56 lines
1.9 KiB
JavaScript
56 lines
1.9 KiB
JavaScript
"use strict";
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Object.defineProperty(exports, "__esModule", { value: true });
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exports.compile = void 0;
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var boolbase_1 = require("boolbase");
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/**
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* Returns a function that checks if an elements index matches the given rule
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* highly optimized to return the fastest solution.
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*
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* @param parsed A tuple [a, b], as returned by `parse`.
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* @returns A highly optimized function that returns whether an index matches the nth-check.
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* @example
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* const check = nthCheck.compile([2, 3]);
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*
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* check(0); // `false`
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* check(1); // `false`
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* check(2); // `true`
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* check(3); // `false`
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* check(4); // `true`
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* check(5); // `false`
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* check(6); // `true`
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*/
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function compile(parsed) {
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var a = parsed[0];
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// Subtract 1 from `b`, to convert from one- to zero-indexed.
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var b = parsed[1] - 1;
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/*
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* When `b <= 0`, `a * n` won't be lead to any matches for `a < 0`.
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* Besides, the specification states that no elements are
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* matched when `a` and `b` are 0.
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*
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* `b < 0` here as we subtracted 1 from `b` above.
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*/
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if (b < 0 && a <= 0)
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return boolbase_1.falseFunc;
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// When `a` is in the range -1..1, it matches any element (so only `b` is checked).
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if (a === -1)
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return function (index) { return index <= b; };
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if (a === 0)
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return function (index) { return index === b; };
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// When `b <= 0` and `a === 1`, they match any element.
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if (a === 1)
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return b < 0 ? boolbase_1.trueFunc : function (index) { return index >= b; };
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/*
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* Otherwise, modulo can be used to check if there is a match.
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*
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* Modulo doesn't care about the sign, so let's use `a`s absolute value.
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*/
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var absA = Math.abs(a);
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// Get `b mod a`, + a if this is negative.
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var bMod = ((b % absA) + absA) % absA;
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return a > 1
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? function (index) { return index >= b && index % absA === bMod; }
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: function (index) { return index <= b && index % absA === bMod; };
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}
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exports.compile = compile;
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