From 7f8bf5f45545ff44cc8489b2ac3288b52d64d9d2 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Sat, 9 May 2026 14:52:13 +0000
Subject: [PATCH] ocaml: phase 5.1 mortgage.ml baseline (monthly payment, 200k
 @ 5% / 30y = $1073)
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Standard amortising-mortgage formula:

  payment = P * r * (1 + r)^n / ((1 + r)^n - 1)

where r = annual_rate / 12, n = years * 12.

  let payment principal annual_rate years =
    let r = annual_rate /. 12.0 in
    let n = years * 12 in
    let pow_r = ref 1.0 in
    for _ = 1 to n do pow_r := !pow_r *. (1.0 +. r) done;
    principal *. r *. !pow_r /. (!pow_r -. 1.0)

For 200,000 at 5% over 30 years: monthly payment ~= $1073.64,
int_of_float -> 1073.

Manual (1+r)^n via for-loop instead of Float.pow keeps the program
portable to any environment where pow is restricted.

Tests float arithmetic precedence, for-loop accumulation in a float
ref, int_of_float on the result.

70 baseline programs total — milestone.
---
 lib/ocaml/baseline/expected.json |  1 +
 lib/ocaml/baseline/mortgage.ml   | 10 ++++++++++
 plans/ocaml-on-sx.md             |  7 +++++++
 3 files changed, 18 insertions(+)
 create mode 100644 lib/ocaml/baseline/mortgage.ml

diff --git a/lib/ocaml/baseline/expected.json b/lib/ocaml/baseline/expected.json
index 019643c9..4b719c8a 100644
--- a/lib/ocaml/baseline/expected.json
+++ b/lib/ocaml/baseline/expected.json
@@ -40,6 +40,7 @@
   "lambda_calc.ml": 7,
   "levenshtein.ml": 11,
   "memo_fib.ml": 75025,
+  "mortgage.ml": 1073,
   "merge_sort.ml": 44,
   "module_use.ml": 3,
   "newton_sqrt.ml": 1414,
diff --git a/lib/ocaml/baseline/mortgage.ml b/lib/ocaml/baseline/mortgage.ml
new file mode 100644
index 00000000..5119c516
--- /dev/null
+++ b/lib/ocaml/baseline/mortgage.ml
@@ -0,0 +1,10 @@
+let payment principal annual_rate years =
+  let r = annual_rate /. 12.0 in
+  let n = years * 12 in
+  let pow_r = ref 1.0 in
+  for _ = 1 to n do pow_r := !pow_r *. (1.0 +. r) done;
+  principal *. r *. !pow_r /. (!pow_r -. 1.0)
+
+;;
+
+int_of_float (payment 200000.0 0.05 30)
diff --git a/plans/ocaml-on-sx.md b/plans/ocaml-on-sx.md
index c36c914a..f47ef114 100644
--- a/plans/ocaml-on-sx.md
+++ b/plans/ocaml-on-sx.md
@@ -407,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — mortgage.ml baseline (monthly mortgage
+  payment formula, 200k @ 5% / 30y → $1073). Manual `(1+r)^n`
+  computed via for-loop because `Float.pow` may not be available
+  for arbitrary args here. Then plugs into `principal * r *
+  (1+r)^n / ((1+r)^n - 1)`. Tests float arithmetic precedence,
+  for-loop accumulation in a float ref, int_of_float on the result.
+  70 baseline programs total — milestone.
 - 2026-05-09 Phase 5.1 — group_consec.ml baseline (group consecutive
   equals into sublists, 5*10 + 3 = 53). Inner `collect cur acc
   tail` walks while head matches `cur`, accumulates into `acc`,