Leapsecs and TAI64 support

[gotai64n.git] / leapsecs.go

diff --git a/leapsecs.go b/leapsecs.go
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+++ b/leapsecs.go
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+/*
+go.cypherpunks.ru/tai64n -- Pure Go TAI64/TAI64N implementation
+Copyright (C) 2020-2021 Sergey Matveev <stargrave@stargrave.org>
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, version 3 of the License.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package tai64n
+
+import (
+       "sort"
+       "time"
+)
+
+const Leapsecs1972 = 10
+
+// Database of Unix timestamps of the time when leap second occurred.
+// Library contains and initializes it with leap seconds up to 2016-12-31.
+var LeapsecsDB []int64
+
+// TAI<->UTC difference for the given Unix timestamp.
+func LeapsecsDiff(t int64) int {
+       for i, leap := range LeapsecsDB {
+               if t > leap {
+                       return len(LeapsecsDB) - i
+               }
+       }
+       return 0
+}
+
+// Add currently known (LeapsecsDB) leap seconds, not including initial
+// 1972-01-01 10-seconds offset.
+func LeapsecsAdd(t time.Time) time.Time {
+       return t.Add(time.Second * time.Duration(LeapsecsDiff(t.Unix())))
+}
+
+// Opposite of LeapsecsAdd().
+func LeapsecsSub(t time.Time) time.Time {
+       return t.Add(-time.Second * time.Duration(LeapsecsDiff(t.Unix())))
+}
+
+type Int64s []int64
+
+func (a Int64s) Len() int           { return len(a) }
+func (a Int64s) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a Int64s) Less(i, j int) bool { return a[i] > a[j] }
+
+// Load "leapsecs.dat"-like database: concatenated TAI64 leap seconds.
+// Function panics if encoding is invalid.
+func LeapsecsDBLoad(buf []byte) {
+       db := make([]int64, 0, len(buf)/TAI64Size)
+       for i := 0; i < len(buf); i += TAI64Size {
+               db = append(db, (ToTime(buf[i:i+TAI64Size]).Unix()/86400)*86400)
+       }
+       sort.Sort(Int64s(db))
+       LeapsecsDB = db
+}