- EPOCH_YEAR - TM_YEAR_BASE, 0, 0, 0, - guessed_offset);
-
- if (TIME_T_MAX / INT_MAX / 366 / 24 / 60 / 60 < 3)
- {
- /* time_t isn't large enough to rule out overflows, so check
- for major overflows. A gross check suffices, since if t0
- has overflowed, it is off by a multiple of TIME_T_MAX -
- TIME_T_MIN + 1. So ignore any component of the difference
- that is bounded by a small value. */
-
- /* Approximate log base 2 of the number of time units per
- biennium. A biennium is 2 years; use this unit instead of
- years to avoid integer overflow. For example, 2 average
- Gregorian years are 2 * 365.2425 * 24 * 60 * 60 seconds,
- which is 63113904 seconds, and rint (log2 (63113904)) is
- 26. */
- int ALOG2_SECONDS_PER_BIENNIUM = 26;
- int ALOG2_MINUTES_PER_BIENNIUM = 20;
- int ALOG2_HOURS_PER_BIENNIUM = 14;
- int ALOG2_DAYS_PER_BIENNIUM = 10;
- int LOG2_YEARS_PER_BIENNIUM = 1;
-
- int approx_requested_biennia =
- (SHR (year_requested, LOG2_YEARS_PER_BIENNIUM)
- - SHR (EPOCH_YEAR - TM_YEAR_BASE, LOG2_YEARS_PER_BIENNIUM)
- + SHR (mday, ALOG2_DAYS_PER_BIENNIUM)
- + SHR (hour, ALOG2_HOURS_PER_BIENNIUM)
- + SHR (min, ALOG2_MINUTES_PER_BIENNIUM)
- + (LEAP_SECONDS_POSSIBLE
- ? 0
- : SHR (sec, ALOG2_SECONDS_PER_BIENNIUM)));
-
- int approx_biennia = SHR (t0, ALOG2_SECONDS_PER_BIENNIUM);
- int diff = approx_biennia - approx_requested_biennia;
- int approx_abs_diff = diff < 0 ? -1 - diff : diff;
-
- /* IRIX 4.0.5 cc miscalculates TIME_T_MIN / 3: it erroneously
- gives a positive value of 715827882. Setting a variable
- first then doing math on it seems to work.
- (ghazi@caip.rutgers.edu) */
- time_t time_t_max = TIME_T_MAX;
- time_t time_t_min = TIME_T_MIN;
- time_t overflow_threshold =
- (time_t_max / 3 - time_t_min / 3) >> ALOG2_SECONDS_PER_BIENNIUM;
-
- if (overflow_threshold < approx_abs_diff)
- {
- /* Overflow occurred. Try repairing it; this might work if
- the time zone offset is enough to undo the overflow. */
- time_t repaired_t0 = -1 - t0;
- approx_biennia = SHR (repaired_t0, ALOG2_SECONDS_PER_BIENNIUM);
- diff = approx_biennia - approx_requested_biennia;
- approx_abs_diff = diff < 0 ? -1 - diff : diff;
- if (overflow_threshold < approx_abs_diff)
- return -1;
- guessed_offset += repaired_t0 - t0;
- t0 = repaired_t0;
- }
- }