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mathjs

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Math.js is an extensive math library for JavaScript and Node.js. It features a flexible expression parser with support for symbolic computation, comes with a large set of built-in functions and constants, and offers an integrated solution to work with dif

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'use strict'; const unique = arr => arr.filter((v, i) => arr.lastIndexOf(v) === i); const compact = arr => unique(arr).filter(Boolean); module.exports = (action, data = {}, value = '') => { let { past = [], present = '' } = data; let rest, prev; switch (action) { case 'prev': case 'undo': rest = past.slice(0, past.length - 1); prev = past[past.length - 1] || ''; return { past: compact([value, ...rest]), present: prev }; case 'next': case 'redo': rest = past.slice(1); prev = past[0] || ''; return { past: compact([...rest, value]), present: prev }; case 'save': return { past: compact([...past, value]), present: '' }; case 'remove': prev = compact(past.filter(v => v !== value)); present = ''; if (prev.length) { present = prev.pop(); } return { past: prev, present }; default: { throw new Error(`Invalid action: "${action}"`); } } };