#ifndef TENSORFLOW_LIB_CORE_BITS_H_
#define TENSORFLOW_LIB_CORE_BITS_H_

#include "tensorflow/core/platform/port.h"

namespace tensorflow {

// Return floor(log2(n)) for positive integer n.  Returns -1 iff n == 0.
int Log2Floor(uint32 n);
int Log2Floor64(uint64 n);

// Return ceiling(log2(n)) for positive integer n.  Returns -1 iff n == 0.
int Log2Ceiling(uint32 n);
int Log2Ceiling64(uint64 n);

// ------------------------------------------------------------------------
// Implementation details follow
// ------------------------------------------------------------------------

#if defined(__GNUC__)

// Return floor(log2(n)) for positive integer n.  Returns -1 iff n == 0.
inline int Log2Floor(uint32 n) {
  return n == 0 ? -1 : 31 ^ __builtin_clz(n);
}

// Return floor(log2(n)) for positive integer n.  Returns -1 iff n == 0.
inline int Log2Floor64(uint64 n) {
  return n == 0 ? -1 : 63 ^ __builtin_clzll(n);
}

#else

// Return floor(log2(n)) for positive integer n.  Returns -1 iff n == 0.
inline int Log2Floor(uint32 n) {
  if (n == 0)
    return -1;
  int log = 0;
  uint32 value = n;
  for (int i = 4; i >= 0; --i) {
    int shift = (1 << i);
    uint32 x = value >> shift;
    if (x != 0) {
      value = x;
      log += shift;
    }
  }
  assert(value == 1);
  return log;
}

// Return floor(log2(n)) for positive integer n.  Returns -1 iff n == 0.
// Log2Floor64() is defined in terms of Log2Floor32()
inline int Log2Floor64(uint64 n) {
  const uint32 topbits = static_cast<uint32>(n >> 32);
  if (topbits == 0) {
    // Top bits are zero, so scan in bottom bits
    return Log2Floor(static_cast<uint32>(n));
  } else {
    return 32 + Log2Floor(topbits);
  }
}

#endif

inline int Log2Ceiling(uint32 n) {
  int floor = Log2Floor(n);
  if (n == (n & ~(n - 1)))  // zero or a power of two
    return floor;
  else
    return floor + 1;
}

inline int Log2Ceiling64(uint64 n) {
  int floor = Log2Floor64(n);
  if (n == (n & ~(n - 1)))  // zero or a power of two
    return floor;
  else
    return floor + 1;
}

}  // namespace tensorflow

#endif  // TENSORFLOW_LIB_CORE_BITS_H_