✨ feat: Add len and _len.

Fixes #49.

Author: make-github-pseudonymous-again

package.json

@@ -202,6 +202,7 @@
     ],
     "rules": {
       "camelcase": "off",
+      "unicorn/prevent-abbreviations": "off",
       "unicorn/filename-case": "off"
     },
     "overrides": [

src/_len.js

@@ -0,0 +1,20 @@
+import assert from 'assert';
+import Node from './Node.js';
+
+/**
+ * Compute the length of a non-empty list.
+ *
+ * @param {Node} x First node of the input list.
+ * @return {number} The length of the input list.
+ */
+export default function _len(x) {
+	assert(x instanceof Node);
+	let n = 1;
+	let y = x.next;
+	while (y !== x) {
+		++n;
+		y = y.next;
+	}
+
+	return n;
+}

src/index.js

@@ -2,11 +2,13 @@ export {default as Node} from './Node.js';
 export {default as _append} from './_append.js';
 export {default as _concat} from './_concat.js';
 export {default as _iter} from './_iter.js';
+export {default as _len} from './_len.js';
 export {default as _remove} from './_remove.js';
 export {default as concat} from './concat.js';
 export {default as empty} from './empty.js';
 export {default as from} from './from.js';
 export {default as iter} from './iter.js';
+export {default as len} from './len.js';
 export {default as pop} from './pop.js';
 export {default as push} from './push.js';
 export {default as shift} from './shift.js';

src/len.js

@@ -0,0 +1,16 @@
+import assert from 'assert';
+import _len from './_len.js';
+import Node from './Node.js';
+
+/**
+ * Compute the length of a list (can be empty).
+ *
+ * @param {Node} x First node of the input list (can be null).
+ * @return {number} The length of the input list.
+ */
+const len = (x) => {
+	assert(x === null || x instanceof Node);
+	return x === null ? 0 : _len(x);
+};
+
+export default len;

test/src/len.js

@@ -0,0 +1,21 @@
+import test from 'ava';
+
+import {list, range} from '@aureooms/js-itertools';
+import {from, len} from '../../src/index.js';
+
+const macro = (t, input) => {
+	const x = from(input);
+	const expected = input.length;
+	t.is(len(x), expected);
+};
+
+macro.title = (title, input) =>
+	title || `len(from(${JSON.stringify(input)})) = ${input.length}`;
+
+test(macro, []);
+test(macro, '');
+test(macro, [{}]);
+test(macro, 'z');
+test(macro, [1, 2, 3]);
+test(macro, 'abracadabra');
+test('len(from(list(range(987)))) = 987', macro, list(range(987)));