forest_disturbance

//VERSION=3 (auto-converted from 1)
// Minimum difference between the two mean NDVIs
var thresold = 0.25;
// Thresold to dismiss clouds to calculate the mean
var blueThresold = 0.18;
// Minimum mean NDVI to consider for month(s) of last year
var minimunNDVI = 0.7;
// to normalize colormap
var stretchMin = 0;
var stretchMax = 0.8;

function setup() {
  return {
    input: [{
      bands: [
          "B02",
          "B03",
          "B04",
          "B05",
          "B08",
          "B12"
      ]
    }],
    output: { bands: 3 },
    mosaicking: "ORBIT"
  }
}


function stretch(val, min, max) {
    return (val - min) / (max - min);
}


function NDVI(sample) {
    let denom = sample.B08 + sample.B04;
    return ((denom != 0) ? (sample.B08 - sample.B04) / denom : 0.0);
}

function NDWI(sample) {
    let denom = sample.B03 + sample.B08;
    return ((denom != 0) ? (sample.B03 - sample.B08) / denom : -1);
}

function mean(array) {
    if (array.length == 0) {
        return 1
    } else {
        var sum = 0;
        for (var i = 0; i < array.length; i++) {
            sum += parseFloat(array[i]); //don't forget to add the base
        }

        var avg = sum / array.length;
        return avg
    }
}

// Manage overlapping years.
// If january is selected and user need 2 months to take
// script needs to take december from last year as second month.
function getNeededDates(sceneMonth, sceneYear, monthsToTake) {
    let monthToGet = [];
    let yearToGet = [];

    for (i = 0; i < monthsToTake; i++) {
        if (sceneMonth - i <= 0) {
            month = 12 - (sceneMonth - i);
            year = sceneYear - 1;
        } else {
            month = sceneMonth - i;
            year = sceneYear;
        }
        monthToGet.push(month);
        yearToGet.push(year);
    }

    return [monthToGet, yearToGet];
}

function evaluatePixel(samples, scenes) {
    let initMonth = scenes[0].date.getMonth();
    let initYear = scenes[0].date.getFullYear();

    let monthsAndYears = getNeededDates(initMonth, initYear, 3);

    let currentYearNDVI = 0;
    let currentYearCount = 0;

    let previousYearNDVI = 0;
    let previousYearCount = 0;
    let lastYearMonth0 = [];
    let lastYearMonth1 = [];
    let lastYearMonth2 = [];

    for (i = 0; i < samples.length; i++) {
        if (!(samples[i].B04 == 0) & !(samples[i].B03 == 0)) {
            sceneMonth = scenes[i].date.getMonth();
            sceneYear = scenes[i].date.getFullYear();
            if (monthsAndYears[0].includes(sceneMonth)) {
                if (samples[i].B02 < blueThresold) {
                    ndvi = NDVI(samples[i]);
                    if (monthsAndYears[1].includes(sceneYear))
                    // if current year (last 12 months max)
                    {
                        currentYearNDVI = currentYearNDVI + ndvi;
                        currentYearCount++;

                    }
                    // if year before
                    else if (monthsAndYears[1].includes(sceneYear + 1)) {
                        previousYearNDVI = previousYearNDVI + ndvi;

                        if (monthsAndYears[0][0] == sceneMonth) {
                            lastYearMonth0.push(ndvi);
                        } else if (monthsAndYears[0][1] == sceneMonth) {
                            lastYearMonth1.push(ndvi);
                        } else if (monthsAndYears[0][2] == sceneMonth) {
                            lastYearMonth2.push(ndvi);
                        }
                        previousYearCount++;
                    }
                }
            }
        }
    }

    // compute the mean
    let avgCurrentYearNDVI = currentYearNDVI / currentYearCount;
    let avgPreviousYearNDVI = previousYearNDVI / previousYearCount;

    // if ndvi decreases from defined thresold in the same months from previous year
    // highlights in red the pixel
    // check also if is not water
    let difference = avgPreviousYearNDVI - avgCurrentYearNDVI;

    if ((NDWI(samples[0]) < 0.5) & (difference >= thresold) & (avgPreviousYearNDVI > minimunNDVI) & (mean(lastYearMonth0) > minimunNDVI) & (mean(lastYearMonth1) > minimunNDVI) & (mean(lastYearMonth2) > minimunNDVI)) {
        // the more the difference is high, the more it is red
        colorMap = [stretch((2.8 * (2 / 3) * 10 * difference * samples[0].B04 + 0.1 * samples[0].B05), stretchMin, stretchMax), stretch((2.8 * samples[0].B03 + 0.15 * samples[0].B08), stretchMin, stretchMax), stretch((2.8 * samples[0].B02), stretchMin, stretchMax)];
    }
    // else show current image
    else {
        colorMap = [stretch((2.8 * samples[0].B04 + 0.1 * samples[0].B05), stretchMin, stretchMax), stretch((2.8 * samples[0].B03 + 0.15 * samples[0].B08), stretchMin, stretchMax), stretch((2.8 * samples[0].B02), stretchMin, stretchMax)];
    }
    return colorMap;
}

function preProcessScenes (collections) {
    collections.scenes.orbits = collections.scenes.orbits.filter(function (orbit) {
        var orbitDateFrom = new Date(orbit.dateFrom)
        return orbitDateFrom.getTime() >= (collections.to.getTime()-(14 * 31 * 24 * 3600 * 1000)); // 14 = 11 months + 3 months
    })
    return collections
}