extracted general methods to zip.c for reuse in other zip* structures

zip.c

@@ -0,0 +1,46 @@
+#define ZIP_BIGLEN 254
+#define ZIP_END 255
+
+/* The following macro returns the number of bytes needed to encode the length
+ * for the integer value _l, that is, 1 byte for lengths < ZIP_BIGLEN and
+ * 5 bytes for all the other lengths. */
+#define ZIP_LEN_BYTES(_l) (((_l) < ZIP_BIGLEN) ? 1 : sizeof(unsigned int)+1)
+
+/* Decode the encoded length pointed by 'p' */
+static unsigned int zipDecodeLength(unsigned char *p) {
+    unsigned int len = *p;
+
+    if (len < ZIP_BIGLEN) return len;
+    memcpy(&len,p+1,sizeof(unsigned int));
+    return len;
+}
+
+/* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
+ * the amount of bytes required to encode such a length. */
+static unsigned int zipEncodeLength(unsigned char *p, unsigned int len) {
+    if (p == NULL) {
+        return ZIP_LEN_BYTES(len);
+    } else {
+        if (len < ZIP_BIGLEN) {
+            p[0] = len;
+            return 1;
+        } else {
+            p[0] = ZIP_BIGLEN;
+            memcpy(p+1,&len,sizeof(len));
+            return 1+sizeof(len);
+        }
+    }
+}
+
+/* Return the total amount used by an entry (encoded length + payload). */
+static unsigned int zipRawEntryLength(unsigned char *p) {
+    unsigned int l = zipDecodeLength(p);
+    return zipEncodeLength(NULL,l) + l;
+}
+
+/* Resize the zip* structure. */
+static unsigned char *zipResize(unsigned char *z, unsigned int len) {
+    z = zrealloc(z,len);
+    z[len-1] = ZIP_END;
+    return z;
+}