RoundPowOfTwo.h

template<typename T>
static inline auto RoundPowOfTwo(T n) -> decltype(n) {
  uint64_t value = n;

  // Fill 1
  value |= value >> 1U;
  value |= value >> 2U;
  value |= value >> 4U;
  value |= value >> 8U;
  value |= value >> 16U;
  value |= value >> 32U;

  // Unable to round-up, take the value of round-down
  if (decltype(n)(value + 1) == 0) {
    value >>= 1U;
  }

  return value + 1;
}

static inline uint16_t RoundUpPowOfTwo(uint16_t n) {
  if (n == 0) return 1;

  // Avoid is already a power of 2
  return RoundPowOfTwo<decltype(n)>(n - 1);
}

static inline uint16_t RoundDownPowOfTwo(uint16_t n) {
  return RoundPowOfTwo<decltype(n)>(n >> 1U);
}

//  EXPECT_EQ(RoundDownPowOfTwo(0), 1);
//  EXPECT_EQ(RoundDownPowOfTwo(1), 1);
//  EXPECT_EQ(RoundDownPowOfTwo(2), 2);
//  EXPECT_EQ(RoundDownPowOfTwo(3), 2);
//  EXPECT_EQ(RoundDownPowOfTwo(4), 4);
//  EXPECT_EQ(RoundDownPowOfTwo(5), 4);
//  EXPECT_EQ(RoundDownPowOfTwo(63), 32);
//  EXPECT_EQ(RoundDownPowOfTwo(64), 64);
//  EXPECT_EQ(RoundDownPowOfTwo(65), 64);
//  EXPECT_EQ(RoundDownPowOfTwo(UINT16_MAX - 1), INT16_MAX + 1);
//  EXPECT_EQ(RoundDownPowOfTwo(UINT16_MAX), INT16_MAX + 1);
//
//  EXPECT_EQ(RoundUpPowOfTwo(0), 1);
//  EXPECT_EQ(RoundUpPowOfTwo(1), 1);
//  EXPECT_EQ(RoundUpPowOfTwo(2), 2);
//  EXPECT_EQ(RoundUpPowOfTwo(3), 4);
//  EXPECT_EQ(RoundUpPowOfTwo(4), 4);
//  EXPECT_EQ(RoundUpPowOfTwo(5), 8);
//  EXPECT_EQ(RoundUpPowOfTwo(63), 64);
//  EXPECT_EQ(RoundUpPowOfTwo(64), 64);
//  EXPECT_EQ(RoundUpPowOfTwo(65), 128);
//  EXPECT_EQ(RoundUpPowOfTwo(UINT16_MAX - 1), INT16_MAX + 1);
//  EXPECT_EQ(RoundUpPowOfTwo(UINT16_MAX), INT16_MAX + 1);