package agents

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"slices"
	"strings"
	"unicode/utf8"

	"g.fc.run/misc/byre/internal/api"
	"g.fc.run/misc/byre/internal/git"
	"g.fc.run/misc/byre/internal/mcp"
	"g.fc.run/misc/byre/internal/store"
)

const maxMergeTitle = 72

func mergeInPrompt(target string) string {
	return fmt.Sprintf("Merge %[1]s into this branch: run `git merge --no-ff --no-commit %[1]s`. "+
		"Resolve any conflicts keeping both sides' intent; regenerate generated files instead of hand-merging them. "+
		"Check that the build and tests pass. Leave the merge uncommitted: byre concludes it when the turn ends. "+
		"If resolving the conflicts took substantive changes rather than combining both sides as written, stop there: "+
		"end your turn with a description of those changes, so the review of the turn's snapshot covers them.", target)
}

func (m *Manager) MergeIn(ctx context.Context, task api.Task, user api.User, target string) (api.AgentSession, error) {
	prompt := mergeInPrompt(target) + fmt.Sprintf(" Finally, if the merge commit title (currently \"%s\") no longer describes all the work on this branch, set a better one with set_merge_title.", task.MergeTitle)
	return m.sendMergeIn(ctx, task, user, prompt)
}

func (m *Manager) sendMergeIn(ctx context.Context, task api.Task, user api.User, prompt string) (api.AgentSession, error) {
	sessions, err := m.store.ListSessions(ctx, task.Id)
	if err != nil {
		return api.AgentSession{}, err
	}

	for _, sess := range sessions {
		if sess.State != api.AgentSessionStateCompleted {
			return m.Send(ctx, sess.Id, "byre", prompt, "")
		}
	}

	return m.create(ctx, task, user, prompt, "", "byre", nil)
}

func (m *Manager) Merge(ctx context.Context, task api.Task, project api.Project, user api.User, title string) (api.Task, error) {
	if user.GitName == "" || user.GitEmail == "" {
		return api.Task{}, ErrNoGitIdentity
	}
	if task.MergeQueuedAt != nil {
		return api.Task{}, fmt.Errorf("%w: task %s is already waiting to merge", ErrConflict, task.Id)
	}
	if task.MergingIntoParent {
		return api.Task{}, fmt.Errorf("%w: task %s is merging into its parent", ErrConflict, task.Id)
	}
	err := m.checkNoSubtasks(ctx, task)
	if err != nil {
		return api.Task{}, err
	}
	title, err = cleanMergeTitle(title)
	if err != nil {
		return api.Task{}, err
	}

	m.mergeMu.Lock()
	defer m.mergeMu.Unlock()

	if m.mergeHolders[project.Id] == task.Id {
		return api.Task{}, fmt.Errorf("%w: task %s is already merging", ErrConflict, task.Id)
	}
	if title != task.MergeTitle {
		err = m.storeMergeTitle(ctx, task.Id, title, user.Login)
		if err != nil {
			return api.Task{}, err
		}
		task.MergeTitle = title
	}

	switch m.mergeHolders[project.Id] {
	case "":
		err := m.startMerge(ctx, task, project, user)
		if err != nil {
			return api.Task{}, err
		}
	default:
		err := m.store.QueueMerge(ctx, task.Id, user.Login)
		if errors.Is(err, store.ErrNotFound) {
			return api.Task{}, fmt.Errorf("%w: task %s is closed, being discarded or already waiting to merge", ErrConflict, task.Id)
		}
		if err != nil {
			return api.Task{}, err
		}
	}
	return m.store.GetTask(ctx, task.Id)
}

func (m *Manager) startMerge(ctx context.Context, task api.Task, project api.Project, user api.User) error {
	err := git.FetchFastForward(project.RepoPath, project.DefaultBranch)
	if err != nil {
		return err
	}

	err = m.store.SetMergingBy(ctx, task.Id, user.Login)
	if errors.Is(err, store.ErrNotFound) {
		return fmt.Errorf("%w: task %s is being discarded", ErrConflict, task.Id)
	}
	if err != nil {
		return err
	}
	m.mergeHolders[project.Id] = task.Id

	prompt := mergeInPrompt(project.DefaultBranch) + fmt.Sprintf(" When this turn ends, byre squashes this branch onto %s and pushes it; if you stop, call stop_merge first so it doesn't. ", project.DefaultBranch) + summaryRequest
	_, err = m.sendMergeIn(ctx, task, user, prompt)
	if err != nil {
		m.takeMerge(task.Id)
		delete(m.mergeHolders, project.Id)
	}
	return err
}

func (m *Manager) recoverMerges(ctx context.Context) error {
	merging, err := m.store.MergingTasks(ctx)
	if err != nil {
		return err
	}

	m.mergeMu.Lock()
	defer m.mergeMu.Unlock()
	for _, t := range merging {
		m.mergeHolders[t.ProjectId] = t.Id
	}
	return nil
}

func (m *Manager) recoverFinalizing(ctx context.Context) error {
	finalizing, err := m.store.FinalizingTasks(ctx)
	if err != nil {
		return err
	}

	for _, t := range finalizing {
		err = m.store.ClearFinalizing(ctx, t.Id)
		if err != nil {
			return err
		}
		sessions, err := m.store.ListSessions(ctx, t.Id)
		if err != nil {
			return err
		}
		slog.Error("merge was finishing at startup", "task", t.Id)
		m.fileTaskError(ctx, t, sessions, "byre restarted while finishing this task's merge, so it may not have been pushed, or the task's worktree may be gone. Check the default branch before merging again.")
	}
	return nil
}

func (m *Manager) startHeldMerges(ctx context.Context) {
	projects, err := m.store.ProjectsWithQueuedMerges(ctx)
	if err != nil {
		slog.Error("list projects with held merges", "err", err)
		return
	}
	for _, p := range projects {
		m.nextMerge(p)
	}
}

func (m *Manager) abandonMerge(taskID, projectID string) {
	if m.takeMerge(taskID) == "" {
		return
	}
	go m.releaseMerge(projectID, taskID)
}

func (m *Manager) releaseMerge(projectID, taskID string) {
	m.mergeMu.Lock()
	defer m.mergeMu.Unlock()
	if m.mergeHolders[projectID] == taskID {
		delete(m.mergeHolders, projectID)
	}
	m.startNextMerge(projectID)
}

func (m *Manager) nextMerge(projectID string) {
	m.mergeMu.Lock()
	defer m.mergeMu.Unlock()
	m.startNextMerge(projectID)
}

func (m *Manager) startNextMerge(projectID string) {
	if m.mergeHolders[projectID] != "" {
		return
	}

	ctx := context.Background()
	held, err := m.store.QueuedMerges(ctx, projectID)
	if err != nil {
		slog.Error("list held merges", "project", projectID, "err", err)
		return
	}

	for _, task := range held {
		sessions, err := m.store.ListSessions(ctx, task.Id)
		if err != nil {
			slog.Error("list sessions for held merge", "task", task.Id, "err", err)
			continue
		}
		if slices.ContainsFunc(sessions, func(s api.AgentSession) bool {
			return s.State == api.AgentSessionStateRunning
		}) {
			continue
		}

		login, err := m.store.TakeQueuedMerge(ctx, task.Id)
		if err != nil {
			slog.Error("take held merge", "task", task.Id, "err", err)
			continue
		}
		if login == "" {
			continue
		}

		err = m.startHeldMerge(ctx, task, login)
		if err == nil {
			slog.Info("started held merge", "task", task.Id, "by", login)
			return
		}
		slog.Error("start held merge", "task", task.Id, "err", err)
		m.fileTaskError(ctx, task, sessions, "Starting the held merge failed: "+err.Error())
	}
}

func (m *Manager) startHeldMerge(ctx context.Context, task api.Task, login string) error {
	project, err := m.store.GetProject(ctx, task.ProjectId)
	if err != nil {
		return err
	}
	user, err := m.store.GetUser(ctx, login)
	if err != nil {
		return err
	}
	if user.GitName == "" || user.GitEmail == "" {
		return ErrNoGitIdentity
	}
	return m.startMerge(ctx, task, project, user)
}

func (m *Manager) fileTaskError(ctx context.Context, task api.Task, sessions []api.AgentSession, msg string) {
	if len(sessions) == 0 {
		slog.Error("no session to file task error on", "task", task.Id, "error", msg)
		return
	}
	last := sessions[len(sessions)-1]
	m.fileError(&live{id: last.Id, taskID: task.Id, projectID: task.ProjectId}, msg)
}

func (m *Manager) takeMerge(taskID string) string {
	login, err := m.store.TakeMergingBy(context.Background(), taskID)
	if err != nil {
		slog.Error("take merge intent", "task", taskID, "err", err)
	}
	return login
}

func (m *Manager) finishMerge(l *live, login string) {
	defer m.releaseMerge(l.projectID, l.taskID)

	ctx := context.Background()
	err := m.squashAndClose(ctx, l.taskID, login)
	if err == nil {
		return
	}

	slog.Error("finish merge", "task", l.taskID, "err", err)
	cerr := m.store.ClearFinalizing(ctx, l.taskID)
	if cerr != nil {
		slog.Error("clear merge finalizing", "task", l.taskID, "err", cerr)
	}
	m.fileError(l, "Merging the task failed: "+err.Error())
	err = m.Deliver(ctx, l.id)
	if err != nil {
		slog.Error("deliver inbox responses", "session", l.id, "err", err)
	}
}

func (m *Manager) squashAndClose(ctx context.Context, taskID, login string) error {
	task, err := m.store.GetTask(ctx, taskID)
	if err != nil {
		return err
	}
	project, err := m.store.GetProject(ctx, task.ProjectId)
	if err != nil {
		return err
	}
	user, err := m.store.GetUser(ctx, login)
	if err != nil {
		return err
	}

	coAuthors, err := m.CoAuthors(ctx, taskID, login)
	if err != nil {
		return err
	}

	env := []string{
		"GIT_AUTHOR_NAME=" + user.GitName,
		"GIT_AUTHOR_EMAIL=" + user.GitEmail,
		"GIT_COMMITTER_NAME=" + user.GitName,
		"GIT_COMMITTER_EMAIL=" + user.GitEmail,
	}
	commit, err := git.SquashCommit(project.RepoPath, task.WorktreePath, project.DefaultBranch, squashMessage(task.MergeTitle, coAuthors), env)
	if err != nil {
		return err
	}

	pushed := ""
	if commit != "" {
		note, err := m.mergeNote(ctx, task, login)
		if err != nil {
			return err
		}
		notes, err := git.NoteCommit(project.RepoPath, commit, note, env)
		if err != nil {
			return err
		}
		err = git.Push(project.RepoPath, commit, project.DefaultBranch, notes)
		if err != nil {
			return err
		}
		pushed = git.Remote + "/" + project.DefaultBranch
		err = git.SetNotes(project.RepoPath, notes)
		if err != nil {
			return fmt.Errorf("pushed %s to %s, but updating %s in the project checkout failed: %w", commit, pushed, git.NotesRef, err)
		}
		err = git.FastForward(project.RepoPath, commit)
		if err != nil {
			return fmt.Errorf("pushed %s to %s, but fast-forwarding the project checkout failed: %w", commit, pushed, err)
		}
	}

	err = m.RemoveTask(ctx, task, project, login)
	if err != nil {
		return err
	}

	_, err = m.store.MergeTask(ctx, taskID, commit, pushed, login)
	return err
}

func (m *Manager) CoAuthors(ctx context.Context, taskID, login string) ([]api.Participant, error) {
	participants, err := m.store.Participants(ctx, taskID)
	if err != nil {
		return nil, err
	}

	coAuthors := []api.Participant{}
	for _, p := range participants {
		if p.Login != login {
			coAuthors = append(coAuthors, p)
		}
	}
	return coAuthors, nil
}

func squashMessage(title string, coAuthors []api.Participant) string {
	trailers := []string{}
	for _, p := range coAuthors {
		if p.GitName == "" || p.GitEmail == "" {
			continue
		}
		trailers = append(trailers, fmt.Sprintf("Co-authored-by: %s <%s>", p.GitName, p.GitEmail))
	}
	if len(trailers) == 0 {
		return title
	}
	return title + "\n\n" + strings.Join(trailers, "\n")
}

func (m *Manager) RemoveTask(ctx context.Context, task api.Task, project api.Project, actor string) error {
	err := m.StopTask(ctx, task.Id, actor)
	if err != nil {
		return err
	}
	m.cancelReviews(ctx, task, project)
	err = m.dropSnapshots(project.RepoPath, task.Id)
	if err != nil {
		return err
	}
	return git.RemoveWorktree(project.RepoPath, task.WorktreePath, task.Branch)
}

type setMergeTitleArgs struct {
	Title string `json:"title"`
}

func (m *Manager) setMergeTitleTool(l *live) mcp.Tool {
	return mcp.Tool{
		Name: "set_merge_title",
		Description: "Set the title of the single commit this task becomes when it is merged onto the default branch. " +
			"It starts as the task title. Write one line in the imperative mood that describes all the work on the branch, " +
			"for example \"Add retry with backoff to webhook delivery\". It replaces any earlier title.",
		InputSchema: json.RawMessage(`{
			"type": "object",
			"properties": {
				"title": {"type": "string", "description": "One line, at most 72 characters."}
			},
			"required": ["title"]
		}`),
		Handler: func(ctx context.Context, raw json.RawMessage) (string, error) {
			return m.setMergeTitle(ctx, l, raw)
		},
	}
}

func (m *Manager) setMergeTitle(ctx context.Context, l *live, raw json.RawMessage) (string, error) {
	args := setMergeTitleArgs{}
	err := json.Unmarshal(raw, &args)
	if err != nil {
		return "", fmt.Errorf("malformed set_merge_title call: %w", err)
	}

	title, err := cleanMergeTitle(args.Title)
	if err != nil {
		return "", err
	}

	err = m.storeMergeTitle(ctx, l.taskID, title, agentActor)
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("Merge commit title set to %q.", title), nil
}

type stopMergeArgs struct {
	Reason string `json:"reason"`
}

func (m *Manager) stopMergeTool(l *live) mcp.Tool {
	return mcp.Tool{
		Name: "stop_merge",
		Description: "Stop the merge byre is to make when this turn ends, so the task stays active and its snapshot is reviewed before anything is pushed. " +
			"Call it during a Merge turn when resolving conflicts took substantive changes, then end your turn describing them. The human merges again after the review.",
		InputSchema: json.RawMessage(`{
			"type": "object",
			"properties": {
				"reason": {"type": "string", "description": "The substantive changes the resolution made, in a sentence or two."}
			},
			"required": ["reason"]
		}`),
		Handler: func(ctx context.Context, raw json.RawMessage) (string, error) {
			return m.stopMerge(ctx, l, raw)
		},
	}
}

func (m *Manager) stopMerge(ctx context.Context, l *live, raw json.RawMessage) (string, error) {
	args := stopMergeArgs{}
	err := json.Unmarshal(raw, &args)
	if err != nil {
		return "", fmt.Errorf("malformed stop_merge call: %w", err)
	}
	reason := strings.TrimSpace(args.Reason)
	if reason == "" {
		return "", fmt.Errorf("%w: reason must say what the resolution changed", ErrInvalid)
	}

	if m.takeMerge(l.taskID) == "" {
		return "No merge is pending for this task; just end your turn.", nil
	}
	go m.releaseMerge(l.projectID, l.taskID)

	err = m.emit(ctx, l, agentActor, "task.merge_stopped", map[string]any{"id": l.taskID, "reason": reason})
	if err != nil {
		return "", err
	}
	return "Merge stopped: byre won't squash or push when this turn ends. End your turn now with a description of the changes.", nil
}

func cleanMergeTitle(raw string) (string, error) {
	title := strings.TrimSpace(raw)
	if title == "" || strings.ContainsAny(title, "\r\n") {
		return "", fmt.Errorf("%w: title must be one non-empty line", ErrInvalid)
	}
	if utf8.RuneCountInString(title) > maxMergeTitle {
		return "", fmt.Errorf("%w: title is %d characters; keep it to %d", ErrInvalid, utf8.RuneCountInString(title), maxMergeTitle)
	}
	return title, nil
}

func (m *Manager) storeMergeTitle(ctx context.Context, taskID, title, actor string) error {
	err := m.store.SetMergeTitle(ctx, taskID, title, actor)
	if errors.Is(err, store.ErrNotFound) {
		return fmt.Errorf("%w: this task is closed, or its merge was already confirmed with its title", ErrConflict)
	}
	return err
}