transmission/libtransmission/bitfield.cc

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// This file Copyright © 2008-2022 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.
#include <algorithm>
#include <array>
#include <climits> // SIZE_MAX
#include <vector>
#include "tr-popcount.h"
#include "transmission.h"
#include "bitfield.h"
#include "tr-assert.h"
/****
*****
****/
namespace
{
[[nodiscard]] constexpr size_t getBytesNeeded(size_t bit_count) noexcept
{
/* NB: If can guarantee bit_count <= SIZE_MAX - 8 then faster logic
is ((bit_count + 7) >> 3). */
return (bit_count >> 3) + ((bit_count & 7) != 0 ? 1 : 0);
}
/* Used only in cases where it can be guaranteed bit_count <= SIZE_MAX - 8 */
[[nodiscard]] constexpr size_t getBytesNeededSafe(size_t bit_count) noexcept
{
return ((bit_count + 7) >> 3);
}
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void setAllTrue(uint8_t* array, size_t bit_count)
{
uint8_t constexpr Val = 0xFF;
/* Only ever called internally with in-use bit counts. Impossible
for bitcount > SIZE_MAX - 8. */
size_t const n = getBytesNeededSafe(bit_count);
if (n > 0)
{
std::fill_n(array, n, Val);
/* -bit_count & 7U. Since bitcount is unsigned do ~bitcount +
1 to replace -bitcount as linters warn about negating
unsigned types. Any compiler will optimize ~x + 1 to -x in
the backend. */
uint32_t const shift = ((~bit_count) + 1) & 7U;
array[n - 1] = Val << shift;
}
}
/* Switch to std::popcount if project upgrades to c++20 or newer */
[[nodiscard]] uint32_t doPopcount(uint8_t flags) noexcept
{
/* If flags are ever expanded to use machine words instead of
uint8_t popcnt64 is also available */
return tr_popcnt<uint8_t>::count(flags);
}
[[nodiscard]] size_t rawCountFlags(uint8_t const* flags, size_t n) noexcept
{
auto ret = size_t{};
for (auto const* const end = flags + n; flags != end; ++flags)
{
ret += doPopcount(*flags);
}
return ret;
}
} // namespace
/****
*****
****/
size_t tr_bitfield::countFlags() const noexcept
{
return rawCountFlags(std::data(flags_), std::size(flags_));
}
size_t tr_bitfield::countFlags(size_t begin, size_t end) const noexcept
{
auto ret = size_t{};
size_t const first_byte = begin >> 3U;
size_t const last_byte = (end - 1) >> 3U;
if (bit_count_ == 0)
{
return 0;
}
if (first_byte >= std::size(flags_))
{
return 0;
}
TR_ASSERT(begin < end);
TR_ASSERT(!std::empty(flags_));
if (first_byte == last_byte)
{
uint8_t val = flags_[first_byte];
auto i = begin & 7U;
val <<= i;
i = (begin - end) & 7U;
val >>= i;
ret = doPopcount(val);
}
else
{
size_t const walk_end = std::min(std::size(flags_), last_byte);
/* first byte */
size_t const first_shift = begin & 7U;
uint8_t val = flags_[first_byte];
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val <<= first_shift;
/* No need to shift back val for correct popcount. */
ret = doPopcount(val);
/* middle bytes */
/* Use 2x accumulators to help alleviate high latency of
popcnt instruction on many architectures. */
size_t tmp_accum = 0;
for (size_t i = first_byte + 1; i < walk_end;)
{
tmp_accum += doPopcount(flags_[i]);
i += 2;
if (i > walk_end)
{
break;
}
ret += doPopcount(flags_[i - 1]);
}
ret += tmp_accum;
/* last byte */
if (last_byte < std::size(flags_))
{
/* -end & 7U. Since bitcount is unsigned do ~end + 1 to
replace -end as linters warn about negating unsigned
types. Any compiler will optimize ~x + 1 to -x in the
backend. */
uint32_t const last_shift = (~end + 1) & 7U;
val = flags_[last_byte];
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val >>= last_shift;
/* No need to shift back val for correct popcount. */
ret += doPopcount(val);
}
}
TR_ASSERT(ret <= (begin - end));
return ret;
}
size_t tr_bitfield::count(size_t begin, size_t end) const
{
if (hasAll())
{
return end - begin;
}
if (hasNone())
{
return 0;
}
return countFlags(begin, end);
}
/***
****
***/
bool tr_bitfield::isValid() const
{
return std::empty(flags_) || true_count_ == countFlags();
}
std::vector<uint8_t> tr_bitfield::raw() const
{
/* Impossible for bit_count_ to exceed SIZE_MAX - 8 */
auto const n = getBytesNeededSafe(bit_count_);
if (!std::empty(flags_))
{
return flags_;
}
auto raw = std::vector<uint8_t>(n);
if (hasAll())
{
setAllTrue(std::data(raw), bit_count_);
}
return raw;
}
void tr_bitfield::ensureBitsAlloced(size_t n)
{
bool const has_all = hasAll();
/* Cant use getBytesNeededSafe as n can be > SIZE_MAX - 8. */
size_t const bytes_needed = has_all ? getBytesNeeded(std::max(n, true_count_)) : getBytesNeeded(n);
if (std::size(flags_) < bytes_needed)
{
flags_.resize(bytes_needed);
if (has_all)
{
setAllTrue(std::data(flags_), true_count_);
}
}
}
bool tr_bitfield::ensureNthBitAlloced(size_t nth)
{
// count is zero-based, so we need to allocate nth+1 bits before setting the nth */
if (nth == SIZE_MAX)
{
return false;
}
ensureBitsAlloced(nth + 1);
return true;
}
void tr_bitfield::freeArray() noexcept
{
flags_ = std::vector<uint8_t>{};
}
void tr_bitfield::setTrueCount(size_t n) noexcept
{
TR_ASSERT(bit_count_ == 0 || n <= bit_count_);
true_count_ = n;
have_all_hint_ = n == bit_count_;
have_none_hint_ = n == 0;
if (hasAll() || hasNone())
{
freeArray();
}
TR_ASSERT(isValid());
}
void tr_bitfield::rebuildTrueCount() noexcept
{
setTrueCount(countFlags());
}
void tr_bitfield::incrementTrueCount(size_t inc) noexcept
{
TR_ASSERT(bit_count_ == 0 || inc <= bit_count_);
TR_ASSERT(bit_count_ == 0 || true_count_ <= bit_count_ - inc);
setTrueCount(true_count_ + inc);
}
void tr_bitfield::decrementTrueCount(size_t dec) noexcept
{
TR_ASSERT(bit_count_ == 0 || dec <= bit_count_);
TR_ASSERT(bit_count_ == 0 || true_count_ >= dec);
setTrueCount(true_count_ - dec);
}
/****
*****
****/
tr_bitfield::tr_bitfield(size_t bit_count)
: bit_count_{ bit_count }
{
TR_ASSERT(isValid());
}
void tr_bitfield::setHasNone() noexcept
{
freeArray();
true_count_ = 0;
have_all_hint_ = false;
have_none_hint_ = true;
TR_ASSERT(isValid());
}
void tr_bitfield::setHasAll() noexcept
{
freeArray();
true_count_ = bit_count_;
have_all_hint_ = true;
have_none_hint_ = false;
TR_ASSERT(isValid());
}
void tr_bitfield::setRaw(uint8_t const* raw, size_t byte_count)
{
flags_.assign(raw, raw + byte_count);
// ensure any excess bits at the end of the array are set to '0'.
if (byte_count == getBytesNeededSafe(bit_count_))
{
auto const excess_bit_count = byte_count * 8 - bit_count_;
TR_ASSERT(excess_bit_count <= 7);
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if (excess_bit_count != 0)
{
flags_.back() &= 0xff << excess_bit_count;
}
}
rebuildTrueCount();
}
void tr_bitfield::setFromBools(bool const* flags, size_t n)
{
size_t trueCount = 0;
freeArray();
ensureBitsAlloced(n);
for (size_t i = 0; i < n; ++i)
{
if (flags[i])
{
++trueCount;
flags_[i >> 3U] |= (0x80 >> (i & 7U));
}
}
setTrueCount(trueCount);
}
void tr_bitfield::set(size_t nth, bool value)
{
if (test(nth) == value)
{
return;
}
if (!ensureNthBitAlloced(nth))
{
return;
}
/* Already tested that val != nth bit so just swap */
auto& byte = flags_[nth >> 3U];
#ifdef TR_ENABLE_ASSERTS
auto const old_byte_pop = doPopcount(byte);
#endif
byte ^= 0x80 >> (nth & 7U);
#ifdef TR_ENABLE_ASSERTS
auto const new_byte_pop = doPopcount(byte);
#endif
if (value)
{
++true_count_;
TR_ASSERT(old_byte_pop + 1 == new_byte_pop);
}
else
{
--true_count_;
TR_ASSERT(new_byte_pop + 1 == old_byte_pop);
}
have_all_hint_ = true_count_ == bit_count_;
have_none_hint_ = true_count_ == 0;
}
/* Sets bit range [begin, end) to 1 */
void tr_bitfield::setSpan(size_t begin, size_t end, bool value)
{
// bounds check
end = std::min(end, bit_count_);
if (end == 0 || begin >= end)
{
return;
}
// NB: count(begin, end) can be quite expensive. Might be worth it
// to fuse the count and set loop
size_t const old_count = count(begin, end);
size_t const new_count = value ? (end - begin) : 0;
// did anything change?
if (old_count == new_count)
{
return;
}
--end;
if (!ensureNthBitAlloced(end))
{
return;
}
auto walk = begin >> 3;
auto const last_byte = end >> 3;
unsigned char first_mask = 0xff >> (begin & 7U);
unsigned char last_mask = 0xff << ((~end) & 7U);
if (value)
{
if (walk == last_byte)
{
flags_[walk] |= first_mask & last_mask;
}
else
{
flags_[walk] |= first_mask;
/* last_byte is expected to be hot in cache due to earlier
count(begin, end) */
flags_[last_byte] |= last_mask;
if (++walk < last_byte)
{
std::fill_n(std::data(flags_) + walk, last_byte - walk, 0xff);
}
}
incrementTrueCount(new_count - old_count);
}
else
{
first_mask = ~first_mask;
last_mask = ~last_mask;
if (walk == last_byte)
{
flags_[walk] &= first_mask | last_mask;
}
else
{
flags_[walk] &= first_mask;
/* last_byte is expected to be hot in cache due to earlier
count(begin, end) */
flags_[last_byte] &= last_mask;
if (++walk < last_byte)
{
std::fill_n(std::data(flags_) + walk, last_byte - walk, 0);
}
}
decrementTrueCount(old_count);
}
}