package session

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"

	"agent-native/runtime/llm"
)

// Stats 是转录文件的轻量元数据（扫描即得，无副作用）。
type Stats struct {
	Lines int
	Bytes int64
}

// Stats 统计当前转录行数与字节数（空行不计）。
func (s *Store) Stats() Stats {
	s.mu.Lock()
	defer s.mu.Unlock()
	st := Stats{}
	f, err := os.Open(s.path)
	if err != nil {
		return st
	}
	defer f.Close()
	sc := bufio.NewScanner(f)
	sc.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
	for sc.Scan() {
		if len(sc.Bytes()) == 0 {
			continue
		}
		st.Lines++
	}
	if info, err := f.Stat(); err == nil {
		st.Bytes = info.Size()
	}
	return st
}

// SaveMeta 把会话元数据写进 <path>.meta.json（创建时间/模型等由调用方定义）。
func (s *Store) SaveMeta(meta map[string]string) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	raw, err := json.MarshalIndent(meta, "", "  ")
	if err != nil {
		return err
	}
	tmp := s.path + ".meta.json.tmp"
	if err := os.WriteFile(tmp, raw, 0o644); err != nil {
		return err
	}
	return os.Rename(tmp, s.path+".meta.json")
}

// ReadMeta 读取会话元数据；不存在时返回 nil（新会话）。
func (s *Store) ReadMeta() map[string]string {
	raw, err := os.ReadFile(s.path + ".meta.json")
	if err != nil {
		return nil
	}
	var meta map[string]string
	if err := json.Unmarshal(raw, &meta); err != nil {
		return nil
	}
	return meta
}

const maxCompactAttempts = 3

// Compact appends a compaction entry. The file keeps every earlier record.
// Load hides the summarized prefix behind the summary plus the last
// keepLastN non-system messages. Returns false when there is nothing to fold.
// A session change during summary is retried from the new transcript.
// Three consecutive summary failures or conflicts trip the breaker and
// leave the file unchanged.
func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (string, error)) (bool, error) {
	var last error
	for attempt := 0; attempt < maxCompactAttempts; attempt++ {
		done, err, retry := s.compactOnce(keepLastN, summarize)
		if !retry {
			return done, err
		}
		last = err
	}
	if last == nil {
		last = fmt.Errorf("compaction conflict: session changed during summary")
	}
	return false, fmt.Errorf("compaction circuit open: %w", last)
}

func (s *Store) compactOnce(keepLastN int, summarize func(messages []llm.Message) (string, error)) (bool, error, bool) {
	s.mu.Lock()
	entries, err := s.readEntriesLocked()
	if err != nil {
		s.mu.Unlock()
		return false, err, false
	}
	leaf := s.leafLocked(entries)
	generation := len(entries)
	s.mu.Unlock()
	path := pathTo(entries, leaf)
	var messages []llm.Message
	var msgEntries []Entry
	for _, e := range path {
		if e.Type == TypeMessage && e.Message != nil {
			messages = append(messages, *e.Message)
			msgEntries = append(msgEntries, e)
		}
	}
	if len(messages) <= keepLastN+1 {
		return false, nil, false
	}
	rest := messages
	restEntries := msgEntries
	if len(rest) > 0 && rest[0].Role == llm.RoleSystem {
		rest = rest[1:]
		restEntries = restEntries[1:]
	}
	tailStart := len(rest) - keepLastN
	if tailStart <= 0 {
		return false, nil, false
	}
	tailStart = alignToolTail(rest, tailStart)
	head := rest[:tailStart]
	summary, err := summarize(head)
	if err != nil {
		return false, err, true
	}
	var tailIDs []string
	for _, e := range restEntries[tailStart:] {
		tailIDs = append(tailIDs, e.ID)
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	now, err := s.readEntriesLocked()
	if err != nil {
		return false, err, false
	}
	if len(now) != generation {
		return false, fmt.Errorf("compaction conflict: session changed during summary"), true
	}
	_, err = s.appendEntryLocked(Entry{Type: TypeCompaction, Summary: summary, TailIDs: tailIDs})
	return err == nil, err, false
}

// alignToolTail 让保留尾巴从一条非 tool 消息开始，避免把 tool_calls 和结果拆开。
func alignToolTail(messages []llm.Message, tailStart int) int {
	for tailStart > 0 && messages[tailStart].Role == llm.RoleTool {
		tailStart--
	}
	return tailStart
}