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* src/timefns.c: Add comments.

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This commit is contained in:
Paul Eggert 2020-03-08 00:16:17 -08:00
parent b16ba4041d
commit 0a3682a566
1 changed files with 24 additions and 8 deletions
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32
src/timefns.c
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@ -491,11 +491,14 @@ timespec_mpz (struct timespec t)
static Lisp_Object
timespec_ticks (struct timespec t)
{
/* For speed, use intmax_t arithmetic if it will do. */
intmax_t accum;
if (FASTER_TIMEFNS
&& !INT_MULTIPLY_WRAPV (t.tv_sec, TIMESPEC_HZ, &accum)
&& !INT_ADD_WRAPV (t.tv_nsec, accum, &accum))
return make_int (accum);
/* Fall back on bignum arithmetic. */
timespec_mpz (t);
return make_integer_mpz ();
}
@ -505,12 +508,17 @@ timespec_ticks (struct timespec t)
static Lisp_Object
lisp_time_hz_ticks (struct lisp_time t, Lisp_Object hz)
{
/* For speed, just return TICKS if T is (TICKS . HZ). */
if (FASTER_TIMEFNS && EQ (t.hz, hz))
return t.ticks;
/* Check HZ for validity. */
if (FIXNUMP (hz))
{
if (XFIXNUM (hz) <= 0)
invalid_hz (hz);
/* For speed, use intmax_t arithmetic if it will do. */
intmax_t ticks;
if (FASTER_TIMEFNS && FIXNUMP (t.ticks) && FIXNUMP (t.hz)
&& !INT_MULTIPLY_WRAPV (XFIXNUM (t.ticks), XFIXNUM (hz), &ticks))
@ -520,6 +528,7 @@ lisp_time_hz_ticks (struct lisp_time t, Lisp_Object hz)
else if (! (BIGNUMP (hz) && 0 < mpz_sgn (*xbignum_val (hz))))
invalid_hz (hz);
/* Fall back on bignum arithmetic. */
mpz_mul (mpz[0],
*bignum_integer (&mpz[0], t.ticks),
*bignum_integer (&mpz[1], hz));
@ -533,9 +542,13 @@ lisp_time_seconds (struct lisp_time t)
{
if (!FASTER_TIMEFNS)
return lisp_time_hz_ticks (t, make_fixnum (1));
/* For speed, use EMACS_INT arithmetic if it will do. */
if (FIXNUMP (t.ticks) && FIXNUMP (t.hz))
return make_fixnum (XFIXNUM (t.ticks) / XFIXNUM (t.hz)
- (XFIXNUM (t.ticks) % XFIXNUM (t.hz) < 0));
/* For speed, inline what lisp_time_hz_ticks would do. */
mpz_fdiv_q (mpz[0],
*bignum_integer (&mpz[0], t.ticks),
*bignum_integer (&mpz[1], t.hz));
@ -1116,21 +1129,22 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
(subtract ? mpz_submul : mpz_addmul) (*iticks, *fa, *nb);
/* Normalize iticks/ihz by dividing both numerator and
denominator by ig = gcd (iticks, ihz). However, if that
would cause the denominator to become less than hzmin,
rescale the denominator upwards from its ordinary value by
multiplying numerator and denominator so that the denominator
becomes at least hzmin. This rescaling avoids returning a
timestamp that is less precise than both a and b, or a
timestamp that looks obsolete when that might be a problem. */
denominator by ig = gcd (iticks, ihz). For speed, though,
skip this division if ihz = 1. */
mpz_t *ig = &mpz[3];
mpz_gcd (*ig, *iticks, *ihz);
if (!FASTER_TIMEFNS || mpz_cmp_ui (*ig, 1) > 0)
{
mpz_divexact (*iticks, *iticks, *ig);
mpz_divexact (*ihz, *ihz, *ig);
/* However, if dividing the denominator by ig would cause the
denominator to become less than hzmin, rescale the denominator
upwards by multiplying the normalized numerator and denominator
so that the resulting denominator becomes at least hzmin.
This rescaling avoids returning a timestamp that is less precise
than both a and b, or a timestamp that looks obsolete when that
might be a problem. */
if (!FASTER_TIMEFNS || mpz_cmp (*ihz, *hzmin) < 0)
{
/* Rescale straightforwardly. Although this might not
@ -1144,6 +1158,8 @@ time_arith (Lisp_Object a, Lisp_Object b, bool subtract)
mpz_mul (*ihz, *ihz, *rescale);
}
}
/* mpz[0] and iticks now correspond to the (HZ . TICKS) pair. */
hz = make_integer_mpz ();
mpz_swap (mpz[0], *iticks);
ticks = make_integer_mpz ();