datetime: add interval support

The patch adds interval [1] support for datetime. Except
encoding/decoding interval values from MessagePack, it adds a several
functions for addition and substraction Interval and Datetime types
in GoLang. Reproducing, thus, arithmetic operations from the Lua
implementation [2].

1. https://github.com/tarantool/tarantool/wiki/Datetime-Internals#interval-arithmetic
2. https://github.com/tarantool/tarantool/wiki/Datetime-Internals#arithmetic-operations

Closes #165

Author: oleg-jukovec

CHANGELOG.md

@@ -11,6 +11,7 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 ### Added
 
 - Optional msgpack.v5 usage (#124)
+- Interval support for datetime (#165)
 
 ### Changed
 

datetime/adjust.go

@@ -0,0 +1,31 @@
+package datetime
+
+// An Adjust is used as a parameter for date adjustions, see:
+// https://github.com/tarantool/tarantool/wiki/Datetime-Internals#date-adjustions-and-leap-years
+type Adjust int
+
+const (
+	NoneAdjust   Adjust = 0 // adjust = "none" in Tarantool
+	ExcessAdjust Adjust = 1 // adjust = "excess" in Tarantool
+	LastAdjust   Adjust = 2 // adjust = "last" in Tarantool
+)
+
+// We need the mappings to make NoneAdjust as a default value instead of
+// dtExcess.
+const (
+	dtExcess = 0 // DT_EXCESS from dt-c/dt_arithmetic.h
+	dtLimit  = 1 // DT_LIMIT
+	dtSnap   = 2 // DT_SNAP
+)
+
+var adjustToDt = map[Adjust]int64{
+	NoneAdjust:   dtLimit,
+	ExcessAdjust: dtExcess,
+	LastAdjust:   dtSnap,
+}
+
+var dtToAdjust = map[int64]Adjust{
+	dtExcess: ExcessAdjust,
+	dtLimit:  NoneAdjust,
+	dtSnap:   LastAdjust,
+}

datetime/config.lua

@@ -61,6 +61,11 @@ local function call_datetime_testdata()
 end
 rawset(_G, 'call_datetime_testdata', call_datetime_testdata)
 
+local function call_interval_testdata(interval)
+    return interval
+end
+rawset(_G, 'call_interval_testdata', call_interval_testdata)
+
 -- Set listen only when every other thing is configured.
 box.cfg{
     listen = os.getenv("TEST_TNT_LISTEN"),

datetime/datetime.go

@@ -94,6 +94,101 @@ func NewDatetime(t time.Time) (*Datetime, error) {
 	return dt, nil
 }
 
+func intervalFromDatetime(dtime *Datetime) (ival Interval) {
+	ival.Year = int64(dtime.time.Year())
+	ival.Month = int64(dtime.time.Month())
+	ival.Day = int64(dtime.time.Day())
+	ival.Hour = int64(dtime.time.Hour())
+	ival.Min = int64(dtime.time.Minute())
+	ival.Sec = int64(dtime.time.Second())
+	ival.Nsec = int64(dtime.time.Nanosecond())
+	ival.Adjust = NoneAdjust
+
+	return ival
+}
+
+func daysInMonth(year int64, month int64) int64 {
+	if month == 12 {
+		year++
+		month = 1
+	} else {
+		month += 1
+	}
+	return int64(time.Date(int(year), time.Month(month), 0, 0, 0, 0, 0, time.UTC).Day())
+}
+
+// C imlementation:
+// https://github.com/tarantool/c-dt/blob/cec6acebb54d9e73ea0b99c63898732abd7683a6/dt_arithmetic.c#L74-L98
+func addMonth(ival *Interval, delta int64, adjust Adjust) {
+	oldYear := ival.Year
+	oldMonth := ival.Month
+
+	ival.Month += delta
+	if ival.Month < 1 || ival.Month > 12 {
+		ival.Year += ival.Month / 12
+		ival.Month %= 12
+		if ival.Month < 1 {
+			ival.Year--
+			ival.Month += 12
+		}
+	}
+	if adjust == ExcessAdjust || ival.Day < 28 {
+		return
+	}
+
+	dim := daysInMonth(ival.Year, ival.Month)
+	if ival.Day > dim ||
+		(adjust == LastAdjust && ival.Day == daysInMonth(oldYear, oldMonth)) {
+		ival.Day = dim
+	}
+}
+
+func (dtime *Datetime) add(ival Interval, positive bool) (*Datetime, error) {
+	newVal := intervalFromDatetime(dtime)
+
+	var direction int64
+	if positive {
+		direction = 1
+	} else {
+		direction = -1
+	}
+
+	addMonth(&newVal, direction*ival.Year*12+direction*ival.Month, ival.Adjust)
+	newVal.Day += direction * 7 * ival.Week
+	newVal.Day += direction * ival.Day
+	newVal.Hour += direction * ival.Hour
+	newVal.Min += direction * ival.Min
+	newVal.Sec += direction * ival.Sec
+	newVal.Nsec += direction * ival.Nsec
+
+	tm := time.Date(int(newVal.Year), time.Month(newVal.Month),
+		int(newVal.Day), int(newVal.Hour), int(newVal.Min),
+		int(newVal.Sec), int(newVal.Nsec), dtime.time.Location())
+
+	return NewDatetime(tm)
+}
+
+// Add creates a new Datetime as addition of the Datetime and Interval. It may
+// return an error if a new Datetime is out of supported range,
+// see [NewDatetime].
+func (dtime *Datetime) Add(ival Interval) (*Datetime, error) {
+	return dtime.add(ival, true)
+}
+
+// Add creates a new Datetime as subtraction of the Datetime and Interval. It
+// may return an error if a new Datetime is out of supported range, see
+// [NewDatetime].
+func (dtime *Datetime) Sub(ival Interval) (*Datetime, error) {
+	return dtime.add(ival, false)
+}
+
+// Interval returns an Interval between a next Datetime value.
+func (dtime *Datetime) Interval(next *Datetime) Interval {
+	curIval := intervalFromDatetime(dtime)
+	nextIval := intervalFromDatetime(next)
+	return nextIval.Sub(curIval)
+}
+
 // ToTime returns a time.Time that Datetime contains.
 func (dtime *Datetime) ToTime() time.Time {
 	return dtime.time