fix(rpn): restore binary exponentiation in FastPower

After dead-code removal, FastPower used math.Pow instead of
binaryExponentiationFloat, losing O(log n) efficiency for large
integer exponents and potentially reducing accuracy.

Restore the square-and-multiply helper in operations_arithmetic.go.

1 files changed, 27 insertions, 1 deletions

diff --git a/internal/rpn/operations_arithmetic.go b/internal/rpn/operations_arithmetic.go
index 3b6a1fd..429a14c 100644
--- a/internal/rpn/operations_arithmetic.go
+++ b/internal/rpn/operations_arithmetic.go
@@ -215,11 +215,37 @@ func (o *Operations) FastPower(stack *Stack) error {
 		stack.Push(NewNumber(1, o.GetMode(), GetCoolMetric()))
 		return nil
 	}
-	resultVal := math.Pow(aF, float64(exp))
+	resultVal := binaryExponentiationFloat(aF, exp)
 	stack.Push(NewNumber(resultVal, o.GetMode(), GetCoolMetric()))
 	return nil
 }
 
+// binaryExponentiationFloat computes base^exp using the square-and-multiply algorithm.
+// Time Complexity: O(log exp)
+// Space Complexity: O(1)
+func binaryExponentiationFloat(base float64, exp int) float64 {
+	if exp == 0 {
+		return 1.0
+	}
+
+	// Handle negative exponents: base^-exp = 1 / (base^exp)
+	if exp < 0 {
+		return 1.0 / binaryExponentiationFloat(base, -exp)
+	}
+
+	res := 1.0
+	for exp > 0 {
+		// If exponent is odd, multiply result by current base
+		if exp%2 == 1 {
+			res *= base
+		}
+		// Square the base and divide exponent by 2
+		base *= base
+		exp /= 2
+	}
+	return res
+}
+
 // Log2 pops one value from stack, computes log base 2 (log₂(a)), and pushes result.
 func (o *Operations) Log2(stack *Stack) error {
 	a, err := popStack(stack, "lg")