package cacheproxy

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"os"
	"path/filepath"
	"regexp"
	"slices"
	"strings"
	"sync"
	"time"
)

type reqCtxKey int

const (
	reqCtxAllowedRedirect reqCtxKey = iota
)

var zeroTime time.Time

var (
	httpClient = http.Client{
		// check allowed redirect
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			lastRequest := via[len(via)-1]
			if allowedRedirect, ok := lastRequest.Context().Value(reqCtxAllowedRedirect).(string); ok {
				if matched, err := regexp.MatchString(allowedRedirect, req.URL.String()); err != nil {
					return err
				} else if !matched {
					return http.ErrUseLastResponse
				}
				return nil
			}
			return http.ErrUseLastResponse
		},
	}
)

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)
	slog.With(
		"start", offset,
		"end", memoryObject.Buffer.Len(),
		"n", memoryObject.Buffer.Len()-offset,
	).Debug("remain bytes")

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

type Server struct {
	Config

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

func NewServer(config Config) *Server {
	return &Server{
		Config: config,

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

type Chunk struct {
	buffer []byte
	error  error
}

func (server *Server) serveFile(w http.ResponseWriter, r *http.Request, path string) {
	if location := r.Header.Get(server.Storage.Accel.EnableByHeader); server.Storage.Accel.EnableByHeader != "" && location != "" {
		relPath, err := filepath.Rel(server.Storage.Local.Path, path)
		if err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
			return
		}
		accelPath := filepath.Join(location, relPath)

		for _, headerKey := range server.Storage.Accel.ResponseWithHeaders {
			w.Header().Set(headerKey, accelPath)
		}

		return
	}

	http.ServeFile(w, r, path)
}

func (server *Server) HandleRequestWithCache(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
	}
	if !strings.HasPrefix(fullpath, server.Storage.Local.Path) {
		http.Error(w, "crossing local directory boundary", http.StatusBadRequest)
		return
	}

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

	localStatus, mtime, err := server.checkLocal(w, r, fullpath)
	slog.With("status", localStatus, "mtime", mtime, "error", err, "key", fullpath).Debug("local status checked")
	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)
				server.serveFile(w, r, fullpath)
			} else {
				server.streamOnline(w, r, mtime, fullpath)
			}
		} else {
			server.serveFile(w, r, fullpath)
		}
	} else {
		if ranged {
			server.streamOnline(nil, r, mtime, fullpath)
			server.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 {
		refreshAfter := server.Cache.RefreshAfter
		refresh := ""

		for _, policy := range server.Cache.Policies {
			if match, err := regexp.MatchString(policy.Match, key); err != nil {
				return 0, zeroTime, err
			} else if match {
				if dur, err := time.ParseDuration(policy.RefreshAfter); err != nil {
					if slices.Contains([]string{"always", "never"}, policy.RefreshAfter) {
						refresh = policy.RefreshAfter
					} else {
						return 0, zeroTime, err
					}
				} else {
					refreshAfter = dur
				}
				break
			}
		}
		mtime := stat.ModTime()
		slog.With("policy", refresh, "after", refreshAfter, "mtime", mtime, "key", key).Debug("refresh policy checked")
		if (mtime.Add(refreshAfter).Before(time.Now()) || refresh == "always") && refresh != "never" {
			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 {
				slog.With("error", err).Warn("failed to stream response with existing memory object")
			}
		}
	} else {
		slog.With("mtime", mtime).Debug("checking fastest upstream")
		selectedIdx, response, chunks, err := server.fastesUpstream(r, mtime)
		if chunks == nil && mtime != zeroTime {
			slog.With("upstreamIdx", selectedIdx, "key", key).Debug("not modified. using local version")
			if w != nil {
				server.serveFile(w, r, key)
			}
			return
		}

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

		slog.With(
			"upstreamIdx", selectedIdx,
		).Debug("found fastest upstream")

		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

		err = nil

		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)
		}

		for chunk := range chunks {
			if chunk.error != nil {
				err = chunk.error
				slog.With("error", 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 response.ContentLength > 0 {
			if memoryObject.Offset == int(response.ContentLength) && err != nil {
				if !(errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF)) {
					slog.With("read-length", memoryObject.Offset, "content-length", response.ContentLength, "error", err, "upstreamIdx", selectedIdx).Debug("something happened during download. but response body is read as whole. so error is reset to nil")
				}
				err = nil
			}
		} else if err == io.EOF {
			err = nil
		}

		if err != nil {
			slog.With("error", err, "upstreamIdx", selectedIdx).Error("something happened during download. will not cache this response")
		}
		go func() {
			defer func() {
				server.lu.Lock()
				defer server.lu.Unlock()

				delete(server.o, r.URL.Path)
				slog.Debug("memory object released")
			}()

			if err == nil {
				slog.Debug("preparing to release memory object")
				mtime := zeroTime
				lastModifiedHeader := response.Header.Get("Last-Modified")
				if lastModified, err := time.Parse(time.RFC1123, lastModifiedHeader); err != nil {
					slog.With(
						"error", err,
						"value", lastModifiedHeader,
						"url", response.Request.URL,
					).Debug("failed to parse last modified header value. set modified time to now")
				} else {
					slog.With(
						"header", lastModifiedHeader,
						"value", lastModified,
						"url", response.Request.URL,
					).Debug("found modified time")
					mtime = lastModified
				}
				if err := os.MkdirAll(server.Storage.Local.Path, 0755); err != nil {
					slog.With("error", err).Warn("failed to create local storage path")
				}

				if server.Config.Storage.Local.TemporaryFilePattern == "" {
					if err := os.WriteFile(key, buffer.Bytes(), 0644); err != nil {
						slog.With("error", err).Warn("failed to write file")
						os.Remove(key)
					}
					return
				}

				fp, err := os.CreateTemp(server.Storage.Local.Path, server.Storage.Local.TemporaryFilePattern)
				if err != nil {
					slog.With(
						"key", key,
						"path", server.Storage.Local.Path,
						"pattern", server.Storage.Local.TemporaryFilePattern,
						"error", err,
					).Warn("failed to create template file")
					return
				}

				name := fp.Name()

				if _, err := fp.Write(buffer.Bytes()); err != nil {
					fp.Close()
					os.Remove(name)

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

					slog.With("error", 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)
				}
			}
		}()

	}
}

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))

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

		logger := slog.With("upstreamIdx", idx)

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

			if err != nil {
				if !errors.Is(err, context.Canceled) && !errors.Is(err, context.DeadlineExceeded) {
					logger.With("error", err).Warn("upstream has error")
				}
				return
			}
			if response == nil {
				return
			}
			if response.StatusCode != http.StatusOK {
				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 {
						continue
					}
					cancel()
				}
			})

			logger.Debug("voted")
		}()
	}

	wg.Wait()

	resultIdx = -1
	select {
	case idx := <-selectedCh:
		resultIdx = idx
		slog.With("upstreamIdx", resultIdx).Debug("upstream selected")
	default:
		slog.Debug("no valid upstream found")
	}

	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]

	newpath, matched, err := upstream.GetPath(r.URL.Path)
	if err != nil {
		return nil, nil, err
	}
	if !matched {
		return nil, nil, nil
	}
	logger := slog.With("upstreamIdx", upstreamIdx, "server", upstream.Server, "path", newpath)

	logger.With(
		"matched", matched,
	).Debug("trying upstream")

	newurl := upstream.Server + newpath
	method := r.Method
	if lastModified != zeroTime {
		method = http.MethodGet
	}

	if upstream.AllowedRedirect != nil {
		ctx = context.WithValue(ctx, reqCtxAllowedRedirect, *upstream.AllowedRedirect)
	}

	request, err := http.NewRequestWithContext(ctx, method, newurl, nil)
	if err != nil {
		return nil, nil, err
	}
	if lastModified != zeroTime {
		request.Header.Set("If-Modified-Since", lastModified.Format(time.RFC1123))
	}

	for _, k := range []string{"User-Agent", "Accept"} {
		if _, exists := r.Header[k]; exists {
			request.Header.Set(k, r.Header.Get(k))
		}
	}
	response, err = httpClient.Do(request)
	if err != nil {
		return nil, nil, err
	}
	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 {
		logger.With(
			"url", newurl,
			"status", response.StatusCode,
		).Warn("unexpected status")
		return response, nil, fmt.Errorf("unexpected status(url=%v): %v: %v", newurl, response.StatusCode, response)
	}

	var currentOffset int64

	ch := make(chan Chunk, 1024)

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

	if err != nil {
		if n == 0 {
			return response, nil, err
		}
	}
	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]}
				currentOffset += int64(n)
			}
			if response.ContentLength > 0 && currentOffset == response.ContentLength && err == io.EOF || err == io.ErrUnexpectedEOF {
				return
			}
			if err != nil {
				ch <- Chunk{error: err}
				return
			}
		}
	}()

	return response, ch, nil
}