package windows

import (
	"errors"
	"strings"
	"unicode/utf8"
)

var ErrInvalidCommandLineArgument = errors.New("Windows command-line argument contains NUL or invalid UTF-8")

// BuildCommandLine constructs the lpCommandLine string for an explicit
// lpApplicationName. It implements the CommandLineToArgvW-compatible escaping
// convention: quotes are escaped by a preceding run of backslashes and trailing
// backslashes inside a quoted argument are doubled.
func BuildCommandLine(arguments []string) (string, error) {
	if len(arguments) == 0 {
		return "", nil
	}
	quoted := make([]string, len(arguments))
	for index, argument := range arguments {
		if strings.IndexByte(argument, 0) >= 0 || !utf8.ValidString(argument) {
			return "", ErrInvalidCommandLineArgument
		}
		quoted[index] = quoteArgument(argument)
	}
	return strings.Join(quoted, " "), nil
}

func quoteArgument(argument string) string {
	if argument != "" && !strings.ContainsAny(argument, " \t\n\v\"") {
		return argument
	}
	var output strings.Builder
	output.Grow(len(argument) + 2)
	output.WriteByte('"')
	backslashes := 0
	for _, character := range argument {
		switch character {
		case '\\':
			backslashes++
		case '"':
			output.WriteString(strings.Repeat("\\", backslashes*2+1))
			output.WriteRune(character)
			backslashes = 0
		default:
			output.WriteString(strings.Repeat("\\", backslashes))
			output.WriteRune(character)
			backslashes = 0
		}
	}
	output.WriteString(strings.Repeat("\\", backslashes*2))
	output.WriteByte('"')
	return output.String()
}