package main

import (
	"bytes"
	"context"
	"flag"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"regexp"
	"sync"
	"time"

	_ "net/http/pprof"

	"github.com/getsentry/sentry-go"

	"github.com/sirupsen/logrus"
	"gopkg.in/yaml.v3"
)

var zeroTime time.Time

type UpstreamMatch struct {
	Match   string `yaml:"match"`
	Replace string `yaml:"replace"`
}

type Upstream struct {
	Server string        `yaml:"server"`
	Match  UpstreamMatch `yaml:"match"`
}

func (upstream Upstream) GetPath(orig string) (string, bool, error) {
	if upstream.Match.Match == "" || upstream.Match.Replace == "" {
		return orig, true, nil
	}
	matcher, err := regexp.Compile(upstream.Match.Match)
	if err != nil {
		return "", false, err
	}
	return matcher.ReplaceAllString(orig, upstream.Match.Replace), matcher.MatchString(orig), nil
}

type LocalStorage struct {
	Path string `yaml:"path"`
}

type Storage struct {
	Type  string        `yaml:"type"`
	Local *LocalStorage `yaml:"local"`
}

type Cache struct {
	Timeout time.Duration `yaml:"timeout"`
}

type MiscConfig struct {
	FirstChunkBytes uint64 `yaml:"first-chunk-bytes"`
	ChunkBytes      uint64 `yaml:"chunk-bytes"`
}

type Config struct {
	Upstreams []Upstream `yaml:"upstream"`
	Storage   Storage    `yaml:"storage"`
	Cache     Cache      `yaml:"cache"`
	Misc      MiscConfig `yaml:"misc"`
}

type StreamObject struct {
	Headers http.Header
	Buffer  *bytes.Buffer
	Offset  int

	ctx context.Context
	wg  *sync.WaitGroup
}

func (memoryObject *StreamObject) StreamTo(w io.Writer, wg *sync.WaitGroup) error {
	defer wg.Done()
	offset := 0
	if w == nil {
		w = io.Discard
	}
OUTER:
	for {
		select {
		case <-memoryObject.ctx.Done():
			break OUTER
		default:
		}

		newOffset := memoryObject.Offset
		if newOffset == offset {
			time.Sleep(time.Millisecond)
			continue
		}
		bytes := memoryObject.Buffer.Bytes()[offset:newOffset]
		written, err := w.Write(bytes)
		if err != nil {
			return err
		}

		offset += written
	}
	time.Sleep(time.Millisecond)
	logrus.WithFields(logrus.Fields{
		"start": offset,
		"end":   memoryObject.Buffer.Len(),
		"n":     memoryObject.Buffer.Len() - offset,
	}).Trace("remain bytes")

	_, err := w.Write(memoryObject.Buffer.Bytes()[offset:])
	return err
}

type Server struct {
	Config

	lu *sync.Mutex
	o  map[string]*StreamObject
}

type Chunk struct {
	buffer []byte
	error  error
}

func configFromFile(path string) (*Config, error) {
	file, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer file.Close()

	config := &Config{
		Upstreams: []Upstream{
			{
				Server: "https://mirrors.ustc.edu.cn",
			},
		},
		Storage: Storage{
			Type: "local",
			Local: &LocalStorage{
				Path: "./data",
			},
		},
		Misc: MiscConfig{
			FirstChunkBytes: 1024 * 1024 * 50,
			ChunkBytes:      1024 * 1024,
		},
		Cache: Cache{
			Timeout: time.Hour,
		},
	}

	if err := yaml.NewDecoder(file).Decode(&config); err != nil {
		return nil, err
	}

	return config, nil
}

func (server *Server) handleRequest(w http.ResponseWriter, r *http.Request) {
	fullpath := filepath.Join(server.Storage.Local.Path, r.URL.Path)
	fullpath, err := filepath.Abs(fullpath)
	if err != nil {
		http.Error(w, err.Error(), http.StatusBadRequest)
		return
	}

	ranged := r.Header.Get("Range") != ""

	localStatus, mtime, err := server.checkLocal(w, r, fullpath)
	if os.IsPermission(err) {
		http.Error(w, err.Error(), http.StatusForbidden)
	} else if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	} else if localStatus != localNotExists {
		if localStatus == localExistsButNeedHead {
			if ranged {
				server.streamOnline(nil, r, mtime, fullpath)
				http.ServeFile(w, r, fullpath)
			} else {
				server.streamOnline(w, r, mtime, fullpath)
			}
		} else {
			http.ServeFile(w, r, fullpath)
		}
	} else {
		if ranged {
			server.streamOnline(nil, r, mtime, fullpath)
			http.ServeFile(w, r, fullpath)
		} else {
			server.streamOnline(w, r, mtime, fullpath)
		}
	}
}

type localStatus int

const (
	localNotExists localStatus = iota
	localExists
	localExistsButNeedHead
)

func (server *Server) checkLocal(w http.ResponseWriter, _ *http.Request, key string) (exists localStatus, mtime time.Time, err error) {
	if stat, err := os.Stat(key); err == nil {
		if mtime := stat.ModTime(); mtime.Add(server.Cache.Timeout).Before(time.Now()) {
			return localExistsButNeedHead, mtime.In(time.UTC), nil
		}
		return localExists, mtime.In(time.UTC), nil
	} else if os.IsPermission(err) {
		http.Error(w, err.Error(), http.StatusForbidden)
	} else if !os.IsNotExist(err) {
		return localNotExists, zeroTime, err
	}

	return localNotExists, zeroTime, nil
}

func (server *Server) streamOnline(w http.ResponseWriter, r *http.Request, mtime time.Time, key string) {
	memoryObject, exists := server.o[r.URL.Path]
	locked := false
	defer func() {
		if locked {
			server.lu.Unlock()
			locked = false
		}
	}()
	if !exists {
		server.lu.Lock()
		locked = true
	}
	memoryObject, exists = server.o[r.URL.Path]
	if exists {
		if locked {
			server.lu.Unlock()
			locked = false
		}

		if w != nil {
			memoryObject.wg.Add(1)
			for k := range memoryObject.Headers {
				v := memoryObject.Headers.Get(k)
				w.Header().Set(k, v)
			}

			if err := memoryObject.StreamTo(w, memoryObject.wg); err != nil {
				logrus.WithError(err).Warn("failed to stream response with existing memory object")
			}
		}
	} else {
		logrus.WithField("mtime", mtime).Trace("checking fastest upstream")
		selectedIdx, response, chunks, err := server.fastesUpstream(r, mtime)
		logrus.WithFields(logrus.Fields{
			"upstreamIdx": selectedIdx,
		}).Trace("fastest upstream")
		if chunks == nil && mtime != zeroTime {
			logrus.WithFields(logrus.Fields{"upstreamIdx": selectedIdx, "key": key}).Trace("not modified. using local version")
			if w != nil {
				http.ServeFile(w, r, key)
			}
			return
		}

		if err != nil {
			logrus.WithError(err).Warn("failed to select fastest upstream")
			http.Error(w, err.Error(), http.StatusInternalServerError)
			return
		}
		if selectedIdx == -1 || response == nil || chunks == nil {
			logrus.Trace("no upstream is selected")
			http.NotFound(w, r)
			return
		}
		if response.StatusCode == http.StatusNotModified {
			logrus.WithField("upstreamIdx", selectedIdx).Trace("not modified. using local version")
			os.Chtimes(key, zeroTime, time.Now())
			http.ServeFile(w, r, key)
			return
		}

		buffer := &bytes.Buffer{}
		ctx, cancel := context.WithCancel(r.Context())
		defer cancel()

		memoryObject = &StreamObject{
			Headers: response.Header,
			Buffer:  buffer,

			ctx: ctx,

			wg: &sync.WaitGroup{},
		}

		server.o[r.URL.Path] = memoryObject
		server.lu.Unlock()
		locked = false

		if w != nil {
			memoryObject.wg.Add(1)

			for k := range memoryObject.Headers {
				v := memoryObject.Headers.Get(k)
				w.Header().Set(k, v)
			}

			go memoryObject.StreamTo(w, memoryObject.wg)
		}

		err = nil
		for chunk := range chunks {
			if chunk.error != nil {
				err = chunk.error
				logrus.WithError(err).Warn("failed to read from upstream")
			}
			if chunk.buffer == nil {
				break
			}
			n, _ := buffer.Write(chunk.buffer)
			memoryObject.Offset += n

		}
		cancel()

		memoryObject.wg.Wait()

		if err != nil {
			logrus.WithError(err).WithField("upstreamIdx", selectedIdx).Error("something happened during download. will not cache this response")
			return
		}

		go func() {
			logrus.Trace("preparing to release memory object")
			mtime := zeroTime
			lastModifiedHeader := response.Header.Get("Last-Modified")
			if lastModified, err := time.Parse(time.RFC1123, lastModifiedHeader); err != nil {
				logrus.WithError(err).WithFields(logrus.Fields{
					"value": lastModifiedHeader,
					"url":   response.Request.URL,
				}).Trace("failed to parse last modified header value")
			} else {
				mtime = lastModified
			}
			if err := os.MkdirAll(server.Storage.Local.Path, 0755); err != nil {
				logrus.Warn(err)
			}
			fp, err := os.CreateTemp(server.Storage.Local.Path, "temp.*")
			name := fp.Name()
			if err != nil {
				logrus.WithFields(logrus.Fields{
					"key":     key,
					"path":    server.Storage.Local.Path,
					"pattern": "temp.*",
				}).WithError(err).Warn("ftime.Time{}ailed to create template file")
			} else if _, err := fp.Write(buffer.Bytes()); err != nil {
				fp.Close()
				os.Remove(name)

				logrus.WithError(err).Warn("failed to write into template file")
			} else if err := fp.Close(); err != nil {
				os.Remove(name)

				logrus.WithError(err).Warn("failed to close template file")
			} else {
				os.Chtimes(name, zeroTime, mtime)
				dirname := filepath.Dir(key)
				os.MkdirAll(dirname, 0755)
				os.Remove(key)
				os.Rename(name, key)
			}

			server.lu.Lock()
			defer server.lu.Unlock()

			delete(server.o, r.URL.Path)
			logrus.Trace("memory object released")
		}()
	}
}

func (server *Server) fastesUpstream(r *http.Request, lastModified time.Time) (resultIdx int, resultResponse *http.Response, resultCh chan Chunk, resultErr error) {
	returnLock := &sync.Mutex{}
	upstreams := len(server.Upstreams)
	cancelFuncs := make([]func(), upstreams)
	selectedCh := make(chan int, 1)
	selectedOnce := &sync.Once{}
	wg := &sync.WaitGroup{}
	wg.Add(len(server.Upstreams))
	logrus.WithField("size", len(server.Upstreams)).Trace("wg")

	defer close(selectedCh)
	for idx := range server.Upstreams {
		idx := idx
		ctx, cancel := context.WithCancel(context.Background())
		cancelFuncs[idx] = cancel

		logger := logrus.WithField("upstreamIdx", idx)

		go func() {
			defer wg.Done()
			response, ch, err := server.tryUpstream(ctx, idx, r, lastModified)
			if err == context.Canceled { // others returned
				logger.Trace("context canceled")
				return
			}

			if err != nil {
				if err != context.Canceled && err != context.DeadlineExceeded {
					logger.WithError(err).Warn("upstream has error")
				}
				return
			}
			if response == nil {
				return
			}
			locked := returnLock.TryLock()
			if !locked {
				return
			}
			defer returnLock.Unlock()

			selectedOnce.Do(func() {
				resultResponse, resultCh, resultErr = response, ch, err
				selectedCh <- idx

				for cancelIdx, cancel := range cancelFuncs {
					if cancelIdx == idx {
						logrus.WithField("upstreamIdx", cancelIdx).Trace("selected thus not canceled")
						continue
					}
					logrus.WithField("upstreamIdx", cancelIdx).Trace("not selected and thus canceled")
					cancel()
				}

				logger.Trace("upstream is selected")
			})

			logger.Trace("voted")

			return
		}()
	}

	wg.Wait()
	logrus.Trace("all upstream tried")

	resultIdx = -1
	select {
	case idx := <-selectedCh:
		resultIdx = idx
	default:
	}

	return
}

func (server *Server) tryUpstream(ctx context.Context, upstreamIdx int, r *http.Request, lastModified time.Time) (response *http.Response, chunks chan Chunk, err error) {
	upstream := server.Upstreams[upstreamIdx]

	logger := logrus.WithField("upstreamIdx", upstreamIdx)

	newpath, matched, err := upstream.GetPath(r.URL.Path)
	logger.WithFields(logrus.Fields{
		"path":    newpath,
		"matched": matched,
	}).Trace("trying upstream")
	if err != nil {
		return nil, nil, err
	}
	if !matched {
		return nil, nil, nil
	}

	newurl := upstream.Server + newpath
	method := r.Method
	if lastModified != zeroTime {
		method = http.MethodGet
	}
	request, err := http.NewRequestWithContext(ctx, method, newurl, nil)
	if err != nil {
		return nil, nil, err
	}
	if lastModified != zeroTime {
		logger.WithFields(logrus.Fields{
			"mtime": lastModified.Format(time.RFC1123),
		}).Trace("check modified since")
		request.Header.Set("If-Modified-Since", lastModified.Format(time.RFC1123))
	}

	for _, k := range []string{"User-Agent"} {
		if _, exists := request.Header[k]; exists {
			request.Header.Set(k, r.Header.Get(k))
		}
	}
	response, err = http.DefaultClient.Do(request)
	if err != nil {
		return nil, nil, err
	}
	logrus.WithField("status", response.StatusCode).Trace("responded")
	if response.StatusCode == http.StatusNotModified {
		return response, nil, nil
	}
	if response.StatusCode >= 400 && response.StatusCode < 500 {
		return nil, nil, nil
	}
	if response.StatusCode < 200 || response.StatusCode >= 500 {
		logrus.WithFields(logrus.Fields{
			"url":    newurl,
			"status": response.StatusCode,
		}).Warn("unexpected status")
		return response, nil, fmt.Errorf("unexpected status(url=%v): %v: %v", newurl, response.StatusCode, response)
	}

	ch := make(chan Chunk, 1024)

	buffer := make([]byte, server.Misc.FirstChunkBytes)
	start := time.Now()
	n, err := io.ReadAtLeast(response.Body, buffer, len(buffer))

	if err != nil {
		if n == 0 {
			return response, nil, err
		}
	}
	logger.WithField("duration", time.Now().Sub(start)).Tracef("first %v bytes", n)
	ch <- Chunk{buffer: buffer[:n]}

	go func() {
		defer close(ch)

		for {
			buffer := make([]byte, server.Misc.ChunkBytes)
			n, err := io.ReadAtLeast(response.Body, buffer, len(buffer))
			if n > 0 {
				ch <- Chunk{buffer: buffer[:n]}
			}
			if err == io.EOF {
				logger.Trace("done")
				return
			}
			if err != nil {
				ch <- Chunk{error: err}
				logger.WithError(err).Trace("failed")
				return
			}
		}
	}()

	return response, ch, nil
}

var (
	configFilePath = "config.yaml"
	logLevel       = "info"
	sentrydsn      = ""
)

func init() {
	if v, ok := os.LookupEnv("CONFIG_PATH"); ok {
		configFilePath = v
	}
	if v, ok := os.LookupEnv("LOG_LEVEL"); ok {
		logLevel = v
	}
	if v, ok := os.LookupEnv("SENTRY_DSN"); ok {
		sentrydsn = v
	}
	flag.StringVar(&configFilePath, "config", configFilePath, "path to config file")
	flag.StringVar(&logLevel, "log-level", logLevel, "log level. (trace, debug, info, warn, error)")
	flag.StringVar(&sentrydsn, "sentry", sentrydsn, "sentry dsn to report errors")
}

func main() {
	flag.Parse()

	if lvl, err := logrus.ParseLevel(logLevel); err != nil {
		logrus.WithError(err).Panic("failed to parse log level")
	} else {
		logrus.SetLevel(lvl)
	}

	if sentrydsn != "" {
		if err := sentry.Init(sentry.ClientOptions{
			Dsn: sentrydsn,
		}); err != nil {
			logrus.WithField("dsn", sentrydsn).WithError(err).Panic("failed to setup sentry")
		}
		defer sentry.Flush(time.Second * 3)
	}

	logrus.SetFormatter(&logrus.TextFormatter{
		FullTimestamp:   true,
		TimestampFormat: "2006-01-02T15:04:05.000",
	})

	config, err := configFromFile(configFilePath)
	if err != nil {
		panic(err)
	}

	ch := make(chan any, 10)
	for idx := 0; idx < 10; idx += 1 {
		ch <- idx
	}

	server := Server{
		Config: *config,

		lu: &sync.Mutex{},
		o:  make(map[string]*StreamObject),
	}

	http.HandleFunc("GET /{path...}", server.handleRequest)
	logrus.WithFields(logrus.Fields{"addr": ":8881"}).Info("serving app")
	http.ListenAndServe(":8881", nil)
}