js 精確計算函數庫

// 精度校準
function strip(num, precision = 12) {
	return parseFloat(num.toPrecision(precision));
}
// 計算小數位長度
function digitLength(num) {
	let eSplit = num.toString().split(/[eE]/),
		len = (eSplit[0].split('.')[1] || '').length - (+ (eSplit[1] || 0));
	return len > 0 ? len : 0;
}
// 小數轉整數
function float2Int(num) {
	if(num.toString().indexOf('e') === -1) {
		return parseInt(num.toString().replace('.', ''))
	}
	let dLen = digitLength(num);
	return strip(num * Math.pow(10, dLen));
}
// 精確加法
function add(...items) {
	return items.reduce((accumulator, current) => {
		let base = Math.pow(10, Math.max(digitLength(accumulator), digitLength(current))),
			num1 = mult(accumulator, base),
			num2 = mult(current, base),
			res = (num1 + num2) / base;
		return res;
	});
}
// 精確減法
function sub(...items) {
	return items.reduce((accumulator, current) => {
		let base = Math.pow(10, Math.max(digitLength(accumulator), digitLength(current))),
			num1 = mult(accumulator, base),
			num2 = mult(current, base),
			res = (num1 - num2) / base;
		return res;
	});
}
// 精確乘法
function mult(...items) {
	return items.reduce((accumulator, current) => {
		let base = Math.pow(10, digitLength(accumulator) + digitLength(current)),
			num1 = float2Int(accumulator),
			num2 = float2Int(current),
			res = (num1 * num2) / base;
		return res;
	});
}
// 精確除法
function div(...items) {
	return items.reduce((accumulator, current) => {
		let base = Math.pow(10, digitLength(current) - digitLength(accumulator)),
			num1 = float2Int(accumulator),
			num2 = float2Int(current),
			res = mult((num1 / num2), base);
		return res;
	});
}

export default {
	strip,
	digitLength,
	float2Int,
	add,
	sub,
	mult,
	div
}