Data commit

Task/Run-length-encoding/C/run-length-encoding.c

@@ -0,0 +1,147 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct stream_t stream_t, *stream;
+struct stream_t {
+	/* get function 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;
+}