+ return (!a != !b) & (0 <= a) & (0 <= b);
+}
+
+/* Return an integer value measuring (YEAR1-YDAY1 HOUR1:MIN1:SEC1) -
+ (YEAR0-YDAY0 HOUR0:MIN0:SEC0) in seconds, assuming that the clocks
+ were not adjusted between the time stamps.
+
+ The YEAR values uses the same numbering as TP->tm_year. Values
+ need not be in the usual range. However, YEAR1 must not be less
+ than 2 * INT_MIN or greater than 2 * INT_MAX.
+
+ The result may overflow. It is the caller's responsibility to
+ detect overflow. */
+
+static inline time_t
+ydhms_diff (long_int year1, long_int yday1, int hour1, int min1, int sec1,
+ int year0, int yday0, int hour0, int min0, int sec0)
+{
+ verify (C99_integer_division, -1 / 2 == 0);
+
+ /* Compute intervening leap days correctly even if year is negative.
+ Take care to avoid integer overflow here. */
+ int a4 = SHR (year1, 2) + SHR (TM_YEAR_BASE, 2) - ! (year1 & 3);
+ int b4 = SHR (year0, 2) + SHR (TM_YEAR_BASE, 2) - ! (year0 & 3);
+ int a100 = a4 / 25 - (a4 % 25 < 0);
+ int b100 = b4 / 25 - (b4 % 25 < 0);
+ int a400 = SHR (a100, 2);
+ int b400 = SHR (b100, 2);
+ int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
+
+ /* Compute the desired time in time_t precision. Overflow might
+ occur here. */
+ time_t tyear1 = year1;
+ time_t years = tyear1 - year0;
+ time_t days = 365 * years + yday1 - yday0 + intervening_leap_days;
+ time_t hours = 24 * days + hour1 - hour0;
+ time_t minutes = 60 * hours + min1 - min0;
+ time_t seconds = 60 * minutes + sec1 - sec0;
+ return seconds;
+}
+
+/* Return the average of A and B, even if A + B would overflow. */
+static time_t
+time_t_avg (time_t a, time_t b)
+{
+ return SHR (a, 1) + SHR (b, 1) + (a & b & 1);
+}
+
+/* Return 1 if A + B does not overflow. If time_t is unsigned and if
+ B's top bit is set, assume that the sum represents A - -B, and
+ return 1 if the subtraction does not wrap around. */
+static int
+time_t_add_ok (time_t a, time_t b)
+{
+ if (! TYPE_SIGNED (time_t))
+ {
+ time_t sum = a + b;
+ return (sum < a) == (TIME_T_MIDPOINT <= b);
+ }
+ else if (WRAPV)
+ {
+ time_t sum = a + b;
+ return (sum < a) == (b < 0);
+ }
+ else
+ {
+ time_t avg = time_t_avg (a, b);
+ return TIME_T_MIN / 2 <= avg && avg <= TIME_T_MAX / 2;
+ }
+}
+
+/* Return 1 if A + B does not overflow. */
+static int
+time_t_int_add_ok (time_t a, int b)
+{
+ verify (int_no_wider_than_time_t, INT_MAX <= TIME_T_MAX);
+ if (WRAPV)
+ {
+ time_t sum = a + b;
+ return (sum < a) == (b < 0);
+ }
+ else
+ {
+ int a_odd = a & 1;
+ time_t avg = SHR (a, 1) + (SHR (b, 1) + (a_odd & b));
+ return TIME_T_MIN / 2 <= avg && avg <= TIME_T_MAX / 2;
+ }
+}
+
+/* Return a time_t value corresponding to (YEAR-YDAY HOUR:MIN:SEC),
+ assuming that *T corresponds to *TP and that no clock adjustments
+ occurred between *TP and the desired time.
+ If TP is null, return a value not equal to *T; this avoids false matches.
+ If overflow occurs, yield the minimal or maximal value, except do not
+ yield a value equal to *T. */
+static time_t
+guess_time_tm (long_int year, long_int yday, int hour, int min, int sec,
+ const time_t *t, const struct tm *tp)
+{
+ if (tp)
+ {
+ time_t d = ydhms_diff (year, yday, hour, min, sec,
+ tp->tm_year, tp->tm_yday,
+ tp->tm_hour, tp->tm_min, tp->tm_sec);
+ if (time_t_add_ok (*t, d))
+ return *t + d;
+ }
+
+ /* Overflow occurred one way or another. Return the nearest result
+ that is actually in range, except don't report a zero difference
+ if the actual difference is nonzero, as that would cause a false
+ match; and don't oscillate between two values, as that would
+ confuse the spring-forward gap detector. */
+ return (*t < TIME_T_MIDPOINT
+ ? (*t <= TIME_T_MIN + 1 ? *t + 1 : TIME_T_MIN)
+ : (TIME_T_MAX - 1 <= *t ? *t - 1 : TIME_T_MAX));
+}
+
+/* Use CONVERT to convert *T to a broken down time in *TP.
+ If *T is out of range for conversion, adjust it so that
+ it is the nearest in-range value and then convert that. */
+static struct tm *
+ranged_convert (struct tm *(*convert) (const time_t *, struct tm *),
+ time_t *t, struct tm *tp)
+{
+ struct tm *r = convert (t, tp);
+
+ if (!r && *t)
+ {
+ time_t bad = *t;
+ time_t ok = 0;
+
+ /* BAD is a known unconvertible time_t, and OK is a known good one.
+ Use binary search to narrow the range between BAD and OK until
+ they differ by 1. */
+ while (bad != ok + (bad < 0 ? -1 : 1))
+ {
+ time_t mid = *t = time_t_avg (ok, bad);
+ r = convert (t, tp);
+ if (r)
+ ok = mid;
+ else
+ bad = mid;
+ }
+
+ if (!r && ok)
+ {
+ /* The last conversion attempt failed;
+ revert to the most recent successful attempt. */
+ *t = ok;
+ r = convert (t, tp);
+ }
+ }
+
+ return r;
+}
+
+
+/* Convert *TP to a time_t value, inverting
+ the monotonic and mostly-unit-linear conversion function CONVERT.
+ Use *OFFSET to keep track of a guess at the offset of the result,
+ compared to what the result would be for UTC without leap seconds.
+ If *OFFSET's guess is correct, only one CONVERT call is needed.
+ This function is external because it is used also by timegm.c. */