transmission/libtransmission/bitfield.h

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// This file Copyright © Mnemosyne LLC.
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// 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.
#pragma once
#ifndef __TRANSMISSION__
#error only libtransmission should #include this header.
#endif
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#include <cstddef> // size_t
#include <cstdint> // uint8_t
#include <vector> // std::vector
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#include "libtransmission/tr-macros.h" // TR_CONSTEXPR20
/**
* @brief Implementation of the BitTorrent spec's Bitfield array of bits.
*
* This is for tracking the pieces a peer has. Its functionality is like
* a bitset or vector<bool> with some added use cases:
*
* - It needs to be able to read/write the left-to-right bitfield format
* specified in the bittorrent spec. This is what raw() and getRaw()
* are for.
*
* - "Have all" is a special case where we know the peer has all the
* pieces and don't need to check the bit array. This is useful since
* (a) it's very common (i.e. seeds) and saves memory and work of
* allocating a bit array and doing lookups, and (b) if we have a
* magnet link and haven't gotten the metainfo yet, we may not know
* how many pieces there are -- but we can still know "this peer has
* all of them".
*
* - "Have none" is another special case that has the same advantages
* and motivations as "Have all".
*/
class tr_bitfield
{
public:
explicit tr_bitfield(size_t bit_count);
void set_has_all() noexcept;
void set_has_none() noexcept;
// set one or more bits
void set(size_t nth, bool value = true);
void set_span(size_t begin, size_t end, bool value = true);
void unset(size_t bit)
{
set(bit, false);
}
void unset_span(size_t begin, size_t end)
{
set_span(begin, end, false);
}
void set_from_bools(bool const* flags, size_t n);
// "raw" here is in BEP0003 format: "The first byte of the bitfield
// corresponds to indices 0 - 7 from high bit to low bit, respectively.
// The next one 8-15, etc. Spare bits at the end are set to zero."
void set_raw(uint8_t const* raw, size_t byte_count);
[[nodiscard]] std::vector<uint8_t> raw() const;
[[nodiscard]] constexpr bool has_all() const noexcept
{
return have_all_hint_ || (bit_count_ > 0 && bit_count_ == true_count_);
}
[[nodiscard]] constexpr bool has_none() const noexcept
{
return have_none_hint_ || (bit_count_ > 0 && true_count_ == 0);
}
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[[nodiscard]] TR_CONSTEXPR20 bool test(size_t bit) const
{
return has_all() || (!has_none() && test_flag(bit));
}
[[nodiscard]] constexpr size_t count() const noexcept
{
return true_count_;
}
[[nodiscard]] size_t count(size_t begin, size_t end) const;
[[nodiscard]] constexpr size_t size() const noexcept
{
return bit_count_;
}
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[[nodiscard]] constexpr bool empty() const noexcept
{
return size() == 0;
}
[[nodiscard]] bool is_valid() const;
[[nodiscard]] constexpr auto percent() const noexcept
{
if (has_all())
{
return 1.0F;
}
if (has_none() || empty())
{
return 0.0F;
}
return static_cast<float>(count()) / size();
}
tr_bitfield& operator|=(tr_bitfield const& that) noexcept;
tr_bitfield& operator&=(tr_bitfield const& that) noexcept;
[[nodiscard]] bool intersects(tr_bitfield const& that) const noexcept;
private:
[[nodiscard]] size_t count_flags() const noexcept;
[[nodiscard]] size_t count_flags(size_t begin, size_t end) const noexcept;
[[nodiscard]] TR_CONSTEXPR20 bool test_flag(size_t n) const
{
if (n >> 3U >= std::size(flags_))
{
return false;
}
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return (flags_[n >> 3U] << (n & 7U) & 0x80) != 0;
}
void ensure_bits_alloced(size_t n);
[[nodiscard]] bool ensure_nth_bit_alloced(size_t nth);
void free_array() noexcept
{
// move-assign to ensure the reserve memory is cleared
flags_ = std::vector<uint8_t>{};
}
void increment_true_count(size_t inc) noexcept;
void decrement_true_count(size_t dec) noexcept;
void set_true_count(size_t n) noexcept;
void rebuild_true_count() noexcept
{
set_true_count(count_flags());
}
std::vector<uint8_t> flags_;
size_t bit_count_ = 0;
size_t true_count_ = 0;
/* Special cases for when full or empty but we don't know the bitCount.
This occurs when a magnet link's peers send have all / have none */
bool have_all_hint_ = false;
bool have_none_hint_ = false;
};