Calculating Histograms in Swift

main.swift

import Accelerate

func calculateHistogram(image: CGImage) -> [[vImagePixelCount]] {
    let format = vImage_CGImageFormat(
        bitsPerComponent: 8,
        bitsPerPixel: 32,
        colorSpace: CGColorSpaceCreateDeviceRGB(),
        bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.first.rawValue),
        renderingIntent: .defaultIntent
    )!
    
    let histogramSourceCGImage = image
    guard var histogramSourceBuffer = try? vImage_Buffer(
        cgImage: histogramSourceCGImage,
        format: format
    ) else {
        return []
    }
    defer { histogramSourceBuffer.free() }
    
    var histogramRed = [vImagePixelCount](repeating: 0, count: 256)
    var histogramGreen = [vImagePixelCount](repeating: 0, count: 256)
    var histogramBlue = [vImagePixelCount](repeating: 0, count: 256)
    var histogramAlpha = [vImagePixelCount](repeating: 0, count: 256)
    
    histogramRed.withUnsafeMutableBufferPointer { red in
        histogramGreen.withUnsafeMutableBufferPointer { green in
            histogramBlue.withUnsafeMutableBufferPointer { blue in
                histogramAlpha.withUnsafeMutableBufferPointer { alpha in
                    var histogramBins = [alpha.baseAddress, red.baseAddress, green.baseAddress, blue.baseAddress]
                    
                    histogramBins.withUnsafeMutableBufferPointer { bins in
                        let error = vImageHistogramCalculation_ARGB8888(
                            &histogramSourceBuffer,
                            bins.baseAddress!,
                            vImage_Flags(kvImageNoFlags)
                        )
                        
                        guard error == kvImageNoError else {
                            print("Failed to calculate histogram!")
                            return
                        }
                    }
                }
            }
        }
    }
    
    let channels = [histogramRed, histogramGreen, histogramBlue, histogramAlpha]
    return channels
}