package compose

import (
	"bufio"
	"bytes"
	"fmt"
	"os"
	"strings"
	"unicode"
	"unicode/utf8"
)

// ParseEnvFile reads a file with environment variables enumerated by lines
//
// ``Environment variable names used by the utilities in the Shell and
// Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase
// letters, digits, and the '_' (underscore) from the characters defined in
// Portable Character Set and do not begin with a digit. *But*, other
// characters may be permitted by an implementation; applications shall
// tolerate the presence of such names.''
// -- http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html
//
// As of #16585, it's up to application inside docker to validate or not
// environment variables, that's why we just strip leading whitespace and
// nothing more.
func ParseEnvFile(filename string) ([]string, error) {
	fh, err := os.Open(filename)
	if err != nil {
		return []string{}, err
	}
	defer fh.Close()

	lines := []string{}
	scanner := bufio.NewScanner(fh)
	currentLine := 0
	utf8bom := []byte{0xEF, 0xBB, 0xBF}
	for scanner.Scan() {
		scannedBytes := scanner.Bytes()
		if !utf8.Valid(scannedBytes) {
			return []string{}, fmt.Errorf("env file %s contains invalid utf8 bytes at line %d: %v", filename, currentLine+1, scannedBytes)
		}
		// We trim UTF8 BOM
		if currentLine == 0 {
			scannedBytes = bytes.TrimPrefix(scannedBytes, utf8bom)
		}
		// trim the line from all leading whitespace first
		line := strings.TrimLeftFunc(string(scannedBytes), unicode.IsSpace)
		currentLine++
		// line is not empty, and not starting with '#'
		if len(line) > 0 && !strings.HasPrefix(line, "#") {
			data := strings.SplitN(line, "=", 2)

			// trim the front of a variable, but nothing else
			variable := strings.TrimLeft(data[0], whiteSpaces)
			if strings.ContainsAny(variable, whiteSpaces) {
				return []string{}, ErrBadEnvVariable{fmt.Sprintf("variable '%s' has white spaces", variable)}
			}

			if len(data) > 1 {

				// pass the value through, no trimming
				lines = append(lines, fmt.Sprintf("%s=%s", variable, data[1]))
			} else {
				// if only a pass-through variable is given, clean it up.
				lines = append(lines, fmt.Sprintf("%s=%s", strings.TrimSpace(line), os.Getenv(line)))
			}
		}
	}
	return lines, scanner.Err()
}

var whiteSpaces = " \t"

// ErrBadEnvVariable typed error for bad environment variable
type ErrBadEnvVariable struct {
	msg string
}

func (e ErrBadEnvVariable) Error() string {
	return fmt.Sprintf("poorly formatted environment: %s", e.msg)
}