Rain Water Volume Task

task

using System;
using Xunit;

namespace RainTask
{
    public class Task
    {
        [Fact]
        public void TestNoLand()
        {
            Assert.Equal(0, Volume(3));
        }

        [Fact]
        public void TestU()
        {
            Assert.Equal(1, Volume(1, 0, 1));
        }

        [Fact]
        public void TestW()
        {
            Assert.Equal(5, Volume(2, 0, 3, 0, 4));
        }

        [Fact]
        public void TestIgors1()
        {
            Assert.Equal(10, Volume(2, 5, 1, 2, 3, 4, 7, 7, 6));
        }

        [Fact]
        public void TestIgors2()
        {
            Assert.Equal(8, Volume(2, 5, 1, 2, 5, 4, 7, 7, 6));
        }

        [Fact]
        public void TestIgors3()
        {
            Assert.Equal(10, Volume(2, 5, 1, 2, 7, 4, 7, 7, 6));
        }

        [Fact]
        public void TestIgors4()
        {
            Assert.Equal(11, Volume(2, 5, 1, 2, 7, 4, 7, 7, 6, 7));
        }

        private int Volume(params int[] land)
        {
            var volume = 0;
            var level = land[0];
            for (var i = 1; i < land.Length; i++)
            {
                if (level <= land[i]) // climb, not water volume
                {
                    level = land[i];
                }
                else // find next top and count water volume between tops 
                {
                    var top = i;
                    for (var j = i+1; j < land.Length; j++)
                    {
                        if (land[j] >= level) // next top found with the same or higher level
                        {
                            top = j;
                            break;
                        }

                        // or we look for the highest top below the current level
                        if (land[j] > land[top])
                            top = j;
                    }

                    level = Math.Min(level, land[top]);

                    for (; i < top; i++)
                        volume += (level - land[i]);

                    if (level < land[top])
                        level = land[top];
                }
            }

            return volume;
        }
    }
}