swOperation/node_modules/@antv/g2/esm/transform/utils/lttb.js

/**
 * Sample data with lttb(Largest-Triangle-Three-Buckets) algo (https://github.com/pingec/downsample-lttb).
 * Return the sampled index array.
 */
export function lttb(I, X, Y, thresholds) {
    const length = I.length;
    if (thresholds >= length || thresholds === 0) {
        return I;
    }
    const x = (i) => X[I[i]] * 1;
    const y = (i) => Y[I[i]] * 1;
    const sampled = [];
    // Bucket size. Leave room for start and end data points.
    const every = (length - 2) / (thresholds - 2);
    let a = 0; // Initially a is the first point in the triangle.
    let maxArea;
    let area;
    let nextA;
    sampled.push(a); // Always add the first point.
    for (let i = 0; i < thresholds - 2; i++) {
        // Calculate point average for next bucket (containing c).
        let avgX = 0;
        let avgY = 0;
        let start = Math.floor((i + 1) * every) + 1;
        let end = Math.floor((i + 2) * every) + 1;
        end = Math.min(end, length);
        const size = end - start;
        for (; start < end; start++) {
            avgX += x(start);
            avgY += y(start);
        }
        avgX /= size;
        avgY /= size;
        // Get the range for this bucket.
        let frameStart = Math.floor((i + 0) * every) + 1;
        const frameEnd = Math.floor((i + 1) * every) + 1;
        // Point a.
        const pointA = [x(a), y(a)];
        maxArea = area = -1;
        for (; frameStart < frameEnd; frameStart++) {
            // Calculate triangle area over three buckets.
            area =
                Math.abs((pointA[0] - avgX) * (x(frameStart) - pointA[1]) -
                    (pointA[0] - y(frameStart)) * (avgY - pointA[0])) * 0.5;
            if (area > maxArea) {
                maxArea = area;
                nextA = frameStart; // Next a is this b.
            }
        }
        sampled.push(nextA); // Pick this point from the bucket.
        a = nextA; // This a is the next a (chosen b).
    }
    sampled.push(length - 1); // Always add last.
    return sampled.map((a) => I[a]);
}
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