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

---
 example/input.txt                |  4 ++--
 lib/elixir_math_parser.ex        | 22 +++++++++++++++++++---
 lib/main.ex                      |  1 +
 src/elixir_math_parser.yrl       |  4 +++-
 src/elixir_math_parser_lexer.xrl |  1 +
 5 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/example/input.txt b/example/input.txt
index a32d267..7efcb9c 100644
--- a/example/input.txt
+++ b/example/input.txt
@@ -2,6 +2,6 @@ a = 7;; b = 4
 (a + b) * 10 / 2
 
 x = 20
-y = 3
-z = 2y 
+y = 2!
+z = y/2 
 10 + 4(z / y)(x / 4)
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
 
diff --git a/lib/main.ex b/lib/main.ex
index 99c25ca..5e7cf55 100644
--- a/lib/main.ex
+++ b/lib/main.ex
@@ -11,6 +11,7 @@ defmodule ElixirMathParser.Main do
 
     case ElixirMathParser.process_tree(tree) do
       {:ok, _} -> :ok
+      {:error, line, reason} -> reason <> " (line " <> to_string(line) <> ")"
       {:error, reason} -> reason
     end
   end
diff --git a/src/elixir_math_parser.yrl b/src/elixir_math_parser.yrl
index 86b36c8..0808683 100644
--- a/src/elixir_math_parser.yrl
+++ b/src/elixir_math_parser.yrl
@@ -10,7 +10,7 @@ Terminals
   int
   var
   break
-  '+' '-' '*' '/'
+  '+' '-' '*' '/' '!'
   '='
   '(' ')'
 .
@@ -24,6 +24,7 @@ Left 300 '+'.
 Left 300 '-'.
 Left 400 '*'.
 Left 400 '/'.
+Right 500 '!'.
 Left 600 '('.
 
 root -> statements : '$1'.
@@ -46,6 +47,7 @@ expr -> exprs '-' exprs : {sub_op, '$1', '$3'}.
 expr -> exprs '*' exprs : {mul_op, '$1', '$3'}.
 expr -> exprs '/' exprs : {div_op, '$1', '$3'}.
 expr -> '(' exprs ')' : '$2'.
+expr -> expr '!' : {factor_op, '$1'}.
 
 Erlang code.
 
diff --git a/src/elixir_math_parser_lexer.xrl b/src/elixir_math_parser_lexer.xrl
index facf323..5bdcbef 100644
--- a/src/elixir_math_parser_lexer.xrl
+++ b/src/elixir_math_parser_lexer.xrl
@@ -13,6 +13,7 @@ Rules.
 \=            : {token, {'=',  TokenLine}}.
 \(            : {token, {'(',  TokenLine}}.
 \)            : {token, {')',  TokenLine}}.
+!             : {token, {'!',  TokenLine}}.
 {BREAK}+      : {token, {break,  TokenLine}}.
 {NAME}        : {token, {var, TokenLine, TokenChars}}.
 {INT}         : {token, {int,  TokenLine, TokenChars}}.
-- 
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