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compile.js
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Created: Sun, Feb 9, 21:02

compile.js
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	import boolbase 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
	*
	* ```js
	* 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) {
	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 boolbase.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 ? boolbase.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;
	}
	/**
	* 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) {
	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;
	}
	//# sourceMappingURL=compile.js.map

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