package cacheproxy

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

type reqCtxKey int

const (
	reqCtxAllowedRedirect reqCtxKey = iota
)

var zeroTime time.Time

var preclosedChan = make(chan struct{})

func init() {
	close(preclosedChan)
}

var (
	httpClient = http.Client{
		// check allowed redirect
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			if allowedRedirect, ok := req.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
	TempFile *os.File // The temporary file holding the download content.
	Offset   int64    // The number of bytes written to TempFile.
	Done     bool
	Error    error

	mu   *sync.Mutex
	cond *sync.Cond

	fileWrittenCh chan struct{} // Closed when the file is fully written and renamed.
}

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),
	}
}

func (server *Server) serveFile(w http.ResponseWriter, r *http.Request, path string) {
	if location := r.Header.Get(server.Storage.Local.Accel.EnableByHeader); server.Storage.Local.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.Local.Accel.RespondWithHeaders {
			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)
		return
	} else if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}

	if localStatus == localExists {
		server.serveFile(w, r, fullpath)
		return
	}

	// localExistsButNeedHead or localNotExists
	// Both need to go online.
	if ranged {
		server.serveRangedRequest(w, r, fullpath, mtime)
	} else {
		// For full requests, we wait for the download to complete and then serve the file.
		// This maintains the original behavior for now.
		ch := server.startOrJoinStream(r, mtime, fullpath)
		if ch != nil {
			<-ch
		}
		server.serveFile(w, r, 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) serveRangedRequest(w http.ResponseWriter, r *http.Request, key string, mtime time.Time) {
	server.lu.Lock()
	memoryObject, exists := server.o[r.URL.Path]
	if !exists {
		server.lu.Unlock()
		// This is the first request for this file, so we start the download.
		server.startOrJoinStream(r, mtime, key)

		// Re-acquire the lock to get the newly created stream object.
		server.lu.Lock()
		memoryObject = server.o[r.URL.Path]
		if memoryObject == nil {
			server.lu.Unlock()
			// This can happen if the upstream fails very quickly.
			http.Error(w, "Failed to start download stream", http.StatusInternalServerError)
			return
		}
	}
	server.lu.Unlock()

	// At this point, we have a memoryObject, and a producer goroutine is downloading the file.
	// Now we implement the consumer logic.

	// Duplicate the file descriptor from the producer's temp file. This creates a new,
	// independent file descriptor that points to the same underlying file description.
	// This is more efficient and robust than opening the file by path.
	fd, err := syscall.Dup(int(memoryObject.TempFile.Fd()))
	if err != nil {
		http.Error(w, "Failed to duplicate file descriptor", http.StatusInternalServerError)
		return
	}
	// We create a new *os.File from the duplicated descriptor. The consumer is now
	// responsible for closing this new file.
	consumerFile := os.NewFile(uintptr(fd), memoryObject.TempFile.Name())
	if consumerFile == nil {
		syscall.Close(fd) // Clean up if NewFile fails
		http.Error(w, "Failed to create file from duplicated descriptor", http.StatusInternalServerError)
		return
	}
	defer consumerFile.Close()

	rangeHeader := r.Header.Get("Range")
	if rangeHeader == "" {
		// This should not happen if called from HandleRequestWithCache, but as a safeguard:
		http.Error(w, "Range header is required", http.StatusBadRequest)
		return
	}

	// Parse the Range header. We only support a single range like "bytes=start-end".
	var start, end int64
	parts := strings.Split(strings.TrimPrefix(rangeHeader, "bytes="), "-")
	if len(parts) != 2 {
		http.Error(w, "Invalid Range header", http.StatusBadRequest)
		return
	}
	start, err = strconv.ParseInt(parts[0], 10, 64)
	if err != nil {
		http.Error(w, "Invalid Range header", http.StatusBadRequest)
		return
	}
	if parts[1] != "" {
		end, err = strconv.ParseInt(parts[1], 10, 64)
		if err != nil {
			http.Error(w, "Invalid Range header", http.StatusBadRequest)
			return
		}
	} else {
		// An empty end means "to the end of the file". We don't know the full size yet.
		// We'll have to handle this dynamically.
		end = -1 // Sentinel value
	}

	memoryObject.mu.Lock()
	defer memoryObject.mu.Unlock()

	// Wait until we have the headers.
	for memoryObject.Headers == nil && !memoryObject.Done {
		memoryObject.cond.Wait()
	}

	if memoryObject.Error != nil {
		http.Error(w, memoryObject.Error.Error(), http.StatusInternalServerError)
		return
	}

	contentLengthStr := memoryObject.Headers.Get("Content-Length")
	totalSize, _ := strconv.ParseInt(contentLengthStr, 10, 64)
	if end == -1 || end >= totalSize {
		end = totalSize - 1
	}

	if start >= totalSize || start > end {
		http.Error(w, "Range not satisfiable", http.StatusRequestedRangeNotSatisfiable)
		return
	}

	var bytesSent int64
	bytesToSend := end - start + 1
	headersWritten := false

	for bytesSent < bytesToSend && memoryObject.Error == nil {
		// Calculate what we need.
		neededStart := start + bytesSent

		// Wait for the data to be available.
		for memoryObject.Offset <= neededStart && !memoryObject.Done {
			memoryObject.cond.Wait()
		}

		// Check for error AFTER waiting. This is the critical fix.
		if memoryObject.Error != nil {
			// If headers haven't been written, we can send a 500 error.
			// If they have, it's too late, the connection will just be closed.
			if !headersWritten {
				http.Error(w, memoryObject.Error.Error(), http.StatusInternalServerError)
			}
			return // Use return instead of break to exit immediately.
		}

		// If we are here, we have some data to send. Write headers if we haven't already.
		if !headersWritten {
			w.Header().Set("Content-Range", "bytes "+strconv.FormatInt(start, 10)+"-"+strconv.FormatInt(end, 10)+"/"+contentLengthStr)
			w.Header().Set("Content-Length", strconv.FormatInt(end-start+1, 10))
			w.Header().Set("Accept-Ranges", "bytes")
			w.WriteHeader(http.StatusPartialContent)
			headersWritten = true
		}

		// Data is available, read from the temporary file.
		// We calculate how much we can read in this iteration.
		readNow := memoryObject.Offset - neededStart
		if readNow <= 0 {
			// This can happen if we woke up but the data isn't what we need.
			// The loop will continue to wait.
			continue
		}

		// Don't read more than the client requested in total.
		remainingToSend := bytesToSend - bytesSent
		if readNow > remainingToSend {
			readNow = remainingToSend
		}

		// Read the chunk from the file at the correct offset.
		buffer := make([]byte, readNow)
		bytesRead, err := consumerFile.ReadAt(buffer, neededStart)
		if err != nil && err != io.EOF {
			if !headersWritten {
				http.Error(w, "Error reading from cache stream", http.StatusInternalServerError)
			}
			return
		}

		if bytesRead > 0 {
			n, err := w.Write(buffer[:bytesRead])
			if err != nil {
				// Client closed connection, just return.
				return
			}
			bytesSent += int64(n)
		}

		if memoryObject.Done && bytesSent >= bytesToSend {
			break
		}
	}
}

// startOrJoinStream ensures a download stream is active for the given key.
// If a stream already exists, it returns the channel that signals completion.
// If not, it starts a new download producer and returns its completion channel.
func (server *Server) startOrJoinStream(r *http.Request, mtime time.Time, key string) <-chan struct{} {
	server.lu.Lock()
	memoryObject, exists := server.o[r.URL.Path]
	if exists {
		// A download is already in progress. Return its completion channel.
		server.lu.Unlock()
		return memoryObject.fileWrittenCh
	}

	// No active stream, create a new one.
	if err := os.MkdirAll(filepath.Dir(key), 0755); err != nil {
		slog.With("error", err).Warn("failed to create local storage path for temp file")
		server.lu.Unlock()
		return preclosedChan // Return a closed channel to prevent blocking
	}

	tempFilePattern := server.Storage.Local.TemporaryFilePattern
	if tempFilePattern == "" {
		tempFilePattern = "cache-proxy-*"
	}
	tempFile, err := os.CreateTemp(filepath.Dir(key), tempFilePattern)
	if err != nil {
		slog.With("error", err).Warn("failed to create temporary file")
		server.lu.Unlock()
		return preclosedChan
	}

	mu := &sync.Mutex{}
	fileWrittenCh := make(chan struct{})
	memoryObject = &StreamObject{
		TempFile:      tempFile,
		mu:            mu,
		cond:          sync.NewCond(mu),
		fileWrittenCh: fileWrittenCh,
	}
	server.o[r.URL.Path] = memoryObject
	server.lu.Unlock()

	// This is the producer goroutine
	go func(mo *StreamObject) {
		var err error
		downloadSucceeded := false
		tempFileName := mo.TempFile.Name()

		defer func() {
			// On completion (or error), update the stream object,
			// wake up all consumers, and then clean up.
			mo.mu.Lock()
			mo.Done = true
			mo.Error = err
			mo.cond.Broadcast()
			mo.mu.Unlock()

			// Close the temp file handle.
			mo.TempFile.Close()

			// If download failed, remove the temp file.
			if !downloadSucceeded {
				os.Remove(tempFileName)
			}

			// The producer's job is done. Remove the object from the central map.
			// Any existing consumers still hold a reference to the object,
			// so it won't be garbage collected until they are done.
			server.lu.Lock()
			delete(server.o, r.URL.Path)
			server.lu.Unlock()
			slog.Debug("memory object released by producer")
			close(mo.fileWrittenCh)
		}()

		slog.With("mtime", mtime).Debug("checking fastest upstream")
		selectedIdx, response, firstChunk, upstreamErr := server.fastestUpstream(r, mtime)
		if upstreamErr != nil {
			if !errors.Is(upstreamErr, io.EOF) && !errors.Is(upstreamErr, io.ErrUnexpectedEOF) {
				slog.With("error", upstreamErr).Warn("failed to select fastest upstream")
				err = upstreamErr
				return
			}
		}

		if selectedIdx == -1 || response == nil {
			slog.Debug("no upstream is selected")
			err = errors.New("no suitable upstream found")
			return
		}

		if response.StatusCode == http.StatusNotModified {
			slog.With("upstreamIdx", selectedIdx).Debug("not modified. using local version")
			os.Chtimes(key, zeroTime, time.Now())
			// In this case, we don't have a new file, so we just exit.
			// The temp file will be cleaned up by the defer.
			return
		}

		defer response.Body.Close()

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

		mo.mu.Lock()
		mo.Headers = response.Header
		mo.cond.Broadcast() // Broadcast headers availability
		mo.mu.Unlock()

		// Write the first chunk that we already downloaded.
		if len(firstChunk) > 0 {
			written, writeErr := mo.TempFile.Write(firstChunk)
			if writeErr != nil {
				err = writeErr
				return
			}
			mo.mu.Lock()
			mo.Offset += int64(written)
			mo.cond.Broadcast()
			mo.mu.Unlock()
		}

		// Download the rest of the file in chunks
		buffer := make([]byte, server.Misc.ChunkBytes)
		for {
			n, readErr := response.Body.Read(buffer)
			if n > 0 {
				written, writeErr := mo.TempFile.Write(buffer[:n])
				if writeErr != nil {
					err = writeErr
					break
				}
				mo.mu.Lock()
				mo.Offset += int64(written)
				mo.cond.Broadcast()
				mo.mu.Unlock()
			}
			if readErr != nil {
				if readErr != io.EOF {
					err = readErr
				}
				break
			}
		}

		// After download, if no critical error, rename the temp file to its final destination.
		if err == nil {
			// Set modification time
			mtime := zeroTime
			lastModifiedHeader := response.Header.Get("Last-Modified")
			if lastModified, lmErr := time.Parse(time.RFC1123, lastModifiedHeader); lmErr == nil {
				mtime = lastModified
			}
			// Close file before Chtimes and Rename
			mo.TempFile.Close()

			os.Chtimes(tempFileName, zeroTime, mtime)

			// Rename the file
			if renameErr := os.Rename(tempFileName, key); renameErr != nil {
				slog.With("error", renameErr, "from", tempFileName, "to", key).Warn("failed to rename temp file")
				err = renameErr
				os.Remove(tempFileName) // Attempt to clean up if rename fails
			} else {
				downloadSucceeded = true
			}
		} else {
			logger := slog.With("upstreamIdx", selectedIdx)
			logger.Error("something happened during download. will not cache this response.", "error", err)
		}
	}(memoryObject)

	return fileWrittenCh
}

func (server *Server) fastestUpstream(r *http.Request, lastModified time.Time) (resultIdx int, resultResponse *http.Response, firstChunk []byte, resultErr error) {
	returnLock := &sync.Mutex{}
	upstreams := len(server.Upstreams)
	cancelFuncs := make([]func(), upstreams)
	updateCh := make(chan int, 1)
	updateOnce := &sync.Once{}
	notModifiedCh := make(chan int, 1)
	notModifiedOnce := &sync.Once{}

	resultIdx = -1

	var resultFirstChunk []byte

	defer close(updateCh)
	defer close(notModifiedCh)
	defer func() {
		for cancelIdx, cancel := range cancelFuncs {
			if cancelIdx == resultIdx || cancel == nil {
				continue
			}
			cancel()
		}
	}()

	groups := make(map[int][]int)
	for upstreamIdx, upstream := range server.Upstreams {
		if _, matched, err := upstream.GetPath(r.URL.Path); err != nil {
			resultErr = err
			return
		} else if !matched {
			continue
		}

		priority := 0
		for _, priorityGroup := range upstream.PriorityGroups {
			if matched, err := regexp.MatchString(priorityGroup.Match, r.URL.Path); err != nil {
				resultErr = err
				return
			} else if matched {
				priority = priorityGroup.Priority
				break
			}
		}
		groups[priority] = append(groups[priority], upstreamIdx)
	}

	priorities := make([]int, 0, len(groups))
	for priority := range groups {
		priorities = append(priorities, priority)
	}
	slices.Sort(priorities)
	slices.Reverse(priorities)

	for _, priority := range priorities {
		upstreams := groups[priority]

		wg := &sync.WaitGroup{}
		wg.Add(len(upstreams))

		logger := slog.With()
		if priority != 0 {
			logger = logger.With("priority", priority)
		}

		for _, idx := range upstreams {
			idx := idx
			ctx, cancel := context.WithCancel(context.Background())
			cancelFuncs[idx] = cancel

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

			go func() {
				defer wg.Done()
				response, chunk, err := server.tryUpstream(ctx, idx, priority, 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 && response.StatusCode != http.StatusNotModified {
					return
				}
				locked := returnLock.TryLock()
				if !locked {
					if response != nil {
						response.Body.Close()
					}
					return
				}
				defer returnLock.Unlock()

				if response.StatusCode == http.StatusNotModified {
					notModifiedOnce.Do(func() {
						resultResponse, resultErr = response, err
						notModifiedCh <- idx
					})
					logger.Debug("voted not modified")
					return
				}

				updateOnce.Do(func() {
					resultResponse, resultFirstChunk, resultErr = response, chunk, err
					updateCh <- idx

					for cancelIdx, cancel := range cancelFuncs {
						if cancelIdx == idx || cancel == nil {
							continue
						}
						cancel()
					}
				})

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

		wg.Wait()

		select {
		case idx := <-updateCh:
			resultIdx = idx
			firstChunk = resultFirstChunk
			logger.With("upstreamIdx", resultIdx).Debug("upstream selected")
			return
		default:
			select {
			case idx := <-notModifiedCh:
				resultIdx = idx
				logger.With("upstreamIdx", resultIdx).Debug("all upstream not modified")
				return
			default:
				logger.Debug("no valid upstream found")
			}
		}
	}

	return
}

func (server *Server) tryUpstream(ctx context.Context, upstreamIdx, priority int, r *http.Request, lastModified time.Time) (response *http.Response, firstChunk []byte, 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)
	if priority != 0 {
		logger = logger.With("priority", priority)
	}

	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
	}
	shouldCloseBody := true
	defer func() {
		if shouldCloseBody && response != nil {
			response.Body.Close()
		}
	}()

	if response.StatusCode == http.StatusNotModified {
		return response, nil, nil
	}

	responseCheckers := upstream.Checkers
	if len(responseCheckers) == 0 {
		responseCheckers = append(responseCheckers, Checker{})
	}

	for _, checker := range responseCheckers {
		if len(checker.StatusCodes) == 0 {
			checker.StatusCodes = append(checker.StatusCodes, http.StatusOK)
		}

		if !slices.Contains(checker.StatusCodes, response.StatusCode) {
			return nil, nil, err
		}

		for _, headerChecker := range checker.Headers {
			if headerChecker.Match == nil {
				// check header exists
				if _, ok := response.Header[headerChecker.Name]; !ok {
					logger.Debug("missing header", "header", headerChecker.Name)
					return nil, nil, nil
				}
			} else {
				// check header match
				value := response.Header.Get(headerChecker.Name)
				if matched, err := regexp.MatchString(*headerChecker.Match, value); err != nil {
					return nil, nil, err
				} else if !matched {
					logger.Debug("invalid header value",
						"header", headerChecker.Name,
						"value", value,
						"matcher", *headerChecker.Match,
					)

					return nil, nil, nil
				}
			}
		}
	}

	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
		}

		if errors.Is(err, io.ErrUnexpectedEOF) || errors.Is(err, io.EOF) {
			err = nil
		}
	}

	shouldCloseBody = false
	return response, buffer[:n], err
}