equalTo: g.centerYAnchor),
            
            infoLabel.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 16),
            infoLabel.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -16),
            infoLabel.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),

        ])
        
        setupSquare()
    }
    
    @objc func newColorTap(_ g: UITapGestureRecognizer) {
        
        guard let v = g.view as? UILabel,
              let t = v.text,
              let idx = Int(t)
        else { return }
        
        for i in 0..<colorStack.arrangedSubviews.count {
            colorStack.arrangedSubviews[i].layer.borderWidth = i == idx ? 1 : 0
        }
        
        newColor = idx
        
    }
    
    @objc func cellTap(_ g: UITapGestureRecognizer) {
        
        guard let v = g.view as? UILabel,
              let t = v.text,
              let oldColor = Int(t),
              let rowStack = v.superview as? UIStackView,
              let c = rowStack.arrangedSubviews.firstIndex(of: v),
              let r = gridStack.arrangedSubviews.firstIndex(of: rowStack)
        else { return }
        

        let st = CFAbsoluteTimeGetCurrent()

        let newGrid = floodFillScanlineStackV4(x: c, y: r, color_Grid: grid, newColor: newColor, oldColor: oldColor, h: gridHeight, w: gridWidth)
        
        let elapsed = CFAbsoluteTimeGetCurrent() - st

        let nf = NumberFormatter()
        nf.maximumFractionDigits = 8

        infoLabel.text = "Elapsed Time: " + nf.string(from: NSNumber(value: elapsed))! + " seconds"

        // if new color equals tapped cell color,
        //  newGrid will be empty
        if newGrid.count != 0 {
            grid = newGrid
            refreshGrid(grid)
        }
    }
    
    func refreshGrid(_ g: [Int]) {
        
        for i in 0..<bufLength {
            let r = i / gridWidth
            guard let rs = gridStack.arrangedSubviews[r] as? UIStackView
            else { fatalError("bad setup") }
            for c in 0..<gridWidth {
                guard let v = rs.arrangedSubviews[c] as? UILabel
                else { fatalError("bad setup") }
                let p = r * gridWidth + c
                v.text = "\(g[p])"
                v.backgroundColor = colors[g[p]]
            }
        }
        
    }
    
    func floodFillScanlineStackV4(x: Int, y: Int,
                                  color_Grid: [Int],
                                  newColor: Int, oldColor: Int,
                                  h: Int, w: Int) -> [Int] {
        
        var screenBuffer = color_Grid
        
        assert(x < w, "p.x \(x) out of range, must be < \(w)")
        assert(y < h, "p.y \(y) out of range, must be < \(h)")
        
        if oldColor == newColor { return [] }
        
        var x1: Int = 0
        
        var stack : [(Int, Int)] = [(x, y)] // 0 is X, 1 is Y
        
        while let pp = stack.popLast() {
            
            x1 = pp.0
            let y = pp.1
            
            while(x1 >= 0 && screenBuffer[y * w + x1] == oldColor) {
                x1 -= 1
            }
            x1 += 1
            
            var spanAbove = false
            var spanBelow = false
            
            while(x1 < w && screenBuffer[y * w + x1] == oldColor) {
                screenBuffer[y * w + x1] = newColor;
                
                if(!spanAbove && y > 0 && screenBuffer[(y - 1) * w + x1] == oldColor) {
                    stack.append((x1, y - 1))
                    spanAbove = true
                }
                else if(spanAbove && y > 0 && screenBuffer[(y - 1) * w + x1] != oldColor) {
                    spanAbove = false
                }
                if(!spanBelow && y < h - 1 && screenBuffer[(y + 1) * w + x1] == oldColor) {
                    stack.append((x1, y + 1))
                    spanBelow = true
                }
                else if(spanBelow && y < h - 1 && screenBuffer[(y + 1) * w + x1] != oldColor) {
                    spanBelow = false
                }
                x1 += 1
            }
        }
        
        return screenBuffer
    }
    
    func compareColor(_ v1: Int, _ v2: Int) -> Bool {
        return v1 == v2
    }

    // MARK: grid setups
    @objc func setupShape(_ sender: Any?) {
        var t: String = "square"
        if let btn = sender as? UIButton {
            t = btn.currentTitle ?? "square"
        }
        switch t {
        case "Square":
            setupSquare()
            ()
        case "Cross":
            setupCross()
            ()
        case "Triangle":
            setupTriangle()
            ()
        default:
            setupRandom()
            ()
        }
    }
    
    @objc func setupSquare() {
        
        // init array to all Zeroes
        grid = Array(repeating: 0, count: bufLength)
        
        let row1: Int = 2
        let row2: Int = gridHeight - (row1 + 1)
        let col1: Int = 2
        let col2: Int = gridWidth - (col1 + 1)
        
        for r in row1...row2 {
            for c in col1...col2 {
                let p = r * gridWidth + c
                grid[p] = 1
            }
        }
        
        refreshGrid(grid)
        
    }
    
    @objc func setupCross() {
        
        // init array to all Zeroes
        grid = Array(repeating: 0, count: bufLength)
        
        var row1: Int = 2
        var row2: Int = gridHeight - (row1 + 1)
        var col1: Int = gridWidth / 2 - 1
        var col2: Int = col1 + 1
        for r in row1...row2 {
            for c in col1...col2 {
                let p = r * gridWidth + c
                grid[p] = 1
            }
        }
        row1 = gridHeight / 2 - 1
        row2 = row1 + 1
        col1 = 2
        col2 = gridWidth - (col1 + 1)
        for r in row1...row2 {
            for c in col1...col2 {
                let p = r * gridWidth + c
                grid[p] = 1
            }
        }
        
        refreshGrid(grid)
        
    }
    
    @objc func setupTriangle() {
        
        // init array to all Zeroes
        grid = Array(repeating: 0, count: bufLength)
        
        var row: Int = 1
        
        var col1: Int = gridWidth / 2 - 1
        var col2: Int = col1 + 1
        
        let p: Int = row * gridWidth + col1
        grid[p] = 1
        grid[p + 1] = 1
        row += 1
        col1 -= 1
        col2 += 1
        
        while col1 > 0 {
            var p1: Int = row * gridWidth + col1
            var p2: Int = row * gridWidth + col2
            grid[p1] = 1
            grid[p2] = 1
            row += 1
            
            p1 = row * gridWidth + col1
            p2 = row * gridWidth + col2
            grid[p1] = 1
            grid[p2] = 1
            row += 1
            
            col1 -= 1
            col2 += 1
        }
        for c in col1...col2 {
            let p: Int = row * gridWidth + c
            grid[p] = 1
        }
        
        refreshGrid(grid)
        
    }
    
    @objc func setupRandom() {
        
        // init array to all Zeroes
        grid = Array(repeating: 0, count: bufLength)
        
        // we'll fill grid with random excluding
        //  the first color, to make it easier to see
        //  the changes
        for r in 0..<gridHeight {
            for c in 0..<gridWidth {
                let p = r * gridWidth + c
                grid[p] = Int.random(in: 1..<colors.count)
            }
        }
        
        let idx: Int = 0
        
        for i in 0..<colorStack.arrangedSubviews.count {
            colorStack.arrangedSubviews[i].layer.borderWidth = i == idx ? 1 : 0
        }
        
        newColor = idx
        
        refreshGrid(grid)
        
    }
    
}

It looks like this when running:

Tapping a color-box at the top will select the "replacement color".

Tapping a box inside the grid will perform the Flood Fill.

Since this would ideally be used directly on a buffer of image data, the "Elapsed Time" value is for the Int Array process only -- it does not include refreshing the UI (the grid of colors/numbers).

Tapping on the labeled buttons sets up an initial pattern:

Use Random and then select various contiguous squares to try out a winding path: