Vegetation Condition Index

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

const NODATA = -32768;

// tolerance in either direction, so i.e. +- 5 days
const toleranceDays = 5;
const msInDay = 24 * 60 * 60 * 1000;
const msInYear = 365.25 * msInDay;
const msInHalfYear = msInYear / 2
const toleranceMs = toleranceDays * msInDay;

var metadata = undefined;

function relDiff(a, b) {
  const diff = Math.abs(a - b);
  return diff > msInHalfYear ? msInYear - diff : diff;
}

function datetimeToYearEpoch(date) {
  return date - new Date(Date.UTC(date.getUTCFullYear(), 0, 1))
}

function sortDatesDescending(d1, d2) {
  const date1 = new Date(d1.dateFrom);
  const date2 = new Date(d2.dateFrom);
  return date2 - date1
}

function preProcessScenes(collections) {
  // sort
  let scenes = collections.scenes.orbits;
  scenes = scenes.sort(sortDatesDescending);
  let newScenes = [];
  // convert first scene to day of year
  const observed = new Date(scenes[0].dateFrom);
  const obsMs = datetimeToYearEpoch(observed)
  for (let i = 0; i < scenes.length; i++) {
    let currentDate = new Date(scenes[i].dateFrom);
    let sceneMs = datetimeToYearEpoch(currentDate)
    let dt = relDiff(obsMs, sceneMs)
    if (dt <= toleranceMs) {
      newScenes.push(scenes[i]);
    }
  }

  metadata = {
    observed: observed.toISOString(),
    historical: newScenes.slice(1).map(scene => scene.dateFrom)
  }

  collections.scenes.orbits = newScenes;
  return collections;
}


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

function calcNDVI(sample) {
  return index(sample.B08, sample.B04);
}

function calcMaxMin(samples) {
  let ndvi = calcNDVI(samples[0])
  let max = ndvi;
  let min = ndvi;
  for (let i = 1; i < samples.length; ++i) {
    ndvi = calcNDVI(samples[i])
    if (ndvi > max) {
      max = ndvi;
    } else if (ndvi < min) {
      min = ndvi;
    }
  }
  return [max, min]
}

function isClear(sample) {
  return sample.CLM == 0 && sample.dataMask == 1;
}

function evaluatePixel(samples) {
  // if the first value isn't clear, stop
  if (!isClear(samples[0])) {
    return [NODATA]
  }
  const clearTs = samples.filter(isClear)
  const observed = index(clearTs[0].B08, clearTs[0].B04);
  let max = NODATA, min = NODATA, vci = NODATA;
  if (clearTs.length > 0) {
    [max, min] = calcMaxMin(clearTs);
    vci = (observed - min) / (max - min)
  }

  return [vci];
}