refactor: Improve blurhash decoding performance (#770)

By Christophe Beyls in https://github.com/tuskyapp/Tusky/pull/4515.
Their commit notes:

Improve the performance of `BlurHashDecoder` while also reducing memory
allocations.

- Precompute cosines tables before composing the image so each cosine
value is only computed once.
- Compute cosines tables once if both are identical (for square images
with the same number of colors in both dimensions).
- Store colors in a one-dimension array instead of a two-dimension array
to reduce memory allocations.
- Use a simple String.indexOf() to find the index of a Base83 char,
which is both faster and needs less memory than a HashMap thanks to
better locality and no boxing of chars.
- No cache is used, so computations may be performed in parallel on
background threads without the need for synchronization which limits
throughput.

core/common/src/main/kotlin/app/pachli/core/common/util/BlurHashDecoder.kt

@@ -19,6 +19,7 @@
  * Blurhash implementation from blurhash project:
  * https://github.com/woltapp/blurhash
  * Minor modifications by charlag
+ * Major performance improvements by cbeyls
  */
 
 package app.pachli.core.common.util
@@ -31,7 +32,6 @@ import kotlin.math.pow
 import kotlin.math.withSign
 
 object BlurHashDecoder {
-
     fun decode(blurHash: String?, width: Int, height: Int, punch: Float = 1f): Bitmap? {
         require(width > 0) { "Width must be greater than zero" }
         require(height > 0) { "height must be greater than zero" }
@@ -41,28 +41,27 @@ object BlurHashDecoder {
         val numCompEnc = decode83(blurHash, 0, 1)
         val numCompX = (numCompEnc % 9) + 1
         val numCompY = (numCompEnc / 9) + 1
-        if (blurHash.length != 4 + 2 * numCompX * numCompY) {
+        val totalComp = numCompX * numCompY
+        if (blurHash.length != 4 + 2 * totalComp) {
             return null
         }
         val maxAcEnc = decode83(blurHash, 1, 2)
         val maxAc = (maxAcEnc + 1) / 166f
-        val colors = Array(numCompX * numCompY) { i ->
+        val colors = FloatArray(totalComp * 3)
-            if (i == 0) {
+        var colorEnc = decode83(blurHash, 2, 6)
-                val colorEnc = decode83(blurHash, 2, 6)
+        decodeDc(colorEnc, colors)
-                decodeDc(colorEnc)
+        for (i in 1 until totalComp) {
-            } else {
             val from = 4 + i * 2
-                val colorEnc = decode83(blurHash, from, from + 2)
+            colorEnc = decode83(blurHash, from, from + 2)
-                decodeAc(colorEnc, maxAc * punch)
+            decodeAc(colorEnc, maxAc * punch, colors, i * 3)
-            }
         }
         return composeBitmap(width, height, numCompX, numCompY, colors)
     }
 
-    private fun decode83(str: String, from: Int = 0, to: Int = str.length): Int {
+    private fun decode83(str: String, from: Int, to: Int): Int {
         var result = 0
         for (i in from until to) {
-            val index = charMap[str[i]] ?: -1
+            val index = CHARS.indexOf(str[i])
             if (index != -1) {
                 result = result * 83 + index
             }
@@ -70,11 +69,13 @@ object BlurHashDecoder {
         return result
     }
 
-    private fun decodeDc(colorEnc: Int): FloatArray {
+    private fun decodeDc(colorEnc: Int, outArray: FloatArray) {
-        val r = colorEnc shr 16
+        val r = (colorEnc shr 16) and 0xFF
-        val g = (colorEnc shr 8) and 255
+        val g = (colorEnc shr 8) and 0xFF
-        val b = colorEnc and 255
+        val b = colorEnc and 0xFF
-        return floatArrayOf(srgbToLinear(r), srgbToLinear(g), srgbToLinear(b))
+        outArray[0] = srgbToLinear(r)
+        outArray[1] = srgbToLinear(g)
+        outArray[2] = srgbToLinear(b)
     }
 
     private fun srgbToLinear(colorEnc: Int): Float {
@@ -86,15 +87,13 @@ object BlurHashDecoder {
         }
     }
 
-    private fun decodeAc(value: Int, maxAc: Float): FloatArray {
+    private fun decodeAc(value: Int, maxAc: Float, outArray: FloatArray, outIndex: Int) {
         val r = value / (19 * 19)
         val g = (value / 19) % 19
         val b = value % 19
-        return floatArrayOf(
+        outArray[outIndex] = signedPow2((r - 9) / 9.0f) * maxAc
-            signedPow2((r - 9) / 9.0f) * maxAc,
+        outArray[outIndex + 1] = signedPow2((g - 9) / 9.0f) * maxAc
-            signedPow2((g - 9) / 9.0f) * maxAc,
+        outArray[outIndex + 2] = signedPow2((b - 9) / 9.0f) * maxAc
-            signedPow2((b - 9) / 9.0f) * maxAc,
-        )
     }
 
     private fun signedPow2(value: Float) = value.pow(2f).withSign(value)
@@ -104,21 +103,29 @@ object BlurHashDecoder {
         height: Int,
         numCompX: Int,
         numCompY: Int,
-        colors: Array<FloatArray>,
+        colors: FloatArray,
     ): Bitmap {
         val imageArray = IntArray(width * height)
+        val cosinesX = createCosines(width, numCompX)
+        val cosinesY = if (width == height && numCompX == numCompY) {
+            cosinesX
+        } else {
+            createCosines(height, numCompY)
+        }
         for (y in 0 until height) {
             for (x in 0 until width) {
                 var r = 0f
                 var g = 0f
                 var b = 0f
                 for (j in 0 until numCompY) {
+                    val cosY = cosinesY[y * numCompY + j]
                     for (i in 0 until numCompX) {
-                        val basis = (cos(PI * x * i / width) * cos(PI * y * j / height)).toFloat()
+                        val cosX = cosinesX[x * numCompX + i]
-                        val color = colors[j * numCompX + i]
+                        val basis = cosX * cosY
-                        r += color[0] * basis
+                        val colorIndex = (j * numCompX + i) * 3
-                        g += color[1] * basis
+                        r += colors[colorIndex] * basis
-                        b += color[2] * basis
+                        g += colors[colorIndex + 1] * basis
+                        b += colors[colorIndex + 2] * basis
                     }
                 }
                 imageArray[x + width * y] = Color.rgb(linearToSrgb(r), linearToSrgb(g), linearToSrgb(b))
@@ -127,6 +134,12 @@ object BlurHashDecoder {
         return Bitmap.createBitmap(imageArray, width, height, Bitmap.Config.ARGB_8888)
     }
 
+    private fun createCosines(size: Int, numComp: Int) = FloatArray(size * numComp) { index ->
+        val x = index / numComp
+        val i = index % numComp
+        cos(PI * x * i / size).toFloat()
+    }
+
     private fun linearToSrgb(value: Float): Int {
         val v = value.coerceIn(0f, 1f)
         return if (v <= 0.0031308f) {
@@ -136,13 +149,5 @@ object BlurHashDecoder {
         }
     }
 
-    private val charMap = listOf(
+    private const val CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{|}~"
-        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G',
-        'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
-        'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o',
-        'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '#', '$', '%', '*', '+', ',',
-        '-', '.', ':', ';', '=', '?', '@', '[', ']', '^', '_', '{', '|', '}', '~',
-    )
-        .mapIndexed { i, c -> c to i }
-        .toMap()
 }