// Copyright 2010 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_windows was copied from https://github.com/golang/go/blob/master/src/path/filepath/path_windows.go
// to be able to verify Windows paths from Go code running on unix.
package filepath_windows

func isSlash(c uint8) bool {
	return c == '\\' || c == '/'
}

// IsAbs reports whether the path is absolute.
func IsAbs(path string) (b bool) {
	l := volumeNameLen(path)
	if l == 0 {
		return false
	}
	// If the volume name starts with a double slash, this is an absolute path.
	if isSlash(path[0]) && isSlash(path[1]) {
		return true
	}
	path = path[l:]
	if path == "" {
		return false
	}
	return isSlash(path[0])
}

func toUpper(c byte) byte {
	if 'a' <= c && c <= 'z' {
		return c - ('a' - 'A')
	}
	return c
}

// pathHasPrefixFold tests whether the path s begins with prefix,
// ignoring case and treating all path separators as equivalent.
// If s is longer than prefix, then s[len(prefix)] must be a path separator.
func pathHasPrefixFold(s, prefix string) bool {
	if len(s) < len(prefix) {
		return false
	}
	for i := 0; i < len(prefix); i++ {
		if isSlash(prefix[i]) {
			if !isSlash(s[i]) {
				return false
			}
		} else if toUpper(prefix[i]) != toUpper(s[i]) {
			return false
		}
	}
	if len(s) > len(prefix) && !isSlash(s[len(prefix)]) {
		return false
	}
	return true
}

// volumeNameLen returns length of the leading volume name on Windows.
// It returns 0 elsewhere.
//
// See:
// https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats
// https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html
func volumeNameLen(path string) int {
	switch {
	case len(path) >= 2 && path[1] == ':':
		// Path starts with a drive letter.
		//
		// Not all Windows functions necessarily enforce the requirement that
		// drive letters be in the set A-Z, and we don't try to here.
		//
		// We don't handle the case of a path starting with a non-ASCII character,
		// in which case the "drive letter" might be multiple bytes long.
		return 2

	case len(path) == 0 || !isSlash(path[0]):
		// Path does not have a volume component.
		return 0

	case pathHasPrefixFold(path, `\\.\UNC`):
		// We're going to treat the UNC host and share as part of the volume
		// prefix for historical reasons, but this isn't really principled;
		// Windows's own GetFullPathName will happily remove the first
		// component of the path in this space, converting
		// \\.\unc\a\b\..\c into \\.\unc\a\c.
		return uncLen(path, len(`\\.\UNC\`))

	case pathHasPrefixFold(path, `\\.`) ||
		pathHasPrefixFold(path, `\\?`) || pathHasPrefixFold(path, `\??`):
		// Path starts with \\.\, and is a Local Device path; or
		// path starts with \\?\ or \??\ and is a Root Local Device path.
		//
		// We treat the next component after the \\.\ prefix as
		// part of the volume name, which means Clean(`\\?\c:\`)
		// won't remove the trailing \. (See #64028.)
		if len(path) == 3 {
			return 3 // exactly \\.
		}
		_, rest, ok := cutPath(path[4:])
		if !ok {
			return len(path)
		}
		return len(path) - len(rest) - 1

	case len(path) >= 2 && isSlash(path[1]):
		// Path starts with \\, and is a UNC path.
		return uncLen(path, 2)
	}
	return 0
}

// uncLen returns the length of the volume prefix of a UNC path.
// prefixLen is the prefix prior to the start of the UNC host;
// for example, for "//host/share", the prefixLen is len("//")==2.
func uncLen(path string, prefixLen int) int {
	count := 0
	for i := prefixLen; i < len(path); i++ {
		if isSlash(path[i]) {
			count++
			if count == 2 {
				return i
			}
		}
	}
	return len(path)
}

// cutPath slices path around the first path separator.
func cutPath(path string) (before, after string, found bool) {
	for i := range path {
		if isSlash(path[i]) {
			return path[:i], path[i+1:], true
		}
	}
	return path, "", false
}