--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/app/src/main/java/com/bartsent/simpleresizer/lib/Resizer.kt	Wed Feb 17 07:24:26 2021 -0800
@@ -0,0 +1,140 @@
+package com.bartsent.simpleresizer.lib
+
+import android.graphics.Bitmap
+import android.graphics.Color
+import java.io.Closeable
+import kotlin.math.ceil
+import kotlin.math.floor
+
+class Resizer: Closeable {
+    private val NWORKERS = maxOf(1, Runtime.getRuntime().availableProcessors() - 2)
+    private val workers = Channel<Worker>(NWORKERS)
+    init {
+        for (i in 0 until NWORKERS)
+            workers.write(Worker(workers))
+    }
+
+    override fun close() {
+        for (i in 0 until NWORKERS)
+            workers.read().run {
+                interrupt()
+                join()
+            }
+    }
+
+    private fun drain(): Unit {
+        val saved = Array<Worker>(NWORKERS) { workers.read() }
+        saved.forEach { workers.write(it) }
+    }
+
+    private data class IndexWeight(var index: Int, var weight: Double)
+
+    private fun precomputeWeights(dstSize: Int, srcSize: Int, filter: ScalingKernel): Array<Array<IndexWeight>> {
+        val du = srcSize.toDouble() / dstSize.toDouble()
+        val scale = maxOf(1.0, du)
+        val ru = ceil(scale * filter.size)
+        val TEMPLATE = arrayOf<IndexWeight>()
+        val out = Array<Array<IndexWeight>>(dstSize) { TEMPLATE }
+        val tmp = ArrayList<IndexWeight>((ru.toInt()+2)*2)
+        val emax = srcSize - 1
+
+        for (v in 0 until dstSize) {
+            val fu = (v.toDouble()+0.5)*du - 0.5
+            val begin = maxOf(0, ceil(fu - ru).toInt())
+            val end = minOf(emax, floor(fu + ru).toInt())
+            var sum: Double = 0.0
+            for (u in begin..end) {
+                val w = filter.weight((u.toDouble() - fu) / scale)
+                if (w != 0.0) {
+                    sum += w
+                    tmp.add(IndexWeight(u, w))
+                }
+            }
+            if (sum != 0.0) {
+                tmp.forEach {
+                    it.weight /= sum
+                }
+            }
+            out[v] = tmp.toArray(TEMPLATE)
+            tmp.clear()
+        }
+        return out
+    }
+
+    private data class WorkUnit(val target: Bitmap, val x: Int, val y: Int,
+                                val weights: Array<IndexWeight>, val scanLine: IntArray)
+
+    private class Worker(private val workers: Channel<Worker>): Thread() {
+        private val _input = Channel<WorkUnit>(1)
+
+        override fun run() {
+            while (true) {
+                val data = _input.read()
+                if (isInterrupted)
+                    return
+                resample(data.target, data.x, data.y, data.weights, data.scanLine)
+                workers.write(this)
+            }
+        }
+
+        fun send(data: WorkUnit) {
+            _input.write(data)
+        }
+
+        private fun clamp(v: Double): Int = minOf(255, maxOf(0, (v + 0.5).toInt()))
+
+        private fun resample(target: Bitmap, x: Int, y: Int, weights: Array<IndexWeight>, scanLine: IntArray): Unit {
+            var r = 0.0; var g = 0.0; var b = 0.0; var a = 0.0
+            weights.forEach {
+                val c = scanLine[it.index]
+                val aw = Color.alpha(c).toDouble() * it.weight
+                r += Color.red(c).toDouble() * aw
+                g += Color.green(c).toDouble() * aw
+                b += Color.blue(c).toDouble() * aw
+                a += aw
+            }
+            if (a == 0.0)
+                return
+            val argb = Color.argb(clamp(a), clamp(r/a), clamp(g/a), clamp(b/a))
+            synchronized(target) {
+                target.setPixel(x, y, argb)
+            }
+        }
+    }
+
+    private fun resample(target: Bitmap, x: Int, y: Int, weights: Array<IndexWeight>, scanLine: IntArray): Unit {
+        workers.read().send(WorkUnit(target, x, y, weights, scanLine))
+    }
+
+    fun horizontal(image: Bitmap, newWidth: Int, kernel: ScalingKernel): Bitmap {
+        val dst = Bitmap.createBitmap(newWidth, image.height, Bitmap.Config.ARGB_8888)
+        val weights = precomputeWeights(newWidth, image.width, kernel)
+        val scanLine = IntArray(image.width)
+        for (y in 0 until image.height) {
+            for (x in 0 until image.width) {
+                scanLine[x] = image.getPixel(x, y)
+            }
+            for (x in weights.indices) {
+                resample(dst, x, y, weights[x], scanLine)
+            }
+        }
+        drain()
+        return dst
+    }
+
+    fun vertical(image: Bitmap, newHeight: Int, kernel: ScalingKernel): Bitmap {
+        val dst = Bitmap.createBitmap(image.width, newHeight, Bitmap.Config.ARGB_8888)
+        val weights = precomputeWeights(newHeight, image.height, kernel)
+        val scanLine = IntArray(image.height)
+        for (x in 0 until image.width) {
+            for (y in 0 until image.height) {
+                scanLine[y] = image.getPixel(x, y)
+            }
+            for (y in weights.indices) {
+                resample(dst, x, y, weights[y], scanLine)
+            }
+        }
+        drain()
+        return dst
+    }
+}
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