package filesystem

import (
	"archive/tar"
	"emperror.dev/errors"
	"github.com/juju/ratelimit"
	"github.com/karrick/godirwalk"
	"github.com/klauspost/pgzip"
	"github.com/pterodactyl/wings/config"
	"github.com/sabhiram/go-gitignore"
	"io"
	"os"
	"path/filepath"
	"strings"
	"sync"
)

const memory = 4 * 1024

var pool = sync.Pool{
	New: func() interface{} {
		b := make([]byte, memory)
		return b
	},
}

type Archive struct {
	// BasePath is the absolute path to create the archive from where Files and Ignore are
	// relative to.
	BasePath string

	// Ignore is a gitignore string (most likely read from a file) of files to ignore
	// from the archive.
	Ignore string

	// Files specifies the files to archive, this takes priority over the Ignore option, if
	// unspecified, all files in the BasePath will be archived unless Ignore is set.
	Files []string
}

// Creates an archive at dst with all of the files defined in the included files struct.
func (a *Archive) Create(dst string) error {
	f, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		return err
	}
	defer f.Close()

	// Select a writer based off of the WriteLimit configuration option. If there is no
	// write limit, use the file as the writer.
	var writer io.Writer
	if writeLimit := int64(config.Get().System.Backups.WriteLimit * 1024 * 1024); writeLimit > 0 {
		// Token bucket with a capacity of "writeLimit" MiB, adding "writeLimit" MiB/s
		// and then wrap the file writer with the token bucket limiter.
		writer = ratelimit.Writer(f, ratelimit.NewBucketWithRate(float64(writeLimit), writeLimit))
	} else {
		writer = f
	}

	// Create a new gzip writer around the file.
	gw, _ := pgzip.NewWriterLevel(writer, pgzip.BestSpeed)
	_ = gw.SetConcurrency(1<<20, 1)
	defer gw.Close()

	// Create a new tar writer around the gzip writer.
	tw := tar.NewWriter(gw)
	defer tw.Close()

	// Configure godirwalk.
	options := &godirwalk.Options{
		FollowSymbolicLinks: false,
		Unsorted:            true,
		Callback:            a.callback(tw),
	}

	// If we're specifically looking for only certain files, or have requested
	// that certain files be ignored we'll update the callback function to reflect
	// that request.
	if len(a.Files) == 0 && len(a.Ignore) > 0 {
		i := ignore.CompileIgnoreLines(strings.Split(a.Ignore, "\n")...)
		options.Callback = a.callback(tw, func(_ string, rp string) error {
			if i.MatchesPath(rp) {
				return godirwalk.SkipThis
			}
			return nil
		})
	} else if len(a.Files) > 0 {
		options.Callback = a.withFilesCallback(tw)
	}

	// Recursively walk the path we are archiving.
	return godirwalk.Walk(a.BasePath, options)
}

// Callback function used to determine if a given file should be included in the archive
// being generated.
func (a *Archive) callback(tw *tar.Writer, opts ...func(path string, relative string) error) func(path string, de *godirwalk.Dirent) error {
	return func(path string, de *godirwalk.Dirent) error {
		// Skip directories because we walking them recursively.
		if de.IsDir() {
			return nil
		}

		relative := filepath.ToSlash(strings.TrimPrefix(path, a.BasePath+string(filepath.Separator)))
		// Call the additional options passed to this callback function. If any of them return
		// a non-nil error we will exit immediately.
		for _, opt := range opts {
			if err := opt(path, relative); err != nil {
				return err
			}
		}

		// Add the file to the archive, if it is nested in a directory,
		// the directory will be automatically "created" in the archive.
		return a.addToArchive(path, relative, tw)
	}
}

// Pushes only files defined in the Files key to the final archive.
func (a *Archive) withFilesCallback(tw *tar.Writer) func(path string, de *godirwalk.Dirent) error {
	return a.callback(tw, func(p string, rp string) error {
		for _, f := range a.Files {
			// If the given doesn't match, or doesn't have the same prefix continue
			// to the next item in the loop.
			if p != f && !strings.HasPrefix(p, f) {
				continue
			}
			// Once we have a match return a nil value here so that the loop stops and the
			// call to this function will correctly include the file in the archive. If there
			// are no matches we'll never make it to this line, and the final error returned
			// will be the godirwalk.SkipThis error.
			return nil
		}
		return godirwalk.SkipThis
	})
}

// Adds a given file path to the final archive being created.
func (a *Archive) addToArchive(p string, rp string, w *tar.Writer) error {
	// Lstat the file, this will give us the same information as Stat except
	// that it will not follow a symlink to it's target automatically.
	s, err := os.Lstat(p)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return errors.WithMessage(err, "failed to Lstat '"+rp+"'")
	}

	// Resolve the symlink target if the file is a symlink.
	var target string
	if s.Mode()&os.ModeSymlink != 0 {
		// Read the target of the symlink.
		target, err = os.Readlink(s.Name())
		if err != nil {
			// Skip symlinks if the target does not exist.
			if os.IsNotExist(err) {
				return nil
			}

			return errors.WithMessagef(err, "failed to read symlink target for '%s'", rp)
		}
	}

	// Get the tar FileInfoHeader in order to add the file to the archive.
	header, err := tar.FileInfoHeader(s, filepath.ToSlash(target))
	if err != nil {
		return errors.WithMessagef(err, "failed to get tar#FileInfoHeader for '%s'", rp)
	}

	// Fix the header name if the file is not a symlink.
	if s.Mode()&os.ModeSymlink == 0 {
		header.Name = rp
	}

	// Write the tar FileInfoHeader to the archive.
	if err := w.WriteHeader(header); err != nil {
		return errors.WithMessagef(err, "failed to write tar#FileInfoHeader for '%s'", rp)
	}

	// If the size of the file is less than 1 (most likely for symlinks), skip writing the file.
	if header.Size < 1 {
		return nil
	}

	// If the buffer size is larger than the file size, create a smaller buffer to hold the file.
	var buf []byte
	if header.Size < memory {
		buf = make([]byte, header.Size)
	} else {
		// Get a fixed-size buffer from the pool to save on allocations.
		buf = pool.Get().([]byte)
		defer func() {
			buf = make([]byte, memory)
			pool.Put(buf)
		}()
	}

	// Open the file.
	f, err := os.Open(p)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return errors.WithMessagef(err, "failed to open '%s' for copying", header.Name)
	}
	defer f.Close()

	// Copy the file's contents to the archive using our buffer.
	if _, err := io.CopyBuffer(w, io.LimitReader(f, header.Size), buf); err != nil {
		return errors.WithMessagef(err, "failed to copy '%s' to archive", header.Name)
	}

	return nil
}