transmission/libtransmission/block-info.cc

/*
 * This file Copyright (C) 2007-2014 Mnemosyne LLC
 *
 * It may be used under the GNU GPL versions 2 or 3
 * or any future license endorsed by Mnemosyne LLC.
 *
 */

#include <algorithm>

#include <event2/util.h>

#include "transmission.h"

#include "block-info.h"
#include "tr-assert.h"

// Decide on a block size. Constraints:
// (1) most clients decline requests over 16 KiB
// (2) pieceSize must be a multiple of block size
uint32_t tr_block_info::bestBlockSize(uint64_t piece_size)
{
    uint32_t b = piece_size;

    auto constexpr MaxBlockSize = uint32_t{ 1024 * 16 };
    while (b > MaxBlockSize)
    {
        b /= 2U;
    }

    if (b == 0 || piece_size % b != 0) // not cleanly divisible
    {
        return 0;
    }

    return b;
}

void tr_block_info::initSizes(uint64_t total_size_in, uint64_t piece_size_in)
{
    total_size = total_size_in;
    piece_size = piece_size_in;
    block_size = bestBlockSize(piece_size);

    if (piece_size == 0 || block_size == 0)
    {
        *this = {};
        return;
    }

    n_pieces = (total_size + piece_size - 1) / piece_size;

    auto remainder = total_size % piece_size;
    final_piece_size = remainder ? remainder : piece_size;

    remainder = total_size % block_size;
    final_block_size = remainder ? remainder : block_size;

    if (block_size != 0)
    {
        n_blocks = (total_size + block_size - 1) / block_size;
        n_blocks_in_piece = piece_size / block_size;
        n_blocks_in_final_piece = (final_piece_size + block_size - 1) / block_size;
    }

#ifdef TR_ENABLE_ASSERTS
    // check our work
    if (block_size != 0)
    {
        TR_ASSERT(piece_size % block_size == 0);
    }

    uint64_t t = n_pieces - 1;
    t *= piece_size;
    t += final_piece_size;
    TR_ASSERT(t == total_size);

    t = n_blocks - 1;
    t *= block_size;
    t += final_block_size;
    TR_ASSERT(t == total_size);

    t = n_pieces - 1;
    t *= n_blocks_in_piece;
    t += n_blocks_in_final_piece;
    TR_ASSERT(t == n_blocks);
#endif
}