* added chroma.stdDeviation() to retrieve the variance in an array of…

… colors as an RGBA component. See [Issue 328] (gka#328 (comment))

Signed-off-by: regorxxx <[email protected]>

src/utils/deviation.js

@@ -0,0 +1,19 @@
+const Color = require('../Color');
+
+module.exports = (colors) => {
+    colors = [...colors].map((color) => new Color(color).rgba())
+    const colorNum = colors.length;
+    const results = Array.from(Array(colorNum), () => new Array(colorNum));
+    const mean = [0, 0, 0, 0];
+    for (let i = 0; i < colorNum; i++) {
+        colors[i].forEach((c, j) => mean[j] += (j === 3 ? c * 255 : c));
+    }
+    mean.forEach((c, i) => mean[i] /= colorNum);
+    let stdDev = 0;
+    for (let i = 0; i < colorNum; i++) {
+        stdDev += colors[i].reduce((acc, curr, j) => acc + (curr - mean[j])**2, 0);
+    }
+    stdDev /= Math.max((colorNum - 1), 1) * 4;
+    stdDev = Math.round(stdDev**(1/2));
+    return stdDev;
+};

test/stddev.test.js

@@ -0,0 +1,28 @@
+const vows = require('vows')
+const assert = require('assert');
+require('es6-shim');
+
+const stdDeviation = require('../src/utils/deviation');
+const Color = require('../src/Color');;
+
+const tests = {
+    hex: {in: ['#D2691E', '#8B4513', '#A0522D','#0000FF', '#668B8B', '#FFC0CB'],   out: 158},
+    rgba: {in: [[210,105,30,1],[139,69,19,1],[160,82,45,1],[0,0,255,1],[102,139,139,1],[255,192,203,1]],   out: 158},
+    chromacolor: {in:  ['#D2691E', '#8B4513', '#A0522D', '#0000FF', '#668B8B', '#FFC0CB'].map((color) => new Color(color)),   out: 158},
+};
+
+const batch = {};
+
+Object.keys(tests).forEach(key => {
+    batch[`stdDeviation ${key}`] = {
+        topic: tests[key],
+        num(topic) {
+            assert.deepStrictEqual(stdDeviation(topic.in), topic.out);
+        },
+    }
+})
+
+vows
+    .describe('Testing standard deviation')
+    .addBatch(batch)
+    .export(module);