src/ziplist.c

/* The ziplist is a specially encoded dually linked list that is designed
 * to be very memory efficient. It stores both strings and integer values,
 * where integers are encoded as actual integers instead of a series of
 * characters. It allows push and pop operations on either side of the list
 * in O(1) time. However, because every operation requires a reallocation of
 * the memory used by the ziplist, the actual complexity is related to the
 * amount of memory used by the ziplist.
 *
 * ----------------------------------------------------------------------------
 *
 * ZIPLIST OVERALL LAYOUT:
 * The general layout of the ziplist is as follows:
 * <zlbytes><zltail><zllen><entry><entry><zlend>
 *
 * <zlbytes> is an unsigned integer to hold the number of bytes that the
 * ziplist occupies. This value needs to be stored to be able to resize the
 * entire structure without the need to traverse it first.
 *
 * <zltail> is the offset to the last entry in the list. This allows a pop
 * operation on the far side of the list without the need for full traversal.
 *
 * <zllen> is the number of entries.When this value is larger than 2**16-2,
 * we need to traverse the entire list to know how many items it holds.
 *
 * <zlend> is a single byte special value, equal to 255, which indicates the
 * end of the list.
 *
 * ZIPLIST ENTRIES:
 * Every entry in the ziplist is prefixed by a header that contains two pieces
 * of information. First, the length of the previous entry is stored to be
 * able to traverse the list from back to front. Second, the encoding with an
 * optional string length of the entry itself is stored.
 *
 * The length of the previous entry is encoded in the following way:
 * If this length is smaller than 254 bytes, it will only consume a single
 * byte that takes the length as value. When the length is greater than or
 * equal to 254, it will consume 5 bytes. The first byte is set to 254 to
 * indicate a larger value is following. The remaining 4 bytes take the
 * length of the previous entry as value.
 *
 * The other header field of the entry itself depends on the contents of the
 * entry. When the entry is a string, the first 2 bits of this header will hold
 * the type of encoding used to store the length of the string, followed by the
 * actual length of the string. When the entry is an integer the first 2 bits
 * are both set to 1. The following 2 bits are used to specify what kind of
 * integer will be stored after this header. An overview of the different
 * types and encodings is as follows:
 *
 * |00pppppp| - 1 byte
 *      String value with length less than or equal to 63 bytes (6 bits).
 * |01pppppp|qqqqqqqq| - 2 bytes
 *      String value with length less than or equal to 16383 bytes (14 bits).
 * |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
 *      String value with length greater than or equal to 16384 bytes.
 * |1100____| - 1 byte
 *      Integer encoded as int16_t (2 bytes).
 * |1101____| - 1 byte
 *      Integer encoded as int32_t (4 bytes).
 * |1110____| - 1 byte
 *      Integer encoded as int64_t (8 bytes).
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <assert.h>
#include <limits.h>
#include "zmalloc.h"
#include "ziplist.h"

int ll2string(char *s, size_t len, long long value);

#define ZIP_END 255
#define ZIP_BIGLEN 254

/* Different encoding/length possibilities */
#define ZIP_STR_06B (0 << 6)
#define ZIP_STR_14B (1 << 6)
#define ZIP_STR_32B (2 << 6)
#define ZIP_INT_16B (0xc0 | 0<<4)
#define ZIP_INT_32B (0xc0 | 1<<4)
#define ZIP_INT_64B (0xc0 | 2<<4)

/* Macro's to determine type */
#define ZIP_IS_STR(enc) (((enc) & 0xc0) < 0xc0)
#define ZIP_IS_INT(enc) (!ZIP_IS_STR(enc) && ((enc) & 0x30) < 0x30)

/* Utility macros */
#define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))
#define ZIPLIST_TAIL_OFFSET(zl) (*((uint32_t*)((zl)+sizeof(uint32_t))))
#define ZIPLIST_LENGTH(zl)      (*((uint16_t*)((zl)+sizeof(uint32_t)*2)))
#define ZIPLIST_HEADER_SIZE     (sizeof(uint32_t)*2+sizeof(uint16_t))
#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)

/* We know a positive increment can only be 1 because entries can only be
 * pushed one at a time. */
#define ZIPLIST_INCR_LENGTH(zl,incr) { \
    if (ZIPLIST_LENGTH(zl) < UINT16_MAX) 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 the encoding pointer to by 'p'. */
static unsigned int zipEntryEncoding(unsigned char *p) {
    /* String encoding: 2 MSBs */
    unsigned char b = p[0] & 0xc0;
    if (b < 0xc0) {
        return b;
    } else {
        /* Integer encoding: 4 MSBs */
        return p[0] & 0xf0;
    }
    assert(NULL);
}

/* Return bytes needed to store integer encoded by 'encoding' */
static unsigned int zipIntSize(unsigned char encoding) {
    switch(encoding) {
    case ZIP_INT_16B: return sizeof(int16_t);
    case ZIP_INT_32B: return sizeof(int32_t);
    case ZIP_INT_64B: return sizeof(int64_t);
    }
    assert(NULL);
}

/* Decode the encoded length pointed by 'p'. If a pointer to 'lensize' is
 * provided, it is set to the number of bytes required to encode the length. */
static unsigned int zipDecodeLength(unsigned char *p, unsigned int *lensize) {
    unsigned char encoding = zipEntryEncoding(p);
    unsigned int len;

    if (ZIP_IS_STR(encoding)) {
        switch(encoding) {
        case ZIP_STR_06B:
            len = p[0] & 0x3f;
            if (lensize) *lensize = 1;
            break;
        case ZIP_STR_14B:
            len = ((p[0] & 0x3f) << 8) | p[1];
            if (lensize) *lensize = 2;
            break;
        case ZIP_STR_32B:
            len = (p[1] << 24) | (p[2] << 16) | (p[3] << 8) | p[4];
            if (lensize) *lensize = 5;
            break;
        default:
            assert(NULL);
        }
    } else {
        len = zipIntSize(encoding);
        if (lensize) *lensize = 1;
    }
    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 char encoding, unsigned int rawlen) {
    unsigned char len = 1, buf[5];

    if (ZIP_IS_STR(encoding)) {
        /* Although encoding is given it may not be set for strings,
         * so we determine it here using the raw length. */
        if (rawlen <= 0x3f) {
            if (!p) return len;
            buf[0] = ZIP_STR_06B | rawlen;
        } else if (rawlen <= 0x3fff) {
            len += 1;
            if (!p) return len;
            buf[0] = ZIP_STR_14B | ((rawlen >> 8) & 0x3f);
            buf[1] = rawlen & 0xff;
        } else {
            len += 4;
            if (!p) return len;
            buf[0] = ZIP_STR_32B;
            buf[1] = (rawlen >> 24) & 0xff;
            buf[2] = (rawlen >> 16) & 0xff;
            buf[3] = (rawlen >> 8) & 0xff;
            buf[4] = rawlen & 0xff;
        }
    } else {
        /* Implies integer encoding, so length is always 1. */
        if (!p) return len;
        buf[0] = encoding;
    }

    /* Store this length at p */
    memcpy(p,buf,len);
    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);
        }
    }
}

/* Encode the length of the previous entry and write it to "p". This only
 * uses the larger encoding (required in __ziplistCascadeUpdate). */
static void zipPrevEncodeLengthForceLarge(unsigned char *p, unsigned int len) {
    if (p == NULL) return;
    p[0] = ZIP_BIGLEN;
    memcpy(p+1,&len,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'. */
static int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long *v, unsigned char *encoding) {
    long long value;
    char *eptr;
    char buf[32];

    if (entrylen >= 32 || entrylen == 0) return 0;
    if (entry[0] == '-' || (entry[0] >= '0' && entry[0] <= '9')) {
        int slen;

        /* Perform a back-and-forth conversion to make sure that
         * the string turned into an integer is not losing any info. */
        memcpy(buf,entry,entrylen);
        buf[entrylen] = '\0';
        value = strtoll(buf,&eptr,10);
        if (eptr[0] != '\0') return 0;
        slen = ll2string(buf,32,value);
        if (entrylen != (unsigned)slen || memcmp(buf,entry,slen)) return 0;

        /* Great, the string can be encoded. Check what's the smallest
         * of our encoding types that can hold this value. */
        if (value >= INT16_MIN && value <= INT16_MAX) {
            *encoding = ZIP_INT_16B;
        } else if (value >= INT32_MIN && value <= INT32_MAX) {
            *encoding = ZIP_INT_32B;
        } else {
            *encoding = ZIP_INT_64B;
        }
        *v = value;
        return 1;
    }
    return 0;
}

/* Store integer 'value' at 'p', encoded as 'encoding' */
static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encoding) {
    int16_t i16;
    int32_t i32;
    int64_t i64;
    if (encoding == ZIP_INT_16B) {
        i16 = value;
        memcpy(p,&i16,sizeof(i16));
    } else if (encoding == ZIP_INT_32B) {
        i32 = value;
        memcpy(p,&i32,sizeof(i32));
    } else if (encoding == ZIP_INT_64B) {
        i64 = value;
        memcpy(p,&i64,sizeof(i64));
    } else {
        assert(NULL);
    }
}

/* Read integer encoded as 'encoding' from 'p' */
static int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) {
    int16_t i16;
    int32_t i32;
    int64_t i64, ret;
    if (encoding == ZIP_INT_16B) {
        memcpy(&i16,p,sizeof(i16));
        ret = i16;
    } else if (encoding == ZIP_INT_32B) {
        memcpy(&i32,p,sizeof(i32));
        ret = i32;
    } else if (encoding == ZIP_INT_64B) {
        memcpy(&i64,p,sizeof(i64));
        ret = i64;
    } else {
        assert(NULL);
    }
    return ret;
}

/* 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 = zipEntryEncoding(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) {
    zlentry e = zipEntry(p);
    return e.headersize + e.len;
}

/* Create a new empty ziplist. */
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;
}

/* Resize the ziplist. */
static unsigned char *ziplistResize(unsigned char *zl, unsigned int len) {
    zl = zrealloc(zl,len);
    ZIPLIST_BYTES(zl) = len;
    zl[len-1] = ZIP_END;
    return zl;
}

/* When an entry is inserted, we need to set the prevlen field of the next
 * entry to equal the length of the inserted entry. It can occur that this
 * length cannot be encoded in 1 byte and the next entry needs to be grow
 * a bit larger to hold the 5-byte encoded prevlen. This can be done for free,
 * because this only happens when an entry is already being inserted (which
 * causes a realloc and memmove). However, encoding the prevlen may require
 * that this entry is grown as well. This effect may cascade throughout
 * the ziplist when there are consecutive entries with a size close to
 * ZIP_BIGLEN, so we need to check that the prevlen can be encoded in every
 * consecutive entry.
 *
 * Note that this effect can also happen in reverse, where the bytes required
 * to encode the prevlen field can shrink. This effect is deliberately ignored,
 * because it can cause a "flapping" effect where a chain prevlen fields is
 * first grown and then shrunk again after consecutive inserts. Rather, the
 * field is allowed to stay larger than necessary, because a large prevlen
 * field implies the ziplist is holding large entries anyway.
 *
 * The pointer "p" points to the first entry that does NOT need to be
 * updated, i.e. consecutive fields MAY need an update. */
static unsigned char *__ziplistCascadeUpdate(unsigned char *zl, unsigned char *p) {
    unsigned int curlen = ZIPLIST_BYTES(zl), rawlen, rawlensize;
    unsigned int offset, noffset, extra;
    unsigned char *np;
    zlentry cur, next;

    while (p[0] != ZIP_END) {
        cur = zipEntry(p);
        rawlen = cur.headersize + cur.len;
        rawlensize = zipPrevEncodeLength(NULL,rawlen);

        /* Abort if there is no next entry. */
        if (p[rawlen] == ZIP_END) break;
        next = zipEntry(p+rawlen);

        /* Abort when "prevlen" has not changed. */
        if (next.prevrawlen == rawlen) break;

        if (next.prevrawlensize < rawlensize) {
            /* The "prevlen" field of "next" needs more bytes to hold
             * the raw length of "cur". */
            offset = p-zl;
            extra = rawlensize-next.prevrawlensize;
            zl = ziplistResize(zl,curlen+extra);
            ZIPLIST_TAIL_OFFSET(zl) += extra;
            p = zl+offset;

            /* Move the tail to the back. */
            np = p+rawlen;
            noffset = np-zl;
            memmove(np+rawlensize,
                np+next.prevrawlensize,
                curlen-noffset-next.prevrawlensize-1);
            zipPrevEncodeLength(np,rawlen);

            /* Advance the cursor */
            p += rawlen;
        } else {
            if (next.prevrawlensize > rawlensize) {
                /* This would result in shrinking, which we want to avoid.
                 * So, set "rawlen" in the available bytes. */
                zipPrevEncodeLengthForceLarge(p+rawlen,rawlen);
            } else {
                zipPrevEncodeLength(p+rawlen,rawlen);
            }

            /* Stop here, as the raw length of "next" has not changed. */
            break;
        }
    }
    return zl;
}

/* Delete "num" entries, starting at "p". Returns pointer to the ziplist. */
static unsigned char *__ziplistDelete(unsigned char *zl, unsigned char *p, unsigned int num) {
    unsigned int i, totlen, deleted = 0;
    int offset, nextdiff = 0;
    zlentry first, tail;

    first = zipEntry(p);
    for (i = 0; p[0] != ZIP_END && i < num; i++) {
        p += zipRawEntryLength(p);
        deleted++;
    }

    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;

            /* When the tail contains more than one entry, we need to take
             * "nextdiff" in account as well. Otherwise, a change in the
             * size of prevlen doesn't have an effect on the *tail* offset. */
            tail = zipEntry(p);
            if (p[tail.headersize+tail.len] != ZIP_END)
                ZIPLIST_TAIL_OFFSET(zl) += 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 */
        offset = first.p-zl;
        zl = ziplistResize(zl, ZIPLIST_BYTES(zl)-totlen+nextdiff);
        ZIPLIST_INCR_LENGTH(zl,-deleted);
        p = zl+offset;

        /* When nextdiff != 0, the raw length of the next entry has changed, so
         * we need to cascade the update throughout the ziplist */
        if (nextdiff != 0)
            zl = __ziplistCascadeUpdate(zl,p);
    }
    return zl;
}

/* Insert item at "p". */
static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
    unsigned int curlen = ZIPLIST_BYTES(zl), reqlen, prevlen = 0;
    unsigned int offset, nextdiff = 0;
    unsigned char encoding = 0;
    long long value;
    zlentry entry, tail;

    /* Find out prevlen for the entry that is inserted. */
    if (p[0] != ZIP_END) {
        entry = zipEntry(p);
        prevlen = entry.prevrawlen;
    } else {
        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
        if (ptail[0] != ZIP_END) {
            prevlen = zipRawEntryLength(ptail);
        }
    }

    /* See if the entry can be encoded */
    if (zipTryEncoding(s,slen,&value,&encoding)) {
        /* 'encoding' is set to the appropriate integer encoding */
        reqlen = zipIntSize(encoding);
    } else {
        /* 'encoding' is untouched, however zipEncodeLength will use the
         * string length to figure out how to encode it. */
        reqlen = slen;
    }
    /* 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;

        /* When the tail contains more than one entry, we need to take
         * "nextdiff" in account as well. Otherwise, a change in the
         * size of prevlen doesn't have an effect on the *tail* offset. */
        tail = zipEntry(p+reqlen);
        if (p[reqlen+tail.headersize+tail.len] != ZIP_END)
            ZIPLIST_TAIL_OFFSET(zl) += nextdiff;
    } else {
        /* This element will be the new tail. */
        ZIPLIST_TAIL_OFFSET(zl) = p-zl;
    }

    /* When nextdiff != 0, the raw length of the next entry has changed, so
     * we need to cascade the update throughout the ziplist */
    if (nextdiff != 0) {
        offset = p-zl;
        zl = __ziplistCascadeUpdate(zl,p+reqlen);
        p = zl+offset;
    }

    /* Write the entry */
    p += zipPrevEncodeLength(p,prevlen);
    p += zipEncodeLength(p,encoding,slen);
    if (ZIP_IS_STR(encoding)) {
        memcpy(p,s,slen);
    } else {
        zipSaveInteger(p,value,encoding);
    }
    ZIPLIST_INCR_LENGTH(zl,1);
    return zl;
}

unsigned char *ziplistPush(unsigned char *zl, unsigned 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);
}

/* 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[0] == ZIP_END || index > 0) ? NULL : p;
}

/* Return pointer to next entry in ziplist. */
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p) {
    ((void) zl);

    /* "p" could be equal to ZIP_END, caused by ziplistDelete,
     * and we should return NULL. Otherwise, we should return NULL
     * when the *next* element is ZIP_END (there is no next entry). */
    if (p[0] == ZIP_END) {
        return NULL;
    } else {
        p = p+zipRawEntryLength(p);
        return (p[0] == ZIP_END) ? NULL : p;
    }
}

/* Return pointer to previous entry in ziplist. */
unsigned char *ziplistPrev(unsigned char *zl, unsigned char *p) {
    zlentry entry;

    /* Iterating backwards from ZIP_END should return the tail. When "p" is
     * equal to the first element of the list, we're already at the head,
     * and should return NULL. */
    if (p[0] == ZIP_END) {
        p = ZIPLIST_ENTRY_TAIL(zl);
        return (p[0] == ZIP_END) ? NULL : p;
    } else if (p == ZIPLIST_ENTRY_HEAD(zl)) {
        return NULL;
    } else {
        entry = zipEntry(p);
        assert(entry.prevrawlen > 0);
        return 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 **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 (ZIP_IS_STR(entry.encoding)) {
        if (sstr) {
            *slen = entry.len;
            *sstr = p+entry.headersize;
        }
    } else {
        if (sval) {
            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);
        }
    }
    return 1;
}

/* Insert an entry at "p". */
unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, unsigned 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;
    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. */
    *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 *sstr, unsigned int slen) {
    zlentry entry;
    unsigned char sencoding;
    long long zval, sval;
    if (p[0] == ZIP_END) return 0;

    entry = zipEntry(p);
    if (ZIP_IS_STR(entry.encoding)) {
        /* Raw compare */
        if (entry.len == slen) {
            return memcmp(p+entry.headersize,sstr,slen) == 0;
        } else {
            return 0;
        }
    } else {
        /* Try to compare encoded values */
        if (zipTryEncoding(sstr,slen,&sval,&sencoding)) {
            if (entry.encoding == sencoding) {
                zval = zipLoadInteger(p+entry.headersize,entry.encoding);
                return zval == sval;
            }
        }
    }
    return 0;
}

/* Return length of ziplist. */
unsigned int ziplistLen(unsigned char *zl) {
    unsigned int len = 0;
    if (ZIPLIST_LENGTH(zl) < UINT16_MAX) {
        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 < UINT16_MAX) ZIPLIST_LENGTH(zl) = len;
    }
    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;
    int index = 0;
    zlentry entry;

    printf(
        "{total bytes %d} "
        "{length %u}\n"
        "{tail offset %u}\n",
        ZIPLIST_BYTES(zl),
        ZIPLIST_LENGTH(zl),
        ZIPLIST_TAIL_OFFSET(zl));
    p = ZIPLIST_ENTRY_HEAD(zl);
    while(*p != ZIP_END) {
        entry = zipEntry(p);
        printf(
            "{"
                "addr 0x%08lx, "
                "index %2d, "
                "offset %5ld, "
                "rl: %5u, "
                "hs %2u, "
                "pl: %5u, "
                "pls: %2u, "
                "payload %5u"
            "} ",
            (long unsigned int)p,
            index,
            p-zl,
            entry.headersize+entry.len,
            entry.headersize,
            entry.prevrawlen,
            entry.prevrawlensize,
            entry.len);
        p += entry.headersize;
        if (ZIP_IS_STR(entry.encoding)) {
            if (entry.len > 40) {
                fwrite(p,40,1,stdout);
                printf("...");
            } else {
                fwrite(p,entry.len,1,stdout);
            }
        } else {
            printf("%lld", (long long) zipLoadInteger(p,entry.encoding));
        }
        printf("\n");
        p += entry.len;
        index++;
    }
    printf("{end}\n\n");
}

#ifdef ZIPLIST_TEST_MAIN
#include <sys/time.h>

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);
    return zl;
}

unsigned char *createIntList() {
    unsigned char *zl = ziplistNew();
    char buf[32];

    sprintf(buf, "100");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    sprintf(buf, "128000");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    sprintf(buf, "-100");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
    sprintf(buf, "4294967296");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
    sprintf(buf, "non integer");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    sprintf(buf, "much much longer non integer");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    return zl;
}

long long usec(void) {
    struct timeval tv;
    gettimeofday(&tv,NULL);
    return (((long long)tv.tv_sec)*1000000)+tv.tv_usec;
}

void stress(int pos, int num, int maxsize, int dnum) {
    int i,j,k;
    unsigned char *zl;
    char posstr[2][5] = { "HEAD", "TAIL" };
    long long start;
    for (i = 0; i < maxsize; i+=dnum) {
        zl = ziplistNew();
        for (j = 0; j < i; j++) {
            zl = ziplistPush(zl,(unsigned char*)"quux",4,ZIPLIST_TAIL);
        }

        /* Do num times a push+pop from pos */
        start = usec();
        for (k = 0; k < num; k++) {
            zl = ziplistPush(zl,(unsigned char*)"quux",4,pos);
            zl = ziplistDeleteRange(zl,0,1);
        }
        printf("List size: %8d, bytes: %8d, %dx push+pop (%s): %6lld usec\n",
            i,ZIPLIST_BYTES(zl),num,posstr[pos],usec()-start);
        zfree(zl);
    }
}

void pop(unsigned char *zl, int where) {
    unsigned char *p, *vstr;
    unsigned int vlen;
    long long vlong;

    p = ziplistIndex(zl,where == ZIPLIST_HEAD ? 0 : -1);
    if (ziplistGet(p,&vstr,&vlen,&vlong)) {
        if (where == ZIPLIST_HEAD)
            printf("Pop head: ");
        else
            printf("Pop tail: ");

        if (vstr)
            fwrite(vstr,vlen,1,stdout);
        else
            printf("%lld", vlong);

        printf("\n");
        ziplistDeleteRange(zl,-1,1);
    } else {
        printf("ERROR: Could not pop\n");
        exit(1);
    }
}

int main(int argc, char **argv) {
    unsigned char *zl, *p;
    unsigned char *entry;
    unsigned int elen;
    long long value;

    /* If an argument is given, use it as the random seed. */
    if (argc == 2)
        srand(atoi(argv[1]));

    zl = createIntList();
    ziplistRepr(zl);

    zl = createList();
    ziplistRepr(zl);

    pop(zl,ZIPLIST_TAIL);
    ziplistRepr(zl);

    pop(zl,ZIPLIST_HEAD);
    ziplistRepr(zl);

    pop(zl,ZIPLIST_TAIL);
    ziplistRepr(zl);

    pop(zl,ZIPLIST_TAIL);
    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();
        p = ziplistIndex(zl, 0);
        while (ziplistGet(p, &entry, &elen, &value)) {
            printf("Entry: ");
            if (entry) {
                fwrite(entry,elen,1,stdout);
            } else {
                printf("%lld", value);
            }
            p = ziplistNext(zl,p);
            printf("\n");
        }
        printf("\n");
    }

    printf("Iterate list from 1 to end:\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 = ziplistNext(zl,p);
            printf("\n");
        }
        printf("\n");
    }

    printf("Iterate list from 2 to end:\n");
    {
        zl = createList();
        p = ziplistIndex(zl, 2);
        while (ziplistGet(p, &entry, &elen, &value)) {
            printf("Entry: ");
            if (entry) {
                fwrite(entry,elen,1,stdout);
            } else {
                printf("%lld", value);
            }
            p = ziplistNext(zl,p);
            printf("\n");
        }
        printf("\n");
    }

    printf("Iterate starting out of range:\n");
    {
        zl = createList();
        p = ziplistIndex(zl, 4);
        if (!ziplistGet(p, &entry, &elen, &value)) {
            printf("No entry\n");
        } else {
            printf("ERROR\n");
        }
        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(zl,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);
            p = ziplistPrev(zl,p);
            printf("\n");
        }
        printf("\n");
    }

    printf("Delete inclusive range 0,0:\n");
    {
        zl = createList();
        zl = ziplistDeleteRange(zl, 0, 1);
        ziplistRepr(zl);
    }

    printf("Delete inclusive range 0,1:\n");
    {
        zl = createList();
        zl = ziplistDeleteRange(zl, 0, 2);
        ziplistRepr(zl);
    }

    printf("Delete inclusive range 1,2:\n");
    {
        zl = createList();
        zl = ziplistDeleteRange(zl, 1, 2);
        ziplistRepr(zl);
    }

    printf("Delete with start index out of range:\n");
    {
        zl = createList();
        zl = ziplistDeleteRange(zl, 5, 1);
        ziplistRepr(zl);
    }

    printf("Delete with num overflow:\n");
    {
        zl = createList();
        zl = ziplistDeleteRange(zl, 1, 5);
        ziplistRepr(zl);
    }

    printf("Delete foo while iterating:\n");
    {
        zl = createList();
        p = ziplistIndex(zl,0);
        while (ziplistGet(p,&entry,&elen,&value)) {
            if (entry && strncmp("foo",(char*)entry,elen) == 0) {
                printf("Delete foo\n");
                zl = ziplistDelete(zl,&p);
            } else {
                printf("Entry: ");
                if (entry) {
                    fwrite(entry,elen,1,stdout);
                } else {
                    printf("%lld",value);
                }
                p = ziplistNext(zl,p);
                printf("\n");
            }
        }
        printf("\n");
        ziplistRepr(zl);
    }

    printf("Regression test for >255 byte strings:\n");
    {
        char v1[257],v2[257];
        memset(v1,'x',256);
        memset(v2,'y',256);
        zl = ziplistNew();
        zl = ziplistPush(zl,(unsigned char*)v1,strlen(v1),ZIPLIST_TAIL);
        zl = ziplistPush(zl,(unsigned char*)v2,strlen(v2),ZIPLIST_TAIL);

        /* Pop values again and compare their value. */
        p = ziplistIndex(zl,0);
        assert(ziplistGet(p,&entry,&elen,&value));
        assert(strncmp(v1,entry,elen) == 0);
        p = ziplistIndex(zl,1);
        assert(ziplistGet(p,&entry,&elen,&value));
        assert(strncmp(v2,entry,elen) == 0);
        printf("SUCCESS\n\n");
    }

    printf("Create long list and check indices:\n");
    {
        zl = ziplistNew();
        char buf[32];
        int i,len;
        for (i = 0; i < 1000; i++) {
            len = sprintf(buf,"%d",i);
            zl = ziplistPush(zl,(unsigned char*)buf,len,ZIPLIST_TAIL);
        }
        for (i = 0; i < 1000; i++) {
            p = ziplistIndex(zl,i);
            assert(ziplistGet(p,NULL,NULL,&value));
            assert(i == value);

            p = ziplistIndex(zl,-i-1);
            assert(ziplistGet(p,NULL,NULL,&value));
            assert(999-i == value);
        }
        printf("SUCCESS\n\n");
    }

    printf("Compare strings with ziplist entries:\n");
    {
        zl = createList();
        p = ziplistIndex(zl,0);
        if (!ziplistCompare(p,(unsigned char*)"hello",5)) {
            printf("ERROR: not \"hello\"\n");
            return 1;
        }
        if (ziplistCompare(p,(unsigned char*)"hella",5)) {
            printf("ERROR: \"hella\"\n");
            return 1;
        }

        p = ziplistIndex(zl,3);
        if (!ziplistCompare(p,(unsigned char*)"1024",4)) {
            printf("ERROR: not \"1024\"\n");
            return 1;
        }
        if (ziplistCompare(p,(unsigned char*)"1025",4)) {
            printf("ERROR: \"1025\"\n");
            return 1;
        }
        printf("SUCCESS\n\n");
    }

    printf("Stress with random payloads of different encoding:\n");
    {
        int i, idx, where, len;
        long long v;
        unsigned char *p;
        char buf[0x4041]; /* max length of generated string */
        zl = ziplistNew();
        for (i = 0; i < 100000; i++) {
            where = (rand() & 1) ? ZIPLIST_HEAD : ZIPLIST_TAIL;
            if (rand() & 1) {
                /* equally likely create a 16, 32 or 64 bit int */
                v = (rand() & INT16_MAX) + ((1ll << 32) >> ((rand() % 3)*16));
                v *= 2*(rand() & 1)-1; /* randomly flip sign */
                sprintf(buf, "%lld", v);
                zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), where);
            } else {
                /* equally likely generate 6, 14 or >14 bit length */
                v = rand() & 0x3f;
                v += 0x4000 >> ((rand() % 3)*8);
                memset(buf, 'x', v);
                zl = ziplistPush(zl, (unsigned char*)buf, v, where);
            }

            /* delete a random element */
            if ((len = ziplistLen(zl)) >= 10) {
                idx = rand() % len;
                // printf("Delete index %d\n", idx);
                // ziplistRepr(zl);
                ziplistDeleteRange(zl, idx, 1);
                // ziplistRepr(zl);
                len--;
            }

            /* iterate from front to back */
            idx = 0;
            p = ziplistIndex(zl, 0);
            while((p = ziplistNext(zl,p)))
                idx++;
            assert(len == idx+1);

            /* iterate from back to front */
            idx = 0;
            p = ziplistIndex(zl, -1);
            while((p = ziplistPrev(zl,p)))
                idx++;
            assert(len == idx+1);
        }
        printf("SUCCESS\n\n");
    }

    printf("Stress with variable ziplist size:\n");
    {
        stress(ZIPLIST_HEAD,100000,16384,256);
        stress(ZIPLIST_TAIL,100000,16384,256);
    }

    return 0;
}

#endif