#include <ctype.h>
#include "postgres.h"
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

int popcount(uint64);

PGDLLEXPORT Datum hamming_distance(PG_FUNCTION_ARGS) {
    uint64 v1  = PG_GETARG_INT64(0);
    uint64 v2  = PG_GETARG_INT64(1);

    PG_RETURN_INT16(popcount(v1^v2));
}

PG_FUNCTION_INFO_V1(hamming_distance);

const uint64 m1  = 0x5555555555555555;
const uint64 m2  = 0x3333333333333333;
const uint64 m4  = 0x0f0f0f0f0f0f0f0f;
const uint64 h01 = 0x0101010101010101;

int popcount(uint64 x) {
    x -= (x >> 1) & m1;
    x = (x & m2) + ((x >> 2) & m2);
    x = (x + (x >> 4)) & m4;
    return (x * h01)>>56;
}

CREATE FUNCTION hamming_distance(bigint, bigint) RETURNS smallint
     AS 'C:\Program Files\PostgreSQL\9.4\lib\TestPostgre.dll', 'hamming_distance'
     LANGUAGE C STRICT;

select hamming_distance(4575515931806203904, 14106333703411661563)

CREATE FUNCTION hamming_distance(numeric, numeric) RETURNS smallint
     AS 'C:\Program Files\PostgreSQL\9.4\lib\TestPostgre.dll', 'hamming_distance'
     LANGUAGE C STRICT;

select hamming_distance(4575515931806203904, 14106333703411661563)