| 1 | ;;; loan-calc.lisp
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| 2 |
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| 3 | #|
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| 4 | The MIT license.
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| 5 |
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| 6 | Copyright (c) 2010 Paul L. Krueger
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| 7 |
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| 8 | Permission is hereby granted, free of charge, to any person obtaining a copy of this software
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| 9 | and associated documentation files (the "Software"), to deal in the Software without restriction,
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| 10 | including without limitation the rights to use, copy, modify, merge, publish, distribute,
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| 11 | sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is
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| 12 | furnished to do so, subject to the following conditions:
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| 13 |
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| 14 | The above copyright notice and this permission notice shall be included in all copies or substantial
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| 15 | portions of the Software.
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| 16 |
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| 17 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
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| 18 | LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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| 19 | IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
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| 20 | WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
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| 21 | SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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| 22 |
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| 23 | |#
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| 24 |
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| 25 | ;;; Sample lisp/Cocoa interface that uses a NIB file defined with interface builder;
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| 26 | ;;; A definition is provided for the "SpeechController" class that was specified to interface builder
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| 27 | ;;; as the class of the NIB file owner.
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| 28 | ;;; We manually create an instance of SpeechController and specify it as the owner for the NIB file.
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| 29 |
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| 30 | (eval-when (:compile-toplevel :load-toplevel :execute)
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| 31 | (require :nib)
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| 32 | (require :date)
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| 33 | (require :decimal))
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| 34 |
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| 35 | (defpackage :loan-calc
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| 36 | (:nicknames :lnc)
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| 37 | (:use :iu :ccl :common-lisp)
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| 38 | (:export test-loan))
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| 39 |
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| 40 | (in-package :lnc)
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| 41 |
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| 42 | ;; The loan-controller class
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| 43 |
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| 44 | (defclass loan-controller (ns:ns-window-controller)
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| 45 | ((loan :foreign-type :id :accessor loan)
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| 46 | (loan-text :foreign-type :id :accessor loan-text)
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| 47 | (int-text :foreign-type :id :accessor int-text)
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| 48 | (dur-text :foreign-type :id :accessor dur-text)
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| 49 | (pay-text :foreign-type :id :accessor pay-text)
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| 50 | (int-slider :foreign-type :id :accessor int-slider)
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| 51 | (dur-slider :foreign-type :id :accessor dur-slider)
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| 52 | (pay-slider :foreign-type :id :accessor pay-slider))
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| 53 | (:metaclass ns:+ns-object))
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| 54 |
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| 55 | (objc:defmethod (#/initWithLoan: :id)
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| 56 | ((self loan-controller) (ln :id))
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| 57 | (setf (loan self) ln)
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| 58 | (let* ((nib-name (ccl::%make-nsstring
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| 59 | (namestring (truename "ip:Loan Calc;loan.nib"))))
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| 60 | (init-self (#/initWithWindowNibPath:owner: self nib-name self)))
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| 61 | init-self))
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| 62 |
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| 63 | ;; Action methods that are called when controls do something
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| 64 |
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| 65 | (objc:defmethod (#/buttonPushed: :void)
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| 66 | ((self loan-controller) (button-matrix :id))
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| 67 | (with-slots (loan loan-text int-text dur-text pay-text int-slider
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| 68 | dur-slider pay-slider) self
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| 69 | (let ((cm (#/selectedRow button-matrix)))
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| 70 | (unless (eql cm (compute-mode loan))
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| 71 | (case (compute-mode loan)
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| 72 | (0 (#/setEnabled: loan-text #$YES))
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| 73 | (1 (#/setEnabled: int-text #$YES)
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| 74 | (#/setEnabled: int-slider #$YES))
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| 75 | (2 (#/setEnabled: dur-text #$YES)
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| 76 | (#/setEnabled: dur-slider #$YES))
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| 77 | (3 (#/setEnabled: pay-text #$YES)
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| 78 | (#/setEnabled: pay-slider #$YES)))
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| 79 | (setf (compute-mode loan) cm)
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| 80 | (case cm
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| 81 | (0 (#/setEnabled: loan-text #$NO))
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| 82 | (1 (#/setEnabled: int-text #$NO)
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| 83 | (#/setEnabled: int-slider #$NO))
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| 84 | (2 (#/setEnabled: dur-text #$NO)
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| 85 | (#/setEnabled: dur-slider #$NO))
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| 86 | (3 (#/setEnabled: pay-text #$NO)
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| 87 | (#/setEnabled: pay-slider #$NO)))
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| 88 | (compute-new-loan-values loan)))))
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| 89 |
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| 90 | (objc:defmethod (#/awakeFromNib :void)
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| 91 | ((self loan-controller))
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| 92 | (#/setEnabled: (loan-text self) #$NO)
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| 93 | ;; set the sliders to update continuously so that the text boxes reflect the current value
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| 94 | ;; Note that we can set this in IB for text boxes, but not, apparently, for sliders
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| 95 | (#/setContinuous: (int-slider self) #$YES)
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| 96 | (#/setContinuous: (dur-slider self) #$YES)
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| 97 | (#/setContinuous: (pay-slider self) #$YES))
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| 98 |
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| 99 | (objc:defmethod (#/windowWillClose: :void)
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| 100 | ((self loan-controller) (notif :id))
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| 101 | (declare (ignore notif))
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| 102 | (when (loan self)
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| 103 | ;; Tell the loan that the window is closing
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| 104 | ;; It will #/autorelease this window-controller)
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| 105 | (window-closed (loan self))))
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| 106 |
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| 107 |
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| 108 | ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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| 109 | ;;;; Everything below is related to the loan data model
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| 110 |
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| 111 | (defconstant $max-interest-rate$ .5)
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| 112 |
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| 113 | ;; Some loan utility functions
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| 114 |
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| 115 | ;; all equations are derived from a basic loan equation for an N month loan:
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| 116 | ;; LoanPrinciple = MonthlyPayment * ( (1 / (1 + MonthlyInterest)) +
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| 117 | ;; (1 / (1 + MonthlyInterest)^2) + ... +
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| 118 | ;; (1 / (1 + MonthlyInterest)^N) )
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| 119 | ;; which can be manipulated to derive:
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| 120 | ;; MonthlyPayment = LoanPrinciple * (MonthlyInterest + (MonthlyInterest / ((1 + MonthlyInterest)^LoanDuration - 1)))
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| 121 |
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| 122 | (defun pay-to-loan-ratio (mo-int loan-dur)
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| 123 | ;; Just computes the MonthlyPayment/Loan ratio from the basic loan equation given above
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| 124 | (if (zerop mo-int) 0
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| 125 | (+ mo-int (/ mo-int (1- (expt (1+ mo-int) loan-dur))))))
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| 126 |
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| 127 | ;; The loan class
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| 128 |
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| 129 | (defclass loan (ns:ns-object)
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| 130 | ((loan-amount :accessor loan-amount :initform 0)
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| 131 | (interest-rate :accessor interest-rate :initform 0)
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| 132 | (loan-duration :accessor loan-duration :initform 0)
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| 133 | (monthly-payment :accessor monthly-payment :initform 0)
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| 134 | (origination-date :accessor origination-date :initform (now))
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| 135 | (first-payment :accessor first-payment :initform (next-month (now)))
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| 136 | (pay-schedule :accessor pay-schedule :initform nil)
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| 137 | (window-controller :accessor window-controller :initform nil)
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| 138 | (compute-mode :accessor compute-mode :initform 0)
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| 139 | (max-dur :foreign-type #>BOOL :accessor max-dur)
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| 140 | (min-dur :foreign-type #>BOOL :accessor min-dur)
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| 141 | (min-pay :foreign-type #>BOOL :accessor min-pay))
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| 142 | (:metaclass ns:+ns-object))
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| 143 |
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| 144 |
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| 145 | ;; Normal lisp methods for our class
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| 146 |
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| 147 | (defmethod initialize-instance :after ((self loan)
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| 148 | &key &allow-other-keys)
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| 149 | (setf (window-controller self)
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| 150 | (make-instance (find-class 'loan-controller)
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| 151 | :with-loan self))
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| 152 | (#/showWindow: (window-controller self) self))
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| 153 |
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| 154 | (defmethod window-closed ((self loan))
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| 155 | ;; called by the window-controller to tell us that the window closed
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| 156 | (when (window-controller self)
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| 157 | (#/autorelease (window-controller self))
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| 158 | (setf (window-controller self) nil)))
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| 159 |
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| 160 | (defmethod close-loan ((self loan))
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| 161 | ;; just tell the window to close
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| 162 | (when (window-controller self)
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| 163 | (let ((win (#/window (window-controller self))))
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| 164 | ;; for reasons mysterious to me, calling #/window seems to
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| 165 | ;; also #/retain the window, so we'll #/release it
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| 166 | (#/release win)
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| 167 | (#/performClose: win self))))
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| 168 |
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| 169 | (defmethod set-pay-schedule ((self loan))
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| 170 | ;; create a detailed payment schedule for the loan using daily compounding of interest
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| 171 | ;; Payments are on the same date of each month, but the number of days between payments
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| 172 | ;; varies because the number of days in each month varies.
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| 173 | ;; We compute accurate interest compounded daily for the actual number of days.
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| 174 | (let ((monthly-interest (/ (interest-rate self) 12))
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| 175 | (payment (monthly-payment self))
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| 176 | (sched nil)
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| 177 | (display-min-pay-banner nil))
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| 178 | (prog1
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| 179 | (do* ((begin (loan-amount self) end)
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| 180 | (begin-date (first-payment self) end-date)
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| 181 | (end-date (next-month begin-date) (next-month begin-date))
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| 182 | (int (round (* begin monthly-interest))
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| 183 | (round (* begin monthly-interest)))
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| 184 | (end (- (+ begin int) payment) (- (+ begin int) payment)))
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| 185 | ((not (plusp end))
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| 186 | (progn
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| 187 | (push (list (short-date-string begin-date)
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| 188 | (/ begin 100)
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| 189 | (/ int 100)
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| 190 | (/ payment 100)
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| 191 | (short-date-string end-date)
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| 192 | (/ end 100)
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| 193 | int)
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| 194 | sched)
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| 195 | (setf (pay-schedule self) (nreverse sched))))
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| 196 | (when (>= end begin)
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| 197 | ;; oops, with this combination of values the loan will never
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| 198 | ;; be paid off, so let's set a minimum payment required
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| 199 | ;; Display a field that tells user the minimum payment was reached
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| 200 | (setf display-min-pay-banner t)
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| 201 | (#/willChangeValueForKey: self #@"monthlyPayment")
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| 202 | (setf (monthly-payment self) (1+ int))
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| 203 | (#/didChangeValueForKey: self #@"monthlyPayment")
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| 204 | ;; now patch up our loop variables and keep going
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| 205 | (setf payment (monthly-payment self))
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| 206 | (setf end (1- begin)))
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| 207 | ;; put the last payment into the list
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| 208 | (push (list (short-date-string begin-date)
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| 209 | (/ begin 100)
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| 210 | (/ int 100)
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| 211 | (/ payment 100)
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| 212 | (short-date-string end-date)
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| 213 | (/ end 100)
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| 214 | int)
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| 215 | sched))
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| 216 | (#/willChangeValueForKey: self #@"totInterest")
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| 217 | ;; we'll make the total interest field call our accessor
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| 218 | ;; to generate a new amount
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| 219 | (#/didChangeValueForKey: self #@"totInterest")
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| 220 | (if display-min-pay-banner
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| 221 | (progn
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| 222 | ;; Set a condition that says the minimum payment was reached
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| 223 | (setf display-min-pay-banner t)
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| 224 | (#/willChangeValueForKey: self #@"minPay")
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| 225 | (setf (min-pay self) #$YES)
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| 226 | (#/didChangeValueForKey: self #@"minPay"))
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| 227 | (progn
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| 228 | ;; otherwise reset that condition
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| 229 | (#/willChangeValueForKey: self #@"minPay")
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| 230 | (setf (min-pay self) #$NO)
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| 231 | (#/didChangeValueForKey: self #@"minPay")))
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| 232 | ;; If we happen to be computing the interest rate, then
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| 233 | ;; the combination of loan-amount and monthly payment will
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| 234 | ;; determine a maximum interest rate. This, in turn,
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| 235 | ;; determines a maximum loan duration. If the duration was set
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| 236 | ;; longer than this by the user, we will reset the
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| 237 | ;; lone duration value to the maximum needed.
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| 238 | ;; If, on the other hand, the monthly payment is set so low that
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| 239 | ;; the interest rate approaches 0, then we may have to adjust the
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| 240 | ;; loan duration up to the minimum needed to pay the loan.
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| 241 | ;; Let's start by resetting our two "duration" conditions and then we'll
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| 242 | ;; set them if conditions dictate.
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| 243 | ;; Reset a condition that indicates the max duration was reached
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| 244 | (#/willChangeValueForKey: self #@"maxDur")
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| 245 | (setf (max-dur self) #$NO)
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| 246 | (#/didChangeValueForKey: self #@"maxDur")
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| 247 | ;; Reset a condition that indicates the min duration was reached
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| 248 | (#/willChangeValueForKey: self #@"minDur")
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| 249 | (setf (min-dur self) #$NO)
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| 250 | (#/didChangeValueForKey: self #@"minDur"))
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| 251 | (let ((duration-diff (- (loan-duration self) (list-length (pay-schedule self)))))
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| 252 | (unless (or (eql (compute-mode self) 2) (zerop duration-diff))
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| 253 | ;; i.e. we're not calling this function just to determine the loan duration
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| 254 | ;; and we have to adjust the loan duration
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| 255 | (if (plusp duration-diff)
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| 256 | (progn
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| 257 | ;; change the loan-duration value to what it must be
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| 258 | (#/willChangeValueForKey: self #@"loanDuration")
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| 259 | (setf (loan-duration self) (list-length (pay-schedule self)))
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| 260 | (#/didChangeValueForKey: self #@"loanDuration")
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| 261 | (when (> duration-diff 2)
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| 262 | ;; If we're one-off just fix it and don't post a message
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| 263 | ;; This can occur almost anytime because of numerical issues
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| 264 | ;; Display a field that tells user the max duration was reached
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| 265 | (#/willChangeValueForKey: self #@"maxDur")
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| 266 | (setf (max-dur self) #$YES)
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| 267 | (#/didChangeValueForKey: self #@"maxDur")))
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| 268 | (progn
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| 269 | ;; change the oan-duration value to what it must be
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| 270 | (#/willChangeValueForKey: self #@"loanDuration")
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| 271 | (setf (loan-duration self) (list-length (pay-schedule self)))
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| 272 | (#/didChangeValueForKey: self #@"loanDuration")
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| 273 | (when (< duration-diff -2)
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| 274 | ;; If we're one-off just fix it and don't post a message
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| 275 | ;; This can occur almost anytime because of numerical issues
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| 276 | ;; Display a field that tells user the min duration was reached
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| 277 | (#/willChangeValueForKey: self #@"minDur")
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| 278 | (setf (min-dur self) #$YES)
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| 279 | (#/didChangeValueForKey: self #@"minDur"))))))))
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| 280 |
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| 281 | (defmethod print-pay-schedule ((self loan) &optional (strm t))
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| 282 | (format strm
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| 283 | "~:{~%On ~a balance = $~$ + interest of $~$ - payment of $~$ = ~a balance of $~$~}"
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| 284 | (pay-schedule self)))
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| 285 |
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| 286 | (defmethod compute-int-rate ((self loan))
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| 287 | ;; Find a monthly interest rate that makes the rest of the values work.
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| 288 | ;; There isn't an algebraic solution for the interest rate, so let's search for it.
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| 289 | ;; Find a suitable search range and do a binary search for it. Even for large interest
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| 290 | ;; rates the number of search iterations should be minimal.
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| 291 |
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| 292 | (with-slots (loan-amount monthly-payment loan-duration interest-rate) self
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| 293 |
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| 294 | ;; First we'll check to see whether the monthly payment is great than the loan amount.
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| 295 | ;; If so we'll set the interest rate directly so that the loan is paid off in one month.
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| 296 | ;; This avoids some ugly arithmetic overflow things that can happen when interest rates
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| 297 | ;; go off the charts
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| 298 | (let ((max-monthly-rate (/ $max-interest-rate$ 12)))
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| 299 | (if (>= monthly-payment loan-amount)
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| 300 | (min max-monthly-rate (1- (/ monthly-payment loan-amount)))
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| 301 | (let ((imin (max 0 (min max-monthly-rate
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| 302 | (/ (- (* monthly-payment loan-duration) loan-amount)
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| 303 | (* loan-duration loan-amount)))))
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| 304 | ;; imin is basically a rate that would result in the first month's interest as
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| 305 | ;; the average interest paid for all months. Since we know it must be greater
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| 306 | ;; than this, we have a guaranteed lower bound. But we cap it at our allowed
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| 307 | ;; monthly maximum interest.
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| 308 | (imax (min max-monthly-rate
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| 309 | (- (/ monthly-payment loan-amount) .000008333)))
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| 310 | ;; imax is a rate that would result in the first month's interest being
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| 311 | ;; minimally smaller than the payment. Since we must pay off in a finite
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| 312 | ;; duration, this is a guaranteed maximum. We cap it the allowed maximum
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| 313 | ;; monthly rate.
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| 314 | (target-p-l-ratio (/ monthly-payment loan-amount)))
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| 315 | (unless (>= imax imin)
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| 316 | (error "Max int = ~8,4f, Min int = ~8,4f" imax imin))
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| 317 | (do* ((i (/ (+ imin imax) 2)
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| 318 | (/ (+ imin imax) 2))
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| 319 | (p-l-ratio (pay-to-loan-ratio i loan-duration)
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| 320 | (pay-to-loan-ratio i loan-duration)))
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| 321 | ((<= (- imax imin) .000001) imax)
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| 322 | (if (>= target-p-l-ratio p-l-ratio)
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| 323 | (setf imin i)
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| 324 | (setf imax i))))))))
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| 325 |
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| 326 | (defmethod compute-new-loan-values ((self loan))
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| 327 | ;; For the sake of expediency we assume monthly componding
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| 328 | ;; The basic equation governing these computations is
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| 329 | (with-slots (compute-mode interest-rate loan-duration monthly-payment
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| 330 | loan-amount pay-schedule) self
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| 331 | (case compute-mode
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| 332 | (0
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| 333 | ;; compute the loan amount
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| 334 | (unless (or (zerop interest-rate)
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| 335 | (zerop loan-duration)
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| 336 | (zerop monthly-payment))
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| 337 | (#/willChangeValueForKey: self #@"loanAmt")
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| 338 | (setf loan-amount
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| 339 | (round (/ monthly-payment
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| 340 | (pay-to-loan-ratio (/ interest-rate 12)
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| 341 | loan-duration))))
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| 342 | (set-pay-schedule self)
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| 343 | (#/didChangeValueForKey: self #@"loanAmt")))
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| 344 | (1
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| 345 | ;; compute the interest rate
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| 346 | (unless (or (zerop loan-amount)
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| 347 | (zerop loan-duration)
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| 348 | (zerop monthly-payment))
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| 349 | (#/willChangeValueForKey: self #@"interestRate")
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| 350 | (setf interest-rate
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| 351 | (* 12 (/ (floor (* 1000000 (compute-int-rate self)))
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| 352 | 1000000)))
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| 353 | (set-pay-schedule self)
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| 354 | (#/didChangeValueForKey: self #@"interestRate")))
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| 355 | (2
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| 356 | ;; compute the loan duration
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| 357 | (unless (or (zerop interest-rate)
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| 358 | (zerop loan-amount)
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| 359 | (zerop monthly-payment))
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| 360 | (#/willChangeValueForKey: self #@"loanDuration")
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| 361 | (set-pay-schedule self)
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| 362 | (setf loan-duration
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| 363 | (list-length pay-schedule))
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| 364 | (#/didChangeValueForKey: self #@"loanDuration")))
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| 365 | (3
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| 366 | ;; compute the monthly payment
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| 367 | (unless (or (zerop interest-rate)
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| 368 | (zerop loan-amount)
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| 369 | (zerop loan-duration))
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| 370 | (#/willChangeValueForKey: self #@"monthlyPayment")
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| 371 | (setf monthly-payment
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| 372 | (round (* loan-amount
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| 373 | (pay-to-loan-ratio (/ interest-rate 12)
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| 374 | loan-duration))))
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| 375 | (set-pay-schedule self)
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| 376 | (#/didChangeValueForKey: self #@"monthlyPayment"))))))
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| 377 |
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| 378 | ;; Accessor functions used by display objects to retrieve and set values as dictated
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| 379 | ;; by the bindings we set up in IB
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| 380 |
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| 381 | (objc:defmethod (#/loanAmt :id)
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| 382 | ((self loan))
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| 383 | (lisp-to-ns-decimal (loan-amount self)))
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| 384 |
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| 385 | (objc:defmethod (#/interestRate :id)
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| 386 | ((self loan))
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| 387 | (#/numberWithFloat: ns:ns-number (float (interest-rate self))))
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| 388 |
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| 389 | (objc:defmethod (#/loanDuration :id)
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| 390 | ((self loan))
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| 391 | (#/numberWithInt: ns:ns-number (loan-duration self)))
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| 392 |
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| 393 | (objc:defmethod (#/monthlyPayment :id)
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| 394 | ((self loan))
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| 395 | (lisp-to-ns-decimal (monthly-payment self)))
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| 396 |
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| 397 | (objc:defmethod (#/originationDate :id)
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| 398 | ((self loan))
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| 399 | (lisp-to-ns-date (origination-date self)))
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| 400 |
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| 401 | (objc:defmethod (#/firstPayment :id)
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| 402 | ((self loan))
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| 403 | (lisp-to-ns-date (first-payment self)))
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| 404 |
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| 405 | (objc:defmethod (#/totInterest :id)
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| 406 | ((self loan))
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| 407 | (lisp-to-ns-decimal (reduce #'+ (pay-schedule self)
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| 408 | :key #'seventh
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| 409 | :initial-value 0)))
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| 410 |
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| 411 | (objc:defmethod (#/setLoanAmt: :void)
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| 412 | ((self loan) (amt :id))
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| 413 | (setf (loan-amount self) (lisp-from-ns-decimal amt))
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| 414 | (compute-new-loan-values self))
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| 415 |
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| 416 | (objc:defmethod (#/setInterestRate: :void)
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| 417 | ((self loan) (rate :id))
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| 418 | (setf (interest-rate self) (#/floatValue rate))
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| 419 | (compute-new-loan-values self))
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| 420 |
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| 421 | (objc:defmethod (#/setLoanDuration: :void)
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| 422 | ((self loan) (dur :id))
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| 423 | (setf (loan-duration self) (#/longValue dur))
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| 424 | (compute-new-loan-values self))
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| 425 |
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| 426 | (objc:defmethod (#/setMonthlyPayment: :void)
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| 427 | ((self loan) (pay :id))
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| 428 | (setf (monthly-payment self) (lisp-from-ns-decimal pay))
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| 429 | (compute-new-loan-values self))
|
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| 430 |
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| 431 | (objc:defmethod (#/setOriginationDate: :void)
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|---|
| 432 | ((self loan) (dt :id))
|
|---|
| 433 | (let ((new-dt (ns-to-lisp-date dt)))
|
|---|
| 434 | (setf (origination-date self) new-dt)
|
|---|
| 435 | (#/willChangeValueForKey: self #@"firstPayment")
|
|---|
| 436 | (setf (first-payment self) (next-month new-dt))
|
|---|
| 437 | (#/didChangeValueForKey: self #@"firstPayment"))
|
|---|
| 438 | (compute-new-loan-values self))
|
|---|
| 439 |
|
|---|
| 440 | (objc:defmethod (#/setFirstPayment: :void)
|
|---|
| 441 | ((self loan) (pay :id))
|
|---|
| 442 | (let ((new-pay (ns-to-lisp-date pay)))
|
|---|
| 443 | (setf (first-payment self) new-pay))
|
|---|
| 444 | (compute-new-loan-values self))
|
|---|
| 445 |
|
|---|
| 446 |
|
|---|
| 447 |
|
|---|
| 448 | ;; test by
|
|---|
| 449 | (defun test-loan ()
|
|---|
| 450 | ;; up to caller to #/release the returned loan instance
|
|---|
| 451 | ;; but only after window is closed or crash will occur
|
|---|
| 452 | (make-instance 'loan))
|
|---|
| 453 |
|
|---|
| 454 | (provide :loan-calc)
|
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