2015 day 25.

src/holt59/aoc/tools/math.py

@@ -0,0 +1,21 @@
+def pow_mod(b: int, e: int, m: int):
+    """
+    Compute (b ** e) % m using right-to-left binary method.
+
+    See https://en.wikipedia.org/wiki/Modular_exponentiation.
+
+    Args:
+        b: Base to exponentiate.
+        e: Exponent.
+        m: Modulus.
+
+    Returns:
+        (b ** e) % m.
+    """
+    r = 1
+    while e > 0:
+        if e % 2 == 1:
+            r = (r * b) % m
+        e >>= 1
+        b = (b * b) % m
+    return r