Using code from tzcode.

[pdclib] / functions / time / difftime.c

diff --git a/functions/time/difftime.c b/functions/time/difftime.c
index fa6eb9c7f741cfeb0fd12fa1a5b520cc6c4a828d..b76efb95e81aa5af04a4c8248e74bc328f293c84 100644 (file)
--- a/functions/time/difftime.c
+++ b/functions/time/difftime.c
@@ -10,7 +10,47 @@
 
 double difftime( time_t time1, time_t time0 )
 {
-    return ( time1 > time0 ) ? (double)( time1 - time0 ) : -(double)( time0 - time1 );
+    /* If we want to avoid rounding errors and overflows, we need to be
+       careful with the exact type of time_t being unknown to us.
+       The code below is based on tzcode's difftime.c, which is in the
+       public domain, so clarified as of 1996-06-05 by Arthur David Olson.
+    */
+
+    /* If double is large enough, simply covert and substract
+       (assuming that the larger type has more precision).
+    */
+    if ( sizeof( time_t ) < sizeof( double ) )
+    {
+        return (double)time1 - (double)time0;
+    }
+
+    /* The difference of two unsigned values cannot overflow if the
+       minuend is greater or equal to the subtrahend.
+    */
+    if ( ! _PDCLIB_TYPE_SIGNED( time_t ) )
+    {
+        return ( time1 >= time0 ) ? (double)( time1 - time0 ) : -(double)( time0 - time1 );
+    }
+
+    /* Use uintmax_t if wide enough. */
+    if ( sizeof( time_t ) <= sizeof( _PDCLIB_uintmax_t ) )
+    {
+        _PDCLIB_uintmax_t t1 = time1, t0 = time0;
+        return ( time1 >= time0 ) ? t1 - t0 : -(double)( t0 - t1 );
+    }
+
+    /* If both times have the same sign, their difference cannot overflow. */
+    if ( ( time1 < 0 ) == ( time0 < 0 ) )
+    {
+        return time1 - time0;
+    }
+
+    /* The times have opposite signs, and uintmax_t is too narrow.
+       This suffers from double rounding; attempt to lessen that
+       by using long double temporaries.
+    */
+    long double t1 = time1, t0 = time0;
+    return t1 - t0;
 }
 
 #endif