// Copyright 2015 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package http supports network connections to HTTP servers.
// This package is not intended for use by end developers. Use the
// google.golang.org/api/option package to configure API clients.
package http

import (
	"context"
	"errors"
	"net/http"

	"go.opencensus.io/plugin/ochttp"
	"golang.org/x/oauth2"
	"google.golang.org/api/googleapi/transport"
	"google.golang.org/api/internal"
	"google.golang.org/api/option"
	"google.golang.org/api/transport/http/internal/propagation"
)

// NewClient returns an HTTP client for use communicating with a Google cloud
// service, configured with the given ClientOptions. It also returns the endpoint
// for the service as specified in the options.
func NewClient(ctx context.Context, opts ...option.ClientOption) (*http.Client, string, error) {
	settings, err := newSettings(opts)
	if err != nil {
		return nil, "", err
	}
	// TODO(cbro): consider injecting the User-Agent even if an explicit HTTP client is provided?
	if settings.HTTPClient != nil {
		return settings.HTTPClient, settings.Endpoint, nil
	}
	trans, err := newTransport(ctx, defaultBaseTransport(ctx), settings)
	if err != nil {
		return nil, "", err
	}
	return &http.Client{Transport: trans}, settings.Endpoint, nil
}

// NewTransport creates an http.RoundTripper for use communicating with a Google
// cloud service, configured with the given ClientOptions. Its RoundTrip method delegates to base.
func NewTransport(ctx context.Context, base http.RoundTripper, opts ...option.ClientOption) (http.RoundTripper, error) {
	settings, err := newSettings(opts)
	if err != nil {
		return nil, err
	}
	if settings.HTTPClient != nil {
		return nil, errors.New("transport/http: WithHTTPClient passed to NewTransport")
	}
	return newTransport(ctx, base, settings)
}

func newTransport(ctx context.Context, base http.RoundTripper, settings *internal.DialSettings) (http.RoundTripper, error) {
	trans := base
	trans = userAgentTransport{
		base:      trans,
		userAgent: settings.UserAgent,
	}
	trans = addOCTransport(trans)
	switch {
	case settings.NoAuth:
		// Do nothing.
	case settings.APIKey != "":
		trans = &transport.APIKey{
			Transport: trans,
			Key:       settings.APIKey,
		}
	default:
		creds, err := internal.Creds(ctx, settings)
		if err != nil {
			return nil, err
		}
		trans = &oauth2.Transport{
			Base:   trans,
			Source: creds.TokenSource,
		}
	}
	return trans, nil
}

func newSettings(opts []option.ClientOption) (*internal.DialSettings, error) {
	var o internal.DialSettings
	for _, opt := range opts {
		opt.Apply(&o)
	}
	if err := o.Validate(); err != nil {
		return nil, err
	}
	if o.GRPCConn != nil {
		return nil, errors.New("unsupported gRPC connection specified")
	}
	return &o, nil
}

type userAgentTransport struct {
	userAgent string
	base      http.RoundTripper
}

func (t userAgentTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	rt := t.base
	if rt == nil {
		return nil, errors.New("transport: no Transport specified")
	}
	if t.userAgent == "" {
		return rt.RoundTrip(req)
	}
	newReq := *req
	newReq.Header = make(http.Header)
	for k, vv := range req.Header {
		newReq.Header[k] = vv
	}
	// TODO(cbro): append to existing User-Agent header?
	newReq.Header["User-Agent"] = []string{t.userAgent}
	return rt.RoundTrip(&newReq)
}

// Set at init time by dial_appengine.go. If nil, we're not on App Engine.
var appengineUrlfetchHook func(context.Context) http.RoundTripper

// defaultBaseTransport returns the base HTTP transport.
// On App Engine, this is urlfetch.Transport, otherwise it's http.DefaultTransport.
func defaultBaseTransport(ctx context.Context) http.RoundTripper {
	if appengineUrlfetchHook != nil {
		return appengineUrlfetchHook(ctx)
	}
	return http.DefaultTransport
}

func addOCTransport(trans http.RoundTripper) http.RoundTripper {
	return &ochttp.Transport{
		Base:        trans,
		Propagation: &propagation.HTTPFormat{},
	}
}