static const char *init_once_and_get_prefix(void)
{
gl_once(relocate_once, _get_dir_once);
return (user_prefix) ? user_prefix : relocated_prefix;
}
/* Initializes self_key. */
static void
init_self_key (void)
{
gl_once_define(static, once)
gl_once (once, do_init_self_key);
}
double
fstrcmp_bounded (const char *string1, const char *string2, double lower_bound)
{
struct context ctxt;
size_t xvec_length = strlen (string1);
size_t yvec_length = strlen (string2);
size_t length_sum = xvec_length + yvec_length;
ptrdiff_t i;
ptrdiff_t fdiag_len;
ptrdiff_t *buffer;
uintptr_t bufmax;
/* short-circuit obvious comparisons */
if (xvec_length == 0 || yvec_length == 0) /* Prob: 1% */
return length_sum == 0;
if (! (xvec_length <= length_sum
&& length_sum <= MIN (UINTPTR_MAX, PTRDIFF_MAX) - 3))
xalloc_die ();
if (lower_bound > 0)
{
/* Compute a quick upper bound.
Each edit is an insertion or deletion of an element, hence modifies
the length of the sequence by at most 1.
Therefore, when starting from a sequence X and ending at a sequence Y,
with N edits, | yvec_length - xvec_length | <= N. (Proof by
induction over N.)
So, at the end, we will have
edit_count >= | xvec_length - yvec_length |.
and hence
result
= (xvec_length + yvec_length - edit_count)
/ (xvec_length + yvec_length)
<= (xvec_length + yvec_length - | yvec_length - xvec_length |)
/ (xvec_length + yvec_length)
= 2 * min (xvec_length, yvec_length) / (xvec_length + yvec_length).
*/
ptrdiff_t length_min = MIN (xvec_length, yvec_length);
volatile double upper_bound = 2.0 * length_min / length_sum;
if (upper_bound < lower_bound) /* Prob: 74% */
/* Return an arbitrary value < LOWER_BOUND. */
return 0.0;
#if CHAR_BIT <= 8
/* When X and Y are both small, avoid the overhead of setting up an
array of size 256. */
if (length_sum >= 20) /* Prob: 99% */
{
/* Compute a less quick upper bound.
Each edit is an insertion or deletion of a character, hence
modifies the occurrence count of a character by 1 and leaves the
other occurrence counts unchanged.
Therefore, when starting from a sequence X and ending at a
sequence Y, and denoting the occurrence count of C in X with
OCC (X, C), with N edits,
sum_C | OCC (X, C) - OCC (Y, C) | <= N.
(Proof by induction over N.)
So, at the end, we will have
edit_count >= sum_C | OCC (X, C) - OCC (Y, C) |,
and hence
result
= (xvec_length + yvec_length - edit_count)
/ (xvec_length + yvec_length)
<= (xvec_length + yvec_length - sum_C | OCC(X,C) - OCC(Y,C) |)
/ (xvec_length + yvec_length).
*/
ptrdiff_t occ_diff[UCHAR_MAX + 1]; /* array C -> OCC(X,C) - OCC(Y,C) */
ptrdiff_t sum;
double dsum;
/* Determine the occurrence counts in X. */
memset (occ_diff, 0, sizeof (occ_diff));
for (i = xvec_length - 1; i >= 0; i--)
occ_diff[(unsigned char) string1[i]]++;
/* Subtract the occurrence counts in Y. */
for (i = yvec_length - 1; i >= 0; i--)
occ_diff[(unsigned char) string2[i]]--;
/* Sum up the absolute values. */
sum = 0;
for (i = 0; i <= UCHAR_MAX; i++)
{
ptrdiff_t d = occ_diff[i];
sum += (d >= 0 ? d : -d);
}
dsum = sum;
upper_bound = 1.0 - dsum / length_sum;
if (upper_bound < lower_bound) /* Prob: 66% */
/* Return an arbitrary value < LOWER_BOUND. */
return 0.0;
}
#endif
}
/* set the info for each string. */
ctxt.xvec = string1;
ctxt.yvec = string2;
/* Set TOO_EXPENSIVE to be approximate square root of input size,
bounded below by 4096. */
ctxt.too_expensive = 1;
for (i = xvec_length + yvec_length; i != 0; i >>= 2)
ctxt.too_expensive <<= 1;
if (ctxt.too_expensive < 4096)
ctxt.too_expensive = 4096;
/* Allocate memory for fdiag and bdiag from a thread-local pool. */
fdiag_len = length_sum + 3;
gl_once (keys_init_once, keys_init);
buffer = gl_tls_get (buffer_key);
bufmax = (uintptr_t) gl_tls_get (bufmax_key);
if (fdiag_len > bufmax)
{
/* Need more memory. */
bufmax = 2 * bufmax;
if (fdiag_len > bufmax)
bufmax = fdiag_len;
/* Calling xrealloc would be a waste: buffer's contents does not need
to be preserved. */
free (buffer);
buffer = xnmalloc (bufmax, 2 * sizeof *buffer);
gl_tls_set (buffer_key, buffer);
gl_tls_set (bufmax_key, (void *) (uintptr_t) bufmax);
}
ctxt.fdiag = buffer + yvec_length + 1;
ctxt.bdiag = ctxt.fdiag + fdiag_len;
/* The edit_count is only ever increased. The computation can be aborted
when
(xvec_length + yvec_length - edit_count) / (xvec_length + yvec_length)
< lower_bound,
or equivalently
edit_count > (xvec_length + yvec_length) * (1 - lower_bound)
or equivalently
edit_count > floor((xvec_length + yvec_length) * (1 - lower_bound)).
We need to add an epsilon inside the floor(...) argument, to neutralize
rounding errors. */
ctxt.edit_count_limit =
(lower_bound < 1.0
? (ptrdiff_t) (length_sum * (1.0 - lower_bound + 0.000001))
: 0);
/* Now do the main comparison algorithm */
ctxt.edit_count = - ctxt.edit_count_limit;
if (compareseq (0, xvec_length, 0, yvec_length, 0, &ctxt)) /* Prob: 98% */
/* The edit_count passed the limit. Hence the result would be
< lower_bound. We can return any value < lower_bound instead. */
return 0.0;
ctxt.edit_count += ctxt.edit_count_limit;
/* The result is
((number of chars in common) / (average length of the strings)).
The numerator is
= xvec_length - (number of calls to NOTE_DELETE)
= yvec_length - (number of calls to NOTE_INSERT)
= 1/2 * (xvec_length + yvec_length - (number of edits)).
This is admittedly biased towards finding that the strings are
similar, however it does produce meaningful results. */
return ((double) (xvec_length + yvec_length - ctxt.edit_count)
/ (xvec_length + yvec_length));
}
