HoneyHoney

1. 计算不够精确。其实这个不能够怪新浪的计算器。实际上每个银行在计算每年1月份利率调整的方法各不一样。比如这篇博客里面解释了工商银行的计算方式，而我按照他的计算方式去算结果总是会有一些偏差。
2. 局限性。他只能够计算到3个利率变化的情况，一般5年以上的贷款利率变化很有可能超过3个。

• 关于等额本息以及相关计算，请转百科和这篇博客

require 'date'

# 贷款总额
total_loan_money = 50 * 10000
# 贷款年限
total_loan_month = 20 * 12
# 初始利率
initial_interest_rate = 4.7
# 第一次还贷时间
initial_pay_dt = '2011-05-08'
# 利率调整时间表
interest_rate_changes = [] 
interest_rate_changes << [initial_pay_dt, initial_interest_rate]
interest_rate_changes << ['2012-01-01', 4.9]
interest_rate_changes << ['2013-01-01', 4.5]
interest_rate_changes << ['2015-01-01', 4.25]
interest_rate_changes << ['2016-01-01', 3.5]

p interest_rate_changes

def averageInterest(n, money, rate, ini_pay_dt)
 list = []
 money_to_pay_per_month = (money * rate * (1 + rate) ** n /((1 + rate) ** n - 1)).round(2)

 n.times do |i|
  remaining_load_money = (i==0) ? money : list[i-1][4]
  interest_to_pay = (remaining_load_money * rate).round(2)
  load_money_to_pay = (money_to_pay_per_month - interest_to_pay).round(2)
  # [date, 本期利息，本期本金，本期本息，剩余本金]
  list << [ini_pay_dt.strftime('%Y-%m-%d'), interest_to_pay, load_money_to_pay, 

money_to_pay_per_month, (remaining_load_money - load_money_to_pay).round(2)]

  ini_pay_dt = ini_pay_dt.next_month
 end
 list
end

def averageInterestAdjust(list, date, total_loan_month, rate1, rate2)
 tmp_list = []
 last_list = list[-2]
 money = last_list[4]
 rate1_per_day = rate1 / 30
 rate2_per_day = rate2 / 30
 ini_pay_dt = Date.parse(last_list[0])

 rate1_days = date - ini_pay_dt
 rate2_days = ini_pay_dt.next_month - date

 # 本月利息
 interest_to_pay = (money * (rate1_per_day * rate1_days + 

rate2_per_day * rate2_days)).round(2)

 # 本月应还金额
 # 农业银行特色 应还本金还是按照上一年的利率计算的，但是利息是分段计算的
 money_to_pay = (interest_to_pay + list[-1][2]).round(2)

 tmp_list = [ini_pay_dt.strftime('%Y-%m-%d'), interest_to_pay, 

(money_to_pay - interest_to_pay), money_to_pay, 

money-(money_to_pay - interest_to_pay)]

 list[-1][1] = tmp_list[1].round(2)
 list[-1][2] = tmp_list[2].round(2)
 list[-1][3] = tmp_list[3].round(2)
 list[-1][4] = tmp_list[4].round(2)

 list
end

list = []
i = 0
ini_pay_dt = Date.parse(initial_pay_dt)

interest_rate_changes.each do |t|

 date = Date.parse(t[0])

 if ini_pay_dt == date
  i += 1
  next
 else
  diff = (date.year - ini_pay_dt.year) * 12 + (date.month - ini_pay_dt.month)
 end

 list += averageInterest(total_loan_month, total_loan_money, 

    interest_rate_changes[i-1][1]/100.0/12.0, ini_pay_dt).slice(0, diff+1)

 total_loan_month -= (diff) 

 # 每年1月1日利率调整，按日计息
 list = averageInterestAdjust(list, date, total_loan_month, 

    interest_rate_changes[i-1][1]/100.0/12.0, interest_rate_changes[i][1]/100.0/12.0)

 total_loan_month -= 1
 ini_pay_dt = Date.parse(list[-1][0]).next_month
 total_loan_money = list[-1][4]
 i += 1
end

list += averageInterest(total_loan_month, total_loan_money, 

    interest_rate_changes[-1][1]/100.0/12.0, ini_pay_dt)

total_interest = 0

list.each do |t|
 p t
 total_interest += t[1]
end

puts total_interest