tax.js

function LimitedTaxRule(from, to, rate) {
	console.assert(from >= 0)
	console.assert(to > from)
	console.assert(rate >= 0)

	//起始
	this.from = from
	//结束
	this.to = to
	//税率
	this.rate = rate

	//返回 判断 income 是否达到了当前税收规则的标准
	this.reach = function(income) {
		return income >= from
	}

	//返回 计算指定的 income 在当前税收规则需要交纳的税务
	this.tax = function(income) {
		if (!this.reach(income)) {
			return 0
		}
		if (income > to) {
			return rate * (to - from)
		} else {
			return rate * (income - from)
		}
	}

	//返回当前规则若缴满所需要的税款
	this.fullTax = function() {
		return (to - from) * rate
	}

	this.toString = function() {
		return "超过 " + from + " 到 " + to + " 按照 " + (rate * 100) + "% 缴税"
	}
}

function UnlimitedTaxRule(from, rate) {
	console.assert(from >= 0)
	console.assert(rate >= 0)

	this.from = from
	this.rate = rate

	this.reach = function(income) {
		return income > from
	}

	this.tax = function(income) {
		if (!this.reach(income)) {
			return 0
		}
		return rate * (income - from)
	}
}

function TaxCal(base, limitedRules, unlimitedTaxRule) {

	var self = this

	this.taxForSalary = function(salary) {
		console.assert(salary > 0)
		var tax = 0
		for (var i = 0; i < limitedRules.length; i++) {
			var currentRule = limitedRules[i]
			if ( !currentRule.reach(salary - base) ) {
				return tax
			}
			tax += currentRule.tax(salary - base)
		}
		if (unlimitedTaxRule) {
			tax += unlimitedTaxRule.tax(salary - base)
		}
		return tax
	}


	// {from, to, rule}

	var taxMap = (function() {
				  var map = []
				  for ( var i = 0; i < limitedRules.length; i++ ) {
				  var currentRule = limitedRules[i]
				  map.push({
						   bottom: currentRule.from - self.taxForSalary(currentRule.from + base) + base,
						   top: currentRule.to - self.taxForSalary(currentRule.to + base) + base,
						   rule: currentRule,
						   fullTax: currentRule.fullTax()
						   })
				  }
				  return map
				  })()

	~function(){
	 console.log("税后低点|税后高点|本层级所缴|对应规则")
	 for ( var i = 0; i < taxMap.length; i++ ) {
	 var item = taxMap[i]
	 console.log(item.bottom + "|" + item.top + "|" + item.fullTax + "|" + item.rule.toString())
	 }
	}()

	//
	this.incomeFromTaxedIncome = function(taxedIncome) {
		var matchedTaxMapItem = null
		for ( var i = taxMap.length - 1; i >= 0; i-- ) {
			var taxMapItem = taxMap[i]
			if (taxedIncome > taxMapItem.bottom && taxedIncome < taxMapItem.top) {
				matchedTaxMapItem = taxMapItem
				break
			}
		}

		if (matchedTaxMapItem) {
			console.log("找到符合中的最高标准 \(matchedTaxMapItem)")
			var rule = matchedTaxMapItem.rule
			return rule.from + (taxedIncome - matchedTaxMapItem.bottom) / (1 - rule.rate) + base
		} else if (!!unlimitedTaxRule) {
			var mapItem = taxMap[taxMap.length - 1]
			return (taxedIncome - mapItem.top) / (1 - unlimitedTaxRule.rate) + mapItem.rule.to + base
		} else {
			console.log("没有符合的规则，且没有指定「无界限规则」")
			console.assert(false)
		}

		console.log(taxMapItem)

	}

}

function testTaxRule() {
	var starRule = new LimitedTaxRule(0, 800, 0)
	console.assert(starRule.reach(200) === true)
	console.assert(starRule.reach(1000) === true)
	console.assert(starRule.tax(200) === 0)
	console.assert(starRule.tax(1000) === 0)

	var middleRule = new LimitedTaxRule(800, 1200, 0.1)
	console.assert(middleRule.reach(200) === false)
	console.assert(middleRule.tax(200) === 0)

	console.assert(middleRule.reach(1000) === true)
	console.assert(middleRule.tax(1000) === 20)
	console.assert(middleRule.reach(1300) === true)
	console.assert(middleRule.tax(1300) === 40)
	console.log("testTaxRule pass")
}


/*
 　　1 不超过1500元的 3
 　　2 超过1500元至4500元的部分 10
 　　3 超过4500元至9000元的部分 20
 　　4 超过9000元至35000元的部分 25
 　　5 超过35000元至55000元的部分 30
 　　6 超过55000元至80000元的部分 35
 　　7 超过80000元的部分 45
 */
function testTaxCal() {
	var cal = new TaxCal(3500,
						 [new LimitedTaxRule(0, 1500, 0.03),
						  new LimitedTaxRule(1500, 4500, 0.10),
						  new LimitedTaxRule(4500, 9000, 0.20),
						  new LimitedTaxRule(9000, 35000, 0.25),
						  new LimitedTaxRule(35000, 55000, 0.30),
						  new LimitedTaxRule(55000, 80000, 0.35)],
						 new UnlimitedTaxRule(80000, 0.45))

	console.assert(cal.taxForSalary(3500) === 0)
	console.assert(cal.taxForSalary(4000) === 15)
	console.assert(cal.taxForSalary(15000) === 1870)
	console.assert(cal.taxForSalary(18000) === 2620)
	console.assert(cal.taxForSalary(150000) === 52420)
	console.log("testTaxCal pass")

	console.log(cal.incomeFromTaxedIncome(90000))
}

testTaxRule()
testTaxCal()