// 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.

// General environment variables.

package os

import (
	
	
)

// Expand replaces ${var} or $var in the string based on the mapping function.
// For example, os.ExpandEnv(s) is equivalent to os.Expand(s, os.Getenv).
func ( string,  func(string) string) string {
	var  []byte
	// ${} is all ASCII, so bytes are fine for this operation.
	 := 0
	for  := 0;  < len(); ++ {
		if [] == '$' && +1 < len() {
			if  == nil {
				 = make([]byte, 0, 2*len())
			}
			 = append(, [:]...)
			,  := getShellName([+1:])
			if  == "" &&  > 0 {
				// Encountered invalid syntax; eat the
				// characters.
			} else if  == "" {
				// Valid syntax, but $ was not followed by a
				// name. Leave the dollar character untouched.
				 = append(, [])
			} else {
				 = append(, ()...)
			}
			 += 
			 =  + 1
		}
	}
	if  == nil {
		return 
	}
	return string() + [:]
}

// ExpandEnv replaces ${var} or $var in the string according to the values
// of the current environment variables. References to undefined
// variables are replaced by the empty string.
func ( string) string {
	return Expand(, Getenv)
}

// isShellSpecialVar reports whether the character identifies a special
// shell variable such as $*.
func isShellSpecialVar( uint8) bool {
	switch  {
	case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return true
	}
	return false
}

// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
func isAlphaNum( uint8) bool {
	return  == '_' || '0' <=  &&  <= '9' || 'a' <=  &&  <= 'z' || 'A' <=  &&  <= 'Z'
}

// getShellName returns the name that begins the string and the number of bytes
// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
// expansion and two more bytes are needed than the length of the name.
func getShellName( string) (string, int) {
	switch {
	case [0] == '{':
		if len() > 2 && isShellSpecialVar([1]) && [2] == '}' {
			return [1:2], 3
		}
		// Scan to closing brace
		for  := 1;  < len(); ++ {
			if [] == '}' {
				if  == 1 {
					return "", 2 // Bad syntax; eat "${}"
				}
				return [1:],  + 1
			}
		}
		return "", 1 // Bad syntax; eat "${"
	case isShellSpecialVar([0]):
		return [0:1], 1
	}
	// Scan alphanumerics.
	var  int
	for  = 0;  < len() && isAlphaNum([]); ++ {
	}
	return [:], 
}

// Getenv retrieves the value of the environment variable named by the key.
// It returns the value, which will be empty if the variable is not present.
// To distinguish between an empty value and an unset value, use LookupEnv.
func ( string) string {
	testlog.Getenv()
	,  := syscall.Getenv()
	return 
}

// LookupEnv retrieves the value of the environment variable named
// by the key. If the variable is present in the environment the
// value (which may be empty) is returned and the boolean is true.
// Otherwise the returned value will be empty and the boolean will
// be false.
func ( string) (string, bool) {
	testlog.Getenv()
	return syscall.Getenv()
}

// Setenv sets the value of the environment variable named by the key.
// It returns an error, if any.
func (,  string) error {
	 := syscall.Setenv(, )
	if  != nil {
		return NewSyscallError("setenv", )
	}
	return nil
}

// Unsetenv unsets a single environment variable.
func ( string) error {
	return syscall.Unsetenv()
}

// Clearenv deletes all environment variables.
func () {
	syscall.Clearenv()
}

// Environ returns a copy of strings representing the environment,
// in the form "key=value".
func () []string {
	return syscall.Environ()
}