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// 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 (
        "errors"
        "syscall"
)

// Expand replaces ${var} or $var in the string based on the mapping function.
// Invocations of undefined variables are replaced with the empty string.
func Expand(s string, mapping func(string) string) string {
        buf := make([]byte, 0, 2*len(s))
        // ${} is all ASCII, so bytes are fine for this operation.
        i := 0
        for j := 0; j < len(s); j++ {
                if s[j] == '$' && j+1 < len(s) {
                        buf = append(buf, s[i:j]...)
                        name, w := getShellName(s[j+1:])
                        buf = append(buf, mapping(name)...)
                        j += w
                        i = j + 1
                }
        }
        return string(buf) + s[i:]
}

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

// isSpellSpecialVar reports whether the character identifies a special
// shell variable such as $*.
func isShellSpecialVar(c uint8) bool {
        switch c {
        case '*', '#', '$', '@', '!', '?', '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(c uint8) bool {
        return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
}

// getName 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(s string) (string, int) {
        switch {
        case s[0] == '{':
                if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
                        return s[1:2], 3
                }
                // Scan to closing brace
                for i := 1; i < len(s); i++ {
                        if s[i] == '}' {
                                return s[1:i], i + 1
                        }
                }
                return "", 1 // Bad syntax; just eat the brace.
        case isShellSpecialVar(s[0]):
                return s[0:1], 1
        }
        // Scan alphanumerics.
        var i int
        for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
        }
        return s[:i], i
}

// ENOENV is the error indicating that an environment variable does not exist.
var ENOENV = errors.New("no such environment variable")

// Getenverror retrieves the value of the environment variable named by the key.
// It returns the value and an error, if any.
func Getenverror(key string) (value string, err error) {
        if len(key) == 0 {
                return "", EINVAL
        }
        val, found := syscall.Getenv(key)
        if !found {
                return "", ENOENV
        }
        return val, nil
}

// 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.
func Getenv(key string) string {
        v, _ := Getenverror(key)
        return v
}

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

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

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