Eggert mods

SCCS-file: difftime.c
SCCS-SID: 7.2

difftime.c

@@ -8,10 +8,62 @@ static char	elsieid[] = "%W%";
 
 #include "private.h"
 
+/*
+** Algorithm courtesy Paul Eggert (eggert@twinsun.com).
+*/
+
 double
 difftime(time1, time0)
 const time_t	time1;
 const time_t	time0;
 {
-	return time1 - time0;
+	time_t	delta;
+	time_t	hibit;
+
+	if (sizeof(time_t) < sizeof(double))
+		return (double) time1 - (double) time0;
+#ifdef HAVE_LONG_DOUBLE
+	if (sizeof(time_t) < sizeof(long double))
+		return (long double) time1 - (long double) time0;
+#endif /* defined HAVE_LONG_DOUBLE */
+	if (time1 < time0)
+		return -difftime(time0, time1);
+	/*
+	** As much as possible, avoid loss of precision
+	** by computing the difference before converting to double.
+	*/
+	delta = time1 - time0;
+	if (delta >= 0)
+		return delta;
+	/*
+	** Repair delta overflow.
+	*/
+	hibit = 1;
+	while ((hibit <<= 1) > 0)
+		continue;
+	/*
+	** The following expression rounds twice, which means
+	** the result may not be the closest to the true answer.
+	** For example, suppose time_t is 64-bit signed int,
+	** double is IEEE 754 double with default rounding,
+	** time1 = 9223372036854775807 and time0 = -1536.
+	** Then the true difference is 9223372036854777343,
+	** which rounds to 9223372036854777856
+	** with a total error of 513.
+	** But delta overflows to -9223372036854774273,
+	** which rounds to -9223372036854774784, and correcting
+	** this by subtracting 2 * (double) hibit
+	** (i.e. by adding 2**64 = 18446744073709551616)
+	** yields 9223372036854776832, which
+	** rounds to 9223372036854775808
+	** with a total error of 1535 instead.
+	** This problem occurs only with very large differences.
+	** It's too painful to fix this portably.
+	** We are not alone in this problem;
+	** many C compilers round twice when converting
+	** large unsigned types to small floating types,
+	** so if time_t is unsigned the "return delta" above
+	** has the same double-rounding problem.
+	*/
+	return delta - 2 * (double) hibit;
 }