package main

import (
	"errors"
	"os"
	"path"
	"sort"
)

// An FS provides access to a hierarchical file system.
//
// The FS interface is the minimum implementation required of the file system.
// A file system may implement additional interfaces,
// such as ReadFileFS, to provide additional or optimized functionality.
type FS interface {
	// Open opens the named file.
	//
	// When Open returns an error, it should be of type *PathError
	// with the Op field set to "open", the Path field set to name,
	// and the Err field describing the problem.
	//
	// Open should reject attempts to open names that do not satisfy
	// ValidPath(name), returning a *PathError with Err set to
	// ErrInvalid or ErrNotExist.
	Open(name string) (*os.File, error)
	Stat(name string) (os.FileInfo, error)
}

// ReadDir reads the named directory,
// returning all its directory entries sorted by filename.
// If an error occurs reading the directory,
// ReadDir returns the entries it was able to read before the error,
// along with the error.
func ReadDir(fsys FS, name string) ([]DirEntry, error) {
	f, err := fsys.Open(name)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	infos, err := f.Readdir(-1)
	dirs := make([]DirEntry, len(infos))
	for i, info := range infos {
		dirs[i] = &statDirEntry{info}
	}
	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
	return dirs, err
}

// DirFS returns a file system (an fs.FS) for the tree of files rooted at the directory dir.
//
// Note that DirFS("/prefix") only guarantees that the Open calls it makes to the
// operating system will begin with "/prefix": DirFS("/prefix").Open("file") is the
// same as os.Open("/prefix/file"). So if /prefix/file is a symbolic link pointing outside
// the /prefix tree, then using DirFS does not stop the access any more than using
// os.Open does. DirFS is therefore not a general substitute for a chroot-style security
// mechanism when the directory tree contains arbitrary content.
func DirFS(dir string) FS {
	return dirFS(dir)
}

type dirFS string

func (dir dirFS) Open(name string) (*os.File, error) {
	//if !fs.ValidPath(name) || runtime.GOOS == "windows" && containsAny(name, `\:`) {
	//	return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
	//}
	f, err := os.Open(string(dir) + "/" + name)
	if err != nil {
		return nil, err
	}
	return f, nil
}

func (dir dirFS) Stat(name string) (os.FileInfo, error) {
	info, err := os.Stat(string(dir) + "/" + name)
	if err != nil {
		return info, err
	}
	return info, nil
}

// SkipDir is used as a return value from WalkDirFuncs to indicate that
// the directory named in the call is to be skipped. It is not returned
// as an error by any function.
var SkipDir = errors.New("skip this directory")

// WalkDirFunc is the type of the function called by WalkDir to visit
// each file or directory.
//
// The path argument contains the argument to WalkDir as a prefix.
// That is, if WalkDir is called with root argument "dir" and finds a file
// named "a" in that directory, the walk function will be called with
// argument "dir/a".
//
// The d argument is the fs.DirEntry for the named path.
//
// The error result returned by the function controls how WalkDir
// continues. If the function returns the special value SkipDir, WalkDir
// skips the current directory (path if d.IsDir() is true, otherwise
// path's parent directory). Otherwise, if the function returns a non-nil
// error, WalkDir stops entirely and returns that error.
//
// The err argument reports an error related to path, signaling that
// WalkDir will not walk into that directory. The function can decide how
// to handle that error; as described earlier, returning the error will
// cause WalkDir to stop walking the entire tree.
//
// WalkDir calls the function with a non-nil err argument in two cases.
//
// First, if the initial fs.Stat on the root directory fails, WalkDir
// calls the function with path set to root, d set to nil, and err set to
// the error from fs.Stat.
//
// Second, if a directory's ReadDir method fails, WalkDir calls the
// function with path set to the directory's path, d set to an
// fs.DirEntry describing the directory, and err set to the error from
// ReadDir. In this second case, the function is called twice with the
// path of the directory: the first call is before the directory read is
// attempted and has err set to nil, giving the function a chance to
// return SkipDir and avoid the ReadDir entirely. The second call is
// after a failed ReadDir and reports the error from ReadDir.
// (If ReadDir succeeds, there is no second call.)
//
// The differences between WalkDirFunc compared to filepath.WalkFunc are:
//
//   - The second argument has type fs.DirEntry instead of fs.FileInfo.
//   - The function is called before reading a directory, to allow SkipDir
//     to bypass the directory read entirely.
//   - If a directory read fails, the function is called a second time
//     for that directory to report the error.
//
type WalkDirFunc func(path string, d DirEntry, err error) error

// walkDir recursively descends path, calling walkDirFn.
func walkDir(fsys FS, name string, d DirEntry, walkDirFn WalkDirFunc) error {
	if err := walkDirFn(name, d, nil); err != nil || !d.IsDir() {
		if err == SkipDir && d.IsDir() {
			// Successfully skipped directory.
			err = nil
		}
		return err
	}

	dirs, err := ReadDir(fsys, name)
	if err != nil {
		// Second call, to report ReadDir error.
		err = walkDirFn(name, d, err)
		if err != nil {
			return err
		}
	}

	for _, d1 := range dirs {
		name1 := path.Join(name, d1.Name())
		if err := walkDir(fsys, name1, d1, walkDirFn); err != nil {
			if err == SkipDir {
				break
			}
			return err
		}
	}
	return nil
}

// WalkDir walks the file tree rooted at root, calling fn for each file or
// directory in the tree, including root.
//
// All errors that arise visiting files and directories are filtered by fn:
// see the fs.WalkDirFunc documentation for details.
//
// The files are walked in lexical order, which makes the output deterministic
// but requires WalkDir to read an entire directory into memory before proceeding
// to walk that directory.
//
// WalkDir does not follow symbolic links found in directories,
// but if root itself is a symbolic link, its target will be walked.
func WalkDir(fsys FS, root string, fn WalkDirFunc) error {
	info, err := fsys.Stat(root)
	if err != nil {
		err = fn(root, nil, err)
	} else {
		err = walkDir(fsys, root, &statDirEntry{info}, fn)
	}
	if err == SkipDir {
		return nil
	}
	return err
}

// A DirEntry is an entry read from a directory
// (using the ReadDir function or a ReadDirFile's ReadDir method).
type DirEntry interface {
	// Name returns the name of the file (or subdirectory) described by the entry.
	// This name is only the final element of the path (the base name), not the entire path.
	// For example, Name would return "hello.go" not "/home/gopher/hello.go".
	Name() string

	// IsDir reports whether the entry describes a directory.
	IsDir() bool

	// Type returns the type bits for the entry.
	// The type bits are a subset of the usual FileMode bits, those returned by the FileMode.Type method.
	Type() os.FileMode

	// Info returns the FileInfo for the file or subdirectory described by the entry.
	// The returned FileInfo may be from the time of the original directory read
	// or from the time of the call to Info. If the file has been removed or renamed
	// since the directory read, Info may return an error satisfying errors.Is(err, ErrNotExist).
	// If the entry denotes a symbolic link, Info reports the information about the link itself,
	// not the link's target.
	Info() (os.FileInfo, error)
}

type statDirEntry struct {
	info os.FileInfo
}

func (d *statDirEntry) Name() string               { return d.info.Name() }
func (d *statDirEntry) IsDir() bool                { return d.info.IsDir() }
func (d *statDirEntry) Type() os.FileMode          { return d.info.Mode() & os.ModeType }
func (d *statDirEntry) Info() (os.FileInfo, error) { return d.info, nil }