subtrack/internal/domain/domain.go

// Package domain 存放与存储无关的纯业务计算：日期、剩余时长、状态判定、追加换算。
package domain

import "time"

const (
	DateLayout  = "2006-01-02"
	StampLayout = "2006-01-02 15:04:05"
)

// Today 返回本地时区当天零点。
func Today() time.Time {
	now := time.Now()
	return time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, now.Location())
}

// ParseDate 解析 YYYY-MM-DD。
func ParseDate(s string) (time.Time, error) {
	return time.ParseInLocation(DateLayout, s, time.Local)
}

// ParseDateOr 解析失败时返回默认值。
func ParseDateOr(s string, fallback time.Time) time.Time {
	if t, err := ParseDate(s); err == nil {
		return t
	}
	return fallback
}

// Date 格式化为 YYYY-MM-DD。
func Date(t time.Time) string { return t.Format(DateLayout) }

// DayDiff 返回 b-a 的自然日天数（不受夏令时影响）。
func DayDiff(a, b time.Time) int {
	ay, am, ad := a.Date()
	by, bm, bd := b.Date()
	ua := time.Date(ay, am, ad, 0, 0, 0, 0, time.UTC)
	ub := time.Date(by, bm, bd, 0, 0, 0, 0, time.UTC)
	return int(ub.Sub(ua).Hours() / 24.0)
}

// AddDays 加 n 天。
func AddDays(t time.Time, n int) time.Time { return t.AddDate(0, 0, n) }

// MonthStart 返回当月 1 日。
func MonthStart(y int, m time.Month) time.Time {
	return time.Date(y, m, 1, 0, 0, 0, 0, time.Local)
}

// MonthEnd 返回当月最后一天。
func MonthEnd(y int, m time.Month) time.Time {
	return time.Date(y, m, 1, 0, 0, 0, 0, time.Local).AddDate(0, 1, -1)
}

// WeekdayIndex 返回以周一为 0 的星期索引。
func WeekdayIndex(t time.Time) int {
	wd := int(t.Weekday())
	if wd == 0 {
		return 6
	}
	return wd - 1
}

// 订阅状态（存储值）
const (
	StatusActive = "active"
	StatusPaused = "paused"
	StatusClosed = "closed"
)

// 套餐类型
const (
	UnitDay   = "day"
	UnitCount = "count"
)

// 派生展示状态
const (
	StateActive  = "active"  // 进行中
	StatePending = "pending" // 未开始
	StatePaused  = "paused"  // 暂停中
	StateExpired = "expired" // 已到期
	StateUsedUp  = "usedup"  // 次数用尽
	StateClosed  = "closed"  // 已结束
)

// 到期预警等级
const (
	LevelOK      = "ok"
	LevelWarn    = "warn"
	LevelToday   = "today"
	LevelExpired = "expired"
	LevelNone    = "none" // 无期限（按次且未设有效期）
)

// Eval 是订阅的计算结果。
type Eval struct {
	State          string
	DaysLeft       int // 距结束日的天数：0=今天到期，>0=未来，-1=无结束日
	RemainingDays  int
	RemainingCount int
	Level          string
}

// EvalSub 计算订阅的展示状态与剩余量。
//   - unit=day：剩余天数 = end - today + 1（含当天）
//   - unit=count：剩余次数 = total - used；end 为可选有效期
func EvalSub(unit, status, startDate, endDate string, total, used int, today time.Time, warnDays int) Eval {
	e := Eval{Level: LevelNone, DaysLeft: -1}
	if warnDays <= 0 {
		warnDays = 3
	}
	start := ParseDateOr(startDate, today)
	end, hasEnd := time.Time{}, false
	if endDate != "" {
		if t, err := ParseDate(endDate); err == nil {
			end, hasEnd = t, true
			e.DaysLeft = DayDiff(today, end)
		}
	}

	if unit == UnitDay {
		if hasEnd {
			e.RemainingDays = e.DaysLeft + 1
		}
		if e.RemainingDays < 0 {
			e.RemainingDays = 0
		}
	} else {
		e.RemainingCount = total - used
		if e.RemainingCount < 0 {
			e.RemainingCount = 0
		}
	}

	expired := false
	if hasEnd && end.Before(today) {
		expired = true
	}
	if unit == UnitCount && e.RemainingCount <= 0 {
		expired = true
	}

	switch {
	case status == StatusClosed:
		e.State = StateClosed
	case status == StatusPaused:
		e.State = StatePaused
	case expired:
		if unit == UnitCount && e.RemainingCount <= 0 {
			e.State = StateUsedUp
		} else {
			e.State = StateExpired
		}
	case start.After(today):
		e.State = StatePending
	default:
		e.State = StateActive
	}

	// 预警等级只对「进行中 / 未开始」有意义
	switch e.State {
	case StateExpired:
		e.Level = LevelExpired
	case StateActive, StatePending:
		switch {
		case !hasEnd:
			e.Level = LevelNone
		case e.DaysLeft <= 0:
			e.Level = LevelToday
		case e.DaysLeft <= warnDays:
			e.Level = LevelWarn
		default:
			e.Level = LevelOK
		}
	}
	return e
}

// ExtendPlan 描述一次「按日追加」的换算结果。
type ExtendPlan struct {
	Base      time.Time // 计算起点
	OldEnd    time.Time
	NewEnd    time.Time
	Days      int
	SameDay   bool // 新结束日等于原结束日（未增加）
	Backwards bool // 新结束日早于计算起点
}

// PlanExtend 根据原结束日、今天与新结束日，算出计算起点与追加天数。
// 规则（PRD R4）：已到期从今天起算，未到期从原结束日次日起算。
func PlanExtend(oldEnd, newEnd, today time.Time) ExtendPlan {
	p := ExtendPlan{OldEnd: oldEnd, NewEnd: newEnd}
	if !oldEnd.Before(today) {
		p.Base = AddDays(oldEnd, 1)
	} else {
		p.Base = today
	}
	p.Days = DayDiff(p.Base, newEnd) + 1
	p.SameDay = DayDiff(oldEnd, newEnd) == 0
	p.Backwards = p.Days <= 0
	if p.Days < 0 {
		p.Days = 0
	}
	return p
}