// Package client provides an HTTP client for the MCQ REST API.
package client

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"regexp"
	"strings"
)

var (
	ErrNotFound     = errors.New("not found")
	ErrUnauthorized = errors.New("unauthorized")
	ErrForbidden    = errors.New("forbidden")
)

// Document represents a document returned by the MCQ API.
type Document struct {
	ID       int64  `json:"id"`
	Slug     string `json:"slug"`
	Title    string `json:"title"`
	Body     string `json:"body"`
	PushedBy string `json:"pushed_by"`
	PushedAt string `json:"pushed_at"`
	Read     bool   `json:"read"`
}

// Client talks to a remote MCQ server's REST API.
type Client struct {
	baseURL    string
	token      string
	httpClient *http.Client
}

// Option configures a Client.
type Option func(*Client)

// WithHTTPClient sets a custom HTTP client (useful for testing).
func WithHTTPClient(hc *http.Client) Option {
	return func(c *Client) {
		c.httpClient = hc
	}
}

// New creates a Client. baseURL is the MCQ server URL (e.g. "https://mcq.example.com:8443").
// token is a Bearer token from MCIAS login.
func New(baseURL, token string, opts ...Option) *Client {
	c := &Client{
		baseURL:    strings.TrimRight(baseURL, "/"),
		token:      token,
		httpClient: http.DefaultClient,
	}
	for _, o := range opts {
		o(c)
	}
	return c
}

func (c *Client) do(ctx context.Context, method, path string, body any) (*http.Response, error) {
	var bodyReader io.Reader
	if body != nil {
		buf, err := json.Marshal(body)
		if err != nil {
			return nil, fmt.Errorf("marshal request: %w", err)
		}
		bodyReader = bytes.NewReader(buf)
	}

	req, err := http.NewRequestWithContext(ctx, method, c.baseURL+path, bodyReader)
	if err != nil {
		return nil, err
	}
	if body != nil {
		req.Header.Set("Content-Type", "application/json")
	}
	if c.token != "" {
		req.Header.Set("Authorization", "Bearer "+c.token)
	}

	resp, err := c.httpClient.Do(req)
	if err != nil {
		return nil, err
	}

	switch resp.StatusCode {
	case http.StatusUnauthorized:
		_ = resp.Body.Close()
		return nil, ErrUnauthorized
	case http.StatusForbidden:
		_ = resp.Body.Close()
		return nil, ErrForbidden
	case http.StatusNotFound:
		_ = resp.Body.Close()
		return nil, ErrNotFound
	}

	return resp, nil
}

func decodeJSON[T any](resp *http.Response) (T, error) {
	defer func() { _ = resp.Body.Close() }()
	var v T
	if err := json.NewDecoder(resp.Body).Decode(&v); err != nil {
		return v, fmt.Errorf("decode response: %w", err)
	}
	return v, nil
}

// Login authenticates with MCIAS and returns a bearer token.
func (c *Client) Login(ctx context.Context, username, password, totpCode string) (string, error) {
	resp, err := c.do(ctx, http.MethodPost, "/v1/auth/login", map[string]string{
		"username":  username,
		"password":  password,
		"totp_code": totpCode,
	})
	if err != nil {
		return "", err
	}
	result, err := decodeJSON[struct {
		Token string `json:"token"`
	}](resp)
	if err != nil {
		return "", err
	}
	return result.Token, nil
}

// ListDocuments returns all documents in the queue.
func (c *Client) ListDocuments(ctx context.Context) ([]Document, error) {
	resp, err := c.do(ctx, http.MethodGet, "/v1/documents", nil)
	if err != nil {
		return nil, err
	}
	result, err := decodeJSON[struct {
		Documents []Document `json:"documents"`
	}](resp)
	if err != nil {
		return nil, err
	}
	return result.Documents, nil
}

// GetDocument fetches a single document by slug.
func (c *Client) GetDocument(ctx context.Context, slug string) (*Document, error) {
	resp, err := c.do(ctx, http.MethodGet, "/v1/documents/"+slug, nil)
	if err != nil {
		return nil, err
	}
	doc, err := decodeJSON[Document](resp)
	if err != nil {
		return nil, err
	}
	return &doc, nil
}

// PutDocument creates or replaces a document.
func (c *Client) PutDocument(ctx context.Context, slug, title, body string) (*Document, error) {
	resp, err := c.do(ctx, http.MethodPut, "/v1/documents/"+slug, map[string]string{
		"title": title,
		"body":  body,
	})
	if err != nil {
		return nil, err
	}
	doc, err := decodeJSON[Document](resp)
	if err != nil {
		return nil, err
	}
	return &doc, nil
}

// DeleteDocument removes a document by slug.
func (c *Client) DeleteDocument(ctx context.Context, slug string) error {
	resp, err := c.do(ctx, http.MethodDelete, "/v1/documents/"+slug, nil)
	if err != nil {
		return err
	}
	_ = resp.Body.Close()
	return nil
}

// MarkRead marks a document as read.
func (c *Client) MarkRead(ctx context.Context, slug string) (*Document, error) {
	resp, err := c.do(ctx, http.MethodPost, "/v1/documents/"+slug+"/read", nil)
	if err != nil {
		return nil, err
	}
	doc, err := decodeJSON[Document](resp)
	if err != nil {
		return nil, err
	}
	return &doc, nil
}

// MarkUnread marks a document as unread.
func (c *Client) MarkUnread(ctx context.Context, slug string) (*Document, error) {
	resp, err := c.do(ctx, http.MethodPost, "/v1/documents/"+slug+"/unread", nil)
	if err != nil {
		return nil, err
	}
	doc, err := decodeJSON[Document](resp)
	if err != nil {
		return nil, err
	}
	return &doc, nil
}

var h1Re = regexp.MustCompile(`(?m)^#\s+(.+)$`)

// ExtractTitle returns the first H1 heading from markdown source.
// If no H1 is found, it returns the fallback string.
func ExtractTitle(markdown, fallback string) string {
	m := h1Re.FindStringSubmatch(markdown)
	if m == nil {
		return fallback
	}
	return strings.TrimSpace(m[1])
}