package iso8601

// ValidFlags is a bitset type used to configure the behavior of the Valid
// function.
type ValidFlags int

const (
	// Strict is a validation flag used to represent a string iso8601 validation
	// (this is the default).
	Strict ValidFlags = 0

	// AllowSpaceSeparator allows the presence of a space instead of a 'T' as
	// separator between the date and time.
	AllowSpaceSeparator ValidFlags = 1 << iota

	// AllowMissingTime allows the value to contain only a date.
	AllowMissingTime

	// AllowMissingSubsecond allows the value to contain only a date and time.
	AllowMissingSubsecond

	// AllowMissingTimezone allows the value to be missing the timezone
	// information.
	AllowMissingTimezone

	// AllowNumericTimezone allows the value to represent timezones in their
	// numeric form.
	AllowNumericTimezone

	// Flexible is a combination of all validation flag that allow for
	// non-strict checking of the input value.
	Flexible = AllowSpaceSeparator | AllowMissingTime | AllowMissingSubsecond | AllowMissingTimezone | AllowNumericTimezone
)

// Valid check value to verify whether or not it is a valid iso8601 time
// representation.
func Valid(value string, flags ValidFlags) bool {
	var ok bool

	// year
	if value, ok = readDigits(value, 4, 4); !ok {
		return false
	}

	if value, ok = readByte(value, '-'); !ok {
		return false
	}

	// month
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	if value, ok = readByte(value, '-'); !ok {
		return false
	}

	// day
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	if len(value) == 0 && (flags&AllowMissingTime) != 0 {
		return true // date only
	}

	// separator
	if value, ok = readByte(value, 'T'); !ok {
		if (flags & AllowSpaceSeparator) == 0 {
			return false
		}
		if value, ok = readByte(value, ' '); !ok {
			return false
		}
	}

	// hour
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	if value, ok = readByte(value, ':'); !ok {
		return false
	}

	// minute
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	if value, ok = readByte(value, ':'); !ok {
		return false
	}

	// second
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	// microsecond
	if value, ok = readByte(value, '.'); !ok {
		if (flags & AllowMissingSubsecond) == 0 {
			return false
		}
	} else {
		if value, ok = readDigits(value, 1, 9); !ok {
			return false
		}
	}

	if len(value) == 0 && (flags&AllowMissingTimezone) != 0 {
		return true // date and time
	}

	// timezone
	if value, ok = readByte(value, 'Z'); ok {
		return len(value) == 0
	}

	if (flags & AllowSpaceSeparator) != 0 {
		value, _ = readByte(value, ' ')
	}

	if value, ok = readByte(value, '+'); !ok {
		if value, ok = readByte(value, '-'); !ok {
			return false
		}
	}

	// timezone hour
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	if value, ok = readByte(value, ':'); !ok {
		if (flags & AllowNumericTimezone) == 0 {
			return false
		}
	}

	// timezone minute
	if value, ok = readDigits(value, 2, 2); !ok {
		return false
	}

	return len(value) == 0
}

func readDigits(value string, min, max int) (string, bool) {
	if len(value) < min {
		return value, false
	}

	i := 0

	for i < max && i < len(value) && isDigit(value[i]) {
		i++
	}

	if i < max && i < min {
		return value, false
	}

	return value[i:], true
}

func readByte(value string, c byte) (string, bool) {
	if len(value) == 0 {
		return value, false
	}
	if value[0] != c {
		return value, false
	}
	return value[1:], true
}

func isDigit(c byte) bool {
	return '0' <= c && c <= '9'
}