implement dn_comp RFC 1035 domain name compression

the input name is validated, the other parameters are assumed to be
valid (the list of already compressed names are not checked for
infinite reference loops or out-of-bound offsets).

names are handled case-sensitively for now.

src/network/dn_comp.c

+#include <string.h>
 #include <resolv.h>
 #include "libc.h"
 
+/* RFC 1035 message compression */
+
+/* label start offsets of a compressed domain name s */
+static int getoffs(short *offs, const unsigned char *base, const unsigned char *s)
+{
+	int i=0;
+	for (;;) {
+		while (*s & 0xc0) {
+			if ((*s & 0xc0) != 0xc0) return 0;
+			s = base + ((s[0]&0x3f)<<8 | s[1]);
+		}
+		if (!*s) return i;
+		if (s-base >= 0x4000) return 0;
+		offs[i++] = s-base;
+		s += *s + 1;
+	}
+}
+
+/* label lengths of an ascii domain name s */
+static int getlens(unsigned char *lens, const char *s, int l)
+{
+	int i=0,j=0,k=0;
+	for (;;) {
+		for (; j<l && s[j]!='.'; j++);
+		if (j-k-1u > 62) return 0;
+		lens[i++] = j-k;
+		if (j==l) return i;
+		k = ++j;
+	}
+}
+
+/* longest suffix match of an ascii domain with a compressed domain name dn */
+static int match(int *offset, const unsigned char *base, const unsigned char *dn,
+	const char *end, const unsigned char *lens, int nlen)
+{
+	int l, o, m=0;
+	short offs[128];
+	int noff = getoffs(offs, base, dn);
+	if (!noff) return 0;
+	for (;;) {
+		l = lens[--nlen];
+		o = offs[--noff];
+		end -= l;
+		if (l != base[o] || memcmp(base+o+1, end, l))
+			return m;
+		*offset = o;
+		m += l;
+		if (nlen) m++;
+		if (!nlen || !noff) return m;
+		end--;
+	}
+}
+
 int __dn_comp(const char *src, unsigned char *dst, int space, unsigned char **dnptrs, unsigned char **lastdnptr)
 {
-	return -1;
+	int i, j, n, m=0, offset, bestlen=0, bestoff;
+	unsigned char lens[127];
+	unsigned char **p;
+	const char *end;
+	size_t l = strnlen(src, 255);
+	if (l && src[l-1] == '.') l--;
+	if (l>253 || space<=0) return -1;
+	if (!l) {
+		*dst = 0;
+		return 1;
+	}
+	end = src+l;
+	n = getlens(lens, src, l);
+	if (!n) return -1;
+
+	p = dnptrs;
+	if (p && *p) for (p++; *p; p++) {
+		m = match(&offset, *dnptrs, *p, end, lens, n);
+		if (m > bestlen) {
+			bestlen = m;
+			bestoff = offset;
+			if (m == l)
+				break;
+		}
+	}
+
+	/* encode unmatched part */
+	if (space < l-bestlen+2+(bestlen-1 < l-1)) return -1;
+	memcpy(dst+1, src, l-bestlen);
+	for (i=j=0; i<l-bestlen; i+=lens[j++]+1)
+		dst[i] = lens[j];
+
+	/* add tail */
+	if (bestlen) {
+		dst[i++] = 0xc0 | bestoff>>8;
+		dst[i++] = bestoff;
+	} else
+		dst[i++] = 0;
+
+	/* save dst pointer */
+	if (i>2 && lastdnptr && dnptrs && *dnptrs) {
+		while (*p) p++;
+		if (p+1 < lastdnptr) {
+			*p++ = dst;
+			*p=0;
+		}
+	}
+	return i;
 }
 
 weak_alias(__dn_comp, dn_comp);