Add largest affordable loan

mortgage-calculator.lisp

@@ -1,16 +1,6 @@
 ;;;; Mortgage Calculators
 
 #|
-loan amount (p)
-interest rate (r)
-years (t)
-payments per year (n)
-loan type: fixed-rate
-
-Payment = P x (r/n) * [1+(r/n)^n * t]/(1+r/n)^n *t - 1
-
-or
-
 P = payment
 PV = Present Value
 r = rate per period
@@ -18,16 +8,45 @@ n = number of periods
 
 P = (r(PV)) / (1-(1+r)^-n)
 
+Okay, but I want to find PV instead
+Guess I need to remember algebra
+
+P = (r(PV)) / (1-(1+r)^-n)
+P * (1-(1+r)^-n) = r * PV
+
+(P * (1-(1+r)^-n) / r = PV
+
+   monthly-payments conversion
+
+  (find-payments loan-total (/ rate 12) (* number-of-payments 12)))
+  find-payments = loan-total
 |#
 
-(defun payments (loan-total rate number-of-payments)
+(defun find-payments (loan-total rate number-of-payments)
+  "Base loan calculation"
   (/ (* rate loan-total)
 	 (- 1 (expt (+ 1 rate) (* number-of-payments -1)))))
-(defun monthly-payments(loan-total rate number-of-payments)
-  "Number of payments in years"
-  (payments loan-total (/ rate 12) (* number-of-payments 12)))
-(defun payment-total-per-year (loan-total rate number-of-payments)
-  (* (payments loan-total (/ rate 12) (* number-of-payments 12)) 12))
+
+(defun find-monthly-payments(loan-total rate number-of-payments)
+  "Modify loan calculation to show monthly payments"
+  (find-payments loan-total (/ rate 12) (* number-of-payments 12)))
+
+(defun find-montly-payment-total-per-year (loan-total rate number-of-payments)
+  "Modify loan calculation to show total montly payments per year"
+  (* (find-payments loan-total (/ rate 12) (* number-of-payments 12)) 12))
+
+
+;; fixme: add function to take monthly-payments rate num-of-payments
+(defun find-biggest-loan (payments rate number-of-payments)
+  "Base loan calculation"
+  (/ (* payments
+	   (- 1 (expt (+ 1 rate)
+				  (* -1 number-of-payments))))
+	 rate))
+
+(defun find-biggest-loan-monthly (payments rate number-of-payments)
+  (find-biggest-loan payments (/ rate 12) (* number-of-payments 12)))
+