Solve more problems road to 200.

python/e188.py

@@ -0,0 +1,38 @@
+from lib_prime import prime_factors
+
+
+def phi(n):
+    # Calculate phi using Euler's totient function
+    c = n
+    fs = set(prime_factors(n))
+    for f in fs:
+        # same as c *= (1 - 1/f)
+        c *= (f - 1)
+        c //= f
+    return c
+
+
+def tetmod(a, k, m):
+    # https://stackoverflow.com/questions/30713648/how-to-compute-ab-mod-m
+    assert k > 0
+    if k == 1:
+        return a
+    if m == 1:
+        if a % 2 == 0:
+            return 0
+        else:
+            return 1
+    phi_m = phi(m)
+    return pow(a, phi_m + tetmod(a, k - 1, phi_m), m)
+
+
+def euler_188():
+    assert phi(100) == 40
+    assert tetmod(14, 2016, 100) == 36
+    return tetmod(1777, 1855, 10**8)
+
+
+if __name__ == "__main__":
+    solution = euler_188()
+    print("e188.py: " + str(solution))
+    assert solution == 95962097