[TarunJain04]

Fixes #56

Edge Cases Fixed

1. modPow(a, 0, 1) returned 1 instead of 0

   • The result now starts at 1 % mod, so a modulus of 1 gives 0.

2. Overflow in base reduction

   • A long holds at most 2^63 − 1.
   • The overflow was in (a % mod) + mod, which can reach 2·mod − 1. That exceeds 2^63 − 1 only when mod > 2^62.
   • The base is now reduced with a %= mod first, and mod is added only if the result is negative.

3. Long.MIN_VALUE exponent caused infinite recursion (StackOverflowError)

   • -Long.MIN_VALUE overflows and stays Long.MIN_VALUE (still negative).
   • So the recursive call modPow(modInv(a, mod), -n, mod) re-entered the n < 0 branch forever.
   • Negative exponents are now handled iteratively.
   • The loop uses n != 0 with >>>=, so the exponent is read as an unsigned 2^63.

Tests

Added tests in ModPowTest.java covering each of the three edge cases above.