Gossip about redis source code 77

Here we mainly talk about bit field operation , You should have a deep understanding of basic bit operations ：

/* 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 (bits < 64 && (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 (overflow 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 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 (bits < 64) {
            uint64_t mask = ((uint64_t)-1) << bits;
            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 or more (server.proto_max_bulk_len).
 *
 * 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, uint64_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 server.proto_max_bulk_len (512MB in bytes by default) */
    if ((loffset < 0) || (loffset >> 3) >= server.proto_max_bulk_len)
    {
        addReplyError(c,err);
        return C_ERR;
    }

    *offset = 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;
}