Function logScaleCalcNiceTicks

src/coord/axisNiceTicks.ts:157–195

(
    intervalStub: IntervalScale,
    opt: Pick<
        ScaleCalcNiceMethodOpt,
        'splitNumber' | 'minInterval' | 'maxInterval' | 'userInterval'
    >
)

// ------ START: LogScale Nice ------

function logScaleCalcNiceTicks(
    intervalStub: IntervalScale,
    opt: Pick<
        ScaleCalcNiceMethodOpt,
        'splitNumber' | 'minInterval' | 'maxInterval' | 'userInterval'
    >
): IntervalScaleConfig {
    // [CAVEAT]: If updating this impl, need to sync it to `axisAlignTicks.ts`.

    const splitNumber = ensureValidSplitNumber(opt.splitNumber, 10);
    // Find nice ticks in the "logarithmic space". Notice that "logarithmic space" is a middle space
    // rather than the innermost linear space when axis breaks exist.
    const intervalExtent = intervalStub.getExtent();
    // But use the span in the innermost linear space to calculate nice ticks.
    const span = getScaleLinearSpanEffective(intervalStub);

    if (__DEV__) {
        assert(isFinite(span) && span > 0); // It should be ensured by `intervalScaleEnsureValidExtent`.
    }

    // Interval should be integer
    let interval = mathMax(quantity(span), 1);
    const err = splitNumber / span * interval;
    // Filter ticks to get closer to the desired count.
    if (err <= 0.5) {
        // TODO: support other bases other than 10?
        interval *= 10;
    }

    const intervalPrecision = getIntervalPrecision(interval);
    // For LogScale, we use a `niceExtent` in the "logarithmic space" rather than
    // the original "pow space", because it is used in `intervalStub.getTicks()` thereafter.
    const niceExtent = [
        round(mathCeil(intervalExtent[0] / interval) * interval, intervalPrecision),
        round(mathFloor(intervalExtent[1] / interval) * interval, intervalPrecision)
    ] as [number, number];

    return {intervalPrecision, interval, niceExtent};
};

// ------ END: LogScale Nice ------