rle.c

Go to the documentation of this file.

#include "rle.h"


int rle(unsigned char *output, const unsigned char *input, int len)
{
    int count, index, i;
    unsigned char first;
    unsigned char *out;


    out = output;
    count = 0;
    while (count < len)
    {
        index = count;
        first = input[index++];

        /* Scan for bytes identical to the first one */
        while ((index < len) && (index - count < 127) && (input[index] == first))
            index++;

        if (index - count == 1)
        {
            /* Failed to "replicate" the current byte. See how many to copy.
             */
            while ((index < len) && (index - count < 127))
            {
                /* Avoid a replicate run of only 2-bytes after a literal run.
                 * There is no gain in this, and there is a risc of loss if the
                 * run after the two identical bytes is another literal run.
                 * So search for 3 identical bytes.
                 */
                if ((input[index] == input[index - 1]) &&
                    ((index > 1) && (input[index] == input[index - 2])))
                {
                    /* Reset the index so we can back up these three identical
                     * bytes in the next run.
                     */
                    index -= 2;
                    break;
                }

                index++;
            }

            /* Output a run of uncompressed bytes: write length and values */
            *out++ = (unsigned char)(count - index);
            for (i = count; i < index; i++)
                *out++ = input[i];
        }
        else
        {
            /* Output a compressed run: write length and value */
            *out++ = (unsigned char)(index - count);
            *out++ = first;
        }

        count = index;
    }

    /* Output EOF marker */
    *out++ = 0;

    return (out - output);
}


int unrle(unsigned char *output, const unsigned char *input)
{
    signed char count;
    unsigned char *out;
    unsigned char value;


    out = output;

    for (;;)
    {
        count = (signed char)*input++;
        if (count > 0)
        {
            /* replicate run */
            value = *input++;
            while (count--)
                *out++ = value;
        }
        else if (count < 0)
        {
            /* literal run */
            while (count++)
                *out++ = *input++;
        }
        else
            /* EOF */
            break;
    }

    return (out - output);
}