package main

// Command substitution: $(command) is replaced by the output of command,
// which runs in a subshell (a new gosh process), so that cd, set etc. in it
// do not change the shell.

import (
	"bytes"
	"errors"
	"os"
	"os/exec"
	"strings"
	"syscall"
)

// subshellPath is the gosh binary used for command substitution; empty means
// the running executable. Tests set it, since there the running executable is
// the test binary.
var subshellPath string

// substStatus is the exit status of the last command substitution of the
// current command (a command that consists only of a substitution has this
// status, like in sh).
var substStatus int

// isSubstStart reports whether a command substitution $( starts at s[i].
func isSubstStart(s string, i int) bool {
	return s[i] == '$' && i+1 < len(s) && s[i+1] == '('
}

// substEnd returns the index of the ) that ends the command substitution
// starting with $( at s[i], or -1 if it is not terminated. Quotes, escapes
// and nested substitutions inside are taken into account.
func substEnd(s string, i int) int {
	depth := 0
	var quote byte
	for j := i + 1; j < len(s); j++ {
		ch := s[j]
		switch {
		case quote == '\'':
			if ch == '\'' {
				quote = 0
			}
		case ch == '\\':
			j++
		case quote == '"':
			if ch == '"' {
				quote = 0
			} else if isSubstStart(s, j) {
				end := substEnd(s, j)
				if end < 0 {
					return -1
				}
				j = end
			}
		case ch == '"' || ch == '\'':
			quote = ch
		case ch == '(':
			depth++
		case ch == ')':
			depth--
			if depth == 0 {
				return j
			}
		}
	}
	return -1
}

// commandSubst runs command in a subshell and returns its output without
// trailing newlines. stdin and stderr are those of the shell.
func commandSubst(command string) (string, error) {
	if strings.HasPrefix(command, "(") {
		return "", errors.New("arithmetic expansion $((...)) is not supported")
	}
	path := subshellPath
	if path == "" {
		var err error
		if path, err = os.Executable(); err != nil {
			return "", err
		}
	}

	var out bytes.Buffer
	cmd := exec.Command(path, "-c", command)
	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, &out, os.Stderr
	err := cmd.Run()
	substStatus = exitStatus(err)
	if substStatus == 128+int(syscall.SIGINT) {
		// Ctrl+C aborts the whole command, like in bash
		return "", statusError(substStatus)
	}
	var exitErr *exec.ExitError
	if err != nil && !errors.As(err, &exitErr) {
		return "", err
	}
	return strings.TrimRight(out.String(), "\n"), nil
}