// Package keyValue implements the basic key/value file format as described in
// desktop-entry-spec version 1.5. At the moment, these files can only be read
// from and not written to.
// The specification can be read at:
// https://specifications.freedesktop.org/desktop-entry-spec/1.5/
package keyValue

// This implementation does not support writing files, because, and I quote:
//   Lines beginning with a # and blank lines are considered comments and will
//   be ignored, however they should be preserved across reads and writes of the
//   desktop entry file.
// MISS ME WITH THAT SHIT!

import "io"
import "fmt"
import "bufio"
import "errors"
import "strings"
import "strconv"
import "unicode"
import "git.tebibyte.media/tomo/xdg/locale"

type keyValueError string
func (err keyValueError) Error () string { return string(err) }

const (
	// ErrUnexpectedRune indicates an unexpected rune was encountered.
	ErrUnexpectedRune = keyValueError("unexpected rune")
	// ErrInvalidGroupHeader indicates a wrongly formatted group header was
	// encountered.
	ErrInvalidGroupHeader = keyValueError("invalid group header")
	// ErrInvalidEntry indicates a wrongly formatted key/value entry was
	// encountered.
	ErrInvalidEntry = keyValueError("invalid entry")
	// ErrDuplicateGroup indicates that two or more groups with the same
	// name were found.
	ErrDuplicateGroup = keyValueError("duplicate group name")
	// ErrDuplicateEntry indicates that two or more keys with the same name
	// were found.
	ErrDuplicateEntry = keyValueError("duplicate entry name")
	// ErrDuplicateLocalization indicates that two or more localized values
	// with the same locale were provided for the same entry.
	ErrDuplicateLocalization = keyValueError("duplicate localization")
	// ErrNoDefaultValue indicates that localized values were provided for
	// an entry with no default value.
	ErrNoDefaultValue = keyValueError("no default value")
	// ErrEntryOutsideGroup indicates that an entry was found.
	ErrEntryOutsideGroup = keyValueError("entry outside group")
	// ErrUnsupportedEscape indicates that an unsupported escape sequence
	// was found.
	ErrUnsupportedEscape = keyValueError("unsupported escape")
	
	// ErrStringNotASCII indicates that a string or iconstring value was not
	// found to contain valid ASCII text.
	ErrStringNotASCII = keyValueError("string not ascii")
	// ErrBooleanNotTrueOrFalse indicates that a boolean value was not found
	// to equal "true" or "false".
	ErrBooleanNotTrueOrFalse = keyValueError("boolean not true or false")
)

// Parse parses a key/value file from a Reader.
func Parse (reader io.Reader) (File, error) {
	buffer := bufio.NewReader(reader)
	var file  = File { }
	var group Group
	var specifiedValues map[string] struct { }
	for {
		line, err := buffer.ReadString('\n')
		if errors.Is(err, io.EOF) { break            }
		if err != nil             { return nil, err  }
		
		line = strings.TrimSpace(line)

		switch {
		// comment
		case line == "", strings.HasPrefix(line, "#"):

		// group header
		case strings.HasPrefix(line, "["):
			// check integrity of prev. group
			if group != nil {
				for key := range group {
					if _, ok := specifiedValues[key]; !ok {
						return nil, ErrNoDefaultValue
					}
				}
			}

			// create new group
			name, err := parseGroupHeader(line)
			if err != nil { return nil, err }
			group           = Group { }
			specifiedValues = map[string] struct { } { }
			_, exists := file[name]
			if exists { return nil, ErrDuplicateGroup }
			file[name] = group

		// key/value pair
		default:
			if group == nil { return nil, ErrEntryOutsideGroup }
			key, value, loc, err := parseEntry(line)
			if err != nil { return nil, err }

			if loc != (locale.Locale { }) {
				// localized value
				entry, ok := group[key]
				if !ok { entry = newEntry() }
				entry.Localized[loc] = value
				group[key] = entry
			} else {
				// default value
				_, specified := specifiedValues[key]
				if specified { return nil, ErrDuplicateEntry }

				entry, exists := group[key]
				if !exists { entry = newEntry() }
				
				entry.Value          = value
				group[key]           = entry
				specifiedValues[key] = struct { } { }
			}
		}
	}

	return file, nil
}

func newEntry () Entry {
	return Entry {
		Localized: map[locale.Locale] string { },
	}
}

func parseGroupHeader (line string) (string, error) {
	line = strings.TrimPrefix(line, "[")
	if !strings.HasSuffix(line, "]") {
		return "", ErrInvalidGroupHeader
	}
	line = strings.TrimSuffix(line, "]")
	if strings.ContainsAny(line, "[]") {
		return "", ErrInvalidGroupHeader
	}
	return line, nil
}

func parseEntry (line string) (key, val string, loc locale.Locale, err error) {
	keyb := strings.Builder { }
	locb := strings.Builder { }
	valb := strings.Builder { }

	state := 0
	const getKey  = 0
	const getLoc  = 1
	const waitVal = 2
	const getVal  = 3
	
	for _, char := range line {
		switch state {
		case getKey:
			switch char {
			case '[': state = getLoc
			case '=': state = getVal
			default:  keyb.WriteRune(char)
			}
		case waitVal:
			if char == '=' { state = getVal }
		case getLoc:
			switch char {
			case ']': state = waitVal
			default:  locb.WriteRune(char)
			}
		case getVal:
			valb.WriteRune(char)
		}
	}
	if state != getVal { err = ErrInvalidEntry; return }

	if locb.Len() > 0 {
		loc, err = locale.Parse(locb.String())
		if err != nil { return}
	}
	key = strings.TrimSpace(keyb.String())
	val = strings.TrimSpace(valb.String())

	if !isKeyValid(key) { err = ErrInvalidEntry; return }
	
	return
}

func isKeyValid (key string) bool {
	for _, char := range key {
		if char == '-'                { continue }
		if 'A' <= char && char <= 'Z' { continue }
		if 'a' <= char && char <= 'z' { continue }
		return false
	}
	return true
}

// File represents a key/value file.
type File map[string] Group

// String returns a string representation of the file.
func (file File) String () string {
	outb := strings.Builder { }
	for name, group := range file {
		fmt.Fprintf(&outb, "[%s]\n", name)
		for key, entry := range group {
			fmt.Fprintf(&outb, "%s=%s\n", key, entry.Value)
			for loc, localized := range entry.Localized {
				fmt.Fprintf(&outb, "%s[%v]=%s\n", key, loc, localized)
			}
		}
	}
	out := outb.String()
	if len(out) > 0 { out = out[:len(out) - 2] }
	return out
}

// Group represents a group of entries.
type Group map[string] Entry

// Entry represents an entry in a group.
type Entry struct {
	Value     string
	Localized map[locale.Locale] string
}

// Localize returns a localized value matching the specified locale, which
// should be LC_MESSAGES for a system-localized value.
//
// The matching is done as follows. If locale is of the form
// lang_COUNTRY.ENCODING@MODIFIER, then it will match a key of the form
// lang_COUNTRY@MODIFIER. If such a key does not exist, it will attempt to match
// lang_COUNTRY followed by lang@MODIFIER. Then, a match against lang by itself
// will be attempted. Finally, if no matching key is found the required key
// without a locale specified is used. The encoding from the locale value is
// ignored when matching.
func (entry Entry) Localize (locale locale.Locale) string {
	withoutEncoding := locale
	withoutEncoding.Encoding = ""
	value, ok := entry.Localized[withoutEncoding]
	if ok { return value }

	withoutModifier := withoutEncoding
	withoutModifier.Modifier = ""
	value, ok = entry.Localized[withoutModifier]
	if ok { return value }

	withoutCountry := withoutEncoding
	withoutCountry.Country = ""
	value, ok = entry.Localized[withoutCountry]
	if ok { return value }

	return entry.Value
}

// Unescape returns a new copy of this entry with its main value parsed and
// unescaped as a string, and its localized values parsed and unescaped as
// localestrings.
func (entry Entry) Unescape () (Entry, error) {
	value, err := ParseString(entry.Value)
	if err != nil { return Entry { }, err }
	localizedValue := Entry {
		Value: value,
		Localized: make(map[locale.Locale] string),
	}
	for name, localized := range entry.Localized {
		unescaped, err := ParseLocaleString(localized)
		if err != nil { return Entry { }, err }
		localizedValue.Localized[name] = unescaped
	}
	return localizedValue, nil
}

// TODO have functions to parse/validate all data types

// ParseString parses a value of type string.
// Values of type string may contain all ASCII characters except for control
// characters.
// The escape sequences \s, \n, \t, \r, and \\ are supported, meaning ASCII
// space, newline, tab, carriage return, and backslash, respectively.
func ParseString (value string) (string, error) {
	value, err := escapeString(value)
	if err != nil { return "", err }
	if !isAsciiText(value) { return "", ErrStringNotASCII }
	return value, nil
}

// ParseLocaleString parses a value of type localestring.
// Values of type localestring are user displayable, and are encoded in UTF-8.
// The escape sequences \s, \n, \t, \r, and \\ are supported, meaning ASCII
// space, newline, tab, carriage return, and backslash, respectively.
func ParseLocaleString (value string) (string, error) {
	value, err := escapeString(value)
	if err != nil { return "", err }
	return value, nil
}

// ParseIconString parses a value of type iconstring.
// Values of type iconstring are the names of icons; these may be absolute
// paths, or symbolic names for icons located using the algorithm described in
// the Icon Theme Specification. Such values are not user-displayable, and are
// encoded in UTF-8.
// The escape sequences \s, \n, \t, \r, and \\ are supported, meaning ASCII
// space, newline, tab, carriage return, and backslash, respectively.
func ParseIconString (value string) (string, error) {
	value, err := escapeString(value)
	if err != nil { return "", err }
	if !isAsciiText(value) { return "", ErrStringNotASCII }
	return value, nil
}

// ParseBoolean parses a value of type boolean.
// Values of type boolean must either be the string true or false.
func ParseBoolean (value string) (bool, error) {
	if value == "true"  { return true,  nil }
	if value == "false" { return false, nil }
	return false, ErrBooleanNotTrueOrFalse
}

// ParseInteger parses a value of type integer.
// The geniuses at freedesktop never explained this type at all, or how it
// should be parsed.
func ParseInteger (value string) (int, error) {
	// TODO ensure this is compliant
	integer, err := strconv.ParseInt(value, 10, 64)
	return int(integer), err
}

// ParseNumeric parses a value of type numeric.
// Values of type numeric must be a valid floating point number as recognized by
// the %f specifier for scanf in the C locale.
func ParseNumeric (value string) (float64, error) {
	// TODO ensure this is compliant
	return strconv.ParseFloat(value, 64)
}

// ParseMultiple parses multiple of a value type. Any value parsing function can
// be specified. The multiple values should be separated by a semicolon and the
// input string may be optionally terminated by a semicolon. Trailing empty
// strings must always be terminated with a semicolon. Semicolons in these
// values need to be escaped using \;.
func ParseMultiple[T any] (parser func (string) (T, error), value string) ([]T, error) {
	return parseMultiple(parser, value, ';')
}

// ParseMultipleComma is like ParseMultiple, but uses a comma as a separator
// instead of a semicolon. This is used to parse icon theme files because the
// freedesktop people haven't yet learned the word "consistency".
func ParseMultipleComma[T any] (parser func (string) (T, error), value string) ([]T, error) {
	return parseMultiple(parser, value, ',')
}

func parseMultiple[T any] (parser func (string) (T, error), value string, sep rune) ([]T, error) {
	values  := []T { }
	builder := strings.Builder { }

	newValue := func () error {
		value, err := parser(builder.String())
		if err != nil { return err }
		values = append(values, value)
		builder.Reset()
		return nil
	}

	backslash := false
	for _, char := range value {
		switch char {
		case '\\':
			backslash = true
		case sep:
			if backslash {
				builder.WriteRune(sep)
				backslash = false
			} else {
				err := newValue()
				if err != nil { return nil, err }
			}
		default:
			if backslash {
				builder.WriteRune('\\')
				backslash = false
			}
			builder.WriteRune(char)
		}
	}
	if backslash {
		builder.WriteRune('\\')
	}
	if builder.Len() > 0 {
		err := newValue()
		if err != nil { return nil, err }
	}
	return values, nil
}

func isAsciiText (value string) bool {
	for _, char := range value {
		// must be an ascii character that isn't a control character.
		if char <= 0x1F || char > unicode.MaxASCII {
			return false
		}
	}
	return true
}

func escapeString (value string) (string, error) {
	builder   := strings.Builder { }
	backslash := false
	for _, char := range value {
		if char == '\\' { backslash = true; continue }

		if backslash {
			switch char {
			case 's':  builder.WriteRune(' ')
			case 'n':  builder.WriteRune('\n')
			case 't':  builder.WriteRune('\t')
			case 'r':  builder.WriteRune('\r')
			case '\\': builder.WriteRune('\\')
			default: return "", ErrUnsupportedEscape
			}
		} else {
			builder.WriteRune(char)
		}
	}
	return builder.String(), nil
}