// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package filepath implements utility routines for manipulating filename paths
// in a way compatible with the target operating system-defined file paths.
//
// The filepath package uses either forward slashes or backslashes,
// depending on the operating system. To process paths such as URLs
// that always use forward slashes regardless of the operating
// system, see the path package.
package downloader

import (
	"io/fs"
	"os"
	"runtime"
	"strings"
)

// A lazybuf is a lazily constructed path buffer.
// It supports append, reading previously appended bytes,
// and retrieving the final string. It does not allocate a buffer
// to hold the output until that output diverges from s.
type lazybuf struct {
	path       string
	buf        []byte
	w          int
	volAndPath string
	volLen     int
}

func (b *lazybuf) index(i int) byte {
	if b.buf != nil {
		return b.buf[i]
	}
	return b.path[i]
}

func (b *lazybuf) append(c byte) {
	if b.buf == nil {
		if b.w < len(b.path) && b.path[b.w] == c {
			b.w++
			return
		}
		b.buf = make([]byte, len(b.path))
		copy(b.buf, b.path[:b.w])
	}
	b.buf[b.w] = c
	b.w++
}

func (b *lazybuf) string() string {
	if b.buf == nil {
		return b.volAndPath[:b.volLen+b.w]
	}
	return b.volAndPath[:b.volLen] + string(b.buf[:b.w])
}

const (
	Separator     = os.PathSeparator
	ListSeparator = os.PathListSeparator
)

// Clean returns the shortest path name equivalent to path
// by purely lexical processing. It applies the following rules
// iteratively until no further processing can be done:
//
//  1. Replace multiple Separator elements with a single one.
//  2. Eliminate each . path name element (the current directory).
//  3. Eliminate each inner .. path name element (the parent directory)
//     along with the non-.. element that precedes it.
//  4. Eliminate .. elements that begin a rooted path:
//     that is, replace "/.." by "/" at the beginning of a path,
//     assuming Separator is '/'.
//
// The returned path ends in a slash only if it represents a root directory,
// such as "/" on Unix or `C:\` on Windows.
//
// Finally, any occurrences of slash are replaced by Separator.
//
// If the result of this process is an empty string, Clean
// returns the string ".".
//
// See also Rob Pike, “Lexical File Names in Plan 9 or
// Getting Dot-Dot Right,”
// https://9p.io/sys/doc/lexnames.html
func Clean(path string) string {
	originalPath := path
	volLen := volumeNameLen(path)
	path = path[volLen:]
	if path == "" {
		if volLen > 1 && os.IsPathSeparator(originalPath[0]) && os.IsPathSeparator(originalPath[1]) {
			// should be UNC
			return FromSlash(originalPath)
		}
		return originalPath + "."
	}
	rooted := os.IsPathSeparator(path[0])

	// Invariants:
	//	reading from path; r is index of next byte to process.
	//	writing to buf; w is index of next byte to write.
	//	dotdot is index in buf where .. must stop, either because
	//		it is the leading slash or it is a leading ../../.. prefix.
	n := len(path)
	out := lazybuf{path: path, volAndPath: originalPath, volLen: volLen}
	r, dotdot := 0, 0
	if rooted {
		out.append(Separator)
		r, dotdot = 1, 1
	}

	for r < n {
		switch {
		case os.IsPathSeparator(path[r]):
			// empty path element
			r++
		case path[r] == '.' && (r+1 == n || os.IsPathSeparator(path[r+1])):
			// . element
			r++
		case path[r] == '.' && path[r+1] == '.' && (r+2 == n || os.IsPathSeparator(path[r+2])):
			// .. element: remove to last separator
			r += 2
			switch {
			case out.w > dotdot:
				// can backtrack
				out.w--
				for out.w > dotdot && !os.IsPathSeparator(out.index(out.w)) {
					out.w--
				}
			case !rooted:
				// cannot backtrack, but not rooted, so append .. element.
				if out.w > 0 {
					out.append(Separator)
				}
				out.append('.')
				out.append('.')
				dotdot = out.w
			}
		default:
			// real path element.
			// add slash if needed
			if rooted && out.w != 1 || !rooted && out.w != 0 {
				out.append(Separator)
			}
			// If a ':' appears in the path element at the start of a Windows path,
			// insert a .\ at the beginning to avoid converting relative paths
			// like a/../c: into c:.
			if runtime.GOOS == "windows" && out.w == 0 && out.volLen == 0 && r != 0 {
				for i := r; i < n && !os.IsPathSeparator(path[i]); i++ {
					if path[i] == ':' {
						out.append('.')
						out.append(Separator)
						break
					}
				}
			}
			// copy element
			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
				out.append(path[r])
			}
		}
	}

	// Turn empty string into "."
	if out.w == 0 {
		out.append('.')
	}

	return FromSlash(out.string())
}

func unixIsLocal(path string) bool {
	if IsAbs(path) || path == "" {
		return false
	}
	hasDots := false
	for p := path; p != ""; {
		var part string
		part, p, _ = strings.Cut(p, "/")
		if part == "." || part == ".." {
			hasDots = true
			break
		}
	}
	if hasDots {
		path = Clean(path)
	}
	if path == ".." || strings.HasPrefix(path, "../") {
		return false
	}
	return true
}

// FromSlash returns the result of replacing each slash ('/') character
// in path with a separator character. Multiple slashes are replaced
// by multiple separators.
func FromSlash(path string) string {
	if Separator == '/' {
		return path
	}
	return strings.ReplaceAll(path, "/", string(Separator))
}

// Join joins any number of path elements into a single path,
// separating them with an OS specific Separator. Empty elements
// are ignored. The result is Cleaned. However, if the argument
// list is empty or all its elements are empty, Join returns
// an empty string.
// On Windows, the result will only be a UNC path if the first
// non-empty element is a UNC path.
func Join(elem ...string) string {
	return join(elem)
}

// nolint
func unixAbs(path string) (string, error) {
	if IsAbs(path) {
		return Clean(path), nil
	}
	wd, err := os.Getwd()
	if err != nil {
		return "", err
	}
	return Join(wd, path), nil
}

// SkipDir is used as a return value from WalkFuncs 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 error = fs.SkipDir

// WalkFunc is the type of the function called by Walk to visit each
// file or directory.
//
// The path argument contains the argument to Walk as a prefix.
// That is, if Walk 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 directory and file are joined with Join, which may clean the
// directory name: if Walk is called with the root argument "x/../dir"
// and finds a file named "a" in that directory, the walk function will
// be called with argument "dir/a", not "x/../dir/a".
//
// The info argument is the fs.FileInfo for the named path.
//
// The error result returned by the function controls how Walk continues.
// If the function returns the special value SkipDir, Walk skips the
// current directory (path if info.IsDir() is true, otherwise path's
// parent directory). If the function returns the special value SkipAll,
// Walk skips all remaining files and directories. Otherwise, if the function
// returns a non-nil error, Walk stops entirely and returns that error.
//
// The err argument reports an error related to path, signaling that Walk
// will not walk into that directory. The function can decide how to
// handle that error; as described earlier, returning the error will
// cause Walk to stop walking the entire tree.
//
// Walk calls the function with a non-nil err argument in two cases.
//
// First, if an os.Lstat on the root directory or any directory or file
// in the tree fails, Walk calls the function with path set to that
// directory or file's path, info set to nil, and err set to the error
// from os.Lstat.
//
// Second, if a directory's Readdirnames method fails, Walk calls the
// function with path set to the directory's path, info, set to an
// fs.FileInfo describing the directory, and err set to the error from
// Readdirnames.
type WalkFunc func(path string, info fs.FileInfo, err error) error