[Feature] Allow optional seeding from getColor

.eslintrc

@@ -1,7 +1,7 @@
 {
   "env": {
-    "node": true,
-    "browser": true
+    "es2021": true,
+    "node": true
   },
   "extends": ["eslint:recommended", "eslint-config-prettier"],
   "rules": {

README.md

@@ -27,6 +27,19 @@ color.getColor();
 color.getColor();
 ```
 
+`getColor` can optionally take a seed to guarantee the same random color regardless of order.
+
+```js
+import toColor from '@mapbox/to-color'
+
+const color = new toColor('trees');
+
+color.getColor('cedar'); // Returns a random color based on `trees`
+color.getColor('birch'); 
+color.getColor('cedar'); // Returns the same color for cedar
+color.getColor('spruce');
+```
+
 ### Options
 
 | Option | Value | Default | Description |

demo/index.html

@@ -14,10 +14,15 @@ <h1 class="txt-h4 txt-fancy block mb6">@mapbox/to-color</h1>
       <p class="txt-l">Procedurally generate a deterministic, perceptually distributed color palette. See <a href="https://github.com/mapbox/to-color" class="link" target="_blank">github.com/mapbox/to-color</a> for more.</p>
     </header>
     <div id="swatches" class="grid grid--gut12 txt-mono"></div>
+    <div class="mt36 mb24">
+      <p class="txt-l"><code class="txt-code">getColor</code> can optionally take a seed to guarantee the same random color regardless of its order.</p>
+    </div>
+    <div id="get-color-option-swatches" class="grid grid--gut12 txt-mono"></div>
   </div>
   <script>
     const wrapOrNah = val => typeof val === 'string' ? `'${val}'` : val;
     const swatches = document.getElementById('swatches');
+    const getColorOptionSwatches = document.getElementById('get-color-option-swatches');
 
     [
       ['mouse', 6],
@@ -61,6 +66,44 @@ <h1 class="txt-h4 txt-fancy block mb6">@mapbox/to-color</h1>
       container.appendChild(swatchSnippet);
       swatches.appendChild(container); 
     });
+
+    [
+      ['trees', ['cedar', 'birch', 'cedar', 'willow', 'birch', 'oak'], { brightness: 0 }],
+      ['trees', ['birch', 'cedar', 'willow', 'birch', 'cedar', 'oak'], { brightness: 1.5 }],
+      ['veneers', ['cedar', 'birch', 'cedar', 'willow', 'birch', 'oak'], { brightness: 0 }],
+      ['veneers', ['cedar', 'birch', 'cedar', 'willow', 'birch', 'oak'], { brightness: 1.5 }]
+    ].forEach(swatch => {
+      const seed = swatch[0];
+      const seededPalette = swatch[1];
+      const options = swatch[2];
+
+      const container = document.createElement('div');
+      const swatchSnippet = document.createElement('div');
+      swatchSnippet.classList = 'bg-gray-dark color-gray-light txt-mono px12 py12 round-b txt-truncate';
+      const swatchContainer = document.createElement('div');
+      swatchContainer.classList = 'round-t h120 txt-xs txt-bold flex-parent scroll-hidden';
+      const s = new toColor(seed, options || {});
+
+      container.classList = `col col--12 ${seededPalette.length < 8 ? 'col--6-ml' : ''} mb12`;
+
+      seededPalette.forEach(p => {
+        const { hsl } = s.getColor(p);
+        const swatchContainerItem = document.createElement('div');
+        swatchContainerItem.classList = `px12 py12 flex-child flex-child--grow h120`;
+        swatchContainerItem.style.flexBasis = 0;
+        swatchContainerItem.style.backgroundColor = hsl.formatted;
+        swatchContainerItem.textContent = swatchContainerItem.title = `getColor ('${p}')`;
+        swatchContainer.appendChild(swatchContainerItem);
+      });
+
+      let optionsString = options ? `, ${JSON.stringify(options)}` : '';
+      let textContent = `${wrapOrNah(seed)}`
+      swatchSnippet.textContent = swatchSnippet.title = `new toColor(${textContent}${optionsString});`;
+
+      container.appendChild(swatchContainer);
+      container.appendChild(swatchSnippet);
+      getColorOptionSwatches.appendChild(container); 
+    });
   </script>
 </body>
 </html>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@mapbox/to-color",
-  "version": "2.2.0",
+  "version": "2.3.0",
   "description": "Procedurally generate a deterministic, perceptually distributed color palette.",
   "main": "dist/to-color.js",
   "scripts": {

src/index.js

@@ -18,43 +18,73 @@ export default class toColor {
   constructor(seed, options) {
     this.options = options || {};
     if (typeof seed === 'string' || typeof seed === 'number') {
-      this.seed = typeof seed === 'string' ? this._stringToInteger(seed) : seed;
+      this.rootSeed =
+        typeof seed === 'string' ? this._stringToInteger(seed) : seed;
     } else {
       throw new TypeError('Seed value must be a number or string');
     }
 
+    this.seed = this.rootSeed;
     this.known = [];
+    this.cache = new Map();
   }
 
-  getColor(count = 0) {
+  getColor(key) {
+    if (typeof key === 'string') {
+      if (this.cache.has(key)) return this.cache.get(key);
+
+      const color = this._getDeterministicColor(key);
+      this.cache.set(key, color);
+      return color;
+    }
+
+    return this._getSequentialColor();
+  }
+
+  _getDeterministicColor(key) {
+    const combined = this._stringToInteger(`${this.rootSeed}:${key}`);
+
+    const h = this._mapIndexToHue(combined);
+    const s = this._mapIndexToRange(combined >> 2, 60, 100);
+    const l = this._mapIndexToRange(combined >> 3, 35, 80);
+
+    return this._colorWithModifiers(h, s, l);
+  }
+
+  _getSequentialColor(count = 0) {
     const h = this._pickHue();
     const s = this._pickSaturation();
     const l = this._pickLightness();
 
     const { hsl } = this._HSLuvify(h, s, l);
     const PASSABLE_DISTANCE = 60;
-
-    // The larger `count` grows, we need to divide actual distance to avoid
-    // hitting a maxiumum call stack error.
     const ACTUAL_DISTANCE = PASSABLE_DISTANCE / Math.pow(1.05, count);
 
-    // Detect color similarity. If values are too close to one another, call
-    // getColor until enough dissimilarity is achieved.
     if (
       this.known.length &&
       this.known.some(
         (v) => differenceCiede2000(v, hsl.formatted) < ACTUAL_DISTANCE
       )
     ) {
-      return this.getColor(count + 1);
+      return this._getSequentialColor(count + 1);
     } else {
       this.known.push(hsl.formatted);
-      // Apply modifiers after distribution check + regeneration to ensure
-      // colors with brightness/saturation adjustments remain the same.
       return this._colorWithModifiers(h, s, l);
     }
   }
 
+  _mapIndexToHue(index) {
+    // A hybrid approach to color distance checking in _getSequentialColor but
+    // for `_getDeterministicColor`. Attempts to “spread” hash values evenly to
+    // reduce the same hues appearing next to one another.
+    const GOLDEN_RATIO_CONJUGATE = (Math.sqrt(5) - 1) / 2; // ≈ 0.61803398875
+    return Math.round(((index * GOLDEN_RATIO_CONJUGATE) % 1) * this.HUE_MAX);
+  }
+
+  _mapIndexToRange(index, min, max) {
+    return min + (index % (max - min));
+  }
+
   _clamp = (n, min, max) => (n <= min ? min : n >= max ? max : n);
 
   _colorWithModifiers = (h, s, l) => {

src/index.test.js

@@ -91,6 +91,52 @@ describe('toColor', () => {
     });
   });
 
+  describe('getColor seeding', () => {
+    it('returns the same color regardless of order', () => {
+      const color = new toColor('genres');
+
+      const a = color.getColor('jazz');
+      const b = color.getColor('fusion');
+      const c = color.getColor('jazz');
+
+      expect(a.hsl.formatted).toEqual(c.hsl.formatted);
+      expect(a.hsl.formatted).not.toEqual(b.hsl.formatted);
+    });
+
+    it('works with root seeding being different', () => {
+      const colorA = new toColor('genres');
+      const colorB = new toColor('dance');
+
+      const aa = colorA.getColor('jazz');
+      const ab = colorA.getColor('fusion');
+      const ac = colorA.getColor('jazz');
+
+      const ba = colorB.getColor('jazz');
+      const bb = colorB.getColor('fusion');
+      const bc = colorB.getColor('jazz');
+
+      expect(aa.hsl.formatted).toEqual(ac.hsl.formatted);
+      expect(aa.hsl.formatted).not.toEqual(ab.hsl.formatted);
+
+      expect(aa.hsl.formatted).not.toEqual(ba.hsl.formatted);
+      expect(ba.hsl.formatted).not.toEqual(bb.hsl.formatted);
+      expect(ba.hsl.formatted).toEqual(bc.hsl.formatted);
+    });
+
+    it('determinisic regardless of order', () => {
+      const colorA = new toColor('dance');
+      const colorB = new toColor('dance');
+
+      const aa = colorA.getColor('jazz'); // Defined first
+      colorA.getColor('fusion');
+
+      colorB.getColor('fusion');
+      const bb = colorB.getColor('jazz'); // Defined last
+
+      expect(aa.hsl.formatted).toEqual(bb.hsl.formatted);
+    });
+  });
+
   describe('distribution drops as recursion of getColor increases', () => {
     const color = new toColor('tristen');