Get N nearest dates

main.go

package main

import (
	"time"
	"fmt"
)

func main() {
	times := NewReservationBuilder().
		SetDate(2018, 9, 3).
		Weekdays(time.Sunday).
		NearestN(10)

	for i := 0; i < len(times); i++ {
		fmt.Println(times[i].Format("2006-01-02"))
	}
}

type ReservationBuilder struct {
	date     time.Time
	weekdays []time.Weekday
}

func NewReservationBuilder() ReservationBuilder {
	return ReservationBuilder{
		date: time.Now().UTC(),
	}
}

func (b ReservationBuilder) SetTime(t time.Time) ReservationBuilder {
	b.date = t
	return b
}

func (b ReservationBuilder) SetDate(year int, month time.Month, day int) ReservationBuilder {
	b.date = time.Date(year, month, day, 0, 0, 0, 0, time.UTC)
	return b
}

func (b ReservationBuilder) Weekdays(weekdays ...time.Weekday) ReservationBuilder {
	b.weekdays = weekdays
	return b
}

func (b ReservationBuilder) isExistWeekday(weekday time.Weekday) bool {
	for _, wd := range b.weekdays {
		if weekday == wd {
			return true
		}
	}
	return false
}

func (b ReservationBuilder) NearestN(count int) []time.Time {
	if len(b.weekdays) == 0 {
		return nil
	}

	times := make([]time.Time, 0, count)
	for {
		if len(times) == count {
			break
		}
		if b.isExistWeekday(b.date.Weekday()) {
			times = append(times, b.date)
		}
		b.date = b.date.AddDate(0, 0, 1)
	}
	return times
}