@stemplayer-js/stemplayer-js/src/lib/downsample.js

A streaming, low latency Stem Player Web-Component

/*
 * Copyright (c) 2018 Alexufo . Original code 2013 by Sveinn Steinarsson
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * @see https://github.com/Alexufo/Responsive-waveform/blob/master/largestTriangleThreeBuckets.js
 */

/* eslint-disable */
export default function largestTriangleThreeBuckets(data, threshold) {
  // data is linear array [90,87,97...]
  var data_length = data.length;
  if (threshold >= data_length || threshold === 0) {
    return data; // Nothing to do
  }
  var sampled = [],
    sampled_index = 0;

  // Bucket size. Leave room for start and end data points
  var every = (data_length - 2) / (threshold - 2);

  var a = 0, // Initially a is the first point in the triangle
    max_area_point,
    max_area,
    area,
    next_a;

  sampled[sampled_index++] = data[a]; // Always add the first point

  for (var i = 0; i < threshold - 2; i++) {
    // Calculate point average for next bucket (containing c)
    var avg_x = 0,
      avg_y = 0,
      avg_range_start = Math.floor((i + 1) * every) + 1,
      avg_range_end = Math.floor((i + 2) * every) + 1;
    avg_range_end = avg_range_end < data_length ? avg_range_end : data_length;

    var avg_range_length = avg_range_end - avg_range_start;

    for (; avg_range_start < avg_range_end; avg_range_start++) {
      avg_x += avg_range_start;
      avg_y += data[avg_range_start] * 1; // * 1 enforces Number (value may be Date)
    }

    avg_x /= avg_range_length;
    avg_y /= avg_range_length;

    // Get the range for this bucket
    var range_offs = Math.floor((i + 0) * every) + 1,
      range_to = Math.floor((i + 1) * every) + 1;

    // Point a
    var point_a_x = a * 1, // enforce Number (value may be Date)
      point_a_y = data[a] * 1;

    max_area = area = -1;

    for (; range_offs < range_to; range_offs++) {
      // Calculate triangle area over three buckets
      area =
        Math.abs(
          (point_a_x - avg_x) * (data[range_offs] - point_a_y) -
            (point_a_x - range_offs) * (avg_y - point_a_y),
        ) * 0.5;
      if (area > max_area) {
        max_area = area;
        max_area_point = data[range_offs];
        next_a = range_offs; // Next a is this b
      }
    }

    sampled[sampled_index++] = max_area_point; // Pick this point from the bucket
    a = next_a; // This a is the next a (chosen b)
  }

  sampled[sampled_index++] = data[data_length - 1]; // Always add last
  return sampled;
}