package rules

import (
	"errors"
	"fmt"
	"sort"
)

// ChangeKind 是差异类型。
type ChangeKind string

const (
	ChangeAdded    ChangeKind = "added"
	ChangeModified ChangeKind = "modified"
	ChangeRemoved  ChangeKind = "removed"
)

// ProfileChange 是一个积分模板的增/改/删。
type ProfileChange struct {
	Code   string     `json:"code"`
	Kind   ChangeKind `json:"kind"`
	Before *Profile   `json:"before,omitempty"`
	After  *Profile   `json:"after,omitempty"`
	// Fields 列出被修改的字段，便于前端只高亮变化项。
	Fields []string `json:"fields,omitempty"`
}

// ChannelChange 是一个项目计分通道的变化。
type ChannelChange struct {
	EventCode string     `json:"eventCode"`
	EventName string     `json:"eventName,omitempty"`
	Kind      ChangeKind `json:"kind"`
	Before    []Channel  `json:"before,omitempty"`
	After     []Channel  `json:"after,omitempty"`
}

// SettingChange 是一条赛事设置的变化。
type SettingChange struct {
	Key    string `json:"key"`
	Before string `json:"before"`
	After  string `json:"after"`
}

// ChangeSet 是导入预览的结果，也是 Apply 的输入。
type ChangeSet struct {
	Profiles []ProfileChange `json:"profiles,omitempty"`
	Channels []ChannelChange `json:"channels,omitempty"`
	Settings []SettingChange `json:"settings,omitempty"`
	// UnknownEvents 是规则包里出现、但赛事库中不存在的项目编码；Apply 会跳过它们。
	UnknownEvents []string `json:"unknownEvents,omitempty"`
	// Next 是目标规则包；Apply 以它为完整目标状态落库。不参与 JSON 序列化。
	Next *RulePack `json:"-"`
}

// Empty 判断这次导入是否真的会改动任何东西。
// 只用于预览展示；Apply 始终以 Next 为目标全量同步，不依赖它做短路。
func (cs *ChangeSet) Empty() bool {
	return cs == nil || (len(cs.Profiles) == 0 && len(cs.Channels) == 0 && len(cs.Settings) == 0)
}

// Diff 计算「当前赛事库 → 目标规则包」的差异，只读不落库。
func Diff(cur *Snapshot, next *RulePack) (*ChangeSet, error) {
	if cur == nil || cur.Pack == nil {
		return nil, errors.New("rules: 当前快照为空，无法比对差异")
	}
	if next == nil {
		return nil, fmt.Errorf("%w: 目标规则包为空", ErrNotRulePack)
	}

	target := next.Canonical()
	if err := target.Validate(); err != nil {
		return nil, err
	}

	cs := &ChangeSet{Next: &target}
	cs.Profiles = diffProfiles(cur.Pack, &target)

	targets, unknown, err := resolveTargets(cur, &target)
	if err != nil {
		return nil, err
	}
	cs.UnknownEvents = unknown
	cs.Channels = diffChannels(cur, targets)
	cs.Settings = diffSettings(cur.Pack, &target)
	return cs, nil
}

func diffProfiles(cur, next *RulePack) []ProfileChange {
	curIndex := cur.ProfileIndex()
	nextIndex := next.ProfileIndex()

	codes := make([]string, 0, len(curIndex)+len(nextIndex))
	seen := map[string]bool{}
	for code := range curIndex {
		if !seen[code] {
			seen[code] = true
			codes = append(codes, code)
		}
	}
	for code := range nextIndex {
		if !seen[code] {
			seen[code] = true
			codes = append(codes, code)
		}
	}
	sort.Strings(codes)

	var out []ProfileChange
	for _, code := range codes {
		before, hasBefore := curIndex[code]
		after, hasAfter := nextIndex[code]
		switch {
		case !hasBefore:
			out = append(out, ProfileChange{Code: code, Kind: ChangeAdded, After: &after})
		case !hasAfter:
			out = append(out, ProfileChange{Code: code, Kind: ChangeRemoved, Before: &before})
		default:
			if fields := profileDiffFields(before, after); len(fields) > 0 {
				out = append(out, ProfileChange{Code: code, Kind: ChangeModified, Before: &before, After: &after, Fields: fields})
			}
		}
	}
	return out
}

func profileDiffFields(a, b Profile) []string {
	var fields []string
	if a.Name != b.Name {
		fields = append(fields, "name")
	}
	if a.AwardRanks != b.AwardRanks {
		fields = append(fields, "awardRanks")
	}
	if a.LastPlaceZero != b.LastPlaceZero {
		fields = append(fields, "lastPlaceZero")
	}
	if a.TieMode != b.TieMode {
		fields = append(fields, "tieMode")
	}
	if a.RecordBonusMode != b.RecordBonusMode {
		fields = append(fields, "recordBonusMode")
	}
	if !rulesEqual(a.Rules, b.Rules) {
		fields = append(fields, "rules")
	}
	return fields
}

func rulesEqual(a, b []Rule) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if a[i] != b[i] {
			return false
		}
	}
	return true
}

// resolveTargets 把规则包展开成「项目编码 → 目标通道」。
// 优先级：eventOverrides 显式给了 channels > 第一条命中的 channelTemplates。
// 没被规则包提到的项目不出现在结果里，表示「保持原样」。
func resolveTargets(cur *Snapshot, next *RulePack) (map[string][]Channel, []string, error) {
	overrideByCode := make(map[string]EventOverride, len(next.EventOverrides))
	for _, o := range next.EventOverrides {
		overrideByCode[o.EventCode] = o
	}

	targets := make(map[string][]Channel)
	for _, ev := range cur.Events {
		if ov, ok := overrideByCode[ev.Code]; ok && ov.Channels != nil {
			targets[ev.Code] = append([]Channel(nil), (*ov.Channels)...)
			continue
		}
		for _, t := range next.ChannelTemplates {
			if matchEvent(t.Match, ev) {
				targets[ev.Code] = append([]Channel(nil), t.Channels...)
				break
			}
		}
	}

	var unknown []string
	for _, o := range next.EventOverrides {
		if _, ok := cur.EventByCode(o.EventCode); !ok {
			unknown = append(unknown, o.EventCode)
		}
	}
	sort.Strings(unknown)
	return targets, unknown, nil
}

func diffChannels(cur *Snapshot, targets map[string][]Channel) []ChannelChange {
	curByCode := make(map[string]EventInfo, len(cur.Events))
	for _, ev := range cur.Events {
		curByCode[ev.Code] = ev
	}

	codes := make([]string, 0, len(targets))
	for code := range targets {
		codes = append(codes, code)
	}
	sort.Strings(codes)

	var out []ChannelChange
	for _, code := range codes {
		ev := curByCode[code]
		before := ev.Channels
		after := targets[code]
		if channelsEqual(before, after) {
			continue
		}
		kind := ChangeModified
		switch {
		case len(before) == 0:
			kind = ChangeAdded
		case len(after) == 0:
			kind = ChangeRemoved
		}
		out = append(out, ChannelChange{
			EventCode: code,
			EventName: ev.Name,
			Kind:      kind,
			Before:    append([]Channel(nil), before...),
			After:     append([]Channel(nil), after...),
		})
	}
	return out
}

func channelsEqual(a, b []Channel) bool {
	if len(a) != len(b) {
		return false
	}
	for i := range a {
		if a[i] != b[i] {
			return false
		}
	}
	return true
}

func diffSettings(cur, next *RulePack) []SettingChange {
	curSettings := settingsFromPack(cur)
	nextSettings := settingsFromPack(next)

	var out []SettingChange
	for _, key := range settingsKeys() {
		before, after := curSettings[key], nextSettings[key]
		if before != after {
			out = append(out, SettingChange{Key: key, Before: before, After: after})
		}
	}
	return out
}