入力した税率となる所得を返すプログラム

目的：（Xにかかる税金）/X = 指定税率 を解く

アイデア

平均税率テーブルを計算しておく

4000万円以上の場合(29.25+27+109.5+67.65+387+1100+(X-4000)*0.55)/X

与えられた税率が出てきたら、テーブルを見てどの範囲に収まるかを確認して計算する

例：

0.15が与えられた場合、テーブルに一致するのは195万円以下なので195万円

0.20が与えられた場合、330-695万円の間なので(29.25+27+(x-330)/xを解く

code:py

from sympy import symbols, Eq, solve

# 入力した税率となる所得の上限を返す関数

def calculate_income_for_tax_rate(tax_rate):

# Tax brackets (income, rate)

tax_brackets = [

(1950000, 0.15),

(3300000, 0.2),

(6950000, 0.3),

(9000000, 0.33),

(18000000, 0.43),

(40000000, 0.5)

]

# Initialize cumulative tax and previous upper limit

cumulative_tax = 0.0

prev_limit = 0.0

# Initialize symbolic variable

x = symbols('x')

# Check for rates below the minimum tax rate

if tax_rate < 0.15:

return None

# Special case for the first tax bracket

if tax_rate == 0.15:

return 1950000.0

# Loop through each tax bracket to find the range where the given average tax rate falls

for upper_limit, rate in tax_brackets:

prev_cumulative_tax = cumulative_tax

# Calculate the total tax for incomes up to the upper limit of this bracket

cumulative_tax += (upper_limit - prev_limit) * rate

# Calculate the average tax rate for incomes up to the upper limit of this bracket

avg_tax_rate = cumulative_tax / upper_limit

# Check if the given tax rate falls within this bracket

if avg_tax_rate >= tax_rate:

# Solve for the income that would result in the given average tax rate

equation = Eq((prev_cumulative_tax + (x - prev_limit) * rate) / x, tax_rate)

income = solve(equation)

# Return the income if a valid solution exists

# Update the previous upper limit for the next iteration

prev_limit = upper_limit

# If we get here, the tax rate is above 50% and should fall into the 55% bracket

equation = Eq((cumulative_tax + (x - 40000000) * 0.55) / x, tax_rate)

income = solve(equation)

# If no solution was found, return None

return None

# Testing the updated function with an average tax rate of 0.5 (50%)

calculate_income_for_tax_rate(0.5)

code:verify.py

# 入力値が正しいかの検算コード

def verify_calculated_income(calculated_income):

tax_brackets_verification = [

(1950000, 0.15),

(3300000, 0.2),

(6950000, 0.3),

(9000000, 0.33),

(18000000, 0.43),

(40000000, 0.5)

]

cumulative_tax_verification = 0.0

prev_limit_verification = 0.0

for upper_limit, rate in tax_brackets_verification:

if calculated_income > upper_limit:

cumulative_tax_verification += (upper_limit - prev_limit_verification) * rate

else:

cumulative_tax_verification += (calculated_income - prev_limit_verification) * rate

break

prev_limit_verification = upper_limit

# Only apply the 55% rate for incomes above 40,000,000 JPY

if calculated_income > 40000000:

cumulative_tax_verification += (calculated_income - 40000000) * 0.55

average_tax_rate_verification = cumulative_tax_verification / calculated_income

return cumulative_tax_verification, average_tax_rate_verification