/*!
 * Copyright 2013 Raymond Hill
 *
 * Project: github.com/gorhill/cronexpr
 * File: cronexpr.go
 * Version: 1.0
 * License: GPL v3 see <https://www.gnu.org/licenses/gpl.html>
 *
 */

// Package cronexpr parses cron time expressions.
package cronexpr

/******************************************************************************/

import (
	"regexp"
	"sort"
	"strconv"
	"strings"
	"time"
)

/******************************************************************************/

// A Expression represents a specific cron time expression as defined at
// <https://github.com/gorhill/cronexpr#implementation>
type Expression struct {
	expression             string
	secondList             []int
	minuteList             []int
	hourList               []int
	daysOfMonth            map[int]bool
	workdaysOfMonth        map[int]bool
	lastDayOfMonth         bool
	daysOfMonthRestricted  bool
	actualDaysOfMonthList  []int
	monthList              []int
	daysOfWeek             map[int]bool
	specificWeekDaysOfWeek map[int]bool
	lastWeekDaysOfWeek     map[int]bool
	daysOfWeekRestricted   bool
	yearList               []int
}

/******************************************************************************/

// Returns a new Expression pointer. It expects a well-formed cron expression.
// If a malformed cron expression is supplied, the result is undefined. See
// <https://github.com/gorhill/cronexpr#implementation> for documentation
// about what is a well-formed cron expression from this library point of view.
func Parse(cronLine string) *Expression {
	cronLineNormalized := cronNormalize(cronLine)

	// Split into fields
	cronFields := regexp.MustCompile(`\s+`).Split(cronLineNormalized, -1)

	// Our cron expression parser expects 7 fields:
	//    second minute hour dayofmonth month dayofweek year
	// Standard cron is 6 fields with year field being optional
	//           minute hour dayofmonth month dayofweek {year}
	// Thus...
	// If we have 5 fields, append wildcard year field
	if len(cronFields) < 6 {
		cronFields = append(cronFields, "*")
	}
	// If we have 6 fields, prepend match-once second field
	if len(cronFields) < 7 {
		cronFields = append(cronFields, "")
		copy(cronFields[1:], cronFields[0:])
		cronFields[0] = "0"
	}
	// At this point, we should have at least 7 fields. Fields beyond the
	// seventh one, if any, are ignored.
	if len(cronFields) < 7 {
		panic("Malformed cron expression\n")
	}

	// Generic parser can be used for most fields
	cronExpr := &Expression{
		expression: cronLine,
		secondList: genericFieldParse(cronFields[0], 0, 59),
		minuteList: genericFieldParse(cronFields[1], 0, 59),
		hourList:   genericFieldParse(cronFields[2], 0, 23),
		monthList:  genericFieldParse(cronFields[4], 1, 12),
		yearList:   genericFieldParse(cronFields[6], 1970, 2099),
	}

	// Days of month/days of week is a bit more complicated, due
	// to their extended syntax, and the fact that days per
	// month is a variable quantity, and relation between
	// days of week and days of month depends on the month/year.
	cronExpr.dayofmonthFieldParse(cronFields[3])
	cronExpr.dayofweekFieldParse(cronFields[5])

	return cronExpr
}

/******************************************************************************/

// Given a time stamp `fromTime`, return the closest following time stamp which
// matches the cron expression `expr`. The `time.Location` of the returned
// time stamp is the same as `fromTime`.
//
// A nil time.Time object is returned if no matching time stamp exists.
func (expr *Expression) Next(fromTime time.Time) time.Time {
	// Special case
	if fromTime.IsZero() {
		return fromTime
	}

	// Since expr.nextSecond()-expr.nextMonth() expects that the
	// supplied time stamp is a perfect match to the underlying cron
	// expression, and since this function is an entry point where `fromTime`
	// does not necessarily matches the underlying cron expression,
	// we first need to ensure supplied time stamp matches
	// the cron expression. If not, this means the supplied time
	// stamp falls in between matching time stamps, thus we move
	// to closest future matching immediately upon encountering a mismatching
	// time stamp.

	// year
	v := fromTime.Year()
	i := sort.SearchInts(expr.yearList, v)
	if i == len(expr.yearList) {
		return time.Time{}
	}
	if v != expr.yearList[i] {
		return expr.nextYear(fromTime)
	}
	// month
	v = int(fromTime.Month())
	i = sort.SearchInts(expr.monthList, v)
	if i == len(expr.monthList) {
		return expr.nextYear(fromTime)
	}
	if v != expr.monthList[i] {
		return expr.nextMonth(fromTime)
	}

	expr.actualDaysOfMonthList = expr.calculateActualDaysOfMonth(fromTime.Year(), int(fromTime.Month()))
	if len(expr.actualDaysOfMonthList) == 0 {
		return expr.nextMonth(fromTime)
	}

	// day of month
	v = fromTime.Day()
	i = sort.SearchInts(expr.actualDaysOfMonthList, v)
	if i == len(expr.actualDaysOfMonthList) {
		return expr.nextMonth(fromTime)
	}
	if v != expr.actualDaysOfMonthList[i] {
		return expr.nextDayOfMonth(fromTime)
	}
	// hour
	v = fromTime.Hour()
	i = sort.SearchInts(expr.hourList, v)
	if i == len(expr.hourList) {
		return expr.nextDayOfMonth(fromTime)
	}
	if v != expr.hourList[i] {
		return expr.nextHour(fromTime)
	}
	// minute
	v = fromTime.Minute()
	i = sort.SearchInts(expr.minuteList, v)
	if i == len(expr.minuteList) {
		return expr.nextHour(fromTime)
	}
	if v != expr.minuteList[i] {
		return expr.nextMinute(fromTime)
	}
	// second
	v = fromTime.Second()
	i = sort.SearchInts(expr.secondList, v)
	if i == len(expr.secondList) {
		return expr.nextMinute(fromTime)
	}

	// If we reach this point, there is nothing better to do
	// than to move to the next second

	return expr.nextSecond(fromTime)
}

/******************************************************************************/

// Given a time stamp `fromTime`, return a slice of `n` closest following time
// stamps which match the cron expression `expr`. The time stamps in the
// returned slice are in chronological ascending order. The `time.Location` of
// the returned time stamps is the same as `fromTime`.
//
// A slice with less than `n` entries (up to zero) is returned if not
// enough existing matching time stamps which exist.
func (expr *Expression) NextN(fromTime time.Time, n int) []time.Time {
	if n <= 0 {
		panic("Expression.NextN(): invalid count")
	}
	nextTimes := make([]time.Time, 0)
	fromTime = expr.Next(fromTime)
	for {
		if fromTime.IsZero() {
			break
		}
		nextTimes = append(nextTimes, fromTime)
		n -= 1
		if n == 0 {
			break
		}
		fromTime = expr.nextSecond(fromTime)
	}
	return nextTimes
}

/******************************************************************************/

func (expr *Expression) nextYear(t time.Time) time.Time {
	// Find index at which item in list is greater or equal to
	// candidate year
	i := sort.SearchInts(expr.yearList, t.Year()+1)
	if i == len(expr.yearList) {
		return time.Time{}
	}
	// Year changed, need to recalculate actual days of month
	expr.actualDaysOfMonthList = expr.calculateActualDaysOfMonth(expr.yearList[i], expr.monthList[0])
	if len(expr.actualDaysOfMonthList) == 0 {
		return expr.nextMonth(time.Date(
			expr.yearList[i],
			time.Month(expr.monthList[0]),
			1,
			expr.hourList[0],
			expr.minuteList[0],
			expr.secondList[0],
			0,
			t.Location()))
	}
	return time.Date(
		expr.yearList[i],
		time.Month(expr.monthList[0]),
		expr.actualDaysOfMonthList[0],
		expr.hourList[0],
		expr.minuteList[0],
		expr.secondList[0],
		0,
		t.Location())
}

/******************************************************************************/

func (expr *Expression) nextMonth(t time.Time) time.Time {
	// Find index at which item in list is greater or equal to
	// candidate month
	i := sort.SearchInts(expr.monthList, int(t.Month())+1)
	if i == len(expr.monthList) {
		return expr.nextYear(t)
	}
	// Month changed, need to recalculate actual days of month
	expr.actualDaysOfMonthList = expr.calculateActualDaysOfMonth(t.Year(), expr.monthList[i])
	if len(expr.actualDaysOfMonthList) == 0 {
		return expr.nextMonth(time.Date(
			t.Year(),
			time.Month(expr.monthList[i]),
			1,
			expr.hourList[0],
			expr.minuteList[0],
			expr.secondList[0],
			0,
			t.Location()))
	}

	return time.Date(
		t.Year(),
		time.Month(expr.monthList[i]),
		expr.actualDaysOfMonthList[0],
		expr.hourList[0],
		expr.minuteList[0],
		expr.secondList[0],
		0,
		t.Location())
}

/******************************************************************************/

func (expr *Expression) calculateActualDaysOfMonth(year, month int) []int {
	actualDaysOfMonthMap := make(map[int]bool)
	timeOrigin := time.Date(year, time.Month(month), 1, 0, 0, 0, 0, time.UTC)
	lastDayOfMonth := timeOrigin.AddDate(0, 1, -1).Day()

	// As per crontab man page (http://linux.die.net/man/5/crontab#):
	//  "The day of a command's execution can be specified by two
	//  "fields - day of month, and day of week. If both fields are
	//  "restricted (ie, aren't *), the command will be run when
	//  "either field matches the current time"
	if expr.daysOfMonthRestricted || expr.daysOfWeekRestricted == false {
		// Last day of month
		if expr.lastDayOfMonth {
			actualDaysOfMonthMap[lastDayOfMonth] = true
		}
		// Days of month
		for v, _ := range expr.daysOfMonth {
			// Ignore days beyond end of month
			if v <= lastDayOfMonth {
				actualDaysOfMonthMap[v] = true
			}
		}
		// Work days of month
		// As per Wikipedia: month boundaries are not crossed.
		for v, _ := range expr.workdaysOfMonth {
			// Ignore days beyond end of month
			if v <= lastDayOfMonth {
				// If saturday, then friday
				if timeOrigin.AddDate(0, 0, v-1).Weekday() == time.Saturday {
					if v > 1 {
						v -= 1
					} else {
						v += 2
					}
					// If sunday, then monday
				} else if timeOrigin.AddDate(0, 0, v-1).Weekday() == time.Sunday {
					if v < lastDayOfMonth {
						v += 1
					} else {
						v -= 2
					}
				}
				actualDaysOfMonthMap[v] = true
			}
		}
	}

	if expr.daysOfWeekRestricted {
		// How far first sunday is from first day of month
		offset := 7 - int(timeOrigin.Weekday())
		// days of week
		//  offset : (7 - day_of_week_of_1st_day_of_month)
		//  target : 1 + (7 * week_of_month) + (offset + day_of_week) % 7
		for w := 0; w <= 4; w += 1 {
			for v, _ := range expr.daysOfWeek {
				v := 1 + w*7 + (offset+v)%7
				if v <= lastDayOfMonth {
					actualDaysOfMonthMap[v] = true
				}
			}
		}
		// days of week of specific week in the month
		//  offset : (7 - day_of_week_of_1st_day_of_month)
		//  target : 1 + (7 * week_of_month) + (offset + day_of_week) % 7
		for v, _ := range expr.specificWeekDaysOfWeek {
			v := 1 + 7*(v/7) + (offset+v)%7
			if v <= lastDayOfMonth {
				actualDaysOfMonthMap[v] = true
			}
		}
		// Last days of week of the month
		lastWeekOrigin := timeOrigin.AddDate(0, 1, -7)
		offset = 7 - int(lastWeekOrigin.Weekday())
		for v, _ := range expr.lastWeekDaysOfWeek {
			v := lastWeekOrigin.Day() + (offset+v)%7
			if v <= lastDayOfMonth {
				actualDaysOfMonthMap[v] = true
			}
		}
	}

	return toList(actualDaysOfMonthMap)
}

/******************************************************************************/

func (expr *Expression) nextDayOfMonth(t time.Time) time.Time {
	// Find index at which item in list is greater or equal to
	// candidate day of month
	i := sort.SearchInts(expr.actualDaysOfMonthList, t.Day()+1)
	if i == len(expr.actualDaysOfMonthList) {
		return expr.nextMonth(t)
	}

	return time.Date(
		t.Year(),
		t.Month(),
		expr.actualDaysOfMonthList[i],
		expr.hourList[0],
		expr.minuteList[0],
		expr.secondList[0],
		0,
		t.Location())
}

/******************************************************************************/

func (expr *Expression) nextHour(t time.Time) time.Time {
	// Find index at which item in list is greater or equal to
	// candidate hour
	i := sort.SearchInts(expr.hourList, t.Hour()+1)
	if i == len(expr.hourList) {
		return expr.nextDayOfMonth(t)
	}

	return time.Date(
		t.Year(),
		t.Month(),
		t.Day(),
		expr.hourList[i],
		expr.minuteList[0],
		expr.secondList[0],
		0,
		t.Location())
}

/******************************************************************************/

func (expr *Expression) nextMinute(t time.Time) time.Time {
	// Find index at which item in list is greater or equal to
	// candidate minute
	i := sort.SearchInts(expr.minuteList, t.Minute()+1)
	if i == len(expr.minuteList) {
		return expr.nextHour(t)
	}

	return time.Date(
		t.Year(),
		t.Month(),
		t.Day(),
		t.Hour(),
		expr.minuteList[i],
		expr.secondList[0],
		0,
		t.Location())
}

/******************************************************************************/

func (expr *Expression) nextSecond(t time.Time) time.Time {
	// nextSecond() assumes all other fields are exactly matched
	// to the cron expression

	// Find index at which item in list is greater or equal to
	// candidate second
	i := sort.SearchInts(expr.secondList, t.Second()+1)
	if i == len(expr.secondList) {
		return expr.nextMinute(t)
	}

	return time.Date(
		t.Year(),
		t.Month(),
		t.Day(),
		t.Hour(),
		t.Minute(),
		expr.secondList[i],
		0,
		t.Location())
}

/******************************************************************************/

var cronNormalizer = strings.NewReplacer(
	// Order is important!
	"@yearly", "0 0 0 1 1 * *",
	"@annually", "0 0 0 1 1 * *",
	"@monthly", "0 0 0 1 * * *",
	"@weekly", "0 0 0 * * 0 *",
	"@daily", "0 0 0 * * * *",
	"@hourly", "0 0 * * * * *",
	"january", "1",
	"february", "2",
	"march", "3",
	"april", "4",
	"may", "5",
	"june", "6",
	"july", "7",
	"august", "8",
	"september", "9",
	"october", "0",
	"november", "1",
	"december", "2",
	"sunday", "0",
	"monday", "1",
	"tuesday", "2",
	"wednesday", "3",
	"thursday", "4",
	"friday", "5",
	"saturday", "6",
	"jan", "1",
	"feb", "2",
	"mar", "3",
	"apr", "4",
	"jun", "6",
	"jul", "7",
	"aug", "8",
	"sep", "9",
	"oct", "10",
	"nov", "11",
	"dec", "12",
	"sun", "0",
	"mon", "1",
	"tue", "2",
	"wed", "3",
	"thu", "4",
	"fri", "5",
	"sat", "6",
	"?", "*")

func cronNormalize(cronLine string) string {
	cronLine = strings.TrimSpace(cronLine)
	cronLine = strings.ToLower(cronLine)
	cronLine = cronNormalizer.Replace(cronLine)
	return cronLine
}

/******************************************************************************/

func (expr *Expression) dayofweekFieldParse(cronField string) error {
	// Defaults
	expr.daysOfWeekRestricted = true
	expr.lastWeekDaysOfWeek = make(map[int]bool)
	expr.daysOfWeek = make(map[int]bool)

	// "You can also mix all of the above, as in: 1-5,10,12,20-30/5"
	cronList := strings.Split(cronField, ",")
	for _, s := range cronList {
		// "/"
		step, s := extractInterval(s)
		// "*"
		if s == "*" {
			expr.daysOfWeekRestricted = (step > 1)
			populateMany(expr.daysOfWeek, 0, 6, step)
			continue
		}
		// "-"
		// week day interval for all weeks
		i := strings.Index(s, "-")
		if i >= 0 {
			min := atoi(s[:i]) % 7
			max := atoi(s[i+1:]) % 7
			populateMany(expr.daysOfWeek, min, max, step)
			continue
		}
		// single value
		// "l": week day for last week
		i = strings.Index(s, "l")
		if i >= 0 {
			populateOne(expr.lastWeekDaysOfWeek, atoi(s[:i])%7)
			continue
		}
		// "#": week day for specific week
		i = strings.Index(s, "#")
		if i >= 0 {
			// v#w
			v := atoi(s[:i]) % 7
			w := atoi(s[i+1:])
			// v domain = [0,7]
			// w domain = [1,5]
			populateOne(expr.specificWeekDaysOfWeek, (w-1)*7+(v%7))
			continue
		}
		// week day interval for all weeks
		if step > 0 {
			v := atoi(s) % 7
			populateMany(expr.daysOfWeek, v, 6, step)
			continue
		}
		// single week day for all weeks
		v := atoi(s) % 7
		populateOne(expr.daysOfWeek, v)
	}

	return nil
}

/******************************************************************************/

func (expr *Expression) dayofmonthFieldParse(cronField string) error {
	// Defaults
	expr.daysOfMonthRestricted = true
	expr.lastDayOfMonth = false

	expr.daysOfMonth = make(map[int]bool)     // days of month map
	expr.workdaysOfMonth = make(map[int]bool) // work day of month map

	// Comma separator is used to mix different allowed syntax
	cronList := strings.Split(cronField, ",")
	for _, s := range cronList {
		// "/"
		step, s := extractInterval(s)
		// "*"
		if s == "*" {
			expr.daysOfMonthRestricted = (step > 1)
			populateMany(expr.daysOfMonth, 1, 31, step)
			continue
		}
		// "-"
		i := strings.Index(s, "-")
		if i >= 0 {
			populateMany(expr.daysOfMonth, atoi(s[:i]), atoi(s[i+1:]), step)
			continue
		}
		// single value
		// "l": last day of month
		if s == "l" {
			expr.lastDayOfMonth = true
			continue
		}
		// "w": week day
		i = strings.Index(s, "w")
		if i >= 0 {
			populateOne(expr.workdaysOfMonth, atoi(s[:i]))
			continue
		}
		// single value with interval
		if step > 0 {
			populateMany(expr.daysOfMonth, atoi(s), 31, step)
			continue
		}
		// single value
		populateOne(expr.daysOfMonth, atoi(s))
	}

	return nil
}

/******************************************************************************/

func genericFieldParse(cronField string, min, max int) []int {
	// Defaults
	values := make(map[int]bool)

	// Comma separator is used to mix different allowed syntax
	cronList := strings.Split(cronField, ",")
	for _, s := range cronList {
		// "/"
		step, s := extractInterval(s)
		// "*"
		if s == "*" {
			populateMany(values, min, max, step)
			continue
		}
		// "-"
		i := strings.Index(s, "-")
		if i >= 0 {
			populateMany(values, atoi(s[:i]), atoi(s[i+1:]), step)
			continue
		}
		// single value with interval
		if step > 0 {
			populateMany(values, atoi(s), max, step)
			continue
		}
		// single value
		populateOne(values, atoi(s))
	}

	return toList(values)
}

/******************************************************************************/

// Local helpers

func extractInterval(s string) (int, string) {
	step := 0
	i := strings.Index(s, "/")
	if i >= 0 {
		step = atoi(s[i+1:])
		s = s[:i]
	}
	return step, s
}

func atoi(s string) int {
	v, err := strconv.Atoi(s)
	if err != nil {
		panic(err)
	}
	return v
}

func populateOne(values map[int]bool, v int) {
	values[v] = true
}

func populateMany(values map[int]bool, min, max, step int) {
	if step == 0 {
		step = 1
	}
	for i := min; i <= max; i += step {
		values[i] = true
	}
}

func toList(set map[int]bool) []int {
	list := make([]int, len(set))
	i := 0
	for k, _ := range set {
		list[i] = k
		i += 1
	}
	sort.Ints(list)
	return list
}