// Create sample data. Data must be sorted large to small.
double[] sortedValues = Enumerable.Range(0, 40)
    .Select(x => (double)Random.Shared.Next(10, 100))
    .OrderByDescending(x => x)
    .ToArray();

// Create an array of labels in the same order as the sorted data.
string[] labels = sortedValues.Select(x => x.ToString()).ToArray();

// Calculate the size and position of all rectangles in the tree map
int width = 600;
int height = 400;
RectangleF[] rectangles = TreeMap.GetRectangles(sortedValues, width, height);

// Create an image to draw on (with 1px extra to make room for the outline)
using Bitmap bmp = new(width + 1, height + 1);
using Graphics gfx = Graphics.FromImage(bmp);
using Font fnt = new("Consolas", 8);
using SolidBrush brush = new(Color.Black);
gfx.Clear(Color.White);

// Draw and label each rectangle
for (int i = 0; i < rectangles.Length; i++)
{
    brush.Color = Color.FromArgb(
        red: Random.Shared.Next(150, 250),
        green: Random.Shared.Next(150, 250),
        blue: Random.Shared.Next(150, 250));

    gfx.FillRectangle(brush, rectangles[i]);
    gfx.DrawRectangle(Pens.Black, rectangles[i]);
    gfx.DrawString(labels[i], fnt, Brushes.Black, rectangles[i].X, rectangles[i].Y);
}

// Save the output
bmp.Save("treemap.bmp");

public static class TreeMap
{
    public static RectangleF[] GetRectangles(double[] values, int width, int height)
    {
        for (int i = 1; i < values.Length; i++)
            if (values[i] > values[i - 1])
                throw new ArgumentException("values must be ordered large to small");

        var slice = GetSlice(values, 1, 0.35);
        var rectangles = GetRectangles(slice, width, height);
        return rectangles.Select(x => x.ToRectF()).ToArray();
    }

    private class Slice
    {
        public double Size { get; }
        public IEnumerable<double> Values { get; }
        public Slice[] Children { get; }

        public Slice(double size, IEnumerable<double> values, Slice sub1, Slice sub2)
        {
            Size = size;
            Values = values;
            Children = new Slice[] { sub1, sub2 };
        }

        public Slice(double size, double finalValue)
        {
            Size = size;
            Values = new double[] { finalValue };
            Children = Array.Empty<Slice>();
        }
    }

    private class SliceResult
    {
        public double ElementsSize { get; }
        public IEnumerable<double> Elements { get; }
        public IEnumerable<double> RemainingElements { get; }

        public SliceResult(double elementsSize, IEnumerable<double> elements, IEnumerable<double> remainingElements)
        {
            ElementsSize = elementsSize;
            Elements = elements;
            RemainingElements = remainingElements;
        }
    }

    private class SliceRectangle
    {
        public Slice Slice { get; set; }
        public float X { get; set; }
        public float Y { get; set; }
        public float Width { get; set; }
        public float Height { get; set; }
        public SliceRectangle(Slice slice) => Slice = slice;
        public RectangleF ToRectF() => new(X, Y, Width, Height);
    }

    private static Slice GetSlice(IEnumerable<double> elements, double totalSize, double sliceWidth)
    {
        if (elements.Count() == 1)
            return new Slice(totalSize, elements.Single());

        SliceResult sr = GetElementsForSlice(elements, sliceWidth);
        Slice child1 = GetSlice(sr.Elements, sr.ElementsSize, sliceWidth);
        Slice child2 = GetSlice(sr.RemainingElements, 1 - sr.ElementsSize, sliceWidth);
        return new Slice(totalSize, elements, child1, child2);
    }

    private static SliceResult GetElementsForSlice(IEnumerable<double> elements, double sliceWidth)
    {
        var elementsInSlice = new List<double>();
        var remainingElements = new List<double>();
        double current = 0;
        double total = elements.Sum();

        foreach (var element in elements)
        {
            if (current > sliceWidth)
                remainingElements.Add(element);
            else
            {
                elementsInSlice.Add(element);
                current += element / total;
            }
        }

        return new SliceResult(current, elementsInSlice, remainingElements);
    }

    private static IEnumerable<SliceRectangle> GetRectangles(Slice slice, int width, int height)
    {
        SliceRectangle area = new(slice) { Width = width, Height = height };

        foreach (var rect in GetRectangles(area))
        {
            if (rect.X + rect.Width > area.Width)
                rect.Width = area.Width - rect.X;

            if (rect.Y + rect.Height > area.Height)
                rect.Height = area.Height - rect.Y;

            yield return rect;
        }
    }

    private static IEnumerable<SliceRectangle> GetRectangles(SliceRectangle sliceRectangle)
    {
        var isHorizontalSplit = sliceRectangle.Width >= sliceRectangle.Height;
        var currentPos = 0;
        foreach (var subSlice in sliceRectangle.Slice.Children)
        {
            var subRect = new SliceRectangle(subSlice);
            int rectSize;

            if (isHorizontalSplit)
            {
                rectSize = (int)Math.Round(sliceRectangle.Width * subSlice.Size);
                subRect.X = sliceRectangle.X + currentPos;
                subRect.Y = sliceRectangle.Y;
                subRect.Width = rectSize;
                subRect.Height = sliceRectangle.Height;
            }
            else
            {
                rectSize = (int)Math.Round(sliceRectangle.Height * subSlice.Size);
                subRect.X = sliceRectangle.X;
                subRect.Y = sliceRectangle.Y + currentPos;
                subRect.Width = sliceRectangle.Width;
                subRect.Height = rectSize;
            }

            currentPos += rectSize;

            if (subSlice.Values.Count() > 1)
            {
                foreach (var sr in GetRectangles(subRect))
                {
                    yield return sr;
                }
            }
            else if (subSlice.Values.Count() == 1)
            {
                yield return subRect;
            }
        }
    }
}