add loan payoff calculations

loan.rkt

@@ -0,0 +1,44 @@
+#lang racket
+
+(provide compute-payment
+         compute-monthly-payment
+         compute-remaining-monthly-payment
+         compute-remaining-daily-payment)
+
+(define (compute-payment initial-balance
+                         annual-interest-rate
+                         payments-per-year
+                         total-number-of-payments)
+  (let ([i (/ (if (> annual-interest-rate 0)
+                  annual-interest-rate
+                  0.0000001)
+              payments-per-year)])
+    (/ (* i initial-balance)
+       (- 1 (expt (+ 1 i) (- total-number-of-payments))))))
+
+(define (compute-monthly-payment initial-balance
+                                 annual-interest-rate
+                                 months-to-pay)
+  (compute-payment initial-balance annual-interest-rate 12 months-to-pay))
+
+(define (compute-remaining-monthly-payment initial-balance
+                                           annual-interest-rate
+                                           months-to-pay
+                                           already-paid-this-month)
+  (- (compute-monthly-payment initial-balance
+                              annual-interest-rate
+                              months-to-pay)
+     already-paid-this-month))
+
+(define average-days-in-month
+  (/ (+ 31 28.25 31 30 31 30 31 31 30 31 30 31) 12))
+
+(define (compute-remaining-daily-payment initial-balance
+                                         annual-interest-rate
+                                         months-to-pay
+                                         already-paid-this-month)
+  (/ (compute-remaining-monthly-payment initial-balance
+                                        annual-interest-rate
+                                        months-to-pay
+                                        already-paid-this-month)
+     average-days-in-month))