-- taxes.hs
-- L. David Baron <dbaron@dbaron.org>, 2008-03-16
--
-- Figure changes in marginal tax rates for a single Californian earning
-- all income from wages.
--
-- This is actually my first Haskell program, so don't be too harsh!

module TaxRates
  where

import Text.Printf

ss_tax income =
  0.062 * (min income 97500)

medicare_tax income =
  0.0145 * income

cal_sdi_tax income =
  0.006 * (min income 83388.333333333333)

cal_deduction income =
  3516

-- Technically more correct, but unused
cal_exemption_discrete income =
  max (94 - (6 * fromInteger (round ((max (income - 155416) 0) / 2500)))) 0

cal_exemption income =
  max (94 - (6 * (max (income - 155416) 0) / 2500)) 0

cal_income_tax income =
  let agi = income - (cal_deduction income)
      ia = min agi 6827
      ib = (min agi 16185) - ia
      ic = (min agi 25544) - ia - ib
      id = (min agi 35460) - ia - ib - ic
      ie = (min agi 44814) - ia - ib - ic - id
      ig = agi - ia - ib - ic - id - ie
      tax = 0.01 * ia + 0.02 * ib + 0.04 * ic + 0.06 * id + 0.08 * ie + 0.093 * ig
    in max (tax - (cal_exemption income)) 0

cal_total_tax income =
  (cal_sdi_tax income) + (cal_income_tax income)

federal_deduction income =
  let itemizedunlimited = cal_total_tax income
      limit = (min (0.8 * itemizedunlimited) ((max 0 (income - 156400)) * 0.03)) * (2 / 3)
      itemized = itemizedunlimited - limit
  in max itemized 5350

-- Technically more correct, but unused
federal_exemption_discrete income =
  if (income > (156400 + 122500))
    then 1133
    else 3400 * (1 - (fromInteger (max 0 (ceiling ((income - 156400) / 2500)))) * (2 / 150))

federal_exemption income =
  max 1133 (3400 * (1 - (max 0 ((income - 156400) / 2500)) * (2 / 150)))

federal_eic income =
  -- This is a lot higher for people with kids.
  min (min (income * 0.0765) 428) (max 0 ((12590 - income) * 0.0765))

federal_regular_income_tax income =
  let ti = income - (federal_deduction income) - (federal_exemption income)
      ia = (min ti 7825)
      ib = (min ti 31850) - ia
      ic = (min ti 77100) - ia - ib
      id = (min ti 160850) - ia - ib - ic
      ie = (min ti 349700) - ia - ib - ic - id
      ig = ti - ia - ib - ic - id - ie
  in ia * 0.1 + ib * 0.15 + ic * 0.25 + id * 0.28 + ie * 0.33 + ig * 0.35

federal_amt_exemption income =
  max 0 (44350 - 0.25 * (max 0 (income - 112500)))

federal_amt income =
  let amti = max 0 (income - (federal_amt_exemption income))
  in
    if amti < 175000
      then 0.26 * amti
      else (0.28 * amti) - 3500

federal_income_tax income =
  max (federal_regular_income_tax income) (federal_amt income)

total_tax income =
  (cal_total_tax income) + (federal_income_tax income) + (ss_tax income) + (medicare_tax income) - (federal_eic income)



rate tax income =
  100 * (tax (income + 0.01) - tax income)

to_nearest_cents dollars =
  (fromInteger (round (dollars * 100))) / 100

marginal_rate_table_points tax incomel incomeh =
  let ratel = rate tax incomel
      rateh = rate tax incomeh
  in
    -- Continue the search if (a) there is a rate difference that we're
    -- trying to narrow, or (b) in case the rate jumps back to the same
    -- amount, the low and high incomes are separated by more than 1%
    -- and more than $100.
    if (abs (rateh - ratel) >= 0.0001) ||
       ((incomeh - incomel > 100) && ((incomeh - incomel) / incomel > 0.01))
      then
        let incomec = to_nearest_cents ((incomeh + incomel) / 2)
        in
          if incomel == incomec || incomec == incomeh
            then
              [incomeh]
            else
              (marginal_rate_table_points tax incomel incomec) ++
              (marginal_rate_table_points tax incomec incomeh)
      else
        []

marginal_rate_table_string tax incomel incomeh =
  let
    point_string tax income =
      printf ("At income $%.2f where tax is $%.2f\n" ++
              "  rate changes from %.5f%%%% to %.5f%%%%.\n")
             income
             (tax income)
             ((rate tax (income - 0.01)) * 100)
             ((rate tax income) * 100)

    between_points_string tax incomea incomeb =
      printf ("Between income $%.2f and income $%.2f where tax is $%.2f\n" ++
              "  rate changes from %.5f%%%% to %.5f%%%%.\n")
             incomea
             incomeb
             (tax incomeb)
             ((rate tax (incomea - 0.01)) * 100)
             ((rate tax incomeb) * 100)

    points_string tax [] = ""
    points_string tax (point:[]) =
      point_string tax point
    points_string tax (pointa:pointb:rest) =
      if pointb - pointa <= 0.011
        then
          (between_points_string tax pointa pointb) ++
          (points_string tax rest)
        else
          (point_string tax pointa) ++
          (points_string tax (pointb:rest))

  in
    points_string tax (marginal_rate_table_points tax incomel incomeh)

-- printf (marginal_rate_table_string total_tax 0 10000000)