script.js

//VERSION=3
function setup() {
  return {
    input: ["B04", "B08"],
    output: { bands: 1 },
    mosaicking: "ORBIT"
  }
}

const msInDay = 24 * 60 * 60 * 1000;
const msInYear = 365.25 * msInDay;
const toleranceDays = 10;
const toleranceMs = toleranceDays * msInDay;

var metadata = undefined;

function preProcessScenes(collections) {
  var scenes = collections.scenes.orbits;
  scenes = scenes.sort((s1, s2) => s2.dateFrom - s1.dateFrom);
  const observed = new Date(scenes[0].dateFrom)
  var newScenes = [scenes[0]];
  for (var historical = observed - msInYear; historical >= new Date(collections.from) - toleranceMs; historical -= msInYear) {
    newScenes.push(findClosest(scenes, historical));
  }
  newScenes = newScenes.filter(scene => scene != null);
  metadata = {
    observed: observed.toISOString(),
    historical: newScenes.slice(1).map(scene => new Date(scene.dateFrom).toISOString())
  }
  collections.scenes.orbits = newScenes
  return collections
}

function findClosest(scenes, date) {
  var closestDt = toleranceMs + 1, closestScene = null;
  for (var i = 0; i < scenes.length; i++) {
    const dt = Math.abs(new Date(scenes[i].dateFrom) - date);
    if (dt < closestDt) {
      closestDt = dt;
      closestScene = scenes[i];
    }
  }
  return closestScene;
}

function updateOutputMetadata(scenes, inputMetadata, outputMetadata) {
  outputMetadata.userData = metadata;
}

function evaluatePixel(samples, scenes) {
  const observed = index(samples[0].B08, samples[0].B04);
  var histMax = Number.NEGATIVE_INFINITY, histMin = Number.POSITIVE_INFINITY;
  for (var i = 1; i < samples.length; i++) {
    const ndvi = index(samples[i].B08, samples[i].B04);
    histMax = Math.max(histMax, ndvi);
    histMin = Math.min(histMin, ndvi);
  }
  return [(observed - histMin) / (histMax - histMin)];
}