pg_hamming/hamming.c

#include "postgresql/server/postgres.h"
#include "postgresql/server/fmgr.h"

PG_MODULE_MAGIC;

PG_FUNCTION_INFO_V1(hash_is_within_distance);

/**
 * Check if the hamming distance of the two raw byte arrays
 * is within the specified distance
 *
 * It is assumed that: the two arrays are exactly 18 bytes long
 *
 * Import with
    CREATE OR REPLACE FUNCTION hash_is_within_distance(bytea, bytea, integer) RETURNS boolean
     AS '/path/to/libhamming.so', 'hash_is_within_distance'
     LANGUAGE C STRICT;'
 *
 * @return the hamming distance between the two arrays
 */
Datum hash_is_within_distance(PG_FUNCTION_ARGS) {

    char *h1 = VARDATA(PG_GETARG_BYTEA_P(0));
    char *h2 = VARDATA(PG_GETARG_BYTEA_P(1));
    int32 max_distance = PG_GETARG_INT32(2);

    int distance = 0;

    distance += __builtin_popcountll(
            *((uint64 *) h1) ^ *((uint64 *) h2)
    );
    if (distance > max_distance) {
        PG_RETURN_BOOL(false);
    }
    distance += __builtin_popcountll(
            *((uint64 *) h1 + 1) ^ *((uint64 *) h2 + 1)
    );
    if (distance > max_distance) {
        PG_RETURN_BOOL(false);
    }
    distance += __builtin_popcount(
            *((uint16 *) h1 + 8) ^ *((uint16 *) h2 + 8)
    );

    PG_RETURN_BOOL(distance <= max_distance);
}

PG_FUNCTION_INFO_V1(hash_distance);

/**
 * Hamming distance of two raw byte arrays
 *
 * It is assumed that: the two arrays are exactly 18 bytes long
 *
 * Import with
    CREATE OR REPLACE FUNCTION hash_distance(bytea, bytea) RETURNS integer
     AS '/path/to/libhamming.so', 'hash_distance'
     LANGUAGE C STRICT;'
 *
 * @return the hamming distance between the two arrays
 */
Datum hash_distance(PG_FUNCTION_ARGS) {

    char *h1 = VARDATA(PG_GETARG_BYTEA_P(0));
    char *h2 = VARDATA(PG_GETARG_BYTEA_P(1));

    int distance = 0;

    distance += __builtin_popcountll(
            *((uint64 *) h1) ^ *((uint64 *) h2)
    );
    distance += __builtin_popcountll(
            *((uint64 *) h1 + 1) ^ *((uint64 *) h2 + 1)
    );
    distance += __builtin_popcount(
            *((uint16 *) h1 + 8) ^ *((uint16 *) h2 + 8)
            );

    PG_RETURN_INT32(distance);
}