package twitterapi

import (
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"time"
)

type Getter interface {
	Get(string) (*http.Response, error)
}

// NewClient returns a new APIClient.
func NewClient(httpclient Getter) APIClient {
	return &apiClient{httpclient}
}

// NewAPIClient returns a new APIClient which will authenticate with the
// provided bearer token.
func NewClientWithBearerToken(bearerToken string) APIClient {
	return &apiClient{&http.Client{
		Timeout:   time.Second * 5,
		Transport: &bearerTokenTransport{bearerToken: bearerToken},
	}}
}

// apiClient implements APIClient.
type apiClient struct {
	Getter
}

// bearerTokenTransport implements http.RoundTripper.
type bearerTokenTransport struct {
	bearerToken string
}

// RoundTrip sets the Authorization header with the provided bearerToken, and
// forwards the request. It will panic if the request includes a body (which
// requires special handling).
func (t *bearerTokenTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	if req.Body != nil {
		panic("bearerTokenTransport does not handle requests with non-nil body")
	}
	req2 := cloneRequest(req)
	req2.Header.Set("Authorization", "Bearer "+t.bearerToken)
	return http.DefaultTransport.RoundTrip(req2)
}

// https://go.googlesource.com/oauth2/+/f95fa95eaa936d9d87489b15d1d18b97c1ba9c28/transport.go#96
func cloneRequest(r *http.Request) *http.Request {
	// shallow copy of the struct
	r2 := new(http.Request)
	*r2 = *r
	// deep copy of the Header
	r2.Header = make(http.Header, len(r.Header))
	for k, s := range r.Header {
		r2.Header[k] = append([]string(nil), s...)
	}
	return r2
}

// GetMe returns the currently authenticated user.
func (c *apiClient) GetMe() (*User, error) {
	type oauthResponse struct {
		Data *User `json:"data"`
	}

	resp, err := c.Get("https://api.twitter.com/2/users/me")
	if err != nil {
		return nil, fmt.Errorf("error fetching resource: %v", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("error fetching resource: status code %d", resp.StatusCode)
	}

	var oauthResp oauthResponse
	if err = json.NewDecoder(resp.Body).Decode(&oauthResp); err != nil {
		return nil, fmt.Errorf("error decoding resource: %v", err)
	}

	return oauthResp.Data, nil
}

var ErrNoTweets = errors.New("no tweets available")

// GetLastTweet returns the most recent tweet for a given user. If no tweets
// are available, ErrNoTweets will be returned.
func (c *apiClient) GetLastTweet(userID string) (*Tweet, error) {
	type oauthResponse struct {
		Data []*Tweet        `json:"data"`
		Meta TwitterMetadata `json:"meta"`
	}

	apiURL := "https://api.twitter.com/2/users/" + userID + "/tweets?tweet.fields=created_at"

	resp, err := c.Get(apiURL)
	if err != nil {
		return nil, fmt.Errorf("error fetching resource: %v", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("error fetching resource: status code %d", resp.StatusCode)
	}

	var oauthResp oauthResponse
	if err = json.NewDecoder(resp.Body).Decode(&oauthResp); err != nil {
		return nil, fmt.Errorf("error decoding resource: %v", err)
	}

	if oauthResp.Meta.ResultCount == 0 {
		return nil, ErrNoTweets
	}

	// the order of returned tweets seems to be chronological, but it isn't
	// documented so use the metadata instead.
	for _, tweet := range oauthResp.Data {
		if tweet.ID == oauthResp.Meta.NewestID {
			return tweet, nil
		}
	}

	return nil, errors.New("error fetching latest tweet: could not match newest_id")
}

// GetTweets returns the latest tweets for a given user, up to the maximum
// batch size of 100 allowable by the Twitter API.
func (c *apiClient) GetTweets(userID string, sinceID string) ([]*Tweet, error) {
	type oauthResponse struct {
		Data []*Tweet `json:"data"`
	}

	apiURL := "https://api.twitter.com/2/users/" + userID + "/tweets?tweet.fields=created_at&max_results=100"
	if sinceID == "" {
	} else {
		apiURL += "&since_id=" + sinceID
	}

	resp, err := c.Get(apiURL)
	if err != nil {
		return nil, fmt.Errorf("error fetching resource: %v", err)
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("error fetching resource: status code %d", resp.StatusCode)
	}

	var oauthResp oauthResponse
	if err = json.NewDecoder(resp.Body).Decode(&oauthResp); err != nil {
		return nil, fmt.Errorf("error decoding resource: %v", err)
	}

	return oauthResp.Data, nil
}