function totalWaterVolume(arr) {

    // first, we find the 'maxHeight’ which is the highest peak in the water collector
    const maxHeight = Math.max(...arr)

    let totalVolume = 0

    // this loop starts at the maxHeight then decrements the height
    for (let horizontalLevel = maxHeight; horizontalLevel > 0; horizontalLevel--) {

        // 'peaksAtHorizontalLevel' is set to the return value of our first helper function 'peakIndicesMaker' which will be an array of indices of rain collector walls that exist at that level
        var peaksAtHeightLevel = peakIndicesMaker(arr, horizontalLevel)

        // 'vol' is then incremented by the volume that exists at that level, returned from our second helper function 'volAtLevel'
        totalVolume += volAtLevel(peaksAtHeightLevel)
    }

    // total volume is returned
    return totalVolume
}

/* This function takes the original array, as well as the height level we are looking at, and returns an array of indices where reservoir walls exist */
function peakIndicesMaker(arr, level) {

    const peakIndices = []

    // loop over the entire array
    for (let i = 0; i < arr.length; i++) {

        // if the wall height present at each index is at least the height of the given level then that index is pushed to the output array
        if(arr[i] >= level) {
            peakIndices.push(i)
        }
    }

    // array of indices is returned
    return peakIndices
}

/* The distance between the two walls at the same height will also be the volume of water held between them. */
function volAtLevel(peakIndices) {

    let levelVol = 0

    // if there is only one wall at the height currently being calculated, there cannot physically be any water at that level. In this case, we return 0 volume.
    if (peakIndices.length === 1) {
        return 0
    } else {

        // levelVol is incremented for each 'pair' of walls at that level. It is important to note that we are comparing each wall to its adjacent neighbor located at the next index in the array. Therefore the last element in the array could not possibly hold water to its right.  This is because no wall exists at that level beyond the last wall
        for (let i = 0; i < peakIndices.length-1; i++) {

            // Measure the right side of one wall (this is why we look at peakIndices[i] + 1 and not just peakIndices[i]) to the left side of its neighbor
            levelVol += (peakIndices[i + 1] - (peakIndices[i] + 1))

        }
    }

    // the level volume is then returned after all pairs have been summed.
    return levelVol
}