🚧 progress: First draft.

package.json

@@ -59,12 +59,22 @@
     "release": "np --message ':hatching_chick: release: Bumping to v%s.'",
     "test": "ava"
   },
-  "dependencies": {},
+  "dependencies": {
+    "@iterable-iterator/filter": "^0.1.2",
+    "@iterable-iterator/iter": "^0.0.2",
+    "@iterable-iterator/list": "^0.0.2",
+    "@iterable-iterator/map": "^0.1.0",
+    "@iterable-iterator/next": "^1.0.1",
+    "@iterable-iterator/reversed": "^0.0.2"
+  },
   "devDependencies": {
     "@babel/core": "7.14.0",
     "@babel/preset-env": "7.14.0",
     "@babel/register": "7.13.16",
     "@commitlint/cli": "12.1.1",
+    "@iterable-iterator/count": "^0.0.1",
+    "@iterable-iterator/range": "^1.0.0",
+    "@iterable-iterator/slice": "^0.0.1",
     "@js-library/commitlint-config": "0.0.4",
     "ava": "3.15.0",
     "babel-plugin-transform-remove-console": "6.9.4",

src/_bitsets.js

@@ -0,0 +1,35 @@
+export default function* _bitsets() {
+	const bitset = [];
+	while (true) {
+		yield new Progress(VALUE, bitset);
+		yield* _increment(bitset);
+	}
+}
+
+function* _increment(bitset) {
+	const n = bitset.length;
+	for (let i = 0; i < n; ++i) {
+		if (bitset[i] === 0) {
+			bitset[i] = 1;
+			yield new Progress(ADD, i);
+			return;
+		}
+
+		bitset[i] = 0;
+		yield new Progress(REMOVE, i);
+	}
+
+	bitset.push(1);
+	yield new Progress(ADD, n);
+}
+
+class Progress {
+	constructor(type, value) {
+		this.type = type;
+		this.value = value;
+	}
+}
+
+export const VALUE = 0;
+export const ADD = 1;
+export const REMOVE = 2;

src/_powerset.js

@@ -0,0 +1,41 @@
+import assert from 'assert';
+import _bitsets, {ADD, REMOVE, VALUE} from './_bitsets.js';
+
+import {iter} from '@iterable-iterator/iter';
+import {next, StopIteration} from '@iterable-iterator/next';
+import {filter} from '@iterable-iterator/filter';
+
+export default function* _powerset(set) {
+	const source = iter(set);
+	const buffer = [];
+	const stack = [];
+	yield stack;
+	for (const progress of filter(({type}) => type !== VALUE, _bitsets())) {
+		switch (progress.type) {
+			case ADD:
+				if (progress.value === buffer.length) {
+					try {
+						buffer.push(next(source));
+					} catch (error) {
+						if (error instanceof StopIteration) {
+							assert(stack.length === 0);
+							return;
+						}
+
+						throw error;
+					}
+				}
+
+				assert(progress.value < buffer.length);
+				stack.push(buffer[progress.value]);
+				yield stack;
+				break;
+			default:
+				assert(progress.type === REMOVE);
+				assert(stack.length > 0);
+				assert(stack[stack.length - 1] === buffer[progress.value]);
+				stack.pop();
+				break;
+		}
+	}
+}

src/index.js

@@ -1,2 +1,3 @@
-const answer = 42;
-export default answer;
+export {default as _bitsets} from './_bitsets.js';
+export {default as _powerset} from './_powerset.js';
+export {default as powerset} from './powerset.js';

src/powerset.js

@@ -0,0 +1,8 @@
+import {map} from '@iterable-iterator/map';
+import {list} from '@iterable-iterator/list';
+import {reversed} from '@iterable-iterator/reversed';
+import _powerset from './_powerset.js';
+
+const powerset = (set) =>
+	map((subset) => list(reversed(subset)), _powerset(set));
+export default powerset;

test/src/powerset.js

@@ -0,0 +1,52 @@
+import test from 'ava';
+
+import {powerset} from '../../src/index.js';
+import {list} from '@iterable-iterator/list';
+import {range} from '@iterable-iterator/range';
+import {count} from '@iterable-iterator/count';
+import {take} from '@iterable-iterator/slice';
+
+const repr = (x) => (Array.isArray(x) ? JSON.stringify(x) : x.toString());
+
+const whole = (t, set, expected) => {
+	const result = list(powerset(set));
+	t.deepEqual(result, expected);
+};
+
+whole.title = (title, set, expected) =>
+	title ?? `powerset(${repr(set)}) is ${repr(expected)}`;
+
+test(whole, range(0), [[]]);
+
+test(whole, range(1), [[], [0]]);
+
+test(whole, range(2), [[], [0], [1], [0, 1]]);
+
+test(whole, range(3), [[], [0], [1], [0, 1], [2], [0, 2], [1, 2], [0, 1, 2]]);
+
+const startsWith = (t, set, expected) => {
+	const result = list(take(powerset(set), expected.length));
+	t.deepEqual(result, expected);
+};
+
+startsWith.title = (title, set, expected) =>
+	title ?? `powerset(${repr(set)}) starts with ${repr(expected)}`;
+
+test('All subsets of NN!', startsWith, count(), [
+	[],
+	[0],
+	[1],
+	[0, 1],
+	[2],
+	[0, 2],
+	[1, 2],
+	[0, 1, 2],
+	[3],
+	[0, 3],
+	[1, 3],
+	[0, 1, 3],
+	[2, 3],
+	[0, 2, 3],
+	[1, 2, 3],
+	[0, 1, 2, 3],
+]);