From 8f3dadb7178a07ebadf782507cf2af5995321c71 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Tue, 2 Jan 2024 19:43:21 +0100
Subject: Initial commit

---
 mortgage.html    | 68 +++++++++++++++++++++++++++++++++++++++++++++++++++
 mortgage.js      | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 mortgage.py      | 55 +++++++++++++++++++++++++++++++++++++++++
 requirements.txt |  1 +
 4 files changed, 198 insertions(+)
 create mode 100644 mortgage.html
 create mode 100644 mortgage.js
 create mode 100644 mortgage.py
 create mode 100644 requirements.txt

diff --git a/mortgage.html b/mortgage.html
new file mode 100644
index 0000000..07de66b
--- /dev/null
+++ b/mortgage.html
@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<html>
+
+<head>
+    <title>Mortgage Calculator</title>
+    <style>
+        #parameters {
+            margin-top: 2em;
+            display: grid;
+            grid-template-columns: 10em min-content;
+            column-gap: 1em;
+            row-gap: 0.5em;
+        }
+        #parameters div {
+            text-align: right;
+        }
+        #parameters input {
+            width: 12em;
+            text-align: right;
+        }
+        #results {
+            margin-top: 2em;
+        }
+        #results-table {
+            border: 1px solid;
+            border-collapse: collapse;
+        }
+        #results-table th {
+            padding-top: 0.5em;
+            padding-bottom: 0.5em;
+        }
+        #results-table th, td {
+            text-align: center;
+            padding-left: 1em;
+            padding-right: 1em;
+        }
+        #results-table tr:nth-child(even){
+            background-color: #f2f2f2;
+        }
+        #results-table tr:hover {background-color: #ddd;}
+    </style>
+</head>
+
+<body>
+    <h1>Mortgage Calculator</h1>
+    <div id="parameters">
+        <div>Principal</div>
+        <div><input id="principal" type="number" value="100000" onchange="update()"></div>
+        <div>Annual Interest (%)</div>
+        <div><input id="interest" type="number" value="5.0" onchange="update()"></div>
+        <div>Installments</div>
+        <div><input id="installments" type="number" value="300" onchange="update()"></div>
+    </div>
+    <div id="results">
+        <table id="results-table">
+            <tr>
+                <th>Time</th>
+                <th>Outstanding Debt</th>
+                <th>Monthly Installment</th>
+                <th>Interest Paid</th>
+                <th>Amount Amortised</th>
+            </tr>
+        </table>
+    </div>
+    <script type="text/javascript" src="mortgage.js"></script>
+</body>
+
+</html>
diff --git a/mortgage.js b/mortgage.js
new file mode 100644
index 0000000..ffc1b1d
--- /dev/null
+++ b/mortgage.js
@@ -0,0 +1,74 @@
+/// Round up the given number two the nearest two decimal places.
+function round(x) {
+    return Math.ceil(x*100)/100;
+}
+
+/// Format the float as a string using two decimal places.
+function fmt(x) {
+    return round(x).toFixed(2);
+}
+
+/// Clear all rows in the table except for the first row (header).
+function clear_table(table) {
+    while (table.rows.length > 1) {
+        table.deleteRow(1);
+    }
+}
+
+/// Insert the values into a new row in the table.
+function insert_row(table, values) {
+    var row = table.insertRow(table.rows.length);
+    values.forEach((value) => {
+        row.insertCell(row.cells.length).innerHTML = value;
+    });
+}
+
+/// Compute the mortgage.
+function compute_mortgage(principal, annual_interest, installments, table) {
+    const p = principal;
+    const i = annual_interest / 100 / 12; // Monthly equivalent.
+    const n = installments;
+
+    const monthly_installment = p * (i * (1 + i)**n) / ((1 + i)**n - 1);
+
+    var debt = principal;
+    var total_interest = 0;
+    var total_amortised = 0;
+
+    clear_table(table);
+
+    for (k = 0; k < n; ++k) {
+        const interest = i * debt;
+        const amortised = monthly_installment - interest;
+
+        const values = [k.toString(), fmt(debt), fmt(monthly_installment), fmt(interest), fmt(amortised)];
+        insert_row(table, values);
+
+        total_interest += interest;
+        total_amortised += amortised;
+        debt -= amortised;
+    }
+
+    const total_interest_pct = total_interest / principal * 100;
+
+    insert_row(table, ["", "", "", "", ""]); // Separator.
+
+    insert_row(table, ["Total", "", "Amount Paid", "Interest Paid", "Amount Amortised"]);
+    insert_row(table, [
+        "",
+        "",
+        fmt(monthly_installment * n),
+        fmt(total_interest) + " (" + fmt(total_interest_pct) + "%)",
+        fmt(total_amortised)]);
+}
+
+function update() {
+    const principal    = parseFloat(document.getElementById("principal").value);
+    const interest     = parseFloat(document.getElementById("interest").value);
+    const installments = parseInt(document.getElementById("installments").value);
+    const results      = document.getElementById("results-table");
+
+    compute_mortgage(principal, interest, installments, results);
+}
+
+update();
diff --git a/mortgage.py b/mortgage.py
new file mode 100644
index 0000000..3b9b958
--- /dev/null
+++ b/mortgage.py
@@ -0,0 +1,55 @@
+import argparse
+import math
+import sys
+from tabulate import tabulate, SEPARATING_LINE
+
+
+def calc_payments(principal, annual_interest, installments):
+    p = principal
+    i = annual_interest / 100 / 12 # Monthly equivalent.
+    n = installments
+
+    def fmt(x):
+        return f"{math.ceil(x*100)/100:.2f}"
+
+    table = [["Time", "Outstanding Debt", "Monthly Installment", "Interest Paid", "Amount Amortised"]]
+
+    monthly_installment = p * (i * (1 + i)**n) / ((1 + i)**n - 1)
+    debt = principal
+    total_interest = 0
+    total_amortised = 0
+
+    for k in range(n):
+        interest = i * debt
+        amortised = monthly_installment - interest
+        table.append([k] + [fmt(x) for x in [debt, monthly_installment, interest, amortised]])
+
+        total_interest += interest
+        total_amortised += amortised
+        debt -= amortised
+
+    total_interest_pct = total_interest / principal * 100
+
+    table.append(SEPARATING_LINE)
+    table.append(["Total", "", "Amount Paid", "Interest Paid", "Amount Amortised"])
+    table.append(["", ""] + [
+        fmt(monthly_installment * n),
+        fmt(total_interest) + f" ({fmt(total_interest_pct)}%)",
+        fmt(total_amortised)])
+    return table
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("-p", "--principal", type=float, default=100000, help="Principal")
+    parser.add_argument("-i", "--interest", type=float, default=5, help="Annual interest %")
+    parser.add_argument("-n", "--installments", type=int, default=300, help="Number of installments")
+    args = parser.parse_args()
+
+    table = calc_payments(args.principal, args.interest, args.installments)
+    print(tabulate(table))
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/requirements.txt b/requirements.txt
new file mode 100644
index 0000000..a5d5159
--- /dev/null
+++ b/requirements.txt
@@ -0,0 +1 @@
+tabulate
-- 
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