add function to retrieve length of ziplist

ziplist.c

@@ -388,6 +388,24 @@ unsigned int ziplistCompare(unsigned char *p, unsigned char *entry, unsigned int
     }
 }
 
+/* Return length of ziplist. */
+unsigned int ziplistLen(unsigned char *zl) {
+    unsigned int len = 0;
+    if (ZIPLIST_LENGTH(zl) < ZIP_BIGLEN) {
+        len = ZIPLIST_LENGTH(zl);
+    } else {
+        unsigned char *p = zl+ZIPLIST_HEADER_SIZE;
+        while (*p != ZIP_END) {
+            p += zipRawEntryLength(p);
+            len++;
+        }
+
+        /* Re-store length if small enough */
+        if (len < ZIP_BIGLEN) ZIPLIST_LENGTH(zl) = len;
+    }
+    return len;
+}
+
 void ziplistRepr(unsigned char *zl) {
     unsigned char *p, encoding;
     unsigned int l, lsize;

ziplist.h

@@ -9,4 +9,5 @@ unsigned char *ziplistNext(unsigned char *p);
 unsigned int ziplistGet(unsigned char *p, unsigned char **e, unsigned int *elen, long long *v);
 unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p);
 unsigned char *ziplistDeleteRange(unsigned char *zl, unsigned int index, unsigned int num);
-unsigned int ziplistCompare(unsigned char *p, unsigned char *entry, unsigned int elen);
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+unsigned int ziplistCompare(unsigned char *p, unsigned char *entry, unsigned int elen);
+unsigned int ziplistLen(unsigned char *zl);
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