using MrGameEng.AI;
using Xunit;

namespace MrGameEng.AI.Tests;

public class ResponseCurveTests
{
    [Fact]
    public void Identity_ReturnsInputUnchanged()
    {
        var curve = ResponseCurve.Identity;

        Assert.Equal(0f, curve.Evaluate(0f));
        Assert.Equal(0.5f, curve.Evaluate(0.5f), 5);
        Assert.Equal(1f, curve.Evaluate(1f));
    }

    [Fact]
    public void Default_HasZeroSlope_SoIdentityMustBeUsedExplicitly()
    {
        // Guards the gotcha that default(ResponseCurve) is NOT the identity curve.
        var zero = default(ResponseCurve);

        Assert.Equal(0f, zero.Evaluate(0.5f));
    }

    [Fact]
    public void Evaluate_ClampsInputAndOutputToUnitRange()
    {
        var curve = ResponseCurve.Linear();

        Assert.Equal(0f, curve.Evaluate(-2f));
        Assert.Equal(1f, curve.Evaluate(5f));
    }

    [Fact]
    public void Linear_NegativeSlope_InvertsTheInput()
    {
        var curve = ResponseCurve.Linear(slope: -1f, yShift: 1f);

        Assert.Equal(1f, curve.Evaluate(0f), 5);
        Assert.Equal(0.75f, curve.Evaluate(0.25f), 5);
        Assert.Equal(0f, curve.Evaluate(1f), 5);
    }

    [Fact]
    public void Polynomial_Quadratic_EasesInFromZero()
    {
        var curve = ResponseCurve.Polynomial(exponent: 2f);

        Assert.Equal(0.25f, curve.Evaluate(0.5f), 5);
        Assert.True(curve.Evaluate(0.25f) < 0.25f); // below the linear line: slow start
    }

    [Fact]
    public void Logistic_IsMonotonicAndCentredOnMidpoint()
    {
        var curve = ResponseCurve.Logistic(steepness: 12f, midpoint: 0.5f);

        Assert.Equal(0.5f, curve.Evaluate(0.5f), 2);
        Assert.True(curve.Evaluate(0.2f) < 0.5f);
        Assert.True(curve.Evaluate(0.8f) > 0.5f);
        Assert.True(curve.Evaluate(0.6f) > curve.Evaluate(0.4f));
    }

    [Fact]
    public void SmoothStep_IsFlatAtTheEnds()
    {
        var curve = ResponseCurve.SmoothStep();

        Assert.Equal(0f, curve.Evaluate(0f), 5);
        Assert.Equal(1f, curve.Evaluate(1f), 5);
        Assert.Equal(0.5f, curve.Evaluate(0.5f), 5);
    }
}