1 files changed, 146 insertions, 0 deletions
diff --git a/Jel/Models/BlurHashDecode.swift b/Jel/Models/BlurHashDecode.swift
new file mode 100644
index 0000000..7fe3b39
--- /dev/null
+++ b/Jel/Models/BlurHashDecode.swift
@@ -0,0 +1,146 @@
+import UIKit
+
+extension UIImage {
+    public convenience init?(blurHash: String, size: CGSize, punch: Float = 1) {
+        guard blurHash.count >= 6 else { return nil }
+
+		let sizeFlag = String(blurHash[0]).decode83()
+		let numY = (sizeFlag / 9) + 1
+		let numX = (sizeFlag % 9) + 1
+
+		let quantisedMaximumValue = String(blurHash[1]).decode83()
+        let maximumValue = Float(quantisedMaximumValue + 1) / 166
+
+        guard blurHash.count == 4 + 2 * numX * numY else { return nil }
+
+        let colours: [(Float, Float, Float)] = (0 ..< numX * numY).map { i in
+            if i == 0 {
+				let value = String(blurHash[2 ..< 6]).decode83()
+                return decodeDC(value)
+            } else {
+                let value = String(blurHash[4 + i * 2 ..< 4 + i * 2 + 2]).decode83()
+                return decodeAC(value, maximumValue: maximumValue * punch)
+            }
+        }
+
+        let width = Int(size.width)
+        let height = Int(size.height)
+        let bytesPerRow = width * 3
+        guard let data = CFDataCreateMutable(kCFAllocatorDefault, bytesPerRow * height) else { return nil }
+        CFDataSetLength(data, bytesPerRow * height)
+        guard let pixels = CFDataGetMutableBytePtr(data) else { return nil }
+
+        for y in 0 ..< height {
+            for x in 0 ..< width {
+                var r: Float = 0
+                var g: Float = 0
+                var b: Float = 0
+
+                for j in 0 ..< numY {
+                    for i in 0 ..< numX {
+                        let basis = cos(Float.pi * Float(x) * Float(i) / Float(width)) * cos(Float.pi * Float(y) * Float(j) / Float(height))
+                        let colour = colours[i + j * numX]
+                        r += colour.0 * basis
+                        g += colour.1 * basis
+                        b += colour.2 * basis
+                    }
+                }
+
+                let intR = UInt8(linearTosRGB(r))
+                let intG = UInt8(linearTosRGB(g))
+                let intB = UInt8(linearTosRGB(b))
+
+                pixels[3 * x + 0 + y * bytesPerRow] = intR
+                pixels[3 * x + 1 + y * bytesPerRow] = intG
+                pixels[3 * x + 2 + y * bytesPerRow] = intB
+            }
+        }
+
+        let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue)
+
+        guard let provider = CGDataProvider(data: data) else { return nil }
+        guard let cgImage = CGImage(width: width, height: height, bitsPerComponent: 8, bitsPerPixel: 24, bytesPerRow: bytesPerRow,
+        space: CGColorSpaceCreateDeviceRGB(), bitmapInfo: bitmapInfo, provider: provider, decode: nil, shouldInterpolate: true, intent: .defaultIntent) else { return nil }
+
+        self.init(cgImage: cgImage)
+    }
+}
+
+private func decodeDC(_ value: Int) -> (Float, Float, Float) {
+    let intR = value >> 16
+    let intG = (value >> 8) & 255
+    let intB = value & 255
+    return (sRGBToLinear(intR), sRGBToLinear(intG), sRGBToLinear(intB))
+}
+
+private func decodeAC(_ value: Int, maximumValue: Float) -> (Float, Float, Float) {
+	let quantR = value / (19 * 19)
+	let quantG = (value / 19) % 19
+	let quantB = value % 19
+
+	let rgb = (
+		signPow((Float(quantR) - 9) / 9, 2) * maximumValue,
+		signPow((Float(quantG) - 9) / 9, 2) * maximumValue,
+		signPow((Float(quantB) - 9) / 9, 2) * maximumValue
+	)
+
+	return rgb
+}
+
+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) }
+}()
+
+private let decodeCharacters: [String: Int] = {
+	var dict: [String: Int] = [:]
+	for (index, character) in encodeCharacters.enumerated() {
+		dict[character] = index
+	}
+	return dict
+}()
+
+extension String {
+	func decode83() -> Int {
+		var value: Int = 0
+		for character in self {
+			if let digit = decodeCharacters[String(character)] {
+				value = value * 83 + digit
+			}
+		}
+		return value
+	}
+}
+
+private extension String {
+	subscript (offset: Int) -> Character {
+		return self[index(startIndex, offsetBy: offset)]
+	}
+
+	subscript (bounds: CountableClosedRange<Int>) -> Substring {
+		let start = index(startIndex, offsetBy: bounds.lowerBound)
+		let end = index(startIndex, offsetBy: bounds.upperBound)
+		return self[start...end]
+	}
+
+	subscript (bounds: CountableRange<Int>) -> Substring {
+		let start = index(startIndex, offsetBy: bounds.lowerBound)
+		let end = index(startIndex, offsetBy: bounds.upperBound)
+		return self[start..<end]
+	}
+}