feat: Add generate and sequence methods

README.md

@@ -4,7 +4,7 @@ Parses and compiles CSS nth-checks to highly optimized functions.
 
 ### 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()`.
+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()`. It can be used to check if a given index matches a given nth-rule, or to generate a sequence of indices matching a given nth-rule.
 
 `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).
 
@@ -64,6 +64,62 @@ check(5); // `false`
 check(6); // `true`
 ```
 
+##### `generate([a, b])`
+
+Returns a function that produces a monotonously increasing sequence of indices.
+
+If the sequence has an end, the returned function will return `null` after the last index in the sequence.
+
+**Example:** An always increasing sequence
+
+```js
+const gen = nthCheck.generate([2, 3]);
+
+gen(); // `1`
+gen(); // `3`
+gen(); // `5`
+gen(); // `8`
+gen(); // `11`
+```
+
+**Example:** With an end value
+
+```js
+const gen = nthCheck.generate([-2, 5]);
+
+gen(); // 0
+gen(); // 2
+gen(); // 4
+gen(); // null
+```
+
+##### `sequence(formula)`
+
+Parses and compiles a formula to a generator that produces a sequence of indices. Combination of `parse` and `generate`.
+
+**Example:** An always increasing sequence
+
+```js
+const gen = nthCheck.sequence("2n+3");
+
+gen(); // `1`
+gen(); // `3`
+gen(); // `5`
+gen(); // `8`
+gen(); // `11`
+```
+
+**Example:** With an end value
+
+```js
+const gen = nthCheck.sequence("-2n+5");
+
+gen(); // 0
+gen(); // 2
+gen(); // 4
+gen(); // null
+```
+
 ---
 
 License: BSD-2-Clause

src/__fixtures__/rules.ts

@@ -33,6 +33,7 @@ export const valid: [string, [number, number]][] = [
     // Surprisingly, neither sizzle, qwery or nwmatcher cover these cases
     ["-4n+13", [-4, 13]],
     ["-2n + 12", [-2, 12]],
+    ["-n", [-1, 0]],
 ];
 
 export const invalid = [

src/compile.spec.ts

@@ -1,4 +1,4 @@
-import nthCheck, { compile } from ".";
+import nthCheck, { compile, generate, sequence } from ".";
 import { valid } from "./__fixtures__/rules";
 
 const valArray = new Array(...Array(2e3)).map((_, i) => i);
@@ -37,3 +37,59 @@ describe("parse", () => {
         }
     });
 });
+
+describe("generate", () => {
+    it("should return a function", () => {
+        expect(generate([1, 2])).toBeInstanceOf(Function);
+    });
+
+    it("should only return valid values", () => {
+        for (const [_, parsed] of valid) {
+            const gen = generate(parsed);
+            const check = compile(parsed);
+            let val = gen();
+
+            for (let i = 0; i < 1e3; i++) {
+                // Should pass the check iff `i` is the next value.
+                expect(val === i).toBe(check(i));
+
+                if (val === i) {
+                    val = gen();
+                }
+            }
+        }
+    });
+
+    it("should produce an increasing sequence", () => {
+        const gen = generate([2, 2]);
+
+        expect(gen()).toBe(1);
+        expect(gen()).toBe(3);
+        expect(gen()).toBe(5);
+        expect(gen()).toBe(7);
+        expect(gen()).toBe(9);
+    });
+
+    it("should produce an increasing sequence for a negative `n`", () => {
+        const gen = generate([-1, 2]);
+
+        expect(gen()).toBe(0);
+        expect(gen()).toBe(1);
+        expect(gen()).toBe(null);
+    });
+
+    it("should not produce any values for `-n`", () => {
+        const gen = generate([-1, 0]);
+
+        expect(gen()).toBe(null);
+    });
+
+    it("should parse selectors with `sequence`", () => {
+        const gen = sequence("-2n+5");
+
+        expect(gen()).toBe(0);
+        expect(gen()).toBe(2);
+        expect(gen()).toBe(4);
+        expect(gen()).toBe(null);
+    });
+});

src/compile.ts

@@ -7,6 +7,8 @@ import { trueFunc, falseFunc } from "boolbase";
  * @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
+ *
+ * ```js
  * const check = nthCheck.compile([2, 3]);
  *
  * check(0); // `false`
@@ -16,6 +18,7 @@ import { trueFunc, falseFunc } from "boolbase";
  * check(4); // `true`
  * check(5); // `false`
  * check(6); // `true`
+ * ```
  */
 export function compile(
     parsed: [a: number, b: number]
@@ -52,3 +55,68 @@ export function compile(
         ? (index) => index >= b && index % absA === bMod
         : (index) => index <= b && index % absA === bMod;
 }
+
+/**
+ * Returns a function that produces a monotonously increasing sequence of indices.
+ *
+ * If the sequence has an end, the returned function will return `null` after
+ * the last index in the sequence.
+ *
+ * @param parsed A tuple [a, b], as returned by `parse`.
+ * @returns A function that produces a sequence of indices.
+ * @example <caption>Always increasing (2n+3)</caption>
+ *
+ * ```js
+ * const gen = nthCheck.generate([2, 3])
+ *
+ * gen() // `1`
+ * gen() // `3`
+ * gen() // `5`
+ * gen() // `8`
+ * gen() // `11`
+ * ```
+ *
+ * @example <caption>With end value (-2n+10)</caption>
+ *
+ * ```js
+ *
+ * const gen = nthCheck.generate([-2, 5]);
+ *
+ * gen() // 0
+ * gen() // 2
+ * gen() // 4
+ * gen() // null
+ * ```
+ */
+export function generate(parsed: [a: number, b: number]): () => number | null {
+    const a = parsed[0];
+    // Subtract 1 from `b`, to convert from one- to zero-indexed.
+    let b = parsed[1] - 1;
+
+    let n = 0;
+
+    // Make sure to always return an increasing sequence
+    if (a < 0) {
+        const aPos = -a;
+        // Get `b mod a`
+        const minValue = ((b % aPos) + aPos) % aPos;
+        return () => {
+            const val = minValue + aPos * n++;
+
+            return val > b ? null : val;
+        };
+    }
+
+    if (a === 0)
+        return b < 0
+            ? // There are no result — always return `null`
+              () => null
+            : // Return `b` exactly once
+              () => (n++ === 0 ? b : null);
+
+    if (b < 0) {
+        b += a * Math.ceil(-b / a);
+    }
+
+    return () => a * n++ + b;
+}

src/index.ts

@@ -1,11 +1,11 @@
 import { parse } from "./parse";
-import { compile } from "./compile";
+import { compile, generate } from "./compile";
 
-export { parse, compile };
+export { parse, compile, generate };
 
 /**
  * Parses and compiles a formula to a highly optimized function.
- * Combination of `parse` and `compile`.
+ * Combination of {@link parse} and {@link compile}.
  *
  * If the formula doesn't match any elements,
  * it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`.
@@ -29,3 +29,37 @@ export { parse, compile };
 export default function nthCheck(formula: string): (index: number) => boolean {
     return compile(parse(formula));
 }
+
+/**
+ * Parses and compiles a formula to a generator that produces a sequence of indices.
+ * Combination of {@link parse} and {@link generate}.
+ *
+ * @param formula The formula to compile.
+ * @returns A function that produces a sequence of indices.
+ * @example <caption>Always increasing</caption>
+ *
+ * ```js
+ * const gen = nthCheck.sequence('2n+3')
+ *
+ * gen() // `1`
+ * gen() // `3`
+ * gen() // `5`
+ * gen() // `8`
+ * gen() // `11`
+ * ```
+ *
+ * @example <caption>With end value</caption>
+ *
+ * ```js
+ *
+ * const gen = nthCheck.sequence('-2n+5');
+ *
+ * gen() // 0
+ * gen() // 2
+ * gen() // 4
+ * gen() // null
+ * ```
+ */
+export function sequence(formula: string): () => number | null {
+    return generate(parse(formula));
+}