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/*----------------------------------------------------------------------------
*
* hamming.c
*
* Hamming Distance is a similarity metric
*
* Hamming distance between two strings of equal length is the number of
* positions for which the correspondings symbols are different, i.e, the
* number of substitutions required to change one into the other.
*
* X is a bit string
* Y is a bit string
*
* For each position i we compare X[i] with Y[i]; if it doesn't match we
* accumulate those mismatches.
*
* For example:
*
* x: 1010101010
* y: 1010111000
* ^ ^
* mismatches
*
* s = 2
*
*
* Copyright (c) 2008-2018, Euler Taveira de Oliveira
*
*----------------------------------------------------------------------------
*/
#include "similarity.h"
#include "utils/varbit.h"
/* GUC variables */
double pgs_hamming_threshold = 0.7;
bool pgs_hamming_is_normalized = true;
PG_FUNCTION_INFO_V1(hamming);
Datum
hamming(PG_FUNCTION_ARGS)
{
VarBit *a, *b;
int alen, blen;
bits8 *pa, *pb;
int maxlen;
float8 res = 0.0;
int i;
int n;
a = PG_GETARG_VARBIT_P(0);
b = PG_GETARG_VARBIT_P(1);
alen = VARBITLEN(a);
blen = VARBITLEN(b);
elog(DEBUG1, "alen: %d; blen: %d", alen, blen);
if (alen != blen)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("bit strings must have the same length")));
/* alen and blen have the same length */
maxlen = alen;
pa = VARBITS(a);
pb = VARBITS(b);
for (i = 0; i < VARBITBYTES(a); i++)
{
n = *pa++ ^ *pb++;
while (n)
{
res += n & 1;
n >>= 1;
}
}
elog(DEBUG1, "is normalized: %d", pgs_hamming_is_normalized);
elog(DEBUG1, "maximum length: %d", maxlen);
/*
* FIXME print string of bits
elog(DEBUG1, "hammingdistance(%s, %s) = %.3f", DatumGetCString(varbit_out(VarBitPGetDatum(a))), DatumGetCString(varbit_out(VarBitPGetDatum(b))), res);
*/
/* if one string has zero length then return one */
if (maxlen == 0)
{
PG_RETURN_FLOAT8(1.0);
}
else if (pgs_hamming_is_normalized)
{
res = 1.0 - (res / maxlen);
/*
* FIXME print string of bits
elog(DEBUG1, "hamming(%s, %s) = %.3f", DatumGetCString(varbit_out(VarBitPGetDatum(a))), DatumGetCString(varbit_out(VarBitPGetDatum(b))), res);
*/
PG_RETURN_FLOAT8(res);
}
else
{
PG_RETURN_FLOAT8(res);
}
}
PG_FUNCTION_INFO_V1(hamming_op);
Datum hamming_op(PG_FUNCTION_ARGS)
{
float8 res;
/*
* store *_is_normalized value temporarily 'cause
* threshold (we're comparing against) is normalized
*/
bool tmp = pgs_hamming_is_normalized;
pgs_hamming_is_normalized = true;
res = DatumGetFloat8(DirectFunctionCall2(
hamming,
PG_GETARG_DATUM(0),
PG_GETARG_DATUM(1)));
/* we're done; back to the previous value */
pgs_hamming_is_normalized = tmp;
PG_RETURN_BOOL(res >= pgs_hamming_threshold);
}
PG_FUNCTION_INFO_V1(hamming_text);
Datum
hamming_text(PG_FUNCTION_ARGS)
{
char *a, *b;
int alen, blen;
char *pa, *pb;
int maxlen;
float8 res = 0.0;
a = DatumGetPointer(DirectFunctionCall1(textout, PointerGetDatum(PG_GETARG_TEXT_P(0))));
b = DatumGetPointer(DirectFunctionCall1(textout, PointerGetDatum(PG_GETARG_TEXT_P(1))));
alen = strlen(a);
blen = strlen(b);
if (alen > PGS_MAX_STR_LEN || blen > PGS_MAX_STR_LEN)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("argument exceeds the maximum length of %d bytes",
PGS_MAX_STR_LEN)));
elog(DEBUG1, "alen: %d; blen: %d", alen, blen);
if (alen != blen)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("text strings must have the same length")));
elog(DEBUG1, "a: %s ; b: %s", a, b);
/* alen and blen have the same length */
maxlen = alen;
pa = a;
pb = b;
while (*pa != '\0')
{
elog(DEBUG4, "a: %c ; b: %c", *pa, *pb);
if (*pa++ != *pb++)
res += 1.0;
}
elog(DEBUG1, "is normalized: %d", pgs_hamming_is_normalized);
elog(DEBUG1, "maximum length: %d", maxlen);
elog(DEBUG1, "hammingdistance(%s, %s) = %.3f", a, b, res);
/* if one string has zero length then return one */
if (maxlen == 0)
{
PG_RETURN_FLOAT8(1.0);
}
else if (pgs_hamming_is_normalized)
{
res = 1.0 - (res / maxlen);
elog(DEBUG1, "hamming(%s, %s) = %.3f", a, b, res);
PG_RETURN_FLOAT8(res);
}
else
{
PG_RETURN_FLOAT8(res);
}
}
PG_FUNCTION_INFO_V1(hamming_text_op);
Datum hamming_text_op(PG_FUNCTION_ARGS)
{
float8 res;
/*
* store *_is_normalized value temporarily 'cause
* threshold (we're comparing against) is normalized
*/
bool tmp = pgs_hamming_is_normalized;
pgs_hamming_is_normalized = true;
res = DatumGetFloat8(DirectFunctionCall2(
hamming_text,
PG_GETARG_DATUM(0),
PG_GETARG_DATUM(1)));
/* we're done; back to the previous value */
pgs_hamming_is_normalized = tmp;
PG_RETURN_BOOL(res >= pgs_hamming_threshold);
}
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