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/* Copyright (c) 2003-2004, Roger Dingledine
* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
* Copyright (c) 2007-2020, The Tor Project, Inc. */
/* See LICENSE for licensing information */
/**
* \file bits.c
*
* \brief Count the bits in an integer, manipulate powers of 2, etc.
**/
#include "lib/intmath/bits.h"
/** Returns floor(log2(u64)). If u64 is 0, (incorrectly) returns 0. */
int
tor_log2(uint64_t u64)
{
int r = 0;
if (u64 >= (UINT64_C(1)<<32)) {
u64 >>= 32;
r = 32;
}
if (u64 >= (UINT64_C(1)<<16)) {
u64 >>= 16;
r += 16;
}
if (u64 >= (UINT64_C(1)<<8)) {
u64 >>= 8;
r += 8;
}
if (u64 >= (UINT64_C(1)<<4)) {
u64 >>= 4;
r += 4;
}
if (u64 >= (UINT64_C(1)<<2)) {
u64 >>= 2;
r += 2;
}
if (u64 >= (UINT64_C(1)<<1)) {
// u64 >>= 1; // not using this any more.
r += 1;
}
return r;
}
/** Return the power of 2 in range [1,UINT64_MAX] closest to <b>u64</b>. If
* there are two powers of 2 equally close, round down. */
uint64_t
round_to_power_of_2(uint64_t u64)
{
int lg2;
uint64_t low;
uint64_t high;
if (u64 == 0)
return 1;
lg2 = tor_log2(u64);
low = UINT64_C(1) << lg2;
if (lg2 == 63)
return low;
high = UINT64_C(1) << (lg2+1);
if (high - u64 < u64 - low)
return high;
else
return low;
}
/** Return the number of bits set in <b>v</b>. */
int
n_bits_set_u8(uint8_t v)
{
static const int nybble_table[] = {
0, /* 0000 */
1, /* 0001 */
1, /* 0010 */
2, /* 0011 */
1, /* 0100 */
2, /* 0101 */
2, /* 0110 */
3, /* 0111 */
1, /* 1000 */
2, /* 1001 */
2, /* 1010 */
3, /* 1011 */
2, /* 1100 */
3, /* 1101 */
3, /* 1110 */
4, /* 1111 */
};
return nybble_table[v & 15] + nybble_table[v>>4];
}
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