bitops.c

/* Bit operations.
 *
 * Copyright (c) 2009-2012, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include "server.h"

/* -----------------------------------------------------------------------------
 * Helpers and low level bit functions.
 * -------------------------------------------------------------------------- */

/* Count number of bits set in the binary array pointed by 's' and long
 * 'count' bytes. The implementation of this function is required to
 * work with a input string length up to 512 MB. */
size_t redisPopcount(void *s, long count) {
    size_t bits = 0;
    unsigned char *p = s;
    uint32_t *p4;
    static const unsigned char bitsinbyte[256] = {0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,1,2,2,3,2,3,3,4,2,3,3,4,3,4,4,5,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,2,3,3,4,3,4,4,5,3,4,4,5,4,5,5,6,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,3,4,4,5,4,5,5,6,4,5,5,6,5,6,6,7,4,5,5,6,5,6,6,7,5,6,6,7,6,7,7,8};

    /* Count initial bytes not aligned to 32 bit. */
    while((unsigned long)p & 3 && count) {
        bits += bitsinbyte[*p++];
        count--;
    }

    /* Count bits 28 bytes at a time */
    p4 = (uint32_t*)p;
    while(count>=28) {
        uint32_t aux1, aux2, aux3, aux4, aux5, aux6, aux7;

        aux1 = *p4++;
        aux2 = *p4++;
        aux3 = *p4++;
        aux4 = *p4++;
        aux5 = *p4++;
        aux6 = *p4++;
        aux7 = *p4++;
        count -= 28;

        aux1 = aux1 - ((aux1 >> 1) & 0x55555555);
        aux1 = (aux1 & 0x33333333) + ((aux1 >> 2) & 0x33333333);
        aux2 = aux2 - ((aux2 >> 1) & 0x55555555);
        aux2 = (aux2 & 0x33333333) + ((aux2 >> 2) & 0x33333333);
        aux3 = aux3 - ((aux3 >> 1) & 0x55555555);
        aux3 = (aux3 & 0x33333333) + ((aux3 >> 2) & 0x33333333);
        aux4 = aux4 - ((aux4 >> 1) & 0x55555555);
        aux4 = (aux4 & 0x33333333) + ((aux4 >> 2) & 0x33333333);
        aux5 = aux5 - ((aux5 >> 1) & 0x55555555);
        aux5 = (aux5 & 0x33333333) + ((aux5 >> 2) & 0x33333333);
        aux6 = aux6 - ((aux6 >> 1) & 0x55555555);
        aux6 = (aux6 & 0x33333333) + ((aux6 >> 2) & 0x33333333);
        aux7 = aux7 - ((aux7 >> 1) & 0x55555555);
        aux7 = (aux7 & 0x33333333) + ((aux7 >> 2) & 0x33333333);
        bits += ((((aux1 + (aux1 >> 4)) & 0x0F0F0F0F) +
                    ((aux2 + (aux2 >> 4)) & 0x0F0F0F0F) +
                    ((aux3 + (aux3 >> 4)) & 0x0F0F0F0F) +
                    ((aux4 + (aux4 >> 4)) & 0x0F0F0F0F) +
                    ((aux5 + (aux5 >> 4)) & 0x0F0F0F0F) +
                    ((aux6 + (aux6 >> 4)) & 0x0F0F0F0F) +
                    ((aux7 + (aux7 >> 4)) & 0x0F0F0F0F))* 0x01010101) >> 24;
    }
    /* Count the remaining bytes. */
    p = (unsigned char*)p4;
    while(count--) bits += bitsinbyte[*p++];
    return bits;
}

/* Return the position of the first bit set to one (if 'bit' is 1) or
 * zero (if 'bit' is 0) in the bitmap starting at 's' and long 'count' bytes.
 *
 * The function is guaranteed to return a value >= 0 if 'bit' is 0 since if
 * no zero bit is found, it returns count*8 assuming the string is zero
 * padded on the right. However if 'bit' is 1 it is possible that there is
 * not a single set bit in the bitmap. In this special case -1 is returned. */
long redisBitpos(void *s, unsigned long count, int bit) {
    unsigned long *l;
    unsigned char *c;
    unsigned long skipval, word = 0, one;
    long pos = 0; /* Position of bit, to return to the caller. */
    unsigned long j;

    /* Process whole words first, seeking for first word that is not
     * all ones or all zeros respectively if we are lookig for zeros
     * or ones. This is much faster with large strings having contiguous
     * blocks of 1 or 0 bits compared to the vanilla bit per bit processing.
     *
     * Note that if we start from an address that is not aligned
     * to sizeof(unsigned long) we consume it byte by byte until it is
     * aligned. */

    /* Skip initial bits not aligned to sizeof(unsigned long) byte by byte. */
    skipval = bit ? 0 : UCHAR_MAX;
    c = (unsigned char*) s;
    while((unsigned long)c & (sizeof(*l)-1) && count) {
        if (*c != skipval) break;
        c++;
        count--;
        pos += 8;
    }

    /* Skip bits with full word step. */
    skipval = bit ? 0 : ULONG_MAX;
    l = (unsigned long*) c;
    while (count >= sizeof(*l)) {
        if (*l != skipval) break;
        l++;
        count -= sizeof(*l);
        pos += sizeof(*l)*8;
    }

    /* Load bytes into "word" considering the first byte as the most significant
     * (we basically consider it as written in big endian, since we consider the
     * string as a set of bits from left to right, with the first bit at position
     * zero.
     *
     * Note that the loading is designed to work even when the bytes left
     * (count) are less than a full word. We pad it with zero on the right. */
    c = (unsigned char*)l;
    for (j = 0; j < sizeof(*l); j++) {
        word <<= 8;
        if (count) {
            word |= *c;
            c++;
            count--;
        }
    }

    /* Special case:
     * If bits in the string are all zero and we are looking for one,
     * return -1 to signal that there is not a single "1" in the whole
     * string. This can't happen when we are looking for "0" as we assume
     * that the right of the string is zero padded. */
    if (bit == 1 && word == 0) return -1;

    /* Last word left, scan bit by bit. The first thing we need is to
     * have a single "1" set in the most significant position in an
     * unsigned long. We don't know the size of the long so we use a
     * simple trick. */
    one = ULONG_MAX; /* All bits set to 1.*/
    one >>= 1;       /* All bits set to 1 but the MSB. */
    one = ~one;      /* All bits set to 0 but the MSB. */

    while(one) {
        if (((one & word) != 0) == bit) return pos;
        pos++;
        one >>= 1;
    }

    /* If we reached this point, there is a bug in the algorithm, since
     * the case of no match is handled as a special case before. */
    serverPanic("End of redisBitpos() reached.");
    return 0; /* Just to avoid warnings. */
}

/* The following set.*Bitfield and get.*Bitfield functions implement setting
 * and getting arbitrary size (up to 64 bits) signed and unsigned integers
 * at arbitrary positions into a bitmap.
 *
 * The representation considers the bitmap as having the bit number 0 to be
 * the most significant bit of the first byte, and so forth, so for example
 * setting a 5 bits unsigned integer to value 23 at offset 7 into a bitmap
 * previously set to all zeroes, will produce the following representation:
 *
 * +--------+--------+
 * |00000001|01110000|
 * +--------+--------+
 *
 * When offsets and integer sizes are aligned to bytes boundaries, this is the
 * same as big endian, however when such alignment does not exist, its important
 * to also understand how the bits inside a byte are ordered.
 *
 * Note that this format follows the same convention as SETBIT and related
 * commands.
 */

void setUnsignedBitfield(unsigned char *p, uint64_t offset, uint64_t bits, uint64_t value) {
    uint64_t byte, bit, byteval, bitval, j;

    for (j = 0; j < bits; j++) {
        bitval = (value & ((uint64_t)1<<(bits-1-j))) != 0;
        byte = offset >> 3;
        bit = 7 - (offset & 0x7);
        byteval = p[byte];
        byteval &= ~(1 << bit);
        byteval |= bitval << bit;
        p[byte] = byteval & 0xff;
        offset++;
    }
}

void setSignedBitfield(unsigned char *p, uint64_t offset, uint64_t bits, int64_t value) {
    uint64_t uv = value; /* Casting will add UINT64_MAX + 1 if v is negative. */
    setUnsignedBitfield(p,offset,bits,uv);
}

uint64_t getUnsignedBitfield(unsigned char *p, uint64_t offset, uint64_t bits) {
    uint64_t byte, bit, byteval, bitval, j, value = 0;

    for (j = 0; j < bits; j++) {
        byte = offset >> 3;
        bit = 7 - (offset & 0x7);
        byteval = p[byte];
        bitval = (byteval >> bit) & 1;
        value = (value<<1) | bitval;
        offset++;
    }
    return value;
}

int64_t getSignedBitfield(unsigned char *p, uint64_t offset, uint64_t bits) {
    int64_t value;
    union {uint64_t u; int64_t i;} conv;

    /* Converting from unsigned to signed is undefined when the value does
     * not fit, however here we assume two's complement and the original value
     * was obtained from signed -> unsigned conversion, so we'll find the
     * most significant bit set if the original value was negative.
     *
     * Note that two's complement is mandatory for exact-width types
     * according to the C99 standard. */
    conv.u = getUnsignedBitfield(p,offset,bits);
    value = conv.i;

    /* If the top significant bit is 1, propagate it to all the
     * higher bits for two's complement representation of signed
     * integers. */
    if (value & ((uint64_t)1 << (bits-1)))
        value |= ((uint64_t)-1) << bits;
    return value;
}

/* The following two functions detect overflow of a value in the context
 * of storing it as an unsigned or signed integer with the specified
 * number of bits. The functions both take the value and a possible increment.
 * If no overflow could happen and the value+increment fit inside the limits,
 * then zero is returned, otherwise in case of overflow, 1 is returned,
 * otherwise in case of underflow, -1 is returned.
 *
 * When non-zero is returned (oferflow or underflow), if not NULL, *limit is
 * set to the value the operation should result when an overflow happens,
 * depending on the specified overflow semantics:
 *
 * For BFOVERFLOW_SAT if 1 is returned, *limit it is set maximum value that
 * you can store in that integer. when -1 is returned, *limit is set to the
 * minimum value that an integer of that size can represent.
 *
 * For BFOVERFLOW_WRAP *limit is set by performing the operation in order to
 * "wrap" around towards zero for unsigned integers, or towards the most
 * negative number that is possible to represent for signed integers. */

#define BFOVERFLOW_WRAP 0
#define BFOVERFLOW_SAT 1
#define BFOVERFLOW_FAIL 2 /* Used by the BITFIELD command implementation. */

int checkUnsignedBitfieldOverflow(uint64_t value, int64_t incr, uint64_t bits, int owtype, uint64_t *limit) {
    uint64_t max = (bits == 64) ? UINT64_MAX : (((uint64_t)1<<bits)-1);
    int64_t maxincr = max-value;
    int64_t minincr = -value;

    if (value > max || (incr > 0 && incr > maxincr)) {
        if (limit) {
            if (owtype == BFOVERFLOW_WRAP) {
                goto handle_wrap;
            } else if (owtype == BFOVERFLOW_SAT) {
                *limit = max;
            }
        }
        return 1;
    } else if (incr < 0 && incr < minincr) {
        if (limit) {
            if (owtype == BFOVERFLOW_WRAP) {
                goto handle_wrap;
            } else if (owtype == BFOVERFLOW_SAT) {
                *limit = 0;
            }
        }
        return -1;
    }
    return 0;

handle_wrap:
    {
        uint64_t mask = ((uint64_t)-1) << bits;
        uint64_t res = value+incr;

        res &= ~mask;
        *limit = res;
    }
    return 1;
}

int checkSignedBitfieldOverflow(int64_t value, int64_t incr, uint64_t bits, int owtype, int64_t *limit) {
    int64_t max = (bits == 64) ? INT64_MAX : (((int64_t)1<<(bits-1))-1);
    int64_t min = (-max)-1;

    /* Note that maxincr and minincr could overflow, but we use the values
     * only after checking 'value' range, so when we use it no overflow
     * happens. */
    int64_t maxincr = max-value;
    int64_t minincr = min-value;

    if (value > max || (bits != 64 && incr > maxincr) || (value >= 0 && incr > 0 && incr > maxincr))
    {
        if (limit) {
            if (owtype == BFOVERFLOW_WRAP) {
                goto handle_wrap;
            } else if (owtype == BFOVERFLOW_SAT) {
                *limit = max;
            }
        }
        return 1;
    } else if (value < min || (bits != 64 && incr < minincr) || (value < 0 && incr < 0 && incr < minincr)) {
        if (limit) {
            if (owtype == BFOVERFLOW_WRAP) {
                goto handle_wrap;
            } else if (owtype == BFOVERFLOW_SAT) {
                *limit = min;
            }
        }
        return -1;
    }
    return 0;

handle_wrap:
    {
        uint64_t mask = ((uint64_t)-1) << bits;
        uint64_t msb = (uint64_t)1 << (bits-1);
        uint64_t a = value, b = incr, c;
        c = a+b; /* Perform addition as unsigned so that's defined. */

        /* If the sign bit is set, propagate to all the higher order
         * bits, to cap the negative value. If it's clear, mask to
         * the positive integer limit. */
        if (c & msb) {
            c |= mask;
        } else {
            c &= ~mask;
        }
        *limit = c;
    }
    return 1;
}

/* Debugging function. Just show bits in the specified bitmap. Not used
 * but here for not having to rewrite it when debugging is needed. */
void printBits(unsigned char *p, unsigned long count) {
    unsigned long j, i, byte;

    for (j = 0; j < count; j++) {
        byte = p[j];
        for (i = 0x80; i > 0; i /= 2)
            printf("%c", (byte & i) ? '1' : '0');
        printf("|");
    }
    printf("\n");
}

/* -----------------------------------------------------------------------------
 * Bits related string commands: GETBIT, SETBIT, BITCOUNT, BITOP.
 * -------------------------------------------------------------------------- */

#define BITOP_AND   0
#define BITOP_OR    1
#define BITOP_XOR   2
#define BITOP_NOT   3

#define BITFIELDOP_GET 0
#define BITFIELDOP_SET 1
#define BITFIELDOP_INCRBY 2

/* This helper function used by GETBIT / SETBIT parses the bit offset argument
 * making sure an error is returned if it is negative or if it overflows
 * Redis 512 MB limit for the string value.
 *
 * If the 'hash' argument is true, and 'bits is positive, then the command
 * will also parse bit offsets prefixed by "#". In such a case the offset
 * is multiplied by 'bits'. This is useful for the BITFIELD command. */
int getBitOffsetFromArgument(client *c, robj *o, size_t *offset, int hash, int bits) {
    long long loffset;
    char *err = "bit offset is not an integer or out of range";
    char *p = o->ptr;
    size_t plen = sdslen(p);
    int usehash = 0;

    /* Handle #<offset> form. */
    if (p[0] == '#' && hash && bits > 0) usehash = 1;

    if (string2ll(p+usehash,plen-usehash,&loffset) == 0) {
        addReplyError(c,err);
        return C_ERR;
    }

    /* Adjust the offset by 'bits' for #<offset> form. */
    if (usehash) loffset *= bits;

    /* Limit offset to 512MB in bytes */
    if ((loffset < 0) || ((unsigned long long)loffset >> 3) >= (512*1024*1024))
    {
        addReplyError(c,err);
        return C_ERR;
    }

    *offset = (size_t)loffset;
    return C_OK;
}

/* This helper function for BITFIELD parses a bitfield type in the form
 * <sign><bits> where sign is 'u' or 'i' for unsigned and signed, and
 * the bits is a value between 1 and 64. However 64 bits unsigned integers
 * are reported as an error because of current limitations of Redis protocol
 * to return unsigned integer values greater than INT64_MAX.
 *
 * On error C_ERR is returned and an error is sent to the client. */
int getBitfieldTypeFromArgument(client *c, robj *o, int *sign, int *bits) {
    char *p = o->ptr;
    char *err = "Invalid bitfield type. Use something like i16 u8. Note that u64 is not supported but i64 is.";
    long long llbits;

    if (p[0] == 'i') {
        *sign = 1;
    } else if (p[0] == 'u') {
        *sign = 0;
    } else {
        addReplyError(c,err);
        return C_ERR;
    }

    if ((string2ll(p+1,strlen(p+1),&llbits)) == 0 ||
        llbits < 1 ||
        (*sign == 1 && llbits > 64) ||
        (*sign == 0 && llbits > 63))
    {
        addReplyError(c,err);
        return C_ERR;
    }
    *bits = llbits;
    return C_OK;
}

/* This is an helper function for commands implementations that need to write
 * bits to a string object. The command creates or pad with zeroes the string
 * so that the 'maxbit' bit can be addressed. The object is finally
 * returned. Otherwise if the key holds a wrong type NULL is returned and
 * an error is sent to the client. */
robj *lookupStringForBitCommand(client *c, size_t maxbit) {
    size_t byte = maxbit >> 3;
    robj *o = lookupKeyWrite(c->db,c->argv[1]);

    if (o == NULL) {
        o = createObject(OBJ_STRING,sdsnewlen(NULL, byte+1));
        dbAdd(c->db,c->argv[1],o);
    } else {
        if (checkType(c,o,OBJ_STRING)) return NULL;
        o = dbUnshareStringValue(c->db,c->argv[1],o);
        o->ptr = sdsgrowzero(o->ptr,byte+1);
    }
    return o;
}

/* Return a pointer to the string object content, and stores its length
 * in 'len'. The user is required to pass (likely stack allocated) buffer
 * 'llbuf' of at least LONG_STR_SIZE bytes. Such a buffer is used in the case
 * the object is integer encoded in order to provide the representation
 * without usign heap allocation.
 *
 * The function returns the pointer to the object array of bytes representing
 * the string it contains, that may be a pointer to 'llbuf' or to the
 * internal object representation. As a side effect 'len' is filled with
 * the length of such buffer.
 *
 * If the source object is NULL the function is guaranteed to return NULL
 * and set 'len' to 0. */
unsigned char *getObjectReadOnlyString(robj *o, long *len, char *llbuf) {
    serverAssert(o->type == OBJ_STRING);
    unsigned char *p = NULL;

    /* Set the 'p' pointer to the string, that can be just a stack allocated
     * array if our string was integer encoded. */
    if (o && o->encoding == OBJ_ENCODING_INT) {
        p = (unsigned char*) llbuf;
        if (len) *len = ll2string(llbuf,LONG_STR_SIZE,(long)o->ptr);
    } else if (o) {
        p = (unsigned char*) o->ptr;
        if (len) *len = sdslen(o->ptr);
    } else {
        if (len) *len = 0;
    }
    return p;
}

/* SETBIT key offset bitvalue */
void setbitCommand(client *c) {
    robj *o;
    char *err = "bit is not an integer or out of range";
    size_t bitoffset;
    ssize_t byte, bit;
    int byteval, bitval;
    long on;

    if (getBitOffsetFromArgument(c,c->argv[2],&bitoffset,0,0) != C_OK)
        return;

    if (getLongFromObjectOrReply(c,c->argv[3],&on,err) != C_OK)
        return;

    /* Bits can only be set or cleared... */
    if (on & ~1) {
        addReplyError(c,err);
        return;
    }

    if ((o = lookupStringForBitCommand(c,bitoffset)) == NULL) return;

    /* Get current values */
    byte = bitoffset >> 3;
    byteval = ((uint8_t*)o->ptr)[byte];
    bit = 7 - (bitoffset & 0x7);
    bitval = byteval & (1 << bit);

    /* Update byte with new bit value and return original value */
    byteval &= ~(1 << bit);
    byteval |= ((on & 0x1) << bit);
    ((uint8_t*)o->ptr)[byte] = byteval;
    signalModifiedKey(c->db,c->argv[1]);
    notifyKeyspaceEvent(NOTIFY_STRING,"setbit",c->argv[1],c->db->id);
    server.dirty++;
    addReply(c, bitval ? shared.cone : shared.czero);
}

/* GETBIT key offset */
void getbitCommand(client *c) {
    robj *o;
    char llbuf[32];
    size_t bitoffset;
    size_t byte, bit;
    size_t bitval = 0;

    if (getBitOffsetFromArgument(c,c->argv[2],&bitoffset,0,0) != C_OK)
        return;

    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.czero)) == NULL ||
        checkType(c,o,OBJ_STRING)) return;

    byte = bitoffset >> 3;
    bit = 7 - (bitoffset & 0x7);
    if (sdsEncodedObject(o)) {
        if (byte < sdslen(o->ptr))
            bitval = ((uint8_t*)o->ptr)[byte] & (1 << bit);
    } else {
        if (byte < (size_t)ll2string(llbuf,sizeof(llbuf),(long)o->ptr))
            bitval = llbuf[byte] & (1 << bit);
    }

    addReply(c, bitval ? shared.cone : shared.czero);
}

/* BITOP op_name target_key src_key1 src_key2 src_key3 ... src_keyN */
void bitopCommand(client *c) {
    char *opname = c->argv[1]->ptr;
    robj *o, *targetkey = c->argv[2];
    unsigned long op, j, numkeys;
    robj **objects;      /* Array of source objects. */
    unsigned char **src; /* Array of source strings pointers. */
    unsigned long *len, maxlen = 0; /* Array of length of src strings,
                                       and max len. */
    unsigned long minlen = 0;    /* Min len among the input keys. */
    unsigned char *res = NULL; /* Resulting string. */

    /* Parse the operation name. */
    if ((opname[0] == 'a' || opname[0] == 'A') && !strcasecmp(opname,"and"))
        op = BITOP_AND;
    else if((opname[0] == 'o' || opname[0] == 'O') && !strcasecmp(opname,"or"))
        op = BITOP_OR;
    else if((opname[0] == 'x' || opname[0] == 'X') && !strcasecmp(opname,"xor"))
        op = BITOP_XOR;
    else if((opname[0] == 'n' || opname[0] == 'N') && !strcasecmp(opname,"not"))
        op = BITOP_NOT;
    else {
        addReply(c,shared.syntaxerr);
        return;
    }

    /* Sanity check: NOT accepts only a single key argument. */
    if (op == BITOP_NOT && c->argc != 4) {
        addReplyError(c,"BITOP NOT must be called with a single source key.");
        return;
    }

    /* Lookup keys, and store pointers to the string objects into an array. */
    numkeys = c->argc - 3;
    src = zmalloc(sizeof(unsigned char*) * numkeys);
    len = zmalloc(sizeof(long) * numkeys);
    objects = zmalloc(sizeof(robj*) * numkeys);
    for (j = 0; j < numkeys; j++) {
        o = lookupKeyRead(c->db,c->argv[j+3]);
        /* Handle non-existing keys as empty strings. */
        if (o == NULL) {
            objects[j] = NULL;
            src[j] = NULL;
            len[j] = 0;
            minlen = 0;
            continue;
        }
        /* Return an error if one of the keys is not a string. */
        if (checkType(c,o,OBJ_STRING)) {
            unsigned long i;
            for (i = 0; i < j; i++) {
                if (objects[i])
                    decrRefCount(objects[i]);
            }
            zfree(src);
            zfree(len);
            zfree(objects);
            return;
        }
        objects[j] = getDecodedObject(o);
        src[j] = objects[j]->ptr;
        len[j] = sdslen(objects[j]->ptr);
        if (len[j] > maxlen) maxlen = len[j];
        if (j == 0 || len[j] < minlen) minlen = len[j];
    }

    /* Compute the bit operation, if at least one string is not empty. */
    if (maxlen) {
        res = (unsigned char*) sdsnewlen(NULL,maxlen);
        unsigned char output, byte;
        unsigned long i;

        /* Fast path: as far as we have data for all the input bitmaps we
         * can take a fast path that performs much better than the
         * vanilla algorithm. */
        j = 0;
        if (minlen >= sizeof(unsigned long)*4 && numkeys <= 16) {
            unsigned long *lp[16];
            unsigned long *lres = (unsigned long*) res;

            /* Note: sds pointer is always aligned to 8 byte boundary. */
            memcpy(lp,src,sizeof(unsigned long*)*numkeys);
            memcpy(res,src[0],minlen);

            /* Different branches per different operations for speed (sorry). */
            if (op == BITOP_AND) {
                while(minlen >= sizeof(unsigned long)*4) {
                    for (i = 1; i < numkeys; i++) {
                        lres[0] &= lp[i][0];
                        lres[1] &= lp[i][1];
                        lres[2] &= lp[i][2];
                        lres[3] &= lp[i][3];
                        lp[i]+=4;
                    }
                    lres+=4;
                    j += sizeof(unsigned long)*4;
                    minlen -= sizeof(unsigned long)*4;
                }
            } else if (op == BITOP_OR) {
                while(minlen >= sizeof(unsigned long)*4) {
                    for (i = 1; i < numkeys; i++) {
                        lres[0] |= lp[i][0];
                        lres[1] |= lp[i][1];
                        lres[2] |= lp[i][2];
                        lres[3] |= lp[i][3];
                        lp[i]+=4;
                    }
                    lres+=4;
                    j += sizeof(unsigned long)*4;
                    minlen -= sizeof(unsigned long)*4;
                }
            } else if (op == BITOP_XOR) {
                while(minlen >= sizeof(unsigned long)*4) {
                    for (i = 1; i < numkeys; i++) {
                        lres[0] ^= lp[i][0];
                        lres[1] ^= lp[i][1];
                        lres[2] ^= lp[i][2];
                        lres[3] ^= lp[i][3];
                        lp[i]+=4;
                    }
                    lres+=4;
                    j += sizeof(unsigned long)*4;
                    minlen -= sizeof(unsigned long)*4;
                }
            } else if (op == BITOP_NOT) {
                while(minlen >= sizeof(unsigned long)*4) {
                    lres[0] = ~lres[0];
                    lres[1] = ~lres[1];
                    lres[2] = ~lres[2];
                    lres[3] = ~lres[3];
                    lres+=4;
                    j += sizeof(unsigned long)*4;
                    minlen -= sizeof(unsigned long)*4;
                }
            }
        }

        /* j is set to the next byte to process by the previous loop. */
        for (; j < maxlen; j++) {
            output = (len[0] <= j) ? 0 : src[0][j];
            if (op == BITOP_NOT) output = ~output;
            for (i = 1; i < numkeys; i++) {
                byte = (len[i] <= j) ? 0 : src[i][j];
                switch(op) {
                case BITOP_AND: output &= byte; break;
                case BITOP_OR:  output |= byte; break;
                case BITOP_XOR: output ^= byte; break;
                }
            }
            res[j] = output;
        }
    }
    for (j = 0; j < numkeys; j++) {
        if (objects[j])
            decrRefCount(objects[j]);
    }
    zfree(src);
    zfree(len);
    zfree(objects);

    /* Store the computed value into the target key */
    if (maxlen) {
        o = createObject(OBJ_STRING,res);
        setKey(c->db,targetkey,o);
        notifyKeyspaceEvent(NOTIFY_STRING,"set",targetkey,c->db->id);
        decrRefCount(o);
    } else if (dbDelete(c->db,targetkey)) {
        signalModifiedKey(c->db,targetkey);
        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",targetkey,c->db->id);
    }
    server.dirty++;
    addReplyLongLong(c,maxlen); /* Return the output string length in bytes. */
}

/* BITCOUNT key [start end] */
void bitcountCommand(client *c) {
    robj *o;
    long start, end, strlen;
    unsigned char *p;
    char llbuf[LONG_STR_SIZE];

    /* Lookup, check for type, and return 0 for non existing keys. */
    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.czero)) == NULL ||
        checkType(c,o,OBJ_STRING)) return;
    p = getObjectReadOnlyString(o,&strlen,llbuf);

    /* Parse start/end range if any. */
    if (c->argc == 4) {
        if (getLongFromObjectOrReply(c,c->argv[2],&start,NULL) != C_OK)
            return;
        if (getLongFromObjectOrReply(c,c->argv[3],&end,NULL) != C_OK)
            return;
        /* Convert negative indexes */
        if (start < 0 && end < 0 && start > end) {
            addReply(c,shared.czero);
            return;
        }
        if (start < 0) start = strlen+start;
        if (end < 0) end = strlen+end;
        if (start < 0) start = 0;
        if (end < 0) end = 0;
        if (end >= strlen) end = strlen-1;
    } else if (c->argc == 2) {
        /* The whole string. */
        start = 0;
        end = strlen-1;
    } else {
        /* Syntax error. */
        addReply(c,shared.syntaxerr);
        return;
    }

    /* Precondition: end >= 0 && end < strlen, so the only condition where
     * zero can be returned is: start > end. */
    if (start > end) {
        addReply(c,shared.czero);
    } else {
        long bytes = end-start+1;

        addReplyLongLong(c,redisPopcount(p+start,bytes));
    }
}

/* BITPOS key bit [start [end]] */
void bitposCommand(client *c) {
    robj *o;
    long bit, start, end, strlen;
    unsigned char *p;
    char llbuf[LONG_STR_SIZE];
    int end_given = 0;

    /* Parse the bit argument to understand what we are looking for, set
     * or clear bits. */
    if (getLongFromObjectOrReply(c,c->argv[2],&bit,NULL) != C_OK)
        return;
    if (bit != 0 && bit != 1) {
        addReplyError(c, "The bit argument must be 1 or 0.");
        return;
    }

    /* If the key does not exist, from our point of view it is an infinite
     * array of 0 bits. If the user is looking for the fist clear bit return 0,
     * If the user is looking for the first set bit, return -1. */
    if ((o = lookupKeyRead(c->db,c->argv[1])) == NULL) {
        addReplyLongLong(c, bit ? -1 : 0);
        return;
    }
    if (checkType(c,o,OBJ_STRING)) return;
    p = getObjectReadOnlyString(o,&strlen,llbuf);

    /* Parse start/end range if any. */
    if (c->argc == 4 || c->argc == 5) {
        if (getLongFromObjectOrReply(c,c->argv[3],&start,NULL) != C_OK)
            return;
        if (c->argc == 5) {
            if (getLongFromObjectOrReply(c,c->argv[4],&end,NULL) != C_OK)
                return;
            end_given = 1;
        } else {
            end = strlen-1;
        }
        /* Convert negative indexes */
        if (start < 0) start = strlen+start;
        if (end < 0) end = strlen+end;
        if (start < 0) start = 0;
        if (end < 0) end = 0;
        if (end >= strlen) end = strlen-1;
    } else if (c->argc == 3) {
        /* The whole string. */
        start = 0;
        end = strlen-1;
    } else {
        /* Syntax error. */
        addReply(c,shared.syntaxerr);
        return;
    }

    /* For empty ranges (start > end) we return -1 as an empty range does
     * not contain a 0 nor a 1. */
    if (start > end) {
        addReplyLongLong(c, -1);
    } else {
        long bytes = end-start+1;
        long pos = redisBitpos(p+start,bytes,bit);

        /* If we are looking for clear bits, and the user specified an exact
         * range with start-end, we can't consider the right of the range as
         * zero padded (as we do when no explicit end is given).
         *
         * So if redisBitpos() returns the first bit outside the range,
         * we return -1 to the caller, to mean, in the specified range there
         * is not a single "0" bit. */
        if (end_given && bit == 0 && pos == bytes*8) {
            addReplyLongLong(c,-1);
            return;
        }
        if (pos != -1) pos += start*8; /* Adjust for the bytes we skipped. */
        addReplyLongLong(c,pos);
    }
}

/* BITFIELD key subcommmand-1 arg ... subcommand-2 arg ... subcommand-N ...
 *
 * Supported subcommands:
 *
 * GET <type> <offset>
 * SET <type> <offset> <value>
 * INCRBY <type> <offset> <increment>
 * OVERFLOW [WRAP|SAT|FAIL]
 */

struct bitfieldOp {
    uint64_t offset;    /* Bitfield offset. */
    int64_t i64;        /* Increment amount (INCRBY) or SET value */
    int opcode;         /* Operation id. */
    int owtype;         /* Overflow type to use. */
    int bits;           /* Integer bitfield bits width. */
    int sign;           /* True if signed, otherwise unsigned op. */
};

void bitfieldCommand(client *c) {
    robj *o;
    size_t bitoffset;
    int j, numops = 0, changes = 0;
    struct bitfieldOp *ops = NULL; /* Array of ops to execute at end. */
    int owtype = BFOVERFLOW_WRAP; /* Overflow type. */
    int readonly = 1;
    size_t higest_write_offset = 0;

    for (j = 2; j < c->argc; j++) {
        int remargs = c->argc-j-1; /* Remaining args other than current. */
        char *subcmd = c->argv[j]->ptr; /* Current command name. */
        int opcode; /* Current operation code. */
        long long i64 = 0;  /* Signed SET value. */
        int sign = 0; /* Signed or unsigned type? */
        int bits = 0; /* Bitfield width in bits. */

        if (!strcasecmp(subcmd,"get") && remargs >= 2)
            opcode = BITFIELDOP_GET;
        else if (!strcasecmp(subcmd,"set") && remargs >= 3)
            opcode = BITFIELDOP_SET;
        else if (!strcasecmp(subcmd,"incrby") && remargs >= 3)
            opcode = BITFIELDOP_INCRBY;
        else if (!strcasecmp(subcmd,"overflow") && remargs >= 1) {
            char *owtypename = c->argv[j+1]->ptr;
            j++;
            if (!strcasecmp(owtypename,"wrap"))
                owtype = BFOVERFLOW_WRAP;
            else if (!strcasecmp(owtypename,"sat"))
                owtype = BFOVERFLOW_SAT;
            else if (!strcasecmp(owtypename,"fail"))
                owtype = BFOVERFLOW_FAIL;
            else {
                addReplyError(c,"Invalid OVERFLOW type specified");
                zfree(ops);
                return;
            }
            continue;
        } else {
            addReply(c,shared.syntaxerr);
            zfree(ops);
            return;
        }

        /* Get the type and offset arguments, common to all the ops. */
        if (getBitfieldTypeFromArgument(c,c->argv[j+1],&sign,&bits) != C_OK) {
            zfree(ops);
            return;
        }

        if (getBitOffsetFromArgument(c,c->argv[j+2],&bitoffset,1,bits) != C_OK){
            zfree(ops);
            return;
        }

        if (opcode != BITFIELDOP_GET) {
            readonly = 0;
            if (higest_write_offset < bitoffset + bits - 1)
                higest_write_offset = bitoffset + bits - 1;
            /* INCRBY and SET require another argument. */
            if (getLongLongFromObjectOrReply(c,c->argv[j+3],&i64,NULL) != C_OK){
                zfree(ops);
                return;
            }
        }

        /* Populate the array of operations we'll process. */
        ops = zrealloc(ops,sizeof(*ops)*(numops+1));
        ops[numops].offset = bitoffset;
        ops[numops].i64 = i64;
        ops[numops].opcode = opcode;
        ops[numops].owtype = owtype;
        ops[numops].bits = bits;
        ops[numops].sign = sign;
        numops++;

        j += 3 - (opcode == BITFIELDOP_GET);
    }

    if (readonly) {
        /* Lookup for read is ok if key doesn't exit, but errors
         * if it's not a string. */
        o = lookupKeyRead(c->db,c->argv[1]);
        if (o != NULL && checkType(c,o,OBJ_STRING)) return;
    } else {
        /* Lookup by making room up to the farest bit reached by
         * this operation. */
        if ((o = lookupStringForBitCommand(c,
            higest_write_offset)) == NULL) return;
    }

    addReplyMultiBulkLen(c,numops);

    /* Actually process the operations. */
    for (j = 0; j < numops; j++) {
        struct bitfieldOp *thisop = ops+j;

        /* Execute the operation. */
        if (thisop->opcode == BITFIELDOP_SET ||
            thisop->opcode == BITFIELDOP_INCRBY)
        {
            /* SET and INCRBY: We handle both with the same code path
             * for simplicity. SET return value is the previous value so
             * we need fetch & store as well. */

            /* We need two different but very similar code paths for signed
             * and unsigned operations, since the set of functions to get/set
             * the integers and the used variables types are different. */
            if (thisop->sign) {
                int64_t oldval, newval, wrapped, retval;
                int overflow;

                oldval = getSignedBitfield(o->ptr,thisop->offset,
                        thisop->bits);

                if (thisop->opcode == BITFIELDOP_INCRBY) {
                    newval = oldval + thisop->i64;
                    overflow = checkSignedBitfieldOverflow(oldval,
                            thisop->i64,thisop->bits,thisop->owtype,&wrapped);
                    if (overflow) newval = wrapped;
                    retval = newval;
                } else {
                    newval = thisop->i64;
                    overflow = checkSignedBitfieldOverflow(newval,
                            0,thisop->bits,thisop->owtype,&wrapped);
                    if (overflow) newval = wrapped;
                    retval = oldval;
                }

                /* On overflow of type is "FAIL", don't write and return
                 * NULL to signal the condition. */
                if (!(overflow && thisop->owtype == BFOVERFLOW_FAIL)) {
                    addReplyLongLong(c,retval);
                    setSignedBitfield(o->ptr,thisop->offset,
                                      thisop->bits,newval);
                } else {
                    addReply(c,shared.nullbulk);
                }
            } else {
                uint64_t oldval, newval, wrapped, retval;
                int overflow;

                oldval = getUnsignedBitfield(o->ptr,thisop->offset,
                        thisop->bits);

                if (thisop->opcode == BITFIELDOP_INCRBY) {
                    newval = oldval + thisop->i64;
                    overflow = checkUnsignedBitfieldOverflow(oldval,
                            thisop->i64,thisop->bits,thisop->owtype,&wrapped);
                    if (overflow) newval = wrapped;
                    retval = newval;
                } else {
                    newval = thisop->i64;
                    overflow = checkUnsignedBitfieldOverflow(newval,
                            0,thisop->bits,thisop->owtype,&wrapped);
                    if (overflow) newval = wrapped;
                    retval = oldval;
                }
                /* On overflow of type is "FAIL", don't write and return
                 * NULL to signal the condition. */
                if (!(overflow && thisop->owtype == BFOVERFLOW_FAIL)) {
                    addReplyLongLong(c,retval);
                    setUnsignedBitfield(o->ptr,thisop->offset,
                                        thisop->bits,newval);
                } else {
                    addReply(c,shared.nullbulk);
                }
            }
            changes++;
        } else {
            /* GET */
            unsigned char buf[9];
            long strlen = 0;
            unsigned char *src = NULL;
            char llbuf[LONG_STR_SIZE];

            if (o != NULL)
                src = getObjectReadOnlyString(o,&strlen,llbuf);

            /* For GET we use a trick: before executing the operation
             * copy up to 9 bytes to a local buffer, so that we can easily
             * execute up to 64 bit operations that are at actual string
             * object boundaries. */
            memset(buf,0,9);
            int i;
            size_t byte = thisop->offset >> 3;
            for (i = 0; i < 9; i++) {
                if (src == NULL || i+byte >= (size_t)strlen) break;
                buf[i] = src[i+byte];
            }

            /* Now operate on the copied buffer which is guaranteed
             * to be zero-padded. */
            if (thisop->sign) {
                int64_t val = getSignedBitfield(buf,thisop->offset-(byte*8),
                                            thisop->bits);
                addReplyLongLong(c,val);
            } else {
                uint64_t val = getUnsignedBitfield(buf,thisop->offset-(byte*8),
                                            thisop->bits);
                addReplyLongLong(c,val);
            }
        }
    }

    if (changes) {
        signalModifiedKey(c->db,c->argv[1]);
        notifyKeyspaceEvent(NOTIFY_STRING,"setbit",c->argv[1],c->db->id);
        server.dirty += changes;
    }
    zfree(ops);
}