Method lttbDownSample

src/data/DataStore.ts:870–962

* Large data down sampling using largest-triangle-three-buckets * @param {string} valueDimension * @param {number} targetCount

(
        valueDimension: DimensionIndex,
        rate: number
    )

     * @param {number} targetCount
     */
    lttbDownSample(
        valueDimension: DimensionIndex,
        rate: number
    ): DataStore {
        const target = this.clone([valueDimension], true);
        const targetStorage = target._chunks;
        const dimStore = targetStorage[valueDimension];
        const len = this.count();

        let sampledIndex = 0;

        const frameSize = Math.floor(1 / rate);

        let currentRawIndex = this.getRawIndex(0);
        let maxArea;
        let area;
        let nextRawIndex;

        const newIndices = new (getIndicesCtor(this._rawCount))(Math.min((Math.ceil(len / frameSize) + 2) * 2, len));

        // First frame use the first data.
        newIndices[sampledIndex++] = currentRawIndex;
        for (let i = 1; i < len - 1; i += frameSize) {
            const nextFrameStart = Math.min(i + frameSize, len - 1);
            const nextFrameEnd = Math.min(i + frameSize * 2, len);

            const avgX = (nextFrameEnd + nextFrameStart) / 2;
            let avgY = 0;

            for (let idx = nextFrameStart; idx < nextFrameEnd; idx++) {
                const rawIndex = this.getRawIndex(idx);
                const y = dimStore[rawIndex] as number;
                if (isNaN(y)) {
                    continue;
                }
                avgY += y as number;
            }
            avgY /= (nextFrameEnd - nextFrameStart);

            const frameStart = i;
            const frameEnd = Math.min(i + frameSize, len);

            const pointAX = i - 1;
            const pointAY = dimStore[currentRawIndex] as number;

            maxArea = -1;

            nextRawIndex = frameStart;

            let firstNaNIndex = -1;
            let countNaN = 0;
            // Find a point from current frame that construct a triangle with largest area with previous selected point
            // And the average of next frame.
            for (let idx = frameStart; idx < frameEnd; idx++) {
                const rawIndex = this.getRawIndex(idx);
                const y = dimStore[rawIndex] as number;
                if (isNaN(y)) {
                    countNaN++;