package agents

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"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, mergeTitle 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. "+
		"Finally, if the merge commit title (currently \"%[2]s\") no longer describes all the work on this branch, set a better one with set_merge_title.", target, mergeTitle)
}

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

	prompt := mergeInPrompt(target, task.MergeTitle)
	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) (api.AgentSession, error) {
	if user.GitName == "" || user.GitEmail == "" {
		return api.AgentSession{}, ErrNoGitIdentity
	}

	err := git.FetchFastForward(project.RepoPath, project.DefaultBranch)
	if err != nil {
		return api.AgentSession{}, err
	}

	err = m.store.SetMergingBy(ctx, task.Id, user.Login)
	if err != nil {
		return api.AgentSession{}, err
	}

	sess, err := m.MergeIn(ctx, task, user, project.DefaultBranch)
	if err != nil {
		m.takeMerge(task.Id)
	}
	return sess, err
}

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

	slog.Error("finish merge", "task", l.taskID, "err", err)
	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
	}

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

	pushed := ""
	if commit != "" {
		err = git.Push(project.RepoPath, commit, project.DefaultBranch)
		if err != nil {
			return err
		}
		pushed = git.Remote + "/" + project.DefaultBranch
		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) Discard(ctx context.Context, task api.Task, project api.Project, actor string) (api.Task, error) {
	err := m.RemoveTask(ctx, task, project, actor)
	if err != nil {
		return api.Task{}, err
	}
	return m.store.DiscardTask(ctx, task.Id, actor)
}

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
	}
	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 := strings.TrimSpace(args.Title)
	if title == "" || strings.ContainsAny(title, "\r\n") {
		return "", errors.New("title must be one non-empty line")
	}
	if utf8.RuneCountInString(title) > maxMergeTitle {
		return "", fmt.Errorf("title is %d characters; keep it to %d", utf8.RuneCountInString(title), maxMergeTitle)
	}

	err = m.store.SetMergeTitle(ctx, l.taskID, title, agentActor)
	if errors.Is(err, store.ErrNotFound) {
		return "", errors.New("this task is no longer active")
	}
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("Merge commit title set to %q.", title), nil
}