The R Project SVN R

///////////////////////////////////////////////////////////////////////////////
//
/// \file       vli_decoder.c
/// \brief      Decodes variable-length integers
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////

#include "common.h"


extern LZMA_API(lzma_ret)
lzma_vli_decode(lzma_vli *restrict vli, size_t *vli_pos,
        const uint8_t *restrict in, size_t *restrict in_pos,
        size_t in_size)
{
    // If we haven't been given vli_pos, work in single-call mode.
    size_t vli_pos_internal = 0;
    if (vli_pos == NULL) {
        vli_pos = &vli_pos_internal;
        *vli = 0;

        // If there's no input, use LZMA_DATA_ERROR. This way it is
        // easy to decode VLIs from buffers that have known size,
        // and get the correct error code in case the buffer is
        // too short.
        if (*in_pos >= in_size)
            return LZMA_DATA_ERROR;

    } else {
        // Initialize *vli when starting to decode a new integer.
        if (*vli_pos == 0)
            *vli = 0;

        // Validate the arguments.
        if (*vli_pos >= LZMA_VLI_BYTES_MAX
                || (*vli >> (*vli_pos * 7)) != 0)
            return LZMA_PROG_ERROR;;

        if (*in_pos >= in_size)
            return LZMA_BUF_ERROR;
    }

    do {
        // Read the next byte. Use a temporary variable so that we
        // can update *in_pos immediately.
        const uint8_t byte = in[*in_pos];
        ++*in_pos;

        // Add the newly read byte to *vli.
        *vli += (lzma_vli)(byte & 0x7F) << (*vli_pos * 7);
        ++*vli_pos;

        // Check if this is the last byte of a multibyte integer.
        if ((byte & 0x80) == 0) {
            // We don't allow using variable-length integers as
            // padding i.e. the encoding must use the most the
            // compact form.
            if (byte == 0x00 && *vli_pos > 1)
                return LZMA_DATA_ERROR;

            return vli_pos == &vli_pos_internal
                    ? LZMA_OK : LZMA_STREAM_END;
        }

        // There is at least one more byte coming. If we have already
        // read maximum number of bytes, the integer is considered
        // corrupt.
        //
        // If we need bigger integers in future, old versions liblzma
        // will confusingly indicate the file being corrupt istead of
        // unsupported. I suppose it's still better this way, because
        // in the foreseeable future (writing this in 2008) the only
        // reason why files would appear having over 63-bit integers
        // is that the files are simply corrupt.
        if (*vli_pos == LZMA_VLI_BYTES_MAX)
            return LZMA_DATA_ERROR;

    } while (*in_pos < in_size);

    return vli_pos == &vli_pos_internal ? LZMA_DATA_ERROR : LZMA_OK;
}