// Copyright 2002 and onwards Google Inc.
//
// A collection of useful (static) bit-twiddling functions.

#include "kudu/gutil/integral_types.h"
#include "kudu/gutil/macros.h"

#ifndef _BITS_H_
#define _BITS_H_

class Bits {
 public:
  // Return the number of one bits in the given integer.
  static int CountOnesInByte(unsigned char n);

  static int CountOnes(uint32 n) {
    n -= ((n >> 1) & 0x55555555);
    n = ((n >> 2) & 0x33333333) + (n & 0x33333333);
    return (((n + (n >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24;
  }

  // Count bits using sideways addition [WWG'57]. See Knuth TAOCP v4 7.1.3(59)
  static inline int CountOnes64(uint64 n) {
#if defined(__x86_64__)
    n -= (n >> 1) & 0x5555555555555555ULL;
    n = ((n >> 2) & 0x3333333333333333ULL) + (n & 0x3333333333333333ULL);
    return (((n + (n >> 4)) & 0xF0F0F0F0F0F0F0FULL)
            * 0x101010101010101ULL) >> 56;
#else
    return CountOnes(n >> 32) + CountOnes(n & 0xffffffff);
#endif
  }

  // Count bits using popcnt instruction (available on argo machines).
  // Doesn't check if the instruction exists.
  // Please use TestCPUFeature(POPCNT) from base/cpuid/cpuid.h before using this.
  static inline int CountOnes64withPopcount(uint64 n) {
#if defined(__x86_64__) && defined __GNUC__
    int64 count = 0;
    asm("popcnt %1,%0" : "=r"(count) : "rm"(n) : "cc");
    return count;
#else
    return CountOnes64(n);
#endif
  }

  // Reverse the bits in the given integer.
  static uint8 ReverseBits8(uint8 n);
  static uint32 ReverseBits32(uint32 n);
  static uint64 ReverseBits64(uint64 n);

  // Return the number of one bits in the byte sequence.
  static int Count(const void *m, int num_bytes);

  // Return the number of different bits in the given byte sequences.
  // (i.e., the Hamming distance)
  static int Difference(const void *m1, const void *m2, int num_bytes);

  // Return the number of different bits in the given byte sequences,
  // up to a maximum.  Values larger than the maximum may be returned
  // (because multiple bits are checked at a time), but the function
  // may exit early if the cap is exceeded.
  static int CappedDifference(const void *m1, const void *m2,
                              int num_bytes, int cap);

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

  // Potentially faster version of Log2Floor() that returns an
  // undefined value if n == 0
  static int Log2FloorNonZero(uint32 n);
  static int Log2FloorNonZero64(uint64 n);

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

  // Return the first set least / most significant bit, 0-indexed.  Returns an
  // undefined value if n == 0.  FindLSBSetNonZero() is similar to ffs() except
  // that it's 0-indexed, while FindMSBSetNonZero() is the same as
  // Log2FloorNonZero().
  static int FindLSBSetNonZero(uint32 n);
  static int FindLSBSetNonZero64(uint64 n);
  static int FindMSBSetNonZero(uint32 n) { return Log2FloorNonZero(n); }
  static int FindMSBSetNonZero64(uint64 n) { return Log2FloorNonZero64(n); }

  // Portable implementations
  static int Log2Floor_Portable(uint32 n);
  static int Log2FloorNonZero_Portable(uint32 n);
  static int FindLSBSetNonZero_Portable(uint32 n);
  static int Log2Floor64_Portable(uint64 n);
  static int Log2FloorNonZero64_Portable(uint64 n);
  static int FindLSBSetNonZero64_Portable(uint64 n);

  // Viewing bytes as a stream of unsigned bytes, does that stream
  // contain any byte equal to c?
  template <class T> static bool BytesContainByte(T bytes, uint8 c);

  // Viewing bytes as a stream of unsigned bytes, does that stream
  // contain any byte b < c?
  template <class T> static bool BytesContainByteLessThan(T bytes, uint8 c);

  // Viewing bytes as a stream of unsigned bytes, are all elements of that
  // stream in [lo, hi]?
  template <class T> static bool BytesAllInRange(T bytes, uint8 lo, uint8 hi);

 private:
  static const char num_bits[];
  static const unsigned char bit_reverse_table[];
  DISALLOW_COPY_AND_ASSIGN(Bits);
};

// A utility class for some handy bit patterns.  The names l and h
// were chosen to match Knuth Volume 4: l is 0x010101... and h is 0x808080...;
// half_ones is ones in the lower half only.  We assume sizeof(T) is 1 or even.
template <class T> struct BitPattern {
  static const T half_ones = (static_cast<T>(1) << (sizeof(T)*4)) - 1;
  static const T l = (sizeof(T) == 1) ? 1 :
                       (half_ones / 0xff * (half_ones + 2));
  static const T h = ~(l * 0x7f);
};

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

// use GNU builtins where available
#if defined(__GNUC__) && \
    ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
inline int Bits::Log2Floor(uint32 n) {
  return n == 0 ? -1 : 31 ^ __builtin_clz(n);
}

inline int Bits::Log2FloorNonZero(uint32 n) {
  return 31 ^ __builtin_clz(n);
}

inline int Bits::FindLSBSetNonZero(uint32 n) {
  return __builtin_ctz(n);
}

inline int Bits::Log2Floor64(uint64 n) {
  return n == 0 ? -1 : 63 ^ __builtin_clzll(n);
}

inline int Bits::Log2FloorNonZero64(uint64 n) {
  return 63 ^ __builtin_clzll(n);
}

inline int Bits::FindLSBSetNonZero64(uint64 n) {
  return __builtin_ctzll(n);
}
#elif defined(_MSC_VER)
#include "kudu/gutil/bits-internal-windows.h"
#else
#include "kudu/gutil/bits-internal-unknown.h"
#endif

inline int Bits::CountOnesInByte(unsigned char n) {
  return num_bits[n];
}

inline uint8 Bits::ReverseBits8(unsigned char n) {
  n = ((n >> 1) & 0x55) | ((n & 0x55) << 1);
  n = ((n >> 2) & 0x33) | ((n & 0x33) << 2);
  return ((n >> 4) & 0x0f)  | ((n & 0x0f) << 4);
}

inline uint32 Bits::ReverseBits32(uint32 n) {
  n = ((n >> 1) & 0x55555555) | ((n & 0x55555555) << 1);
  n = ((n >> 2) & 0x33333333) | ((n & 0x33333333) << 2);
  n = ((n >> 4) & 0x0F0F0F0F) | ((n & 0x0F0F0F0F) << 4);
  n = ((n >> 8) & 0x00FF00FF) | ((n & 0x00FF00FF) << 8);
  return ( n >> 16 ) | ( n << 16);
}

inline uint64 Bits::ReverseBits64(uint64 n) {
#if defined(__x86_64__)
  n = ((n >> 1) & 0x5555555555555555ULL) | ((n & 0x5555555555555555ULL) << 1);
  n = ((n >> 2) & 0x3333333333333333ULL) | ((n & 0x3333333333333333ULL) << 2);
  n = ((n >> 4) & 0x0F0F0F0F0F0F0F0FULL) | ((n & 0x0F0F0F0F0F0F0F0FULL) << 4);
  n = ((n >> 8) & 0x00FF00FF00FF00FFULL) | ((n & 0x00FF00FF00FF00FFULL) << 8);
  n = ((n >> 16) & 0x0000FFFF0000FFFFULL) | ((n & 0x0000FFFF0000FFFFULL) << 16);
  return ( n >> 32 ) | ( n << 32);
#else
  return ReverseBits32( n >> 32 ) |
         (static_cast<uint64>(ReverseBits32( n &  0xffffffff )) << 32);
#endif
}

inline int Bits::Log2FloorNonZero_Portable(uint32 n) {
  // Just use the common routine
  return Log2Floor(n);
}

// Log2Floor64() is defined in terms of Log2Floor32(), Log2FloorNonZero32()
inline int Bits::Log2Floor64_Portable(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 + Log2FloorNonZero(topbits);
  }
}

// Log2FloorNonZero64() is defined in terms of Log2FloorNonZero32()
inline int Bits::Log2FloorNonZero64_Portable(uint64 n) {
  const uint32 topbits = static_cast<uint32>(n >> 32);
  if (topbits == 0) {
    // Top bits are zero, so scan in bottom bits
    return Log2FloorNonZero(static_cast<uint32>(n));
  } else {
    return 32 + Log2FloorNonZero(topbits);
  }
}

// FindLSBSetNonZero64() is defined in terms of FindLSBSetNonZero()
inline int Bits::FindLSBSetNonZero64_Portable(uint64 n) {
  const uint32 bottombits = static_cast<uint32>(n);
  if (bottombits == 0) {
    // Bottom bits are zero, so scan in top bits
    return 32 + FindLSBSetNonZero(static_cast<uint32>(n >> 32));
  } else {
    return FindLSBSetNonZero(bottombits);
  }
}

template <class T>
inline bool Bits::BytesContainByteLessThan(T bytes, uint8 c) {
  T l = BitPattern<T>::l;
  T h = BitPattern<T>::h;
  // The c <= 0x80 code is straight out of Knuth Volume 4.
  // Usually c will be manifestly constant.
  return c <= 0x80 ?
      ((h & (bytes - l * c) & ~bytes) != 0) :
      ((((bytes - l * c) | (bytes ^ h)) & h) != 0);
}

template <class T> inline bool Bits::BytesContainByte(T bytes, uint8 c) {
  // Usually c will be manifestly constant.
  return Bits::BytesContainByteLessThan<T>(bytes ^ (c * BitPattern<T>::l), 1);
}

template <class T>
inline bool Bits::BytesAllInRange(T bytes, uint8 lo, uint8 hi) {
  T l = BitPattern<T>::l;
  T h = BitPattern<T>::h;
  // In the common case, lo and hi are manifest constants.
  if (lo > hi) {
    return false;
  }
  if (hi - lo < 128) {
    T x = bytes - l * lo;
    T y = bytes + l * (127 - hi);
    return ((x | y) & h) == 0;
  }
  return !Bits::BytesContainByteLessThan(bytes + (255 - hi) * l,
                                         lo + (255 - hi));
}

#endif // _BITS_H_