Add auto optimizer

cli/src/auto-optimizer.js

@@ -0,0 +1,67 @@
+import { instantiateEmscriptenWasm } from "./emscripten-utils.js";
+
+import visdif from "../../codecs/visdif/visdif.js";
+import visdifWasm from "asset-url:../../codecs/visdif/visdif.wasm";
+
+// `measure` is a (async) function that takes exactly one numeric parameter and
+// returns a value. The function is assumed to be monotonic (an increase in `parameter`
+// will result in an increase in the return value. The function uses binary search
+// to find `parameter` such that `measure` returns `measureGoal`, within an error
+// of `epsilon`. It will use at most `maxRounds` attempts.
+export async function binarySearch(
+  measureGoal,
+  measure,
+  { min = 0, max = 100, epsilon = 0.1, maxRounds = 8 } = {}
+) {
+  let parameter = (max - min) / 2 + min;
+  let delta = (max - min) / 4;
+  let value;
+  let round = 0;
+  do {
+    value = await measure(parameter);
+    if (value > measureGoal) {
+      parameter -= delta;
+    } else if (value < measureGoal) {
+      parameter += delta;
+    }
+    delta /= 2;
+    round++;
+  } while (Math.abs(value - measureGoal) > epsilon && round < maxRounds);
+  return { parameter, round, value };
+}
+export async function autoOptimize(
+  bitmapIn,
+  encode,
+  decode,
+  { butteraugliDistanceGoal = 1.4, ...otherOpts } = {}
+) {
+  const { VisDiff } = await instantiateEmscriptenWasm(visdif, visdifWasm);
+
+  const comparator = new VisDiff(
+    bitmapIn.data,
+    bitmapIn.width,
+    bitmapIn.height
+  );
+
+  let bitmapOut;
+  let binaryOut;
+  // Increasing quality means _decrease_ in Butteraugli distance.
+  // `binarySearch` assumes that increasing `parameter` will
+  // increase the metric value. So multipliy Butteraugli values by -1.
+  const { parameter } = await binarySearch(
+    -1 * butteraugliDistanceGoal,
+    async quality => {
+      binaryOut = await encode(bitmapIn, quality);
+      bitmapOut = await decode(binaryOut);
+      return -1 * comparator.distance(bitmapOut.data);
+    },
+    otherOpts
+  );
+  comparator.delete();
+
+  return {
+    bitmap: bitmapOut,
+    binary: binaryOut,
+    quality: parameter
+  };
+}