X-Git-Url: https://git.saurik.com/redis.git/blobdiff_plain/6205b46387ef47e0fcb9bd6c9f371c9eb5cdd7e5..9ae6b0be4ac05c919b943ed29ffa20e5560855a2:/ziplist.c

diff --git a/ziplist.c b/ziplist.c
index 3dd3c1e8..71d965a8 100644
--- a/ziplist.c
+++ b/ziplist.c
@@ -23,6 +23,11 @@
 #include "sds.h"
 #include "ziplist.h"
 
+/* Important note: the ZIP_END value is used to depict the end of the
+ * ziplist structure. When a pointer contains an entry, the first couple
+ * of bytes contain the encoded length of the previous entry. This length
+ * is encoded as ZIP_ENC_RAW length, so the first two bits will contain 00
+ * and the byte will therefore never have a value of 255. */
 #define ZIP_END 255
 #define ZIP_BIGLEN 254
 
@@ -40,11 +45,23 @@
 
 /* Utility macros */
 #define ZIPLIST_BYTES(zl) (*((unsigned int*)(zl)))
-#define ZIPLIST_LENGTH(zl) (*((zl)+sizeof(unsigned int)))
-#define ZIPLIST_HEADER_SIZE (sizeof(unsigned int)+1)
+#define ZIPLIST_TAIL_OFFSET(zl) (*((zl)+sizeof(unsigned int)))
+#define ZIPLIST_LENGTH(zl) (*((zl)+2*sizeof(unsigned int)))
+#define ZIPLIST_HEADER_SIZE (2*sizeof(unsigned int)+1)
+#define ZIPLIST_ENTRY_HEAD(zl) ((zl)+ZIPLIST_HEADER_SIZE)
+#define ZIPLIST_ENTRY_TAIL(zl) ((zl)+ZIPLIST_TAIL_OFFSET(zl))
+#define ZIPLIST_ENTRY_END(zl) ((zl)+ZIPLIST_BYTES(zl)-1)
 #define ZIPLIST_INCR_LENGTH(zl,incr) { \
     if (ZIPLIST_LENGTH(zl) < ZIP_BIGLEN) ZIPLIST_LENGTH(zl)+=incr; }
 
+typedef struct zlentry {
+    unsigned int prevrawlensize, prevrawlen;
+    unsigned int lensize, len;
+    unsigned int headersize;
+    unsigned char encoding;
+    unsigned char *p;
+} zlentry;
+
 /* Return bytes needed to store integer encoded by 'encoding' */
 static unsigned int zipEncodingSize(char encoding) {
     if (encoding == ZIP_ENC_SHORT) {
@@ -116,10 +133,47 @@ static unsigned int zipEncodeLength(unsigned char *p, char encoding, unsigned in
     return len;
 }
 
+/* Decode the length of the previous element stored at "p". */
+static unsigned int zipPrevDecodeLength(unsigned char *p, unsigned int *lensize) {
+    unsigned int len = *p;
+    if (len < ZIP_BIGLEN) {
+        if (lensize) *lensize = 1;
+    } else {
+        if (lensize) *lensize = 1+sizeof(len);
+        memcpy(&len,p+1,sizeof(len));
+    }
+    return len;
+}
+
+/* Encode the length of the previous entry and write it to "p". Return the
+ * number of bytes needed to encode this length if "p" is NULL. */
+static unsigned int zipPrevEncodeLength(unsigned char *p, unsigned int len) {
+    if (p == NULL) {
+        return (len < ZIP_BIGLEN) ? 1 : sizeof(len)+1;
+    } 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 difference in number of bytes needed to store the new length
+ * "len" on the entry pointed to by "p". */
+static int zipPrevLenByteDiff(unsigned char *p, unsigned int len) {
+    unsigned int prevlensize;
+    zipPrevDecodeLength(p,&prevlensize);
+    return zipPrevEncodeLength(NULL,len)-prevlensize;
+}
+
 /* Check if string pointed to by 'entry' can be encoded as an integer.
  * Stores the integer value in 'v' and its encoding in 'encoding'.
  * Warning: this function requires a NULL-terminated string! */
-static int zipTryEncoding(unsigned char *entry, long long *v, char *encoding) {
+static int zipTryEncoding(char *entry, long long *v, char *encoding) {
     long long value;
     char *eptr;
 
@@ -178,11 +232,21 @@ static long long zipLoadInteger(unsigned char *p, char encoding) {
     return ret;
 }
 
-/* Return the total amount used by an entry (encoded length + payload). */
+/* Return a struct with all information about an entry. */
+static zlentry zipEntry(unsigned char *p) {
+    zlentry e;
+    e.prevrawlen = zipPrevDecodeLength(p,&e.prevrawlensize);
+    e.len = zipDecodeLength(p+e.prevrawlensize,&e.lensize);
+    e.headersize = e.prevrawlensize+e.lensize;
+    e.encoding = ZIP_ENCODING(p+e.prevrawlensize);
+    e.p = p;
+    return e;
+}
+
+/* Return the total number of bytes used by the entry at "p". */
 static unsigned int zipRawEntryLength(unsigned char *p) {
-    unsigned int lensize, len;
-    len = zipDecodeLength(p, &lensize);
-    return lensize + len;
+    zlentry e = zipEntry(p);
+    return e.headersize + e.len;
 }
 
 /* Create a new empty ziplist. */
@@ -190,6 +254,7 @@ unsigned char *ziplistNew(void) {
     unsigned int bytes = ZIPLIST_HEADER_SIZE+1;
     unsigned char *zl = zmalloc(bytes);
     ZIPLIST_BYTES(zl) = bytes;
+    ZIPLIST_TAIL_OFFSET(zl) = ZIPLIST_HEADER_SIZE;
     ZIPLIST_LENGTH(zl) = 0;
     zl[bytes-1] = ZIP_END;
     return zl;
@@ -203,189 +268,254 @@ static unsigned char *ziplistResize(unsigned char *zl, unsigned int len) {
     return zl;
 }
 
-static unsigned char *ziplistHead(unsigned char *zl) {
-    return zl+ZIPLIST_HEADER_SIZE;
-}
-
-static unsigned char *ziplistTail(unsigned char *zl) {
-    unsigned char *p, *q;
-    p = q = ziplistHead(zl);
-    while (*p != ZIP_END) {
-        q = p;
+/* Delete "num" entries, starting at "p". Returns pointer to the ziplist. */
+static unsigned char *__ziplistDelete(unsigned char *zl, unsigned char *p, int num) {
+    unsigned int i, totlen, deleted = 0;
+    int nextdiff = 0;
+    zlentry first = zipEntry(p);
+    for (i = 0; p[0] != ZIP_END && i < num; i++) {
         p += zipRawEntryLength(p);
+        deleted++;
     }
-    return q;
+
+    totlen = p-first.p;
+    if (totlen > 0) {
+        if (p[0] != ZIP_END) {
+            /* Tricky: storing the prevlen in this entry might reduce or
+             * increase the number of bytes needed, compared to the current
+             * prevlen. Note that we can always store this length because
+             * it was previously stored by an entry that is being deleted. */
+            nextdiff = zipPrevLenByteDiff(p,first.prevrawlen);
+            zipPrevEncodeLength(p-nextdiff,first.prevrawlen);
+
+            /* Update offset for tail */
+            ZIPLIST_TAIL_OFFSET(zl) -= totlen+nextdiff;
+
+            /* Move tail to the front of the ziplist */
+            memmove(first.p,p-nextdiff,ZIPLIST_BYTES(zl)-(p-zl)-1+nextdiff);
+        } else {
+            /* The entire tail was deleted. No need to move memory. */
+            ZIPLIST_TAIL_OFFSET(zl) = (first.p-zl)-first.prevrawlen;
+        }
+
+        /* Resize and update length */
+        zl = ziplistResize(zl, ZIPLIST_BYTES(zl)-totlen+nextdiff);
+        ZIPLIST_INCR_LENGTH(zl,-deleted);
+    }
+    return zl;
 }
 
-unsigned char *ziplistPush(unsigned char *zl, unsigned char *entry, unsigned int elen, int where) {
-    unsigned int curlen = ZIPLIST_BYTES(zl), reqlen;
-    unsigned char *p;
+/* Insert item at "p". */
+static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, char *s, unsigned int slen) {
+    unsigned int curlen = ZIPLIST_BYTES(zl), reqlen, prevlen = 0;
+    unsigned int offset, nextdiff = 0;
+    unsigned char *tail;
     char encoding = ZIP_ENC_RAW;
     long long value;
+    zlentry entry;
+
+    /* Find out prevlen for the entry that is inserted. */
+    if (p[0] != ZIP_END) {
+        entry = zipEntry(p);
+        prevlen = entry.prevrawlen;
+    } else {
+        tail = ZIPLIST_ENTRY_TAIL(zl);
+        if (tail[0] != ZIP_END) {
+            prevlen = zipRawEntryLength(tail);
+        }
+    }
 
     /* See if the entry can be encoded */
-    if (zipTryEncoding(entry,&value,&encoding)) {
+    if (zipTryEncoding(s,&value,&encoding)) {
         reqlen = zipEncodingSize(encoding);
     } else {
-        reqlen = elen;
+        reqlen = slen;
     }
-    reqlen += zipEncodeLength(NULL,encoding,elen);
 
-    /* Resize the ziplist and move if needed */
-    zl = ziplistResize(zl,curlen+reqlen);
-    if (where == ZIPLIST_HEAD) {
-        p = zl+ZIPLIST_HEADER_SIZE;
-        if (*p != ZIP_END) {
-            /* Subtract one because of the ZIP_END bytes */
-            memmove(p+reqlen,p,curlen-ZIPLIST_HEADER_SIZE-1);
-        }
+    /* We need space for both the length of the previous entry and
+     * the length of the payload. */
+    reqlen += zipPrevEncodeLength(NULL,prevlen);
+    reqlen += zipEncodeLength(NULL,encoding,slen);
+
+    /* When the insert position is not equal to the tail, we need to
+     * make sure that the next entry can hold this entry's length in
+     * its prevlen field. */
+    nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;
+
+    /* Store offset because a realloc may change the address of zl. */
+    offset = p-zl;
+    zl = ziplistResize(zl,curlen+reqlen+nextdiff);
+    p = zl+offset;
+
+    /* Apply memory move when necessary and update tail offset. */
+    if (p[0] != ZIP_END) {
+        /* Subtract one because of the ZIP_END bytes */
+        memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);
+        /* Encode this entry's raw length in the next entry. */
+        zipPrevEncodeLength(p+reqlen,reqlen);
+        /* Update offset for tail */
+        ZIPLIST_TAIL_OFFSET(zl) += reqlen+nextdiff;
     } else {
-        p = zl+curlen-1;
+        /* This element will be the new tail. */
+        ZIPLIST_TAIL_OFFSET(zl) = p-zl;
     }
 
     /* Write the entry */
-    p += zipEncodeLength(p,encoding,elen);
+    p += zipPrevEncodeLength(p,prevlen);
+    p += zipEncodeLength(p,encoding,slen);
     if (encoding != ZIP_ENC_RAW) {
         zipSaveInteger(p,value,encoding);
     } else {
-        memcpy(p,entry,elen);
+        memcpy(p,s,slen);
     }
     ZIPLIST_INCR_LENGTH(zl,1);
     return zl;
 }
 
+unsigned char *ziplistPush(unsigned char *zl, char *s, unsigned int slen, int where) {
+    unsigned char *p;
+    p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
+    return __ziplistInsert(zl,p,s,slen);
+}
+
 unsigned char *ziplistPop(unsigned char *zl, sds *target, int where) {
-    unsigned int curlen = ZIPLIST_BYTES(zl), rawlen;
-    unsigned int len, lensize;
+    zlentry entry;
     unsigned char *p;
     long long value;
     if (target) *target = NULL;
 
     /* Get pointer to element to remove */
-    p = (where == ZIPLIST_HEAD) ? ziplistHead(zl) : ziplistTail(zl);
+    p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_TAIL(zl);
     if (*p == ZIP_END) return zl;
-    len = zipDecodeLength(p,&lensize);
+
+    entry = zipEntry(p);
     if (target) {
-        if (ZIP_ENCODING(p) == ZIP_ENC_RAW) {
-            *target = sdsnewlen(p+lensize,len);
+        if (entry.encoding == ZIP_ENC_RAW) {
+            *target = sdsnewlen(p+entry.headersize,entry.len);
         } else {
-            value = zipLoadInteger(p+lensize,ZIP_ENCODING(p));
+            value = zipLoadInteger(p+entry.headersize,entry.encoding);
             *target = sdscatprintf(sdsempty(), "%lld", value);
         }
     }
 
-    /* Move list to front when popping from the head */
-    rawlen = lensize+len;
-    if (where == ZIPLIST_HEAD) {
-        memmove(p,p+rawlen,curlen-ZIPLIST_HEADER_SIZE-len);
-    }
-
-    /* Resize and update length */
-    zl = ziplistResize(zl,curlen-rawlen);
-    ZIPLIST_INCR_LENGTH(zl,-1);
+    zl = __ziplistDelete(zl,p,1);
     return zl;
 }
 
-/* Returns an offset to use for iterating with ziplistNext. */
-unsigned char *ziplistIndex(unsigned char *zl, unsigned int index) {
-    unsigned char *p = zl+ZIPLIST_HEADER_SIZE;
-    unsigned int i = 0;
-    for (; i < index; i++) {
-        if (*p == ZIP_END) break;
-        p += zipRawEntryLength(p);
+/* Returns an offset to use for iterating with ziplistNext. When the given
+ * index is negative, the list is traversed back to front. When the list
+ * doesn't contain an element at the provided index, NULL is returned. */
+unsigned char *ziplistIndex(unsigned char *zl, int index) {
+    unsigned char *p;
+    zlentry entry;
+    if (index < 0) {
+        index = (-index)-1;
+        p = ZIPLIST_ENTRY_TAIL(zl);
+        if (p[0] != ZIP_END) {
+            entry = zipEntry(p);
+            while (entry.prevrawlen > 0 && index--) {
+                p -= entry.prevrawlen;
+                entry = zipEntry(p);
+            }
+        }
+    } else {
+        p = ZIPLIST_ENTRY_HEAD(zl);
+        while (p[0] != ZIP_END && index--) {
+            p += zipRawEntryLength(p);
+        }
     }
-    return p;
+    return (p[0] == ZIP_END || index > 0) ? NULL : p;
 }
 
 /* Return pointer to next entry in ziplist. */
 unsigned char *ziplistNext(unsigned char *p) {
-    return *p == ZIP_END ? p : p+zipRawEntryLength(p);
+    return (p[0] == ZIP_END) ? NULL : p+zipRawEntryLength(p);
+}
+
+/* Return pointer to previous entry in ziplist. */
+unsigned char *ziplistPrev(unsigned char *p) {
+    zlentry entry = zipEntry(p);
+    return (entry.prevrawlen == 0) ? NULL : p-entry.prevrawlen;
 }
 
 /* Get entry pointer to by 'p' and store in either 'e' or 'v' depending
  * on the encoding of the entry. 'e' is always set to NULL to be able
  * to find out whether the string pointer or the integer value was set.
  * Return 0 if 'p' points to the end of the zipmap, 1 otherwise. */
-unsigned int ziplistGet(unsigned char *p, unsigned char **e, unsigned int *elen, long long *v) {
-    unsigned int len, lensize;
-    if (*p == ZIP_END) return 0;
-    if (e) *e = NULL;
-    len = zipDecodeLength(p,&lensize);
-    if (ZIP_ENCODING(p) == ZIP_ENC_RAW) {
-        if (e) {
-            *elen = len;
-            *e = p+lensize;
+unsigned int ziplistGet(unsigned char *p, char **sstr, unsigned int *slen, long long *sval) {
+    zlentry entry;
+    if (p == NULL || p[0] == ZIP_END) return 0;
+    if (sstr) *sstr = NULL;
+
+    entry = zipEntry(p);
+    if (entry.encoding == ZIP_ENC_RAW) {
+        if (sstr) {
+            *slen = entry.len;
+            *sstr = (char*)p+entry.headersize;
         }
     } else {
-        if (v) {
-            *v = zipLoadInteger(p+lensize,ZIP_ENCODING(p));
+        if (sval) {
+            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);
         }
     }
     return 1;
 }
 
-/* Delete a range of entries from the ziplist. */
-unsigned char *ziplistDeleteRange(unsigned char *zl, unsigned int index, unsigned int num) {
-    unsigned char *p, *first = ziplistIndex(zl, index);
-    unsigned int i, deleted = 0, totlen, newlen;
-    for (p = first, i = 0; *p != ZIP_END && i < num; i++) {
-        p += zipRawEntryLength(p);
-        deleted++;
-    }
-
-    totlen = p-first;
-    if (totlen > 0) {
-        /* Move current tail to the new tail when there *is* a tail */
-        if (*p != ZIP_END) memmove(first,p,ZIPLIST_BYTES(zl)-(p-zl)-1);
-
-        /* Resize and update length */
-        zl = ziplistResize(zl, ZIPLIST_BYTES(zl)-totlen);
-        ZIPLIST_INCR_LENGTH(zl,-deleted);
-    }
-    return zl;
+/* Insert an entry at "p". */
+unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, char *s, unsigned int slen) {
+    return __ziplistInsert(zl,p,s,slen);
 }
 
 /* Delete a single entry from the ziplist, pointed to by *p.
  * Also update *p in place, to be able to iterate over the
  * ziplist, while deleting entries. */
-unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p) {
-    unsigned int offset = *p-zl, tail, len;
-    len = zipRawEntryLength(*p);
-    tail = ZIPLIST_BYTES(zl)-offset-len-1;
-
-    /* Move current tail to the new tail when there *is* a tail */
-    if (tail > 0) memmove(*p,*p+len,tail);
-
-    /* Resize and update length */
-    zl = ziplistResize(zl, ZIPLIST_BYTES(zl)-len);
-    ZIPLIST_INCR_LENGTH(zl,-1);
-
-    /* Store new pointer to current element in p.
-     * This needs to be done because zl can change on realloc. */
-    *p = zl+offset;
+unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p, int direction) {
+    unsigned int offset = *p-zl;
+    zl = __ziplistDelete(zl,*p,1);
+
+    /* Store pointer to current element in p, because ziplistDelete will
+     * do a realloc which might result in a different "zl"-pointer.
+     * When the delete direction is back to front, we might delete the last
+     * entry and end up with "p" pointing to ZIP_END, so check this. */
+    if (*(zl+offset) == ZIP_END && direction == ZIPLIST_HEAD) {
+        *p = ZIPLIST_ENTRY_TAIL(zl);
+    } else {
+        *p = zl+offset;
+    }
     return zl;
 }
 
+/* Delete a range of entries from the ziplist. */
+unsigned char *ziplistDeleteRange(unsigned char *zl, unsigned int index, unsigned int num) {
+    unsigned char *p = ziplistIndex(zl,index);
+    return (p == NULL) ? zl : __ziplistDelete(zl,p,num);
+}
+
 /* Compare entry pointer to by 'p' with 'entry'. Return 1 if equal. */
-unsigned int ziplistCompare(unsigned char *p, unsigned char *entry, unsigned int elen) {
-    unsigned int zlen, lensize;
-    char encoding;
-    long long zval, eval;
-    if (*p == ZIP_END) return 0;
-
-    zlen = zipDecodeLength(p,&lensize);
-    if (zipTryEncoding(entry,&eval,&encoding)) {
-        /* Do integer compare */
-        zval = zipLoadInteger(p+lensize,ZIP_ENCODING(p));
-        return zval == eval;
-    } else {
+unsigned int ziplistCompare(unsigned char *p, char *sstr, unsigned int slen) {
+    zlentry entry;
+    char sencoding;
+    long long val, sval;
+    if (p[0] == ZIP_END) return 0;
+
+    entry = zipEntry(p);
+    if (entry.encoding == ZIP_ENC_RAW) {
         /* Raw compare */
-        if (zlen == elen) {
-            return memcmp(p+lensize,entry,elen) == 0;
+        if (entry.len == slen) {
+            return memcmp(p+entry.headersize,sstr,slen) == 0;
         } else {
             return 0;
         }
+    } else {
+        /* Try to compare encoded values */
+        if (zipTryEncoding(sstr,&sval,&sencoding)) {
+            if (entry.encoding == sencoding) {
+                val = zipLoadInteger(p+entry.headersize,entry.encoding);
+                return val == sval;
+            }
+        }
     }
+    return 0;
 }
 
 /* Return length of ziplist. */
@@ -406,25 +536,28 @@ unsigned int ziplistLen(unsigned char *zl) {
     return len;
 }
 
+/* Return size in bytes of ziplist. */
+unsigned int ziplistSize(unsigned char *zl) {
+    return ZIPLIST_BYTES(zl);
+}
+
 void ziplistRepr(unsigned char *zl) {
-    unsigned char *p, encoding;
-    unsigned int l, lsize;
-    long long value;
+    unsigned char *p;
+    zlentry entry;
 
     printf("{total bytes %d} {length %u}\n",ZIPLIST_BYTES(zl), ZIPLIST_LENGTH(zl));
-    p = ziplistHead(zl);
+    p = ZIPLIST_ENTRY_HEAD(zl);
     while(*p != ZIP_END) {
-        l = zipDecodeLength(p,&lsize);
-        printf("{header %u, payload %u} ",lsize,l);
-        encoding = ZIP_ENCODING(p);
-        p += lsize;
-        if (encoding == ZIP_ENC_RAW) {
-            fwrite(p,l,1,stdout);
+        entry = zipEntry(p);
+        printf("{offset %ld, header %u, payload %u} ",p-zl,entry.headersize,entry.len);
+        p += entry.headersize;
+        if (entry.encoding == ZIP_ENC_RAW) {
+            fwrite(p,entry.len,1,stdout);
         } else {
-            printf("%lld", zipLoadInteger(p,encoding));
+            printf("%lld", zipLoadInteger(p,entry.encoding));
         }
         printf("\n");
-        p += l;
+        p += entry.len;
     }
     printf("{end}\n\n");
 }
@@ -433,10 +566,10 @@ void ziplistRepr(unsigned char *zl) {
 
 unsigned char *createList() {
     unsigned char *zl = ziplistNew();
-    zl = ziplistPush(zl, (unsigned char*)"foo", 3, ZIPLIST_TAIL);
-    zl = ziplistPush(zl, (unsigned char*)"quux", 4, ZIPLIST_TAIL);
-    zl = ziplistPush(zl, (unsigned char*)"hello", 5, ZIPLIST_HEAD);
-    zl = ziplistPush(zl, (unsigned char*)"1024", 4, ZIPLIST_TAIL);
+    zl = ziplistPush(zl, "foo", 3, ZIPLIST_TAIL);
+    zl = ziplistPush(zl, "quux", 4, ZIPLIST_TAIL);
+    zl = ziplistPush(zl, "hello", 5, ZIPLIST_HEAD);
+    zl = ziplistPush(zl, "1024", 4, ZIPLIST_TAIL);
     return zl;
 }
 
@@ -460,7 +593,8 @@ unsigned char *createIntList() {
 }
 
 int main(int argc, char **argv) {
-    unsigned char *zl, *p, *q, *entry;
+    unsigned char *zl, *p;
+    char *entry;
     unsigned int elen;
     long long value;
     sds s;
@@ -479,6 +613,91 @@ int main(int argc, char **argv) {
     printf("Pop head: %s (length %ld)\n", s, sdslen(s));
     ziplistRepr(zl);
 
+    zl = ziplistPop(zl, &s, ZIPLIST_TAIL);
+    printf("Pop tail: %s (length %ld)\n", s, sdslen(s));
+    ziplistRepr(zl);
+
+    zl = ziplistPop(zl, &s, ZIPLIST_TAIL);
+    printf("Pop tail: %s (length %ld)\n", s, sdslen(s));
+    ziplistRepr(zl);
+
+    printf("Get element at index 3:\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, 3);
+        if (!ziplistGet(p, &entry, &elen, &value)) {
+            printf("ERROR: Could not access index 3\n");
+            return 1;
+        }
+        if (entry) {
+            fwrite(entry,elen,1,stdout);
+            printf("\n");
+        } else {
+            printf("%lld\n", value);
+        }
+        printf("\n");
+    }
+
+    printf("Get element at index 4 (out of range):\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, 4);
+        if (p == NULL) {
+            printf("No entry\n");
+        } else {
+            printf("ERROR: Out of range index should return NULL, returned offset: %ld\n", p-zl);
+            return 1;
+        }
+        printf("\n");
+    }
+
+    printf("Get element at index -1 (last element):\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, -1);
+        if (!ziplistGet(p, &entry, &elen, &value)) {
+            printf("ERROR: Could not access index -1\n");
+            return 1;
+        }
+        if (entry) {
+            fwrite(entry,elen,1,stdout);
+            printf("\n");
+        } else {
+            printf("%lld\n", value);
+        }
+        printf("\n");
+    }
+
+    printf("Get element at index -4 (first element):\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, -4);
+        if (!ziplistGet(p, &entry, &elen, &value)) {
+            printf("ERROR: Could not access index -4\n");
+            return 1;
+        }
+        if (entry) {
+            fwrite(entry,elen,1,stdout);
+            printf("\n");
+        } else {
+            printf("%lld\n", value);
+        }
+        printf("\n");
+    }
+
+    printf("Get element at index -5 (reverse out of range):\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, -5);
+        if (p == NULL) {
+            printf("No entry\n");
+        } else {
+            printf("ERROR: Out of range index should return NULL, returned offset: %ld\n", p-zl);
+            return 1;
+        }
+        printf("\n");
+    }
+
     printf("Iterate list from 0 to end:\n");
     {
         zl = createList();
@@ -542,6 +761,40 @@ int main(int argc, char **argv) {
         printf("\n");
     }
 
+    printf("Iterate from back to front:\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, -1);
+        while (ziplistGet(p, &entry, &elen, &value)) {
+            printf("Entry: ");
+            if (entry) {
+                fwrite(entry,elen,1,stdout);
+            } else {
+                printf("%lld", value);
+            }
+            p = ziplistPrev(p);
+            printf("\n");
+        }
+        printf("\n");
+    }
+
+    printf("Iterate from back to front, deleting all items:\n");
+    {
+        zl = createList();
+        p = ziplistIndex(zl, -1);
+        while (ziplistGet(p, &entry, &elen, &value)) {
+            printf("Entry: ");
+            if (entry) {
+                fwrite(entry,elen,1,stdout);
+            } else {
+                printf("%lld", value);
+            }
+            zl = ziplistDelete(zl, &p, ZIPLIST_HEAD);
+            printf("\n");
+        }
+        printf("\n");
+    }
+
     printf("Delete inclusive range 0,0:\n");
     {
         zl = createList();
@@ -584,7 +837,7 @@ int main(int argc, char **argv) {
         while (ziplistGet(p, &entry, &elen, &value)) {
             if (entry && strncmp("foo", entry, elen) == 0) {
                 printf("Delete foo\n");
-                zl = ziplistDelete(zl, &p);
+                zl = ziplistDelete(zl, &p, ZIPLIST_TAIL);
             } else {
                 printf("Entry: ");
                 if (entry) {
@@ -605,22 +858,22 @@ int main(int argc, char **argv) {
         zl = createList();
         p = ziplistIndex(zl, 0);
         if (!ziplistCompare(p,"hello",5)) {
-            printf("ERROR\n");
-            return;
+            printf("ERROR: not \"hello\"\n");
+            return 1;
         }
         if (ziplistCompare(p,"hella",5)) {
-            printf("ERROR\n");
-            return;
+            printf("ERROR: \"hella\"\n");
+            return 1;
         }
 
         p = ziplistIndex(zl, 3);
         if (!ziplistCompare(p,"1024",4)) {
-            printf("ERROR\n");
-            return;
+            printf("ERROR: not \"1024\"\n");
+            return 1;
         }
         if (ziplistCompare(p,"1025",4)) {
-            printf("ERROR\n");
-            return;
+            printf("ERROR: \"1025\"\n");
+            return 1;
         }
         printf("SUCCESS\n");
     }