package appservice

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"time"

	"github.com/dariuszjaniak/warp/server/internal/tracing"
)

// Errors of the Application Service API client. The ping endpoint reports
// each with its own Matrix error code.
var (
	// ErrNoURL is a service registered without a URL, which cannot be
	// sent anything.
	ErrNoURL = errors.New("application service has no URL")

	// ErrConnectionFailed means the service could not be reached.
	ErrConnectionFailed = errors.New("could not connect to the application service")

	// ErrConnectionTimeout means it did not answer in time.
	ErrConnectionTimeout = errors.New("the application service did not answer in time")
)

// BadStatusError is an answer from the service that was not a success.
type BadStatusError struct {
	Status int

	// Body is the start of what the service answered, for whoever has to
	// work out why.
	Body string
}

func (e *BadStatusError) Error() string {
	return fmt.Sprintf("the application service answered %d: %s", e.Status, e.Body)
}

// maxErrorBody caps how much of a failed answer is kept.
const maxErrorBody = 4 << 10

// Client speaks the Application Service API: the requests a homeserver
// makes to a service.
type Client struct {
	http    *http.Client
	timeout time.Duration
}

// NewClient builds a client. The timeout bounds one request; transactions
// that time out are retried like any other failure.
func NewClient(timeout time.Duration) *Client {
	return &Client{
		http: &http.Client{
			Transport: tracing.Transport(nil),
			// A service is reached at the URL its registration names and
			// nowhere else: a redirect would take the hs_token with it.
			CheckRedirect: func(*http.Request, []*http.Request) error { return http.ErrUseLastResponse },
		},
		timeout: timeout,
	}
}

// transactionBody is PUT /_matrix/app/v1/transactions/{txnId}.
type transactionBody struct {
	Events []json.RawMessage `json:"events"`
}

// SendTransaction delivers a batch of events. The same transaction ID is
// resent with the same events until the service accepts it, and the
// service deduplicates by ID.
func (c *Client) SendTransaction(ctx context.Context, svc *Service, txnID int64, events []json.RawMessage) error {
	body, err := json.Marshal(transactionBody{Events: events})
	if err != nil {
		return err
	}
	_, err = c.do(ctx, svc, http.MethodPut, "/_matrix/app/v1/transactions/"+strconv.FormatInt(txnID, 10), body)
	return err
}

// QueryUser asks a service whether a user in its namespace exists, which
// it answers by creating the user if it wants it to. It reports whether
// the service said yes.
func (c *Client) QueryUser(ctx context.Context, svc *Service, userID string) (bool, error) {
	return c.query(ctx, svc, "/_matrix/app/v1/users/"+url.PathEscape(userID))
}

// QueryAlias asks a service whether an alias in its namespace exists,
// which it answers by creating the room and the alias.
func (c *Client) QueryAlias(ctx context.Context, svc *Service, alias string) (bool, error) {
	return c.query(ctx, svc, "/_matrix/app/v1/rooms/"+url.PathEscape(alias))
}

// query makes a GET whose answer is yes (200) or no (404).
func (c *Client) query(ctx context.Context, svc *Service, path string) (bool, error) {
	_, err := c.do(ctx, svc, http.MethodGet, path, nil)
	var status *BadStatusError
	switch {
	case err == nil:
		return true, nil
	case errors.As(err, &status) && status.Status == http.StatusNotFound:
		return false, nil
	default:
		return false, err
	}
}

// Ping makes POST /_matrix/app/v1/ping and reports how long it took.
func (c *Client) Ping(ctx context.Context, svc *Service, transactionID string) (time.Duration, error) {
	body, err := json.Marshal(map[string]string{"transaction_id": transactionID})
	if transactionID == "" {
		body, err = []byte(`{}`), nil
	}
	if err != nil {
		return 0, err
	}
	start := time.Now()
	if _, err := c.do(ctx, svc, http.MethodPost, "/_matrix/app/v1/ping", body); err != nil {
		return 0, err
	}
	return time.Since(start), nil
}

// do makes one request to a service, authenticated with its hs_token.
func (c *Client) do(ctx context.Context, svc *Service, method, path string, body []byte) ([]byte, error) {
	if svc.URL == "" {
		return nil, ErrNoURL
	}
	ctx, cancel := context.WithTimeout(ctx, c.timeout)
	defer cancel()

	var reader io.Reader
	if body != nil {
		reader = bytes.NewReader(body)
	}
	req, err := http.NewRequestWithContext(ctx, method, strings.TrimRight(svc.URL, "/")+path, reader)
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrConnectionFailed, err)
	}
	// The header form only. The spec deprecated ?access_token= for this in
	// v1.4, and a token in a URL ends up in the service's access log.
	req.Header.Set("Authorization", "Bearer "+svc.HSToken)
	if body != nil {
		req.Header.Set("Content-Type", "application/json")
	}

	resp, err := c.http.Do(req)
	if err != nil {
		var netErr net.Error
		if errors.Is(err, context.DeadlineExceeded) || (errors.As(err, &netErr) && netErr.Timeout()) {
			return nil, fmt.Errorf("%w: %v", ErrConnectionTimeout, err)
		}
		return nil, fmt.Errorf("%w: %v", ErrConnectionFailed, err)
	}
	defer func() { _ = resp.Body.Close() }()

	if resp.StatusCode < 200 || resp.StatusCode > 299 {
		head, _ := io.ReadAll(io.LimitReader(resp.Body, maxErrorBody))
		return nil, &BadStatusError{Status: resp.StatusCode, Body: string(head)}
	}
	out, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return nil, fmt.Errorf("%w: %v", ErrConnectionFailed, err)
	}
	return out, nil
}