package channelbridge

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"strings"

	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/model"
)

// This file holds the production LightJudge used by the participation engine
// (design/18 §3). It is deliberately separate from flowgate's Reviewer: the Reviewer
// prunes queued messages from a stack; this judge decides, once, at a cost-controlled
// entrance gate, whether a triggered group/stranger message is worth participating in.
// It only ever decides 值不值得回 — never who the sender is (a project red line).

const (
	// engageMaxTokens bounds the judge output so a noisy group context never produces a
	// giant completion (design/18 §3 cost guard).
	engageMaxTokens = 200
	// engageSnippetLen is how many bytes of each recent message the context renderer
	// shows the judge — enough to weigh the topic without paging the whole group.
	engageSnippetLen = 80
)

// NewLLMLightJudge wraps a model.Adapter into a LightJudge (design/18 §3). It asks the
// LLM, via a single cheap turn, whether the triggered message is worth participating
// in, and routes the structured answer back to the engage engine. A nil adapter returns
// a judge that always fails closed (so the engine stays silent rather than panicking).
func NewLLMLightJudge(llm model.Adapter, log *slog.Logger) LightJudge {
	if log == nil {
		log = slog.Default()
	}
	return &llmLightJudge{llm: llm, log: log}
}

type llmLightJudge struct {
	llm model.Adapter
	log *slog.Logger
}

// Judge performs one cost-controlled light-turn on a triggered participation message
// and returns the structured verdict. Any error (upstream, parse) is returned so the
// engage engine fails closed to silence. It never reads sender identity — name is
// passed only as a label, identity was already resolved structurally.
func (j *llmLightJudge) Judge(ctx context.Context, in LightInput) (LightDecision, error) {
	if j.llm == nil {
		return LightDecision{}, errors.New("channelbridge: light judge: nil model adapter")
	}
	resp, err := j.llm.Chat(ctx, model.Request{
		Messages: []model.Message{
			model.TextMessage(model.RoleSystem, engageJudgeSystemPrompt),
			model.TextMessage(model.RoleUser, buildEngageUserContent(in)),
		},
		Purpose:   "engage_judge",
		MaxTokens: engageMaxTokens,
	}, model.Options{})
	if err != nil {
		return LightDecision{}, fmt.Errorf("channelbridge: light judge: %w", err)
	}
	d, err := parseEngageDecision(resp.Content)
	if err != nil {
		j.log.Warn("light judge output unparsable, fail-closed to silence", "error", err)
		return LightDecision{}, err
	}
	// Map the participation verdict onto the legacy design/12 §6 Respond field so the
	// same decision drives both the engage engine (WorthReply/Interesting) and any
	// legacy consumer that only reads Respond.
	d.Respond = d.WorthReply || d.Interesting
	return d, nil
}

// engageJudgeSystemPrompt instructs the judge to weigh only participation value, never
// identity (design/18 §0 red line), and to answer with a tight JSON object.
const engageJudgeSystemPrompt = "你是群聊参与度轻思考器。只判断「她值不值得参与这条群聊消息」，" +
	"绝不判断消息发送者是谁（谁说的已由系统元数据决定）。" +
	"只输出一个 JSON 对象，字段为：" +
	"{\"interested\": bool(她对这话题感兴趣吗), " +
	"\"worthReply\": bool(值得回吗), " +
	"\"avoidTopic\": bool(该话题她应回避吗,如敏感/隐私/成人红线), " +
	"\"reason\": string(一句话理由)}。" +
	"不要输出其它文字。"

// buildEngageUserContent renders the persona prompt, the recent group context and the
// triggering message for the judge. The persona block carries no owner secret; the
// context is bounded by the engage renderer. When Engage is false (a legacy design/12
// §6 call) only the message and sender label are present, so the judge degrades to a
// plain value judgement.
func buildEngageUserContent(in LightInput) string {
	var b strings.Builder
	if in.Engage && in.PersonaPrompt != "" {
		b.WriteString("【人格设定】\n" + in.PersonaPrompt + "\n\n")
	}
	if in.Engage && in.TriggerWords != nil {
		b.WriteString("【角色提醒词】" + strings.Join(in.TriggerWords, " / ") + "\n\n")
	}
	if in.Engage && in.RecentContext != "" {
		b.WriteString("【群聊最近上下文】\n" + in.RecentContext + "\n\n")
	}
	b.WriteString("【本轮触发消息】\n")
	if in.SenderName != "" {
		b.WriteString(in.SenderName + ": ")
	}
	b.WriteString(in.Msg.Content)
	b.WriteString("\n\n请只输出 JSON 对象，不要其它文字。")
	return b.String()
}

// engageDecisionJSON mirrors the judge's structured verdict for decoding.
type engageDecisionJSON struct {
	Interested bool   `json:"interested"`
	WorthReply bool   `json:"worthReply"`
	AvoidTopic bool   `json:"avoidTopic"`
	Reason     string `json:"reason"`
}

// parseEngageDecision decodes the judge's JSON object, tolerating a fenced/code-block
// wrapper. A malformed answer returns an error so the caller fails closed to silence.
func parseEngageDecision(s string) (LightDecision, error) {
	trimmed := strings.TrimSpace(s)
	if strings.HasPrefix(trimmed, "```") {
		trimmed = strings.TrimPrefix(trimmed, "```")
		if i := strings.IndexByte(trimmed, '\n'); i >= 0 {
			trimmed = trimmed[i+1:]
		}
		trimmed = strings.TrimSuffix(strings.TrimSpace(trimmed), "```")
	}
	// The model may wrap the object with explanatory prose; fall back to the first
	// balanced JSON object if the whole payload is not pure JSON.
	var raw engageDecisionJSON
	if err := json.Unmarshal([]byte(trimmed), &raw); err != nil {
		if obj := firstJSONObject(trimmed); obj != "" {
			if err2 := json.Unmarshal([]byte(obj), &raw); err2 == nil {
				return toEngageDecision(raw), nil
			}
		}
		return LightDecision{}, fmt.Errorf("channelbridge: parse engage judge output: %w", err)
	}
	return toEngageDecision(raw), nil
}

func toEngageDecision(raw engageDecisionJSON) LightDecision {
	return LightDecision{
		Interesting: raw.Interested,
		WorthReply:  raw.WorthReply,
		AvoidTopic:  raw.AvoidTopic,
		Reason:      raw.Reason,
	}
}

// firstJSONObject returns the first balanced {...} region of s, or "" when none exists.
// It is a cheap tolerance for a model that prepends/appends prose to the JSON answer.
func firstJSONObject(s string) string {
	start := strings.IndexByte(s, '{')
	if start < 0 {
		return ""
	}
	depth := 0
	for i := start; i < len(s); i++ {
		switch s[i] {
		case '{':
			depth++
		case '}':
			depth--
			if depth == 0 {
				return s[start : i+1]
			}
		}
	}
	return ""
}