package processor

import (
	"errors"
	"fmt"
	"sort"
	"strings"
	"time"

	"apskel-pos-be/internal/constants"
	"apskel-pos-be/internal/models"
)

// Expiry rules of docs/prd-point-coin.md F12, with the model of note N4 decided: both
// FIXED_DATE and ROLLING are supported and the owner picks one per currency.
//
// A lot expires at the last second of its expiry day in the customer's time zone, so
// "expires on 31 Dec" means it can still be spent all of 31 Dec, and the day the apps
// group it under is that day.

const (
	// A longer grace would push a balance past a whole cycle of fixed dates.
	loyaltyExpiryMaxGraceMonths = 24
	// A refund gives the EnakPoint back at least this long, even when the lot they
	// came from has expired or is about to (note N4, decided).
	refundMinimumValidity = 7 * 24 * time.Hour
)

// ExpirySettings picks the settings of one currency.
func ExpirySettings(s *models.OrganizationLoyaltySettings, currency string) models.LoyaltyExpirySettings {
	if currency == constants.WalletCurrencyCoin {
		return s.CoinExpiry
	}
	return s.PointExpiry
}

// ComputeExpiry is when a balance received at receivedAt expires, or nil when it never
// does because expiry is off. It is the expiry of every lot that is not carried over
// from another one: EARN, ADJUSTMENT and, capped by its origin, EXCHANGE_IN.
func ComputeExpiry(s models.LoyaltyExpirySettings, receivedAt time.Time) *time.Time {
	if !s.Enabled {
		return nil
	}
	day := walletDay(receivedAt)
	if s.Mode == constants.LoyaltyExpiryModeRolling {
		if s.Unit == constants.LoyaltyExpiryUnitDay {
			day = day.AddDate(0, 0, int(s.Period))
		} else {
			day = addMonthsClamped(day, int(s.Period))
		}
		if s.EndOfMonth {
			day = lastDayOfMonth(day)
		}
		return endOfWalletDay(day)
	}
	return endOfWalletDay(fixedDateOnOrAfter(s.FixedDates, addMonthsClamped(day, int(s.GraceMonths))))
}

// ActivationExpiry is the expiry given to the balances that already exist, without an
// expiry, when expiry is turned on (note N4, decided): a full period from activation
// when ROLLING, the second fixed date after activation when FIXED_DATE. Either way no
// customer loses a balance soon after the rule is announced.
func ActivationExpiry(s models.LoyaltyExpirySettings, at time.Time) *time.Time {
	if s.Mode == constants.LoyaltyExpiryModeRolling {
		s.Enabled = true
		return ComputeExpiry(s, at)
	}
	first := fixedDateOnOrAfter(s.FixedDates, walletDay(at))
	return endOfWalletDay(fixedDateOnOrAfter(s.FixedDates, first.AddDate(0, 0, 1)))
}

// RefundExpiry is the expiry of EnakPoint given back by a refund: that of the lot they
// came from, but at least seven days from now (note N4, decided). A lot that never
// expires stays so.
func RefundExpiry(origin *time.Time, now time.Time) *time.Time {
	if origin == nil {
		return nil
	}
	minimum := endOfWalletDay(walletDay(now.Add(refundMinimumValidity)))
	if origin.Before(*minimum) {
		return minimum
	}
	return origin
}

// EarlierExpiry is the sooner of two expiries, where nil means never.
func EarlierExpiry(a, b *time.Time) *time.Time {
	switch {
	case a == nil:
		return b
	case b == nil:
		return a
	case b.Before(*a):
		return b
	}
	return a
}

// normalizeDaysOfYear checks days written as MM-DD and returns them sorted without
// duplicates. 29 February is refused, as it does not come every year.
func normalizeDaysOfYear(days []string) ([]string, error) {
	seen := map[string]bool{}
	var out []string
	for _, d := range days {
		d = strings.TrimSpace(d)
		if d == "" {
			continue
		}
		parsed, err := time.Parse("01-02", d)
		if err != nil || parsed.Format("01-02") != d {
			return nil, fmt.Errorf("has %q, which is not a day of the year written as MM-DD", d)
		}
		if d == "02-29" {
			return nil, errors.New("cannot use 02-29, which is not in every year")
		}
		if !seen[d] {
			seen[d] = true
			out = append(out, d)
		}
	}
	if len(out) == 0 {
		return nil, errors.New("must list at least one day of the year, as MM-DD")
	}
	sort.Strings(out)
	return out, nil
}

// fixedDateOnOrAfter is the first of the days of the year that falls on or after day.
func fixedDateOnOrAfter(days []string, day time.Time) time.Time {
	days, err := normalizeDaysOfYear(days)
	if err != nil {
		days = []string{constants.LoyaltyExpiryFixedDatesDefault}
	}
	for year := day.Year(); ; year++ {
		for _, d := range days {
			parsed, _ := time.Parse("01-02", d)
			candidate := time.Date(year, parsed.Month(), parsed.Day(), 0, 0, 0, 0, walletDisplayLocation)
			if !candidate.Before(day) {
				return candidate
			}
		}
	}
}

// walletDay is the calendar day of t in the customer's time zone, at midnight.
func walletDay(t time.Time) time.Time {
	local := t.In(walletDisplayLocation)
	return time.Date(local.Year(), local.Month(), local.Day(), 0, 0, 0, 0, walletDisplayLocation)
}

func endOfWalletDay(day time.Time) *time.Time {
	end := time.Date(day.Year(), day.Month(), day.Day(), 23, 59, 59, 0, walletDisplayLocation)
	return &end
}

// addMonthsClamped adds months, keeping to the last day of a shorter month: 31 Jan
// plus one month is 28 or 29 Feb, not 3 Mar.
func addMonthsClamped(day time.Time, months int) time.Time {
	first := time.Date(day.Year(), day.Month(), 1, 0, 0, 0, 0, day.Location()).AddDate(0, months, 0)
	last := lastDayOfMonth(first).Day()
	d := day.Day()
	if d > last {
		d = last
	}
	return time.Date(first.Year(), first.Month(), d, 0, 0, 0, 0, day.Location())
}

func lastDayOfMonth(day time.Time) time.Time {
	return time.Date(day.Year(), day.Month()+1, 0, 0, 0, 0, 0, day.Location())
}