@antv/g2
Version:
the Grammar of Graphics in Javascript
60 lines • 2.19 kB
JavaScript
;
Object.defineProperty(exports, "__esModule", { value: true });
exports.lttb = void 0;
/**
* Sample data with lttb(Largest-Triangle-Three-Buckets) algo (https://github.com/pingec/downsample-lttb).
* Return the sampled index array.
*/
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]);
}
exports.lttb = lttb;
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