// Original code from https://gist.github.com/josephbk117/a43f5335cea9b3e12a44f196dc81f30f
Shader "Custom/InteriorReflectionShader"
{
    Properties
    {
        _Glossiness ("Smoothness", Range(0,1)) = 0.8
        _ReflectionProbePosition ("Reflection Probe Position", Vector) = (0,0,0,0)
        _ReflectionProbeBoxMin ("Box Min", Vector) = (0,0,0,0)
        _ReflectionProbeBoxMax ("Box Max", Vector) = (0,0,0,0)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 200

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
            #include "UnityStandardUtils.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
            };

            struct v2f
            {
                float4 pos : SV_POSITION;
                float3 worldNormal : TEXCOORD0;
                float3 worldPos : TEXCOORD1;
            };

            float4 _ReflectionProbePosition;
            float4 _ReflectionProbeBoxMin;
            float4 _ReflectionProbeBoxMax;
            half _Glossiness;

            v2f vert (appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.worldNormal = UnityObjectToWorldNormal(v.normal);
                o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
                return o;
            }

            float3 BoxProjectReflectionDir(float3 worldPos, float3 worldRefl)
            {
                // Calculate relative position within the box
                float3 boxMin = _ReflectionProbeBoxMin.xyz;
                float3 boxMax = _ReflectionProbeBoxMax.xyz;
                float3 boxCenter = 0.5 * (boxMin + boxMax);
                float3 boxSize = boxMax - boxMin;

                float3 localPos = worldPos - boxCenter;
                float3 absLocalRefl = abs(worldRefl);

                // Scale the reflection vector based on which side of the box it hits
                float3 scale = boxSize / absLocalRefl;
                float scaleMin = min(min(scale.x, scale.y), scale.z);
                return localPos + worldRefl * scaleMin;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // Get reflection vector
                float3 worldViewDir = normalize(_WorldSpaceCameraPos - i.worldPos);
                float3 worldRefl = reflect(worldViewDir, i.worldNormal);

                // Apply box projection to the reflection vector
                float3 projectedReflDir = BoxProjectReflectionDir(i.worldPos, worldRefl);
                fixed4 reflectionColor = UNITY_SAMPLE_TEXCUBE(unity_SpecCube0, projectedReflDir);

                return reflectionColor;
            }
            ENDCG
        }
    }
    FallBack "Diffuse"
}