function addLevelTicks()

in src/scale/Time.ts [478:573]
88 lines of code
23 McCabe index (conditional complexity)

    function addLevelTicks(
        unitName: TimeUnit,
        lastLevelTicks: InnerTimeTick[],
        levelTicks: InnerTimeTick[]
    ) {
        const newAddedTicks: ScaleTick[] = [];
        const isFirstLevel = !lastLevelTicks.length;

        if (isUnitValueSame(getPrimaryTimeUnit(unitName), extent[0], extent[1], isUTC)) {
            return;
        }

        if (isFirstLevel) {
            lastLevelTicks = [{
                // TODO Optimize. Not include so may ticks.
                value: getFirstTimestampOfUnit(new Date(extent[0]), unitName, isUTC)
            }, {
                value: extent[1]
            }];
        }

        for (let i = 0; i < lastLevelTicks.length - 1; i++) {
            const startTick = lastLevelTicks[i].value;
            const endTick = lastLevelTicks[i + 1].value;
            if (startTick === endTick) {
                continue;
            }

            let interval: number;
            let getterName;
            let setterName;
            let isDate = false;

            switch (unitName) {
                case 'year':
                    interval = Math.max(1, Math.round(approxInterval / ONE_DAY / 365));
                    getterName = fullYearGetterName(isUTC);
                    setterName = fullYearSetterName(isUTC);
                    break;
                case 'half-year':
                case 'quarter':
                case 'month':
                    interval = getMonthInterval(approxInterval);
                    getterName = monthGetterName(isUTC);
                    setterName = monthSetterName(isUTC);
                    break;
                case 'week':    // PENDING If week is added. Ignore day.
                case 'half-week':
                case 'day':
                    interval = getDateInterval(approxInterval, 31); // Use 32 days and let interval been 16
                    getterName = dateGetterName(isUTC);
                    setterName = dateSetterName(isUTC);
                    isDate = true;
                    break;
                case 'half-day':
                case 'quarter-day':
                case 'hour':
                    interval = getHourInterval(approxInterval);
                    getterName = hoursGetterName(isUTC);
                    setterName = hoursSetterName(isUTC);
                    break;
                case 'minute':
                    interval = getMinutesAndSecondsInterval(approxInterval, true);
                    getterName = minutesGetterName(isUTC);
                    setterName = minutesSetterName(isUTC);
                    break;
                case 'second':
                    interval = getMinutesAndSecondsInterval(approxInterval, false);
                    getterName = secondsGetterName(isUTC);
                    setterName = secondsSetterName(isUTC);
                    break;
                case 'millisecond':
                    interval = getMillisecondsInterval(approxInterval);
                    getterName = millisecondsGetterName(isUTC);
                    setterName = millisecondsSetterName(isUTC);
                    break;
            }

            addTicksInSpan(
                interval, startTick, endTick, getterName, setterName, isDate, newAddedTicks
            );

            if (unitName === 'year' && levelTicks.length > 1 && i === 0) {
                // Add nearest years to the left extent.
                levelTicks.unshift({
                    value: levelTicks[0].value - interval
                });
            }
        }

        for (let i = 0; i < newAddedTicks.length; i++) {
            levelTicks.push(newAddedTicks[i]);
        }
        // newAddedTicks.length && console.log(unitName, newAddedTicks);
        return newAddedTicks;
    }