// Copyright 2015 The Prometheus Authors
// 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 api provides clients for the HTTP APIs.
package api

import (
	"context"
	"io/ioutil"
	"net"
	"net/http"
	"net/url"
	"path"
	"strings"
	"time"
)

// DefaultRoundTripper is used if no RoundTripper is set in Config.
var DefaultRoundTripper http.RoundTripper = &http.Transport{
	Proxy: http.ProxyFromEnvironment,
	DialContext: (&net.Dialer{
		Timeout:   30 * time.Second,
		KeepAlive: 30 * time.Second,
	}).DialContext,
	TLSHandshakeTimeout: 10 * time.Second,
}

// Config defines configuration parameters for a new client.
type Config struct {
	// The address of the Prometheus to connect to.
	Address string

	// RoundTripper is used by the Client to drive HTTP requests. If not
	// provided, DefaultRoundTripper will be used.
	RoundTripper http.RoundTripper
}

func (cfg *Config) roundTripper() http.RoundTripper {
	if cfg.RoundTripper == nil {
		return DefaultRoundTripper
	}
	return cfg.RoundTripper
}

// Client is the interface for an API client.
type Client interface {
	URL(ep string, args map[string]string) *url.URL
	Do(context.Context, *http.Request) (*http.Response, []byte, error)
}

// DoGetFallback will attempt to do the request as-is, and on a 405 it will fallback to a GET request.
func DoGetFallback(c Client, ctx context.Context, u *url.URL, args url.Values) (*http.Response, []byte, error) {
	req, err := http.NewRequest(http.MethodPost, u.String(), strings.NewReader(args.Encode()))
	if err != nil {
		return nil, nil, err
	}
	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

	resp, body, err := c.Do(ctx, req)
	if resp != nil && resp.StatusCode == http.StatusMethodNotAllowed {
		u.RawQuery = args.Encode()
		req, err = http.NewRequest(http.MethodGet, u.String(), nil)
		if err != nil {
			return nil, nil, err
		}

	} else {
		return resp, body, err
	}
	return c.Do(ctx, req)
}

// NewClient returns a new Client.
//
// It is safe to use the returned Client from multiple goroutines.
func NewClient(cfg Config) (Client, error) {
	u, err := url.Parse(cfg.Address)
	if err != nil {
		return nil, err
	}
	u.Path = strings.TrimRight(u.Path, "/")

	return &httpClient{
		endpoint: u,
		client:   http.Client{Transport: cfg.roundTripper()},
	}, nil
}

type httpClient struct {
	endpoint *url.URL
	client   http.Client
}

func (c *httpClient) URL(ep string, args map[string]string) *url.URL {
	p := path.Join(c.endpoint.Path, ep)

	for arg, val := range args {
		arg = ":" + arg
		p = strings.Replace(p, arg, val, -1)
	}

	u := *c.endpoint
	u.Path = p

	return &u
}

func (c *httpClient) Do(ctx context.Context, req *http.Request) (*http.Response, []byte, error) {
	if ctx != nil {
		req = req.WithContext(ctx)
	}
	resp, err := c.client.Do(req)
	defer func() {
		if resp != nil {
			resp.Body.Close()
		}
	}()

	if err != nil {
		return nil, nil, err
	}

	var body []byte
	done := make(chan struct{})
	go func() {
		body, err = ioutil.ReadAll(resp.Body)
		close(done)
	}()

	select {
	case <-ctx.Done():
		<-done
		err = resp.Body.Close()
		if err == nil {
			err = ctx.Err()
		}
	case <-done:
	}

	return resp, body, err
}
