Function Decode

include/bmpread/bmpread.c:813–885

Selects an above decoder and runs it for each scan line of the file. * Returns 0 if there's an error or 1 if it's gravy. */

 * Returns 0 if there's an error or 1 if it's gravy.
 */
static int Decode(read_context * p_ctx)
{
    void (* decoder)(uint8_t *, const uint8_t *, const uint8_t *,
                     const read_context *);

    uint8_t * p_out;      /* Pointer to current scan line in output buffer. */
    uint8_t * p_out_end;  /* End marker for output buffer. */
    uint8_t * p_line_end; /* Pointer to end of current scan line in output. */

    /* out_inc is an incrementor for p_out to advance it one scan line.  I'm
     * not exactly sure what the correct type for it would be, perhaps ssize_t,
     * but that's not C standard.  I went with ptrdiff_t because its value
     * will be equivalent to the difference between two pointers, whether it
     * was derived that way or not.
     */
    ptrdiff_t out_inc;

    /* Double check this won't overflow.  Who knows, man. */
#if SIZE_MAX > PTRDIFF_MAX
    if(p_ctx->out_line_len > PTRDIFF_MAX) return 0;
#endif
    out_inc = p_ctx->out_line_len;

    if(!(p_ctx->info.height < 0) == !(p_ctx->flags & BMPREAD_TOP_DOWN))
    {
        /* We're keeping scan lines in order.  These and subsequent operations
         * have all been checked earlier.
         */
        p_out     = p_ctx->data_out;
        p_out_end = p_ctx->data_out +
                    ((size_t)p_ctx->lines * p_ctx->out_line_len);
    }
    else /* We're reversing scan lines. */
    {
        /* TODO: I'm not 100% sure about the legality, purely C spec-wise, of
         * this subtraction.
         */
        p_out_end = p_ctx->data_out - p_ctx->out_line_len;
        p_out     = p_ctx->data_out +
                    (((size_t)p_ctx->lines - 1) * p_ctx->out_line_len);

        /* Always safe, given two's complement, since it was positive. */
        out_inc = -out_inc;
    }

    p_line_end = p_out + (size_t)p_ctx->info.width * p_ctx->out_channels;

    switch(p_ctx->info.bits)
    {
        case 32: decoder = Decode32; break;
        case 24: decoder = Decode24; break;
        case 16: decoder = Decode16; break;
        case 8:  decoder = Decode8;  break;
        case 4:  decoder = Decode4;  break;
        case 1:  decoder = Decode1;  break;
        default: return 0;
    }