From 3b0cc79dee76244da55d2326cc4244a384d85e44 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?William=20Herg=C3=A8s?= <william@herges.fr>
Date: Sun, 9 Nov 2025 13:23:05 +0100
Subject: feat(calc)): supports factorial

---
 lib/elixir_math_parser.ex | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

(limited to 'lib/elixir_math_parser.ex')

diff --git a/lib/elixir_math_parser.ex b/lib/elixir_math_parser.ex
index f7f916f..863ecd8 100644
--- a/lib/elixir_math_parser.ex
+++ b/lib/elixir_math_parser.ex
@@ -10,9 +10,9 @@ defmodule ElixirMathParser do
     {:ok, value <~> 1}
   end
 
-  defp reduce_to_value({:var, _line, var}, state) do
+  defp reduce_to_value({:var, line, var}, state) do
     if !Map.has_key?(state, var) do
-      {:error, "value not found for " <> to_string(var)}
+      {:error, line, "value not found for " <> to_string(var)}
     else
       {:ok, state[var]}
     end
@@ -46,9 +46,25 @@ defmodule ElixirMathParser do
     end
   end
 
-  defp evaluate_tree([{:assign, {:var, _line, lhs}, rhs} | tail], state) do
+  defp reduce_to_value({:factor_op, lhs}, state) do
+    with {:ok, op} <- reduce_to_value(lhs, state),
+         true <- Ratio.denominator(op) == 1,
+         true <- Ratio.numerator(op) >= 0 do
+      {:ok, factor(Ratio.numerator(op), 1)}
+    else
+      {:error, line, reason} -> {:error, line, reason}
+      false -> {:error, "must have a positive integer for the factorial"}
+    end
+  end
+
+  defp factor(n, acc) when n > 0, do: factor(n - 1, acc * n)
+  defp factor(0, acc), do: acc
+
+  defp evaluate_tree([{:assign, {:var, line, lhs}, rhs} | tail], state) do
     with {:ok, val} <- reduce_to_value(rhs, state) do
       evaluate_tree(tail, Map.merge(state, %{lhs => val}))
+    else
+      {:error, reason} -> {:error, line, reason}
     end
   end
 
-- 
cgit v1.2.3