Anonymous user
Run-length encoding: Difference between revisions
→{{header|C}}: semi serious method
(Added K Version) |
(→{{header|C}}: semi serious method) |
||
Line 361:
=={{header|C}}==
Encoder that can deal with byte streams. Can encode/decode any byte values and any length with reasonable efficiency. Also showing OO and polymophism with structs.
<lang C>#include <stdio.h>
#include <stdlib.h>
typedef struct stream_t stream_t, *stream;
struct stream_t {
/* get funciton is supposed to return a byte value (0-255),
or -1 to signify end of input */
int (*get)(stream);
/* put function does output, one byte at a time */
int (*put)(stream, int);
};
/* next two structs inherit from stream_t */
typedef struct {
int (*get)(stream);
int (*put)(stream, int);
char *string;
int pos;
} string_stream;
typedef struct {
int (*get)(stream);
int (*put)(stream, int);
FILE *fp;
} file_stream;
/* methods for above streams */
int sget(stream in)
{
int c;
string_stream* s = (string_stream*) in;
c = (unsigned char)(s->string[s->pos]);
if (c == '\0') return -1;
s->pos++;
return c;
}
int sput(stream out, int c)
{
string_stream* s = (string_stream*) out;
s->string[s->pos++] = (c == -1) ? '\0' : c;
if (c == -1) s->pos = 0;
return 0;
}
int file_put(stream out, int c)
{
file_stream *f = (file_stream*) out;
return fputc(c, f->fp);
}
/* helper function */
void output(stream out, unsigned char* buf, int len)
{
int i;
out->put(out, 128 + len);
for (i = 0; i < len; i++)
out->put(out, buf[i]);
}
/* Specification: encoded stream are unsigned bytes consisting of sequences.
* First byte of each sequence is the length, followed by a number of bytes.
* If length <=128, the next byte is to be repeated length times;
* If length > 128, the next (length - 128) bytes are not repeated.
* this is to improve efficiency for long non-repeating sequences.
* This scheme can encode arbitrary byte values efficiently.
* c.f. Adobe PDF spec RLE stream encoding (not exactly the same)
*/
void encode(stream in, stream out)
{
unsigned char buf[256];
int len = 0, repeat = 0, end = 0, c;
int (*get)(stream) = in->get;
int (*put)(stream, int) = out->put;
while (!end) {
end = ((c = get(in)) == -1);
if (!end) {
buf[len++] = c;
if (len <= 1) continue;
}
if (repeat) {
if (buf[len - 1] != buf[len - 2])
repeat = 0;
if (!repeat || len == 129 || end) {
/* write out repeating bytes */
put(out, end ? len : len - 1);
put(out, buf[0]);
buf[0] = buf[len - 1];
len = 1;
}
} else {
if (buf[len - 1] == buf[len - 2]) {
repeat = 1;
if (len > 2) {
output(out, buf, len - 2);
buf[0] = buf[1] = buf[len - 1];
len = 2;
}
continue;
}
if (len == 128 || end) {
output(out, buf, len);
len = 0;
repeat = 0;
}
}
}
put(out, -1);
}
void decode(stream in, stream out)
{
int c, i, cnt;
while (1) {
c = in->get(in);
if (c == -1) return;
if (c > 128) {
cnt = c - 128;
for (i = 0; i < cnt; i++)
out->put(out, in->get(in));
} else {
cnt = c;
c = in->get(in);
for (i = 0; i < cnt; i++)
out->put(out, c);
}
}
}
int main()
{
char buf[256];
string_stream str_in = { sget, 0,
"WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW", 0};
string_stream str_out = { sget, sput, buf, 0 };
file_stream file = { 0, file_put, stdout };
/* encode from str_in to str_out */
encode((stream)&str_in, (stream)&str_out);
/* decode from str_out to file (stdout) */
decode((stream)&str_out, (stream)&file);
return 0;
}</lang>
See [[Run-length encoding/C]]
| |||