/*
Copyright 2019 The Knative 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 metrics

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"
	"sync"

	network "knative.dev/networking/pkg"
)

var errUnsupportedMetricType = errors.New("unsupported metric type")

type httpScrapeClient struct {
	httpClient *http.Client
}

var pool = sync.Pool{
	New: func() interface{} {
		return new(bytes.Buffer)
	},
}

func newHTTPScrapeClient(httpClient *http.Client) *httpScrapeClient {
	return &httpScrapeClient{
		httpClient: httpClient,
	}
}

func (c *httpScrapeClient) Scrape(ctx context.Context, url string) (Stat, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return emptyStat, err
	}

	req.Header.Add("Accept", network.ProtoAcceptContent)
	resp, err := c.httpClient.Do(req)
	if err != nil {
		return emptyStat, err
	}
	defer resp.Body.Close()
	if resp.StatusCode < http.StatusOK || resp.StatusCode >= http.StatusMultipleChoices {
		return emptyStat, fmt.Errorf("GET request for URL %q returned HTTP status %v", url, resp.StatusCode)
	}
	if resp.Header.Get("Content-Type") != network.ProtoAcceptContent {
		return emptyStat, errUnsupportedMetricType
	}
	return statFromProto(resp.Body)
}

func statFromProto(body io.Reader) (Stat, error) {
	var stat Stat
	b := pool.Get().(*bytes.Buffer)
	b.Reset()
	defer pool.Put(b)
	_, err := b.ReadFrom(body)
	if err != nil {
		return emptyStat, fmt.Errorf("reading body failed: %w", err)
	}
	err = stat.Unmarshal(b.Bytes())
	if err != nil {
		return emptyStat, fmt.Errorf("unmarshalling failed: %w", err)
	}
	return stat, nil
}
