Support finance.el

Create a finance module to help me cheaply calculate things like the future
value of a Spotify subscription or Dropbox subscription or Jiu Jitsu
membership.

configs/shared/.emacs.d/wpc/finance.el

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