module time

struct DateTimeParser {
	datetime string
	format   string
mut:
	current_pos_datetime int
}

fn new_date_time_parser(datetime string, format string) DateTimeParser {
	return DateTimeParser{
		datetime: datetime
		format: format
	}
}

fn (mut p DateTimeParser) next(length int) !string {
	if p.current_pos_datetime + length > p.datetime.len {
		return error('end of string')
	}
	val := p.datetime[p.current_pos_datetime..p.current_pos_datetime + length]
	p.current_pos_datetime += length
	return val
}

fn (mut p DateTimeParser) peek(length int) !string {
	if p.current_pos_datetime + length > p.datetime.len {
		return error('end of string')
	}
	return p.datetime[p.current_pos_datetime..p.current_pos_datetime + length]
}

fn (mut p DateTimeParser) must_be_int(length int) !int {
	val := p.next(length)!
	if !val.contains_only('0123456789') {
		return error('expected int, found: ${val}')
	}
	return val.int()
}

fn (mut p DateTimeParser) must_be_int_with_minimum_length(min int, max int, allow_leading_zero bool) !int {
	mut length := max + 1 - min
	mut val := ''
	for _ in 0 .. length {
		tok := p.peek(1) or { break }
		if !tok.contains_only('0123456789') {
			break
		}
		p.next(1)!
		val += tok
	}
	if val.len < min {
		return error('expected int with a minimum length of ${min}, found: ${val.len}')
	}
	if !allow_leading_zero && val.starts_with('0') {
		return error('0 is not allowed for this format')
	}
	return val.int()
}

fn (mut p DateTimeParser) must_be_single_int_with_optional_leading_zero() !int {
	mut val := p.next(1)!
	if val == '0' {
		next := p.next(1) or { '' }
		if !next.contains_only('0123456789') {
			return error('expected int, found: ${next}')
		}
		val += next
	}
	return val.int()
}

fn (mut p DateTimeParser) must_be_string(must string) ! {
	val := p.next(must.len)!
	if val != must {
		return error('invalid string: "${val}"!="${must}" at: ${p.current_pos_datetime}')
	}
}

fn (mut p DateTimeParser) must_be_string_one_of(oneof []string) !string {
	for must in oneof {
		val := p.peek(must.len) or { continue }
		if val == must {
			return must
		}
	}
	return error('invalid string: must be one of ${oneof}, at ${p.current_pos_datetime}')
}

fn (mut p DateTimeParser) must_be_valid_month() !int {
	for v in long_months {
		if p.current_pos_datetime + v.len < p.datetime.len {
			month_name := p.datetime[p.current_pos_datetime..p.current_pos_datetime + v.len]
			if v == month_name {
				p.current_pos_datetime += v.len
				return long_months.index(month_name) + 1
			}
		}
	}
	return error_invalid_time(0, 'invalid month name')
}

fn (mut p DateTimeParser) must_be_valid_week_day(letters int) !string {
	val := p.next(letters)!
	for v in long_days {
		if v[0..letters] == val {
			return v
		}
	}
	return error_invalid_time(0, 'invalid month name')
}

fn extract_tokens(s string) ![]string {
	mut tokens := []string{}
	mut current := ''
	for r in s {
		if current.contains_only(r.ascii_str()) || current == '' {
			current += r.ascii_str()
		} else {
			tokens << current
			current = r.ascii_str()
		}
	}
	if current != '' {
		tokens << current
	}
	return tokens
}

// parse_format parses the string `s`, as a custom `format`, containing the following specifiers:
// YYYY - 4 digit year, 0000..9999
// YY - 2 digit year, 00..99
// M - month, 1..12
// MM - month, 2 digits, 01..12
// MMMM - name of month
// D - day of the month, 1..31
// DD - day of the month, 01..31
// H - hour, 0..23
// HH - hour, 00..23
// h - hour, 0..23
// hh - hour, 0..23
// k - hour, 0..23
// kk - hour, 0..23
// m - minute, 0..59
// mm - minute, 0..59
// s - second, 0..59
// ss - second, 0..59
fn (mut p DateTimeParser) parse() !Time {
	mut year_ := 0
	mut month_ := 0
	mut day_in_month := 0
	mut hour_ := 0
	mut minute_ := 0
	mut second_ := 0
	tokens := extract_tokens(p.format) or {
		return error_invalid_time(0, 'malformed format string: ${err}')
	}
	for token in tokens {
		match token {
			'YYYY' {
				year_ = p.must_be_int(4) or {
					return error_invalid_time(0, 'end of string reached before the full year was specified')
				}
			}
			'YY' {
				year_ = now().year / 100 * 100 + p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before the full year was specified')
				}
			}
			'M' {
				month_ = p.must_be_int_with_minimum_length(1, 2, false) or {
					return error_invalid_time(0, 'end of string reached before the month was specified')
				}
				if month_ < 1 || month_ > 12 {
					return error_invalid_time(0, 'month must be  between 1 and 12')
				}
			}
			'MM' {
				month_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before the month was specified')
				}
				if month_ < 1 || month_ > 12 {
					return error_invalid_time(0, 'month must be  between 01 and 12')
				}
			}
			'MMMM' {
				month_ = p.must_be_valid_month() or { return err }
			}
			'D' {
				day_in_month = p.must_be_int_with_minimum_length(1, 2, false) or {
					return error_invalid_time(0, 'end of string reached before the day was specified')
				}
				if day_in_month < 1 || day_in_month > 31 {
					return error_invalid_time(0, 'day must be  between 1 and 31')
				}
			}
			'DD' {
				day_in_month = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before the month was specified')
				}
				if day_in_month < 1 || day_in_month > 31 {
					return error_invalid_time(0, 'day must be  between 01 and 31')
				}
			}
			'H' {
				hour_ = p.must_be_int_with_minimum_length(1, 2, false) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 0 and 23')
				}
			}
			'HH' {
				hour_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 00 and 23')
				}
			}
			'h' {
				hour_ = p.must_be_int_with_minimum_length(1, 2, false) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 0 and 23')
				}
			}
			'hh' {
				hour_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 00 and 23')
				}
			}
			'k' {
				hour_ = p.must_be_int(1) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 0 and 23')
				}
			}
			'kk' {
				hour_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before hours where specified')
				}
				if hour_ < 0 || hour_ > 23 {
					return error_invalid_time(0, 'hour must be  between 00 and 23')
				}
			}
			'm' {
				minute_ = p.must_be_int(1) or {
					return error_invalid_time(0, 'end of string reached before minutes where specified')
				}
				if minute_ < 0 || minute_ > 59 {
					return error_invalid_time(0, 'minute must be between 0 and 59')
				}
			}
			'mm' {
				minute_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before minutes where specified')
				}
				if minute_ < 0 || minute_ > 59 {
					return error_invalid_time(0, 'minute must be between 00 and 59')
				}
			}
			's' {
				second_ = p.must_be_int(1) or {
					return error_invalid_time(0, 'end of string reached before seconds where specified')
				}
				if second_ < 0 || second_ > 59 {
					return error_invalid_time(0, 'second must be between 0 and 59')
				}
			}
			'ss' {
				second_ = p.must_be_int(2) or {
					return error_invalid_time(0, 'end of string reached before seconds where specified')
				}
				if second_ < 0 || second_ > 59 {
					return error_invalid_time(0, 'second must be between 00 and 59')
				}
			}
			else {
				p.must_be_string(token) or { return error_invalid_time(0, '${err}') }
			}
		}
	}

	return new_time(
		year: year_
		month: month_
		day: day_in_month
		hour: hour_
		minute: minute_
		second: second_
	)
}