fb55 · Apr 25, 2015 · Sep 11, 2015 · Nov 1, 2016 · Dec 10, 2016 · Jul 25, 2017
diff --git a/.eslintignore b/.eslintignore
@@ -0,0 +1,3 @@
+node_modules/
+coverage/
+lib/
diff --git a/.eslintrc.json b/.eslintrc.json
@@ -0,0 +1,62 @@
+{
+    "extends": ["eslint:recommended", "prettier"],
+    "env": {
+        "node": true,
+        "es6": true
+    },
+    "rules": {
+        "eqeqeq": [2, "smart"],
+        "no-caller": 2,
+        "dot-notation": 2,
+        "no-var": 2,
+        "prefer-const": 2,
+        "prefer-arrow-callback": [2, { "allowNamedFunctions": true }],
+        "arrow-body-style": [2, "as-needed"],
+        "object-shorthand": 2,
+        "prefer-template": 2,
+        "one-var": [2, "never"],
+        "prefer-destructuring": [2, { "object": true }],
+        "capitalized-comments": 2,
+        "multiline-comment-style": [2, "starred-block"],
+        "spaced-comment": 2,
+        "yoda": [2, "never"],
+        "curly": [2, "multi-line"],
+        "no-else-return": 2
+    },
+    "overrides": [
+        {
+            "files": "*.ts",
+            "extends": [
+                "plugin:@typescript-eslint/eslint-recommended",
+                "plugin:@typescript-eslint/recommended",
+                "prettier"
+            ],
+            "parserOptions": {
+                "sourceType": "module",
+                "project": "./tsconfig.eslint.json"
+            },
+            "rules": {
+                "@typescript-eslint/prefer-for-of": 0,
+                "@typescript-eslint/member-ordering": 0,
+                "@typescript-eslint/explicit-function-return-type": 0,
+                "@typescript-eslint/no-unused-vars": 0,
+                "@typescript-eslint/no-use-before-define": [
+                    2,
+                    { "functions": false }
+                ],
+                "@typescript-eslint/consistent-type-definitions": [
+                    2,
+                    "interface"
+                ],
+                "@typescript-eslint/prefer-function-type": 2,
+                "@typescript-eslint/no-unnecessary-type-arguments": 2,
+                "@typescript-eslint/prefer-string-starts-ends-with": 2,
+                "@typescript-eslint/prefer-readonly": 2,
+                "@typescript-eslint/prefer-includes": 2,
+                "@typescript-eslint/no-unnecessary-condition": 2,
+                "@typescript-eslint/switch-exhaustiveness-check": 2,
+                "@typescript-eslint/prefer-nullish-coalescing": 2
+            }
+        }
+    ]
+}
diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
@@ -0,0 +1,2 @@
+github: [fb55]
+tidelift: "npm/nth-check"
diff --git a/.github/dependabot.yml b/.github/dependabot.yml
@@ -0,0 +1,12 @@
+version: 2
+updates:
+    - package-ecosystem: npm
+      directory: "/"
+      schedule:
+          interval: daily
+      open-pull-requests-limit: 10
+      versioning-strategy: increase
+    - package-ecosystem: "github-actions"
+      directory: "/"
+      schedule:
+          interval: daily
diff --git a/.github/workflows/codeql-analysis.yml b/.github/workflows/codeql-analysis.yml
@@ -0,0 +1,52 @@
+name: "Code scanning - action"
+
+on:
+    push:
+        branches: [master]
+    pull_request:
+        # The branches below must be a subset of the branches above
+        branches: [master]
+    schedule:
+        - cron: "0 7 * * 0"
+
+jobs:
+    CodeQL-Build:
+        runs-on: ubuntu-latest
+
+        steps:
+            - name: Checkout repository
+              uses: actions/checkout@v2
+              with:
+                  # We must fetch at least the immediate parents so that if this is
+                  # a pull request then we can checkout the head.
+                  fetch-depth: 2
+
+            # If this run was triggered by a pull request event, then checkout
+            # the head of the pull request instead of the merge commit.
+            - run: git checkout HEAD^2
+              if: ${{ github.event_name == 'pull_request' }}
+
+            # Initializes the CodeQL tools for scanning.
+            - name: Initialize CodeQL
+              uses: github/codeql-action/init@v1
+              # Override language selection by uncommenting this and choosing your languages
+              # with:
+              #   languages: go, javascript, csharp, python, cpp, java
+            # Autobuild attempts to build any compiled languages  (C/C++, C#, or Java).
+            # If this step fails, then you should remove it and run the build manually (see below)
+            - name: Autobuild
+              uses: github/codeql-action/autobuild@v1
+
+            # ℹ️ Command-line programs to run using the OS shell.
+            # 📚 https://git.io/JvXDl
+
+            # ✏️ If the Autobuild fails above, remove it and uncomment the following three lines
+            #    and modify them (or add more) to build your code if your project
+            #    uses a compiled language
+
+            #- run: |
+            #   make bootstrap
+            #   make release
+
+            - name: Perform CodeQL Analysis
+              uses: github/codeql-action/analyze@v1
diff --git a/.github/workflows/dependabot-automerge.yml b/.github/workflows/dependabot-automerge.yml
@@ -0,0 +1,28 @@
+# Based on https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/automating-dependabot-with-github-actions#enable-auto-merge-on-a-pull-request
+name: Dependabot auto-merge
+on: pull_request_target
+
+permissions:
+    pull-requests: write
+    contents: write
+
+jobs:
+    dependabot:
+        runs-on: ubuntu-latest
+        if: ${{ github.actor == 'dependabot[bot]' }}
+        steps:
+            - name: Dependabot metadata
+              id: metadata
+              uses: dependabot/fetch-metadata@v1.1.1
+              with:
+                  github-token: "${{ secrets.GITHUB_TOKEN }}"
+            - name: Enable auto-merge for Dependabot PRs
+              # Automatically merge semver-patch and semver-minor PRs
+              if: "${{ steps.metadata.outputs.update-type ==
+                  'version-update:semver-minor' ||
+                  steps.metadata.outputs.update-type ==
+                  'version-update:semver-patch' }}"
+              run: gh pr merge --auto --squash "$PR_URL"
+              env:
+                  PR_URL: ${{github.event.pull_request.html_url}}
+                  GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
diff --git a/.github/workflows/nodejs-test.yml b/.github/workflows/nodejs-test.yml
@@ -0,0 +1,57 @@
+name: Node.js CI
+
+on: [push, pull_request]
+
+env:
+    CI: true
+    FORCE_COLOR: 2
+    NODE_COV: 16 # The Node.js version to run coveralls on
+
+jobs:
+    lint:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v2
+            - uses: actions/setup-node@v2
+              with:
+                  node-version: 16
+                  cache: npm
+            - run: npm ci
+            - run: npm run lint
+
+    test:
+        name: Node ${{ matrix.node }}
+        runs-on: ubuntu-latest
+
+        strategy:
+            fail-fast: false
+            matrix:
+                node:
+                    - 10
+                    - 12
+                    - 14
+                    - 16
+
+        steps:
+            - uses: actions/checkout@v2
+            - name: Use Node.js ${{ matrix.node }}
+              uses: actions/setup-node@v2
+              with:
+                  node-version: ${{ matrix.node }}
+                  cache: npm
+            - run: npm ci
+            - run: npm run build --if-present
+
+            - name: Run Jest
+              run: npm run test:jest
+              if: matrix.node != env.NODE_COV
+
+            - name: Run Jest with coverage
+              run: npm run test:jest -- --coverage
+              if: matrix.node == env.NODE_COV
+
+            - name: Run Coveralls
+              uses: coverallsapp/github-action@master
+              if: matrix.node == env.NODE_COV
+              with:
+                  github-token: "${{ secrets.GITHUB_TOKEN }}"
diff --git a/.gitignore b/.gitignore
@@ -0,0 +1,4 @@
+.vscode/
+node_modules/
+coverage/
+lib/
diff --git a/.prettierignore b/.prettierignore
@@ -0,0 +1,3 @@
+node_modules/
+coverage/
+lib/
diff --git a/.travis.yml b/.travis.yml
diff --git a/LICENSE b/LICENSE
@@ -0,0 +1,11 @@
+Copyright (c) Felix Böhm
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
+
+Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
+
+Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
+
+THIS IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS,
+EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/README.md b/README.md
@@ -1,51 +1,80 @@
-#nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.png)](https://travis-ci.org/fb55/nth-check)
+# nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.svg)](https://travis-ci.org/fb55/nth-check)
 
-A performant nth-check parser & compiler.
+Parses and compiles CSS nth-checks to highly optimized functions.
 
-###About
+### About
 
 This module can be used to parse & compile nth-checks, as they are found in CSS 3's `nth-child()` and `nth-last-of-type()`.
 
 `nth-check` focusses on speed, providing optimized functions for different kinds of nth-child formulas, while still following the [spec](http://www.w3.org/TR/css3-selectors/#nth-child-pseudo).
 
-###API
+### API
 
 ```js
-var nthCheck = require("nth-check");
+import nthCheck, { parse, compile } from "nth-check";
 ```
 
-#####`nthCheck(formula)`
+##### `nthCheck(formula)`
 
-First parses, then compiles the formula.
+Parses and compiles a formula to a highly optimized function. Combination of `parse` and `compile`.
 
-#####`nthCheck.parse(formula)`
+If the formula doesn't match any elements, it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`. Otherwise, a function accepting an _index_ is returned, which returns whether or not the passed _index_ matches the formula.
 
-Parses the expression, throws a `SyntaxError` if it fails, otherwise returns an array containing two elements.
+**Note**: The nth-rule starts counting at `1`, the returned function at `0`.
 
-__Example:__
+**Example:**
 
 ```js
-nthCheck.parse("2n+3") //[2, 3]
+const check = nthCheck("2n+3");
+
+check(0); // `false`
+check(1); // `false`
+check(2); // `true`
+check(3); // `false`
+check(4); // `true`
+check(5); // `false`
+check(6); // `true`
 ```
 
-#####`nthCheck.compile([a, b])`
+##### `parse(formula)`
+
+Parses the expression, throws an `Error` if it fails. Otherwise, returns an array containing the integer step size and the integer offset of the nth rule.
+
+**Example:**
+
+```js
+parse("2n+3"); // [2, 3]
+```
+
+##### `compile([a, b])`
 
 Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.
 
-If the formula doesn't match any elements, it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`, otherwise, a function accepting an _index_ is returned, which returns whether or not a passed _index_ matches the formula. (Note: The spec starts counting at `1`, the returned function at `0`).
+**Example:**
 
-__Example:__
 ```js
-var check = nthCheck.compile([2, 3]);
-
-check(0) //false
-check(1) //false
-check(2) //true
-check(3) //false
-check(4) //true
-check(5) //false
-check(6) //true
+const check = compile([2, 3]);
+
+check(0); // `false`
+check(1); // `false`
+check(2); // `true`
+check(3); // `false`
+check(4); // `true`
+check(5); // `false`
+check(6); // `true`
 ```
 
 ---
-License: BSD
+
+License: BSD-2-Clause
+
+## Security contact information
+
+To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).
+Tidelift will coordinate the fix and disclosure.
+
+## `nth-check` for enterprise
+
+Available as part of the Tidelift Subscription
+
+The maintainers of `nth-check` and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. [Learn more.](https://tidelift.com/subscription/pkg/npm-nth-check?utm_source=npm-nth-check&utm_medium=referral&utm_campaign=enterprise&utm_term=repo)
diff --git a/compile.js b/compile.js
diff --git a/index.js b/index.js
diff --git a/package-lock.json b/package-lock.json
diff --git a/package.json b/package.json
@@ -1,27 +1,67 @@
 {
-  "name": "nth-check",
-  "version": "1.0.1",
-  "description": "performant nth-check parser & compiler",
-  "main": "index.js",
-  "scripts": {
-    "test": "node test"
-  },
-  "repository": {
-    "type": "git",
-    "url": "https://github.com/fb55/nth-check"
-  },
-  "keywords": [
-    "nth-child",
-    "nth",
-    "css"
-  ],
-  "author": "Felix Boehm <me@feedic.com>",
-  "license": "BSD",
-  "bugs": {
-    "url": "https://github.com/fb55/nth-check/issues"
-  },
-  "homepage": "https://github.com/fb55/nth-check",
-  "dependencies": {
-    "boolbase": "~1.0.0"
-  }
+    "name": "nth-check",
+    "version": "2.0.1",
+    "description": "Parses and compiles CSS nth-checks to highly optimized functions.",
+    "author": "Felix Boehm <me@feedic.com>",
+    "license": "BSD-2-Clause",
+    "sideEffects": false,
+    "funding": {
+        "url": "https://github.com/fb55/nth-check?sponsor=1"
+    },
+    "directories": {
+        "lib": "lib/"
+    },
+    "main": "lib/index.js",
+    "types": "lib/index.d.ts",
+    "files": [
+        "lib/**/*"
+    ],
+    "scripts": {
+        "test": "npm run test:jest && npm run lint",
+        "test:jest": "jest",
+        "lint": "npm run lint:es && npm run lint:prettier",
+        "lint:es": "eslint .",
+        "lint:prettier": "npm run prettier -- --check",
+        "format": "npm run format:es && npm run format:prettier",
+        "format:es": "npm run lint:es -- --fix",
+        "format:prettier": "npm run prettier -- --write",
+        "prettier": "prettier '**/*.{ts,md,json,yml}'",
+        "build": "tsc",
+        "prepare": "npm run build"
+    },
+    "repository": {
+        "type": "git",
+        "url": "https://github.com/fb55/nth-check"
+    },
+    "keywords": [
+        "nth-child",
+        "nth",
+        "css"
+    ],
+    "bugs": {
+        "url": "https://github.com/fb55/nth-check/issues"
+    },
+    "homepage": "https://github.com/fb55/nth-check",
+    "dependencies": {
+        "boolbase": "^1.0.0"
+    },
+    "devDependencies": {
+        "@types/jest": "^27.0.1",
+        "@types/node": "^16.9.1",
+        "@typescript-eslint/eslint-plugin": "^4.31.1",
+        "@typescript-eslint/parser": "^4.31.1",
+        "eslint": "^7.32.0",
+        "eslint-config-prettier": "^8.3.0",
+        "jest": "^27.2.0",
+        "prettier": "^2.4.1",
+        "ts-jest": "^27.0.5",
+        "typescript": "^4.4.3"
+    },
+    "jest": {
+        "preset": "ts-jest",
+        "testEnvironment": "node"
+    },
+    "prettier": {
+        "tabWidth": 4
+    }
 }
diff --git a/parse.js b/parse.js
diff --git a/src/__fixtures__/rules.ts b/src/__fixtures__/rules.ts
@@ -0,0 +1,49 @@
+export const valid: [string, [number, number]][] = [
+    ["1", [0, 1]],
+    ["2", [0, 2]],
+    ["3", [0, 3]],
+    ["5", [0, 5]],
+    [" 1 ", [0, 1]],
+    [" 5 ", [0, 5]],
+    ["+2n + 1", [2, 1]],
+    ["-1", [0, -1]],
+    ["-1n + 3", [-1, 3]],
+    ["-1n+3", [-1, 3]],
+    ["-n+2", [-1, 2]],
+    ["-n+3", [-1, 3]],
+    ["0n+3", [0, 3]],
+    ["1n", [1, 0]],
+    ["1n+0", [1, 0]],
+    ["2n", [2, 0]],
+    ["2n + 1", [2, 1]],
+    ["2n+1", [2, 1]],
+    ["3n", [3, 0]],
+    ["3n+0", [3, 0]],
+    ["3n+1", [3, 1]],
+    ["3n+2", [3, 2]],
+    ["3n+3", [3, 3]],
+    ["3n-1", [3, -1]],
+    ["3n-2", [3, -2]],
+    ["3n-3", [3, -3]],
+    ["even", [2, 0]],
+    ["n", [1, 0]],
+    ["n+2", [1, 2]],
+    ["odd", [2, 1]],
+
+    // Surprisingly, neither sizzle, qwery or nwmatcher cover these cases
+    ["-4n+13", [-4, 13]],
+    ["-2n + 12", [-2, 12]],
+];
+
+export const invalid = [
+    "-",
+    "- 1n",
+    "-1 n",
+    "2+0",
+    "2n+-0",
+    "an+b",
+    "asdf",
+    "b",
+    "expr",
+    "odd|even|x",
+];
diff --git a/src/compile.spec.ts b/src/compile.spec.ts
@@ -0,0 +1,39 @@
+import nthCheck, { compile } from ".";
+import { valid } from "./__fixtures__/rules";
+
+const valArray = new Array(...Array(2e3)).map((_, i) => i);
+
+/**
+ * Iterate through all possible values. This is adapted from qwery,
+ * and uses a more intuitive way to process all elements.
+ */
+function slowNth([a, b]: [number, number]): number[] {
+    if (a === 0 && b > 0) return [b - 1];
+
+    return valArray.filter((val) => {
+        for (let i = b; a > 0 ? i <= valArray.length : i >= 1; i += a) {
+            if (val === valArray[i - 1]) return true;
+        }
+        return false;
+    });
+}
+
+describe("parse", () => {
+    it("compile & run all valid", () => {
+        for (const [_, parsed] of valid) {
+            const filtered = valArray.filter(compile(parsed));
+            const iterated = slowNth(parsed);
+
+            expect(filtered).toStrictEqual(iterated);
+        }
+    });
+
+    it("parse, compile & run all valid", () => {
+        for (const [rule, parsed] of valid) {
+            const filtered = valArray.filter(nthCheck(rule));
+            const iterated = slowNth(parsed);
+
+            expect([filtered, rule]).toStrictEqual([iterated, rule]);
+        }
+    });
+});
diff --git a/src/compile.ts b/src/compile.ts
@@ -0,0 +1,54 @@
+import { trueFunc, falseFunc } from "boolbase";
+
+/**
+ * Returns a function that checks if an elements index matches the given rule
+ * highly optimized to return the fastest solution.
+ *
+ * @param parsed A tuple [a, b], as returned by `parse`.
+ * @returns A highly optimized function that returns whether an index matches the nth-check.
+ * @example
+ * const check = nthCheck.compile([2, 3]);
+ *
+ * check(0); // `false`
+ * check(1); // `false`
+ * check(2); // `true`
+ * check(3); // `false`
+ * check(4); // `true`
+ * check(5); // `false`
+ * check(6); // `true`
+ */
+export function compile(
+    parsed: [a: number, b: number]
+): (index: number) => boolean {
+    const a = parsed[0];
+    // Subtract 1 from `b`, to convert from one- to zero-indexed.
+    const b = parsed[1] - 1;
+
+    /*
+     * When `b <= 0`, `a * n` won't be lead to any matches for `a < 0`.
+     * Besides, the specification states that no elements are
+     * matched when `a` and `b` are 0.
+     *
+     * `b < 0` here as we subtracted 1 from `b` above.
+     */
+    if (b < 0 && a <= 0) return falseFunc;
+
+    // When `a` is in the range -1..1, it matches any element (so only `b` is checked).
+    if (a === -1) return (index) => index <= b;
+    if (a === 0) return (index) => index === b;
+    // When `b <= 0` and `a === 1`, they match any element.
+    if (a === 1) return b < 0 ? trueFunc : (index) => index >= b;
+
+    /*
+     * Otherwise, modulo can be used to check if there is a match.
+     *
+     * Modulo doesn't care about the sign, so let's use `a`s absolute value.
+     */
+    const absA = Math.abs(a);
+    // Get `b mod a`, + a if this is negative.
+    const bMod = ((b % absA) + absA) % absA;
+
+    return a > 1
+        ? (index) => index >= b && index % absA === bMod
+        : (index) => index <= b && index % absA === bMod;
+}
diff --git a/src/declarations/boolbase.d.ts b/src/declarations/boolbase.d.ts
@@ -0,0 +1,4 @@
+declare module "boolbase" {
+    export function trueFunc(...args: unknown[]): true;
+    export function falseFunc(...args: unknown[]): false;
+}
diff --git a/src/index.ts b/src/index.ts
@@ -0,0 +1,31 @@
+import { parse } from "./parse";
+import { compile } from "./compile";
+
+export { parse, compile };
+
+/**
+ * Parses and compiles a formula to a highly optimized function.
+ * Combination of `parse` and `compile`.
+ *
+ * If the formula doesn't match any elements,
+ * it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`.
+ * Otherwise, a function accepting an _index_ is returned, which returns
+ * whether or not the passed _index_ matches the formula.
+ *
+ * Note: The nth-rule starts counting at `1`, the returned function at `0`.
+ *
+ * @param formula The formula to compile.
+ * @example
+ * const check = nthCheck("2n+3");
+ *
+ * check(0); // `false`
+ * check(1); // `false`
+ * check(2); // `true`
+ * check(3); // `false`
+ * check(4); // `true`
+ * check(5); // `false`
+ * check(6); // `true`
+ */
+export default function nthCheck(formula: string): (index: number) => boolean {
+    return compile(parse(formula));
+}
diff --git a/src/parse.spec.ts b/src/parse.spec.ts
@@ -0,0 +1,16 @@
+import { parse } from "./parse";
+import { valid, invalid } from "./__fixtures__/rules";
+
+describe("parse", () => {
+    it("parse invalid", () => {
+        for (const formula of invalid) {
+            expect(() => parse(formula)).toThrowError(Error);
+        }
+    });
+
+    it("parse valid", () => {
+        for (const [formula, result] of valid) {
+            expect(parse(formula)).toStrictEqual(result);
+        }
+    });
+});
diff --git a/src/parse.ts b/src/parse.ts
@@ -0,0 +1,92 @@
+// Following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo
+
+// Whitespace as per https://www.w3.org/TR/selectors-3/#lex is " \t\r\n\f"
+const whitespace = new Set([9, 10, 12, 13, 32]);
+const ZERO = "0".charCodeAt(0);
+const NINE = "9".charCodeAt(0);
+
+/**
+ * Parses an expression.
+ *
+ * @throws An `Error` if parsing fails.
+ * @returns An array containing the integer step size and the integer offset of the nth rule.
+ * @example nthCheck.parse("2n+3"); // returns [2, 3]
+ */
+export function parse(formula: string): [a: number, b: number] {
+    formula = formula.trim().toLowerCase();
+
+    if (formula === "even") {
+        return [2, 0];
+    } else if (formula === "odd") {
+        return [2, 1];
+    }
+
+    // Parse [ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
+
+    let idx = 0;
+
+    let a = 0;
+    let sign = readSign();
+    let number = readNumber();
+
+    if (idx < formula.length && formula.charAt(idx) === "n") {
+        idx++;
+        a = sign * (number ?? 1);
+
+        skipWhitespace();
+
+        if (idx < formula.length) {
+            sign = readSign();
+            skipWhitespace();
+            number = readNumber();
+        } else {
+            sign = number = 0;
+        }
+    }
+
+    // Throw if there is anything else
+    if (number === null || idx < formula.length) {
+        throw new Error(`n-th rule couldn't be parsed ('${formula}')`);
+    }
+
+    return [a, sign * number];
+
+    function readSign() {
+        if (formula.charAt(idx) === "-") {
+            idx++;
+            return -1;
+        }
+
+        if (formula.charAt(idx) === "+") {
+            idx++;
+        }
+
+        return 1;
+    }
+
+    function readNumber() {
+        const start = idx;
+        let value = 0;
+
+        while (
+            idx < formula.length &&
+            formula.charCodeAt(idx) >= ZERO &&
+            formula.charCodeAt(idx) <= NINE
+        ) {
+            value = value * 10 + (formula.charCodeAt(idx) - ZERO);
+            idx++;
+        }
+
+        // Return `null` if we didn't read anything.
+        return idx === start ? null : value;
+    }
+
+    function skipWhitespace() {
+        while (
+            idx < formula.length &&
+            whitespace.has(formula.charCodeAt(idx))
+        ) {
+            idx++;
+        }
+    }
+}
diff --git a/test.js b/test.js
diff --git a/tsconfig.eslint.json b/tsconfig.eslint.json
@@ -0,0 +1,5 @@
+{
+    "extends": "./tsconfig.json",
+    "include": ["src", "test"],
+    "exclude": []
+}
diff --git a/tsconfig.json b/tsconfig.json
@@ -0,0 +1,38 @@
+{
+    "compilerOptions": {
+        /* Basic Options */
+        "target": "es5" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', 'ES2018', 'ES2019' or 'ESNEXT'. */,
+        "module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */,
+        // "lib": [],                             /* Specify library files to be included in the compilation. */
+        "declaration": true /* Generates corresponding '.d.ts' file. */,
+        "declarationMap": true /* Generates a sourcemap for each corresponding '.d.ts' file. */,
+        // "sourceMap": true,                     /* Generates corresponding '.map' file. */
+        "outDir": "lib" /* Redirect output structure to the directory. */,
+        // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
+
+        /* Strict Type-Checking Options */
+        "strict": true /* Enable all strict type-checking options. */,
+
+        /* Additional Checks */
+        "noUnusedLocals": true /* Report errors on unused locals. */,
+        "noUnusedParameters": true /* Report errors on unused parameters. */,
+        "noImplicitReturns": true /* Report error when not all code paths in function return a value. */,
+        "noFallthroughCasesInSwitch": true /* Report errors for fallthrough cases in switch statement. */,
+
+        /* Module Resolution Options */
+        "baseUrl": "./" /* Base directory to resolve non-absolute module names. */,
+        "esModuleInterop": true /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */,
+        "resolveJsonModule": true,
+
+        "paths": {
+            "*": ["src/declarations/*", "*"]
+        }
+    },
+    "include": ["src"],
+    "exclude": [
+        "**/*.spec.ts",
+        "**/__fixtures__/*",
+        "**/__tests__/*",
+        "**/__snapshots__/*"
+    ]
+}