using UnityEngine;
using UnityEngine.Rendering;

/// <summary>
/// Turns a flat textured card (Quad MeshRenderer) into a zero-thickness
/// "paper" cutout that casts and receives shadows. The colour pass is alpha
/// blended so anti-aliased sprite edges stay soft; the shadow caster pass uses
/// alpha clip so the shadow follows the silhouette. Works with URP/Lit and
/// the built-in Standard shader. Pure renderer setup; no gameplay logic.
/// </summary>
[ExecuteAlways]
[DisallowMultipleComponent]
public class PaperCardShadows : MonoBehaviour
{
    [Tooltip("Alpha below this is clipped in the shadow pass and discarded in the colour pass.")]
    [Range(0f, 1f)] public float alphaClipThreshold = 0.15f;

    [Tooltip("Reapply material settings on Awake/OnValidate. Turn off to manage the material by hand.")]
    public bool enforceMaterialSettings = true;

    Renderer rend;

    void Awake() { Apply(); }
    void OnValidate() { Apply(); }
    void Reset() { Apply(); }

    public void Apply()
    {
        if (rend == null) rend = GetComponent<Renderer>();
        if (rend == null) return;

        rend.shadowCastingMode = ShadowCastingMode.TwoSided;
        rend.receiveShadows = true;

        if (!enforceMaterialSettings) return;
        var mat = Application.isPlaying ? rend.material : rend.sharedMaterial;
        if (mat == null) return;
        ConfigureSoftCutout(mat, alphaClipThreshold);
    }

    /// <summary>Alpha-blended colour with alpha-clipped shadows: soft edges, crisp silhouette shadow.</summary>
    public static void ConfigureSoftCutout(Material mat, float cutoff)
    {
        bool urp = mat.HasProperty("_BaseMap");
        if (urp)
        {
            // Opaque + alpha clip + alpha-to-coverage: MSAA turns the sprite's soft alpha
            // into anti-aliased coverage with correct depth, so silhouettes never show
            // the blend/depth hairline that transparent rendering leaves at the edge.
            mat.SetFloat("_Surface", 0f);               // Opaque
            mat.SetFloat("_Blend", 0f);
            mat.SetFloat("_SrcBlend", (float)BlendMode.One);
            mat.SetFloat("_DstBlend", (float)BlendMode.Zero);
            mat.SetFloat("_SrcBlendAlpha", (float)BlendMode.One);
            mat.SetFloat("_DstBlendAlpha", (float)BlendMode.Zero);
            mat.SetFloat("_ZWrite", 1f);
            mat.SetFloat("_AlphaClip", 1f);
            mat.SetFloat("_Cutoff", cutoff);
            mat.SetFloat("_AlphaToMask", 1f);
            mat.SetFloat("_Cull", (float)CullMode.Off);    // draw both faces so a clipped rim can never look hollow
            mat.SetFloat("_ReceiveShadows", 1f);
            mat.DisableKeyword("_SURFACE_TYPE_TRANSPARENT");
            mat.EnableKeyword("_ALPHATEST_ON");
            mat.DisableKeyword("_ALPHAPREMULTIPLY_ON");
            mat.DisableKeyword("_ALPHAMODULATE_ON");
            mat.SetOverrideTag("RenderType", "TransparentCutout");
            mat.renderQueue = (int)RenderQueue.AlphaTest;
        }
        else
        {
            // Built-in Standard: Fade mode
            mat.SetFloat("_Mode", 2f);
            mat.SetFloat("_Cutoff", cutoff);
            mat.SetInt("_SrcBlend", (int)BlendMode.SrcAlpha);
            mat.SetInt("_DstBlend", (int)BlendMode.OneMinusSrcAlpha);
            mat.SetInt("_ZWrite", 1);
            mat.EnableKeyword("_ALPHABLEND_ON");
            mat.EnableKeyword("_ALPHATEST_ON");
            mat.SetOverrideTag("RenderType", "Transparent");
            mat.renderQueue = (int)RenderQueue.Transparent;
        }
    }
}