Genera los días festivos de colombia dado un año

Go - Genera los días festivos de Colombia

// Genera los días festivos de colombia dado un año
// Basado en los artículos:
// https://www.festivos.com.co/calculo y https://elpais.com/elpais/2017/04/12/el_aleph/1492008750_544261.html
package main

import (
	"fmt"
	"time"
)

const (
	// Formato de fecha
	dateFormat = "2006-01-02"
)

const (
	static typeHoliday = iota + 1
	easter
	nextMonday
)

type typeHoliday byte

type holiday struct {
	name        string
	month       int
	day         int
	typeHoliday typeHoliday
}

func (h *holiday) time(year int) time.Time {
	return shortDate(year, h.month, h.day)
}

func (h *holiday) celebration(year int) time.Time {
	if h.typeHoliday != nextMonday || h.time(year).Weekday() == time.Monday {
		return h.time(year)
	}
	r := time.Monday - h.time(year).Weekday()
	if r < 0 {
		r += 7
	}

	return addDays(h.time(year), int(r))
}

type dayFromEaster struct {
	name    string
	addDays int8
}

func main() {
	var year int = 2019
	fmt.Println(easterDay(year))
	days := append(holidays(), easterDaysToHoliday(year)...)
	show(year, days)
}

func holidays() []holiday {
	return []holiday{
		{"Año nuevo", 1, 1, static},
		{"Día del trabajo", 5, 1, static},
		{"Día de la independencia", 7, 20, static},
		{"Batalla de Boyacá", 8, 7, static},
		{"Inmaculada concepción", 12, 8, static},
		{"Navidad", 12, 25, static},
		{"Reyes Magos", 1, 6, nextMonday},
		{"Día de San José", 3, 19, nextMonday},
		{"San Pedro y San Pablo", 6, 29, nextMonday},
		{"Asunción de la virgen", 8, 15, nextMonday},
		{"Día de la raza", 10, 12, nextMonday},
		{"Todos los santos", 11, 1, nextMonday},
		{"Independencia de Cartagena", 11, 11, nextMonday},
	}
}

func easterDays() []dayFromEaster {
	return []dayFromEaster{
		{"Jueves santo", -3},
		{"Viernes santo", -2},
		{"Ascención de Jesús", 43},
		{"Corpus Christi", 64},
		{"Sagrado corazón de Jesús", 71},
	}
}

func easterDay(year int) time.Time {
	a := year % 19
	b := year % 4
	c := year % 7
	k := year / 100
	p := (13 + 8*k) / 25
	q := k / 4
	M := (15 - p + k - q) % 30
	N := (4 + k - q) % 7
	d := (19*a + M) % 30
	e := (2*b + 4*c + 6*d + N) % 7
	easterDay := time.Now()
	if d+e < 10 {
		easterDay = addDays(shortDate(year, 3, 22), d+e)
	} else {
		easterDay = addDays(shortDate(year, 4, d+e), -9)
	}
	if easterDay.Equal(shortDate(year, 4, 26)) {
		easterDay = shortDate(year, 4, 19)
	}
	if d == 28 && e == 6 && a > 10 && easterDay.Equal(shortDate(year, 4, 25)) {
		easterDay = shortDate(year, 4, 18)
	}

	return easterDay
}

func shortDate(year, month, day int) time.Time {
	return time.Date(year, time.Month(month), day, 0, 0, 0, 0, time.UTC)
}

func addDays(date time.Time, days int) time.Time {
	return date.Add(time.Hour * 24 * time.Duration(days))
}

func show(year int, hs []holiday) {
	for _, v := range hs {
		fmt.Print(v.name)
		fmt.Print(" - ")
		fmt.Print(v.time(year).Format(dateFormat))
		fmt.Print(" - ")
		fmt.Print(v.celebration(year).Format(dateFormat))
		fmt.Println()
	}
}

func easterDaysToHoliday(year int) []holiday {
	r := []holiday{}
	ed := easterDay(year)
	for _, v := range easterDays() {
		day := addDays(ed, int(v.addDays))
		r = append(r, holiday{v.name, int(day.Month()), day.Day(), easter})
	}
	
	return r
}