578ed1ba98
Moving all of my Emacs-related files into their own directory at the root of this repository.
119 lines
4.2 KiB
EmacsLisp
119 lines
4.2 KiB
EmacsLisp
;;; finance.el --- Functions to help me organize my finances -*- lexical-binding: t -*-
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;; Author: William Carroll <wpcarro@gmail.com>
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;;; Commentary:
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;; Using functions to organize my financial thinking.
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;;; Code:
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Dependencies
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(require 'prelude)
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(require 'math)
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Library
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(defvar finance/enable-tests? t
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"When t, run the tests defined herein.")
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;; TODO: Support printing an org-table of these amount in a similar format to:
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;; https://keisan.casio.com/exec/system/1234231998
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(cl-defun finance/future-value (amt
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&key
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num-years
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(frequency 'monthly)
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(interest-rate 0.06)
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(payment-due-at 'beg)
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(present-value 0))
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"Compute the Future Value of AMT.
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This function assumes that the interest rate is applied annually and not
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monthly.
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This function will attempt to provide the following defaults:
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- frequency: 'monthly
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- interest-rate: 6%
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- payment-due-at: 'beg
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- present-value: 0.00"
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(prelude/assert (set/contains? payment-due-at (set/new 'beg 'end)))
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(prelude/assert (set/contains? frequency (set/new 'annually
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'semiannually
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'quarterly
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'monthly)))
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(let ((pmt amt)
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(k (alist/get frequency '((annually . 1)
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(semiannually . 2)
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(quarterly . 4)
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(monthly . 12))))
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(r interest-rate)
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(n num-years)
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(pv present-value))
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(if (= 0 r)
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(+ pv (* pmt n k))
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(if (equal 'beg payment-due-at)
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(+ (* pv (math/exp (+ 1 (/ r k)) (* n k)))
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(* pmt
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(/ (- (math/exp (+ 1 (/ r k)) (* n k)) 1)
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(/ r k))
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(+ 1 (/ r k))))
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(+ (* pv (math/exp (+ 1 (/ r k)) (* n k)))
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(* pmt
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(/ (- (math/exp (+ 1 (/ r k)) (* n k)) 1)
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(/ r k))))))))
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;; Tests
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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(when finance/enable-tests?
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(prelude/assert
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(equal "1551.27"
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(string/format "%0.2f"
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(finance/future-value
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9.99
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:interest-rate 0.05
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:num-years 10
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:frequency 'monthly
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:payment-due-at 'end
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:present-value 0))))
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(prelude/assert
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(equal "14318.34"
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(string/format "%0.2f"
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(finance/future-value 10.0 :num-years 35))))
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(prelude/assert
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(equal "4200.00"
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(string/format "%0.2f"
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(finance/future-value
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10.0
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:interest-rate 0.0
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:num-years 35
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:frequency 'monthly
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:payment-due-at 'beg
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:present-value 0))))
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(prelude/assert
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(equal "14318.34"
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(string/format "%0.2f"
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(finance/future-value
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10.0
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:interest-rate 0.06
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:num-years 35
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:frequency 'monthly
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:payment-due-at 'beg
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:present-value 0))))
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(prelude/assert
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(equal "38282.77"
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(string/format "%0.2f"
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(finance/future-value
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10.0
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:interest-rate 0.1
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:num-years 35
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:frequency 'monthly
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:payment-due-at 'beg
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:present-value 0)))))
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(provide 'finance)
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;;; finance.el ends here
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