perf(calc): use exponentiation by squaring

move calc things in new module

1 files changed, 28 insertions, 0 deletions

diff --git a/lib/math/Calc.ex b/lib/math/Calc.ex
new file mode 100644
index 0000000..7dc5fcc
--- /dev/null
+++ b/lib/math/Calc.ex
@@ -0,0 +1,28 @@
+defmodule ElixirMathParser.Math.Calc do
+  alias ElixirMathParser.Math.Rational
+  def factorial(n), do: factorial_rec(n, 1)
+  defp factorial_rec(n, acc) when n > 0, do: factorial_rec(n - 1, acc * n)
+  defp factorial_rec(0, acc), do: acc
+
+  def pow(value, exponent), do: pow_rec(value, exponent, Rational.new(1))
+
+  defp pow_rec(value, exponent, acc) when is_integer(exponent) do
+    case exponent do
+      0 ->
+        acc
+
+      _ when exponent < 0 ->
+        pow_rec(Rational.new(1, value), -exponent, acc)
+
+      _ when Kernel.rem(exponent, 2) == 0 ->
+        pow_rec(Rational.mult(value, value), Kernel.div(exponent, 2), acc)
+
+      _ ->
+        pow_rec(
+          Rational.mult(value, value),
+          Kernel.div(exponent - 1, 2),
+          Rational.mult(acc, value)
+        )
+    end
+  end
+end