--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/app/src/main/java/com/bartsent/simpleresizer/lib/Resizer.kt	Thu Feb 18 14:15:26 2021 -0800
@@ -0,0 +1,151 @@
+package com.bartsent.simpleresizer.lib
+
+import android.graphics.Bitmap
+import android.graphics.Color
+import kotlin.math.ceil
+import kotlin.math.floor
+
+/**
+ * Bitmapped images designed for efficient resampling and per-pixel access via
+ * subscripting.
+ */
+class Resizer(private val bits: IntArray, val width: Int, val height: Int) {
+    constructor(width: Int, height: Int) : this(IntArray(width * height), width, height)
+
+    companion object {
+        /**
+         *  Make a Resizer from a standard Bitmap.
+         *  @param      image       Bitmap object.
+         *  @return                 A new Resizer.
+         */
+        fun fromBitmap(image: Bitmap): Resizer {
+            val basedOn = IntArray(image.width * image.height)
+            image.getPixels(basedOn, 0, image.width, 0, 0, image.width, image.height)
+            return Resizer(basedOn, image.width, image.height)
+        }
+    }
+
+    private interface Band {
+        operator fun get(i: Int): Int
+        operator fun set(i: Int, v: Int): Unit
+        val index: Int
+        val size: Int
+    }
+
+    private class Row(val basedOn: Resizer, override val index: Int): Band {
+        override operator fun get(i: Int) = basedOn[i, index]
+        override operator fun set(i: Int, v: Int): Unit {
+            basedOn[i, index] = v
+        }
+        override val size; get() = basedOn.width
+    }
+
+    private class Column(val basedOn: Resizer, override val index: Int): Band {
+        override operator fun get(i: Int) = basedOn[index, i]
+        override operator fun set(i: Int, v: Int): Unit {
+            basedOn[index, i] = v
+        }
+        override val size; get() = basedOn.height
+    }
+
+    private data class IndexWeight(var index: Int, var weight: Double)
+
+    operator fun get(x: Int, y: Int): Int = bits[x + y*width]
+
+    operator fun set(x: Int, y: Int, v: Int): Unit {
+        bits[x + y*width] = v
+    }
+
+    /**
+     * Return a standard Bitmap based on this Resizer's image.
+     * @return                 A new Bitmap, which may share this Resizer's image storage.
+     */
+    fun toBitmap(): Bitmap = Bitmap.createBitmap(bits, width, height, Bitmap.Config.ARGB_8888)
+
+    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 fun clamp(v: Double): Int = minOf(255, maxOf(0, (v + 0.5).toInt()))
+
+    private fun resample(target: Band, i: Int, weights: Array<IndexWeight>, source: Band): Unit {
+        var r = 0.0; var g = 0.0; var b = 0.0; var a = 0.0
+        weights.forEach {
+            val c = source[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
+        target[i] = Color.argb(clamp(a), clamp(r/a), clamp(g/a), clamp(b/a))
+    }
+
+    private fun resampleBand(target: Band, weights: Array<Array<IndexWeight>>, source: Band) {
+        for (i in 0 until target.size) {
+            resample(target, i, weights[i], source)
+        }
+    }
+
+    private val DEFAULT_KERNEL = LanczosKernel()
+
+    /**
+     * Resize the horizontal aspect.
+     * @param       newWidth    Width of new image
+     * @param       kernel      Scaling kernel to use (default: LanczosKernel)
+     * @return                  A new Resizer object, resampled per specifications
+     */
+    fun horizontal(newWidth: Int, kernel: ScalingKernel = DEFAULT_KERNEL): Resizer {
+        val dst = Resizer(newWidth, height)
+        val weights = precomputeWeights(newWidth, width, kernel)
+        for (y in 0 until height) {
+            resampleBand(Row(dst, y), weights, Row(this, y))
+        }
+        return dst
+    }
+
+    /**
+     * Resize the vertical aspect.
+     * @param       newHeight   Height of new image
+     * @param       kernel      Scaling kernel to use (default: LanczosKernel)
+     * @return                  A new Resizer object, resampled per specifications
+     */
+    fun vertical(newHeight: Int, kernel: ScalingKernel = DEFAULT_KERNEL): Resizer {
+        val dst = Resizer(width, newHeight)
+        val weights = precomputeWeights(newHeight, height, kernel)
+        for (x in 0 until width) {
+            resampleBand(Column(dst, x), weights, Column(this, x))
+        }
+        return dst
+    }
+}
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