go/zodb: Tid connection with time

Since in ZODB TIDs are corresponding to time, provide functionality to
convert a tid to timestamp. Do so in exactly the same way as ZODB/py
does for interoperability.

go/zodb/time.go

+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package zodb
+// tid connection with time
+
+import (
+	"time"
+)
+
+// TimeStamp is the same as time.Time only .String() is adjusted to be the same as in ZODB/py.
+//
+// XXX get rid eventually of this and just use time.Time.
+type TimeStamp struct {
+	time.Time
+}
+
+func (t TimeStamp) String() string {
+	return string(t.XFmtString(nil))
+}
+
+func (t TimeStamp) XFmtString(b []byte) []byte {
+	// NOTE UTC() in case we get TimeStamp with modified from-outside location
+	return t.UTC().AppendFormat(b, "2006-01-02 15:04:05.000000")
+}
+
+
+// Time converts tid to time
+func (tid Tid) Time() TimeStamp {
+	// the same as _parseRaw in TimeStamp/py
+	// https://github.com/zopefoundation/persistent/blob/aba23595/persistent/timestamp.py#L75
+	a := uint64(tid) >> 32
+	b := uint64(tid) & (1 << 32 - 1)
+	min   := a % 60
+	hour  := a / 60 % 24
+	day   := a / (60 * 24) % 31 + 1
+	month := a / (60 * 24 * 31) % 12 + 1
+	year  := a / (60 * 24 * 31 * 12) + 1900
+	sec   := b * 60 / (1 << 32)
+	nsec  := (b * 60 - (sec << 32)) * 1E9 / (1 << 32)
+
+	t := time.Date(
+		int(year),
+		time.Month(month),
+		int(day),
+		int(hour),
+		int(min),
+		int(sec),
+		int(nsec),
+		time.UTC)
+
+	// round to microsecond: zodb/py does this, and without rounding it is sometimes
+	// not exactly bit-to-bit the same in text output compared to zodb/py. Example:
+	// 037969f722a53488: timeStr = "2008-10-24 05:11:08.119999"  ; want "2008-10-24 05:11:08.120000"
+	t = t.Round(time.Microsecond)
+
+	return TimeStamp{t}
+}
+
+
+// TODO TidFromTime()
+// TODO TidFromTimeStamp()
+// TODO TidForNow() ?

go/zodb/time_test.go

+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package zodb
+
+import "testing"
+
+func TestTidTime(t *testing.T) {
+	var testv = []struct {tid Tid; timeStr string} {
+		{0x0000000000000000, "1900-01-01 00:00:00.000000"},
+		{0x0285cbac258bf266, "1979-01-03 21:00:08.800000"},
+		{0x0285cbad27ae14e6, "1979-01-03 21:01:09.300001"},
+		{0x037969f722a53488, "2008-10-24 05:11:08.120000"},
+		{0x03b84285d71c57dd, "2016-07-01 09:41:50.416574"},
+	}
+
+	for _, tt := range testv {
+		timeStr := tt.tid.Time().String()
+		if timeStr != tt.timeStr {
+			t.Errorf("%v: timeStr = %q  ; want %q", tt.tid, timeStr, tt.timeStr)
+		}
+	}
+}