damus

BlurHashEncode.swift (5058B)

---

 import UIKit
 
 extension UIImage {
     public func blurHash(numberOfComponents components: (Int, Int)) -> String? {
 		let pixelWidth = Int(round(size.width * scale))
 		let pixelHeight = Int(round(size.height * scale))
 
 		let context = CGContext(
 			data: nil,
 			width: pixelWidth,
 			height: pixelHeight,
 			bitsPerComponent: 8,
 			bytesPerRow: pixelWidth * 4,
 			space: CGColorSpace(name: CGColorSpace.sRGB)!,
 			bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue
 		)!
 		context.scaleBy(x: scale, y: -scale)
 		context.translateBy(x: 0, y: -size.height)
 
 		UIGraphicsPushContext(context)
 		draw(at: .zero)
 		UIGraphicsPopContext()
 
 		guard let cgImage = context.makeImage(),
 		let dataProvider = cgImage.dataProvider,
 		let data = dataProvider.data,
 		let pixels = CFDataGetBytePtr(data) else {
 			assertionFailure("Unexpected error!")
 			return nil
 		}
 
         let width = cgImage.width
         let height = cgImage.height
         let bytesPerRow = cgImage.bytesPerRow
 
         var factors: [(Float, Float, Float)] = []
         for y in 0 ..< components.1 {
             for x in 0 ..< components.0 {
                 let normalisation: Float = (x == 0 && y == 0) ? 1 : 2
                 let factor = multiplyBasisFunction(pixels: pixels, width: width, height: height, bytesPerRow: bytesPerRow, bytesPerPixel: cgImage.bitsPerPixel / 8, pixelOffset: 0) {
                     normalisation * cos(Float.pi * Float(x) * $0 / Float(width)) as Float * cos(Float.pi * Float(y) * $1 / Float(height)) as Float
                 }
                 factors.append(factor)
             }
         }
 
         let dc = factors.first!
         let ac = factors.dropFirst()
 
         var hash = ""
 
         let sizeFlag = (components.0 - 1) + (components.1 - 1) * 9
         hash += sizeFlag.encode83(length: 1)
 
 		let maximumValue: Float
 		if ac.count > 0 {
 			let actualMaximumValue = ac.map({ max(abs($0.0), abs($0.1), abs($0.2)) }).max()!
 			let quantisedMaximumValue = Int(max(0, min(82, floor(actualMaximumValue * 166 - 0.5))))
 			maximumValue = Float(quantisedMaximumValue + 1) / 166
 			hash += quantisedMaximumValue.encode83(length: 1)
 		} else {
 			maximumValue = 1
 			hash += 0.encode83(length: 1)
 		}
 
         hash += encodeDC(dc).encode83(length: 4)
 
         for factor in ac {
             hash += encodeAC(factor, maximumValue: maximumValue).encode83(length: 2)
         }
 
         return hash
     }
 
     private func multiplyBasisFunction(pixels: UnsafePointer<UInt8>, width: Int, height: Int, bytesPerRow: Int, bytesPerPixel: Int, pixelOffset: Int, basisFunction: (Float, Float) -> Float) -> (Float, Float, Float) {
         var r: Float = 0
         var g: Float = 0
         var b: Float = 0
 
         let buffer = UnsafeBufferPointer(start: pixels, count: height * bytesPerRow)
 
         for x in 0 ..< width {
             for y in 0 ..< height {
                 let basis = basisFunction(Float(x), Float(y))
                 r += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 0 + y * bytesPerRow])
                 g += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 1 + y * bytesPerRow])
                 b += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 2 + y * bytesPerRow])
             }
         }
 
         let scale = 1 / Float(width * height)
 
         return (r * scale, g * scale, b * scale)
     }
 }
 
 private func encodeDC(_ value: (Float, Float, Float)) -> Int {
     let roundedR = linearTosRGB(value.0)
     let roundedG = linearTosRGB(value.1)
     let roundedB = linearTosRGB(value.2)
     return (roundedR << 16) + (roundedG << 8) + roundedB
 }
 
 private func encodeAC(_ value: (Float, Float, Float), maximumValue: Float) -> Int {
 	let quantR = Int(max(0, min(18, floor(signPow(value.0 / maximumValue, 0.5) * 9 + 9.5))))
 	let quantG = Int(max(0, min(18, floor(signPow(value.1 / maximumValue, 0.5) * 9 + 9.5))))
 	let quantB = Int(max(0, min(18, floor(signPow(value.2 / maximumValue, 0.5) * 9 + 9.5))))
 
 	return quantR * 19 * 19 + quantG * 19 + quantB
 }
 
 private func signPow(_ value: Float, _ exp: Float) -> Float {
     return copysign(pow(abs(value), exp), value)
 }
 
 private func linearTosRGB(_ value: Float) -> Int {
     let v = max(0, min(1, value))
     if v <= 0.0031308 { return Int(v * 12.92 * 255 + 0.5) }
     else { return Int((1.055 * pow(v, 1 / 2.4) - 0.055) * 255 + 0.5) }
 }
 
 private func sRGBToLinear<Type: BinaryInteger>(_ value: Type) -> Float {
     let v = Float(Int64(value)) / 255
     if v <= 0.04045 { return v / 12.92 }
     else { return pow((v + 0.055) / 1.055, 2.4) }
 }
 
 private let encodeCharacters: [String] = {
     return "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{|}~".map { String($0) }
 }()
 
 extension BinaryInteger {
 	func encode83(length: Int) -> String {
 		var result = ""
 		for i in 1 ... length {
 			let digit = (Int(self) / pow(83, length - i)) % 83
 			result += encodeCharacters[Int(digit)]
 		}
 		return result
 	}
 }
 
 private func pow(_ base: Int, _ exponent: Int) -> Int {
     return (0 ..< exponent).reduce(1) { value, _ in value * base }
 }