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; * src/fns.c (fixnum_float_cmp): Explain argument constraint.

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Suggested by Pip Cet.
This commit is contained in:
Mattias Engdegård 2024-07-04 17:48:16 +02:00
parent 0b061ab2ed
commit fa6f088a48
1 changed files with 6 additions and 4 deletions
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10
src/fns.c
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@ -3012,16 +3012,18 @@ bool_vector_cmp (Lisp_Object a, Lisp_Object b)
return (d & aw) ? 1 : -1;
}
/* Return -1 if a<b, 1 if a>b, 0 if a=b or if b is NaN. */
/* Return -1 if a<b, 1 if a>b, 0 if a=b or if b is NaN (a must be a fixnum). */
static inline int
fixnum_float_cmp (EMACS_INT a, double b)
{
double fa = (double)a;
if (fa == b)
{
/* This doesn't mean that a=b because the conversion may have
rounded, but b must be an integer that fits in an EMACS_INT
and we can compare in the integer domain instead. */
/* This doesn't mean that a=b because the conversion may have rounded.
However, b must be an integer that fits in an EMACS_INT,
because |b| ≤ 2|a| and EMACS_INT has at least one bit more than
needed to represent any fixnum.
Thus we can compare in the integer domain instead. */
EMACS_INT ib = b; /* lossless conversion */
return a < ib ? -1 : a > ib;
}