🚧 progress: First draft.

Author: make-github-pseudonymous-again

README.md

@@ -4,12 +4,12 @@
 String searching failure function for JavaScript.
 See [docs](https://string-data-structure.github.io/failure-function/index.html).
 
-> :building_construction: Caveat emptor! This is work in progress. Code may be
-> working. Documentation may be present. Coherence may be. Maybe.
-
-> :warning: Depending on your environment, the code may require
-> `regeneratorRuntime` to be defined, for instance by importing
-> [regenerator-runtime/runtime](https://www.npmjs.com/package/regenerator-runtime).
+```js
+import {build} from '@string-data-structure/failure-function';
+const s = 'abracadabra';
+const t = new Int32Array(s.length + 1);
+build(s, 0, s.length, t, 0);
+```
 
 [![License](https://img.shields.io/github/license/string-data-structure/failure-function.svg)](https://raw.githubusercontent.com/string-data-structure/failure-function/main/LICENSE)
 [![Version](https://img.shields.io/npm/v/@string-data-structure/failure-function.svg)](https://www.npmjs.org/package/@string-data-structure/failure-function)

doc/manual/usage.md

@@ -1,19 +1,8 @@
 # Usage
 
-> :warning: Depending on your environment, the code may require
-> `regeneratorRuntime` to be defined, for instance by importing
-> [regenerator-runtime/runtime](https://www.npmjs.com/package/regenerator-runtime).
-
-First, require the polyfill at the entry point of your application
-```js
-require( 'regenerator-runtime/runtime' ) ;
-// or
-import 'regenerator-runtime/runtime.js' ;
-```
-
-Then, import the library where needed
+Import the library where needed
 ```js
-const failureFunction = require( '@string-data-structure/failure-function' ) ;
+const {build} = require( '@string-data-structure/failure-function' ) ;
 // or
-import * as failureFunction from '@string-data-structure/failure-function' ;
+import {build} from '@string-data-structure/failure-function' ;
 ```

src/build.js

@@ -0,0 +1,50 @@
+/**
+ * Computes the failure function for input string.
+ *
+ * This is the "next[j]" table found in
+ * "Fast pattern matching in strings" by Knuth, Morris, and Pratt,
+ * although here indices are 0-based hence all indices and inputs are one less
+ * than in that paper.
+ *
+ * NOTE The main loop is somewhat unrolled for faster execution. This was not
+ * benchmarked.
+ *
+ * @param {ArrayLike} p
+ * @param {number} pi
+ * @param {number} pj
+ * @param {number[]} t
+ * @param {number} ti
+ */
+const build = (p, pi, pj, t, ti) => {
+	t[ti] = -1;
+	if (pi === pj) return;
+	const p0 = pi;
+	if (++pi === pj) {
+		t[++ti] = 0;
+		return;
+	}
+
+	const t0 = ti;
+	t[++ti] = p[pi] === p[p0] ? -1 : 0;
+	let m = 0;
+	--pj;
+	while (pi < pj) {
+		while (p[pi] !== p[p0 + m]) {
+			m = t[t0 + m];
+			if (m === -1) break;
+		}
+
+		++m;
+		t[++ti] = p[++pi] === p[p0 + m] ? t[t0 + m] : m;
+	}
+
+	while (p[pj] !== p[p0 + m]) {
+		m = t[t0 + m];
+		if (m === -1) break;
+	}
+
+	++m;
+	t[++ti] = m;
+};
+
+export default build;

src/index.js

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

test/src/_fixtures.js

@@ -0,0 +1,10 @@
+import {build} from '../../src/index.js';
+
+const _table = (p, pi, pj) => {
+	// eslint-disable-next-line unicorn/no-new-array
+	const next = new Array(pj - pi + 1);
+	build(p, pi, pj, next, 0);
+	return next;
+};
+
+export const table = (input) => _table(input, 0, input.length);

test/src/api.js

@@ -0,0 +1,67 @@
+import test from 'ava';
+
+import {table} from './_fixtures.js';
+
+const isNext = (t, table, input) => {
+	if (input === '') {
+		t.deepEqual(table, [-1]);
+		return;
+	}
+
+	// TODO this test is not complete
+
+	t.is(table[0], -1);
+	t.not(table[input.length], -1);
+	for (let j = 1; j <= input.length; ++j) {
+		const i = table[j];
+		// TODO test when i === -1
+		if (i !== -1) {
+			// Prefix and suffix match
+			t.true(input.slice(0, i) === input.slice(j - i, j));
+			// Next character does not match
+			t.true(j === input.length || input[i] !== input[j]);
+			// A longer proper prefix/suffix pair does not match
+			// TODO More possibilities need to be checked
+			t.true(
+				i + 1 === j ||
+					input.slice(0, i + 1) !== input.slice(j - (i + 1), j) ||
+					i + 2 === j ||
+					j === input.length ||
+					input[i + 1] === input[j],
+			);
+		}
+	}
+};
+
+const macro = (t, input, expected) => {
+	const next = table(input);
+	t.deepEqual(next, expected);
+	isNext(t, next, input);
+};
+
+macro.title = (title, input, expected) =>
+	title ?? `next(${input}) is ${JSON.stringify(expected)}`;
+
+const auto = (t, input) => {
+	const next = table(input);
+	isNext(t, next, input);
+};
+
+auto.title = (title, input) => title ?? `isNext(next(${input}))`;
+
+test(macro, '', [-1]);
+test(macro, 'z', [-1, 0]);
+test(macro, 'abcd', [-1, 0, 0, 0, 0]);
+test(macro, 'aaaa', [-1, -1, -1, -1, 3]);
+test(macro, 'axax', [-1, 0, -1, 0, 2]);
+test(macro, 'axxa', [-1, 0, 0, -1, 1]);
+test(macro, 'aaaab', [-1, -1, -1, -1, 3, 0]);
+test(macro, 'abracadabra', [-1, 0, 0, -1, 1, -1, 1, -1, 0, 0, -1, 4]);
+test(
+	macro,
+	'abaababaabaababaababa',
+	[-1, 0, -1, 1, 0, -1, 3, -1, 1, 0, -1, 6, 0, -1, 3, -1, 1, 0, -1, 11, -1, 8],
+);
+
+test(auto, 'eifoiwhfeldkasjflkdshfldshflkkdadkkkkkkkkkkkkasjfdljfdleifo');
+test(auto, 'aaaaaaaaaaaabbbbbbbbbbaaaaaaaaabbbbbbbb');