src/pkg/path/filepath/path_windows.go

// 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

import (
	"strings"
)

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

// IsAbs returns true if the path is absolute.
func IsAbs(path string) (b bool) {
	l := volumeNameLen(path)
	if l == 0 {
		return false
	}
	path = path[l:]
	if path == "" {
		return false
	}
	return isSlash(path[0])
}

// volumeNameLen returns length of the leading volume name on Windows.
// It returns 0 elsewhere.
func volumeNameLen(path string) int {
	if len(path) < 2 {
		return 0
	}
	// with drive letter
	c := path[0]
	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
		return 2
	}
	// is it UNC
	if l := len(path); l >= 5 && isSlash(path[0]) && isSlash(path[1]) &&
		!isSlash(path[2]) && path[2] != '.' {
		// first, leading `\\` and next shouldn't be `\`. its server name.
		for n := 3; n < l-1; n++ {
			// second, next '\' shouldn't be repeated.
			if isSlash(path[n]) {
				n++
				// third, following something characters. its share name.
				if !isSlash(path[n]) {
					if path[n] == '.' {
						break
					}
					for ; n < l; n++ {
						if isSlash(path[n]) {
							break
						}
					}
					return n
				}
				break
			}
		}
	}
	return 0
}

// HasPrefix exists for historical compatibility and should not be used.
func HasPrefix(p, prefix string) bool {
	if strings.HasPrefix(p, prefix) {
		return true
	}
	return strings.HasPrefix(strings.ToLower(p), strings.ToLower(prefix))
}