using System;
using NUnit.Framework;

namespace Manipulation
{
    public class TriangleTask
    {
        public static double GetABAngle(double a, double b, double c)
        {
            if (double.IsNaN(a) || double.IsNaN(b) || double.IsNaN(c)
                || a < 0 || b < 0 || c < 0
                || (c == 0 && (a == 0 || b == 0)))
            {
                return double.NaN;
            }
            return Math.Acos((b * b + a * a - c * c) / (2 * a * b));
        }
    }

    [TestFixture]
    public class TriangleTask_Tests
    {
        [TestCase(3, 4, 5, Math.PI / 2)]
        [TestCase(1, 1, 1, Math.PI / 3)]
        [TestCase(1, 1, 3, double.NaN)]
        [TestCase(-1, 1, 1, double.NaN)]
        [TestCase(1, 0, 1, double.NaN)]
        [TestCase(1, 1, 0, 0)]
        [TestCase(1, 0, 0, double.NaN)]
        [TestCase(0, 1, 0, double.NaN)]
        [TestCase(0, 0, 1, double.NaN)]
        [TestCase(0, 0, 0, double.NaN)]
        public void TestGetABAngle(double a, double b, double c, double expectedAngle)
        {
            Assert.AreEqual(TriangleTask.GetABAngle(a, b, c), expectedAngle, 1e-5);
        }
    }
}