package sessions

import (
	"encoding/json"
	"fmt"
	"log"
	"strings"
	"unicode/utf8"
)

type ExecMode string

const (
	ModeNew    ExecMode = "new"
	ModeResume ExecMode = "resume"
	ModeFork   ExecMode = "fork"
)

type ExecError struct {
	message string
}

func (err ExecError) Error() string {
	return err.message
}

type PrepareExecOptions struct {
	Store            *Store
	SessionID        string
	Title            string
	Cwd              string
	ModelID          string
	Provider         string
	Tag              string
	Depth            int
	CallingSessionID string
	CallingToolUseID string
	AgentName        string
	TaskID           string
	Resume           string
	ResumeLatest     bool
	Fork             string
}

type PreparedExec struct {
	Mode          ExecMode
	Session       Metadata
	ContextEvents []Event
	Store         *Store
}

func PrepareExec(options PrepareExecOptions) (PreparedExec, error) {
	resumeID := strings.TrimSpace(options.Resume)
	forkID := strings.TrimSpace(options.Fork)
	if (resumeID != "" || options.ResumeLatest) && forkID != "" {
		return PreparedExec{}, ExecError{"Use either --resume or --fork, not both."}
	}

	store := options.Store
	if store == nil {
		store = NewStore(StoreOptions{})
	}

	if forkID != "" {
		parent, err := store.Get(forkID)
		if err != nil {
			return PreparedExec{}, err
		}
		if parent == nil {
			return PreparedExec{}, ExecError{"Zero session not found: " + forkID}
		}
		contextEvents, err := readExecContextEvents(store, parent.SessionID)
		if err != nil {
			return PreparedExec{}, err
		}
		session, err := store.Fork(parent.SessionID, ForkInput{
			SessionID: options.SessionID,
			Title:     firstNonEmpty(options.Title, forkTitle(parent.Title)),
			Cwd:       options.Cwd,
			ModelID:   options.ModelID,
			Provider:  options.Provider,
		})
		if err != nil {
			return PreparedExec{}, err
		}
		return PreparedExec{Mode: ModeFork, Session: session, ContextEvents: contextEvents, Store: store}, nil
	}

	if resumeID != "" || options.ResumeLatest {
		sessionID := resumeID
		if sessionID == "" && options.ResumeLatest {
			latest, err := store.Latest()
			if err != nil {
				return PreparedExec{}, err
			}
			if latest == nil {
				return PreparedExec{}, ExecError{"No Zero sessions available to resume."}
			}
			sessionID = latest.SessionID
		}
		session, err := store.Get(sessionID)
		if err != nil {
			return PreparedExec{}, err
		}
		if session == nil {
			return PreparedExec{}, ExecError{"Zero session not found: " + sessionID}
		}
		contextEvents, err := readExecContextEvents(store, session.SessionID)
		if err != nil {
			return PreparedExec{}, err
		}
		return PreparedExec{Mode: ModeResume, Session: *session, ContextEvents: contextEvents, Store: store}, nil
	}

	createInput := CreateInput{
		SessionID: options.SessionID,
		Title:     options.Title,
		Cwd:       options.Cwd,
		ModelID:   options.ModelID,
		Provider:  options.Provider,
		Tag:       options.Tag,
		Depth:     options.Depth,
	}
	if strings.TrimSpace(options.CallingSessionID) != "" {
		parentSessionID := strings.TrimSpace(options.CallingSessionID)
		parent, err := store.Get(parentSessionID)
		if err != nil {
			return PreparedExec{}, err
		}
		if parent == nil {
			return PreparedExec{}, ExecError{"Zero parent session not found: " + parentSessionID}
		}
		createInput.SessionKind = SessionKindChild
		createInput.ParentSessionID = parent.SessionID
		createInput.RootSessionID = firstNonEmpty(parent.RootSessionID, parent.SessionID)
		createInput.AgentName = strings.TrimSpace(options.AgentName)
		createInput.TaskID = strings.TrimSpace(firstNonEmpty(options.TaskID, options.SessionID))
		createInput.SpawnedFromEventID = strings.TrimSpace(options.CallingToolUseID)
	}
	session, err := store.Create(createInput)
	if err != nil {
		return PreparedExec{}, err
	}
	return PreparedExec{Mode: ModeNew, Session: session, ContextEvents: []Event{}, Store: store}, nil
}

func readExecContextEvents(store *Store, sessionID string) ([]Event, error) {
	contextEvents, err := store.ReadRehydratedEvents(sessionID)
	if err == nil {
		return contextEvents, nil
	}
	rawEvents, rawErr := store.ReadEvents(sessionID)
	if rawErr != nil {
		return nil, err
	}
	log.Printf("zero sessions: failed to rehydrate compaction events for %s; falling back to raw events: %v", sessionID, err)
	return rawEvents, nil
}

func FormatExecPrompt(prompt string, prepared PreparedExec) string {
	if prepared.Mode == ModeNew || len(prepared.ContextEvents) == 0 {
		return prompt
	}
	events := promptContextEvents(prepared.ContextEvents)

	lines := []string{}
	for _, event := range events {
		lines = append(lines, fmt.Sprintf("- #%d %s: %s", event.Sequence, event.Type, summarizePayload(event.Payload)))
	}
	label := "Continuing"
	sessionID := prepared.Session.SessionID
	if prepared.Mode == ModeFork {
		label = "Forked from"
		if prepared.Session.ParentSessionID != "" {
			sessionID = prepared.Session.ParentSessionID
		}
	}
	return strings.Join([]string{
		fmt.Sprintf("%s Zero session %s.", label, sessionID),
		"Previous session context:",
		strings.Join(lines, "\n"),
		"",
		"Current user request:",
		prompt,
	}, "\n")
}

func promptContextEvents(events []Event) []Event {
	const maxPromptContextEvents = 80

	filtered := make([]Event, 0, len(events))
	for _, event := range events {
		switch event.Type {
		case EventMessage, EventCompaction, EventSessionFork, EventSessionChild, EventSpecialistStart, EventSpecialistStop, EventError:
			filtered = append(filtered, event)
		}
	}
	if len(filtered) == 0 {
		filtered = append(filtered, events...)
	}
	if len(filtered) > maxPromptContextEvents {
		filtered = filtered[len(filtered)-maxPromptContextEvents:]
	}
	return filtered
}

func forkTitle(title string) string {
	if title == "" {
		return ""
	}
	return title + " (fork)"
}

func summarizePayload(payload any) string {
	text := extractText(payload)
	text = strings.Join(strings.Fields(text), " ")
	if text == "" {
		data, err := json.Marshal(payload)
		if err != nil {
			return "{}"
		}
		text = string(data)
	}
	if len(text) > 500 {
		return truncateUTF8(text, 500)
	}
	return text
}

// truncateUTF8 returns the longest prefix of s that is at most n bytes,
// backing off to the nearest rune boundary so a multi-byte character isn't
// split — a split rune here would embed invalid UTF-8 into the exec prompt.
func truncateUTF8(s string, n int) string {
	if len(s) <= n {
		return s
	}
	for n > 0 && !utf8.RuneStart(s[n]) {
		n--
	}
	return s[:n]
}

func extractText(value any) string {
	switch typed := value.(type) {
	case string:
		return typed
	case json.RawMessage:
		var decoded any
		if err := json.Unmarshal(typed, &decoded); err == nil {
			return extractText(decoded)
		}
		return string(typed)
	case float64, bool, int:
		return fmt.Sprint(typed)
	case []any:
		parts := []string{}
		for _, item := range typed {
			if text := extractText(item); text != "" {
				parts = append(parts, text)
			}
		}
		return strings.Join(parts, " ")
	case map[string]any:
		if summary, ok := typed["summary"].(string); ok && strings.TrimSpace(summary) != "" {
			return summary
		}
		parts := []string{}
		for _, item := range typed {
			if text := extractText(item); text != "" {
				parts = append(parts, text)
			}
		}
		return strings.Join(parts, " ")
	default:
		return ""
	}
}