#pragma kernel ComputeNeighbors

static const int NumThreads = 8;

struct Triangle {
    uint VertexIndexA;
    uint VertexIndexB;
    uint VertexIndexC;
    uint EdgeCount;
    uint Edges[3];
};

struct Edge {
    uint VertexIndexA;
    uint VertexIndexB;
};

RWStructuredBuffer<Triangle> PreTriangles;
RWStructuredBuffer<Edge> PreEdges;

uint PreTrianglesCount;
uint PreEdgesCount;

bool GetEdgeIndex(uint vertexIndexA, uint vertexIndexB, inout uint edgeIndex) {
    for (uint i = 0; i < PreEdgesCount; i++) {
        Edge edge = PreEdges[i];
        if (edge.VertexIndexA == vertexIndexA || edge.VertexIndexB == vertexIndexA || edge.VertexIndexB == vertexIndexA || edge.VertexIndexB == vertexIndexB) {
            edgeIndex = i;
            return true;
        }
    }
    edgeIndex = 0;
    return false;
}

[numthreads(NumThreads, NumThreads, 1)]
void ComputeNeighbors(uint3 id:SV_DispatchThreadID) {
    uint triangleIndexA = id.x;
    uint triangleIndexB = id.y;
    if (triangleIndexA == triangleIndexB || triangleIndexA >= PreTrianglesCount || triangleIndexB >= PreTrianglesCount)
    {
        return;
    }
    Triangle triangleA = PreTriangles[triangleIndexA];
    Triangle triangleB = PreTriangles[triangleIndexB];
    uint sharedVertexIndexA = -1;
    uint sharedVertexIndexB = -1;
    if (triangleA.VertexIndexA == triangleB.VertexIndexA || triangleA.VertexIndexA == triangleB.VertexIndexB || triangleA.VertexIndexA == triangleB.VertexIndexC)
    {
        if (sharedVertexIndexA == -1)
        {
            sharedVertexIndexA = triangleA.VertexIndexA;
        }
        else if (sharedVertexIndexB == -1)
        {
            sharedVertexIndexB = triangleA.VertexIndexA;
        }
    }
    if (triangleA.VertexIndexB == triangleB.VertexIndexA || triangleA.VertexIndexB == triangleB.VertexIndexB || triangleA.VertexIndexB == triangleB.VertexIndexC)
    {
        if (sharedVertexIndexA == -1)
        {
            sharedVertexIndexA = triangleA.VertexIndexB;
        }
        else if (sharedVertexIndexB == -1)
        {
            sharedVertexIndexB = triangleA.VertexIndexB;
        }
    }
    if (triangleA.VertexIndexC == triangleB.VertexIndexA || triangleA.VertexIndexC == triangleB.VertexIndexB || triangleA.VertexIndexC == triangleB.VertexIndexC)
    {
        if (sharedVertexIndexA == -1)
        {
            sharedVertexIndexA = triangleA.VertexIndexC;
        }
        else if (sharedVertexIndexB == -1)
        {
            sharedVertexIndexB = triangleA.VertexIndexC;
        }
    }
    if (sharedVertexIndexA > -1 && sharedVertexIndexB > -1)
    {
        uint edgeIndex;
        if (!GetEdgeIndex(sharedVertexIndexA, sharedVertexIndexB, edgeIndex)) {
            Edge edge;
            edge.VertexIndexA = sharedVertexIndexA;
            edge.VertexIndexB = sharedVertexIndexB;
            PreEdges[PreEdgesCount] = edge;
            edgeIndex = PreEdgesCount++;
        }
        triangleA.Edges[triangleA.EdgeCount++] = edgeIndex;
    }
}