package chatd

import (
	"context"
	"strings"
	"time"

	"charm.land/fantasy"
	"golang.org/x/xerrors"

	"cdr.dev/slog/v3"
	"github.com/coder/coder/v2/coderd/chatd/chatprompt"
	"github.com/coder/coder/v2/coderd/database"
	coderdpubsub "github.com/coder/coder/v2/coderd/pubsub"
)

const titleGenerationPrompt = "Generate a concise title (max 8 words, under 128 characters) for " +
	"the user's first message. Return plain text only — no quotes, no emoji, " +
	"no markdown, no special characters."

// maybeGenerateChatTitle generates an AI title for the chat when
// appropriate (first user message, no assistant reply yet, and the
// current title is either empty or still the fallback truncation).
// It is a best-effort operation that logs and swallows errors.
func (p *Server) maybeGenerateChatTitle(
	ctx context.Context,
	chat database.Chat,
	messages []database.ChatMessage,
	model fantasy.LanguageModel,
	logger slog.Logger,
) {
	input, ok := titleInput(chat, messages)
	if !ok {
		return
	}

	titleCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	title, err := generateTitle(titleCtx, model, input)
	if err != nil {
		logger.Debug(ctx, "failed to generate chat title",
			slog.F("chat_id", chat.ID),
			slog.Error(err),
		)
		return
	}
	if title == "" || title == chat.Title {
		return
	}

	_, err = p.db.UpdateChatByID(ctx, database.UpdateChatByIDParams{
		ID:    chat.ID,
		Title: title,
	})
	if err != nil {
		logger.Warn(ctx, "failed to update generated chat title",
			slog.F("chat_id", chat.ID),
			slog.Error(err),
		)
		return
	}
	chat.Title = title
	p.publishChatPubsubEvent(chat, coderdpubsub.ChatEventKindTitleChange)
}

// generateTitle calls the model with a title-generation system prompt
// and returns the normalized result.
func generateTitle(
	ctx context.Context,
	model fantasy.LanguageModel,
	input string,
) (string, error) {
	prompt := []fantasy.Message{
		{
			Role: fantasy.MessageRoleSystem,
			Content: []fantasy.MessagePart{
				fantasy.TextPart{Text: titleGenerationPrompt},
			},
		},
		{
			Role: fantasy.MessageRoleUser,
			Content: []fantasy.MessagePart{
				fantasy.TextPart{Text: input},
			},
		},
	}
	toolChoice := fantasy.ToolChoiceNone
	response, err := model.Generate(ctx, fantasy.Call{
		Prompt:     prompt,
		ToolChoice: &toolChoice,
	})
	if err != nil {
		return "", xerrors.Errorf("generate title text: %w", err)
	}

	title := normalizeTitleOutput(contentBlocksToText(response.Content))
	if title == "" {
		return "", xerrors.New("generated title was empty")
	}
	return title, nil
}

// titleInput returns the first user message text and whether title
// generation should proceed. It returns false when the chat already
// has assistant/tool replies, has more than one visible user message,
// or the current title doesn't look like a candidate for replacement.
func titleInput(
	chat database.Chat,
	messages []database.ChatMessage,
) (string, bool) {
	userCount := 0
	firstUserText := ""

	for _, message := range messages {
		if message.Visibility == database.ChatMessageVisibilityModel {
			continue
		}

		switch message.Role {
		case string(fantasy.MessageRoleAssistant), string(fantasy.MessageRoleTool):
			return "", false
		case string(fantasy.MessageRoleUser):
			userCount++
			if firstUserText == "" {
				parsed, err := chatprompt.ParseContent(
					string(fantasy.MessageRoleUser), message.Content,
				)
				if err != nil {
					return "", false
				}
				firstUserText = strings.TrimSpace(
					contentBlocksToText(parsed),
				)
			}
		}
	}

	if userCount != 1 || firstUserText == "" {
		return "", false
	}

	currentTitle := strings.TrimSpace(chat.Title)
	if currentTitle == "" {
		return firstUserText, true
	}

	if currentTitle != fallbackChatTitle(firstUserText) {
		return "", false
	}

	return firstUserText, true
}

func normalizeTitleOutput(title string) string {
	title = strings.TrimSpace(title)
	if title == "" {
		return ""
	}

	title = strings.Trim(title, "\"'`")
	title = strings.Join(strings.Fields(title), " ")
	return truncateRunes(title, 80)
}

func fallbackChatTitle(message string) string {
	const maxWords = 6
	const maxRunes = 80

	words := strings.Fields(message)
	if len(words) == 0 {
		return "New Chat"
	}

	truncated := false
	if len(words) > maxWords {
		words = words[:maxWords]
		truncated = true
	}

	title := strings.Join(words, " ")
	if truncated {
		title += "…"
	}

	return truncateRunes(title, maxRunes)
}

// contentBlocksToText concatenates the text parts of content blocks
// into a single space-separated string.
func contentBlocksToText(content []fantasy.Content) string {
	parts := make([]string, 0, len(content))
	for _, block := range content {
		textBlock, ok := fantasy.AsContentType[fantasy.TextContent](block)
		if !ok {
			continue
		}
		text := strings.TrimSpace(textBlock.Text)
		if text == "" {
			continue
		}
		parts = append(parts, text)
	}
	return strings.Join(parts, " ")
}

func truncateRunes(value string, maxLen int) string {
	if maxLen <= 0 {
		return ""
	}
	runes := []rune(value)
	if len(runes) <= maxLen {
		return value
	}
	return string(runes[:maxLen])
}