import { Vector3 } from 'three';

export type DepthFace<T>={vertices:Vector3[];edges:boolean[];data:T};
/** BSP painter for an orthographic camera at negative depth. Split crossing faces,
 * preserving only original edges so partition boundaries never become CAD outlines. */
export function visibleOrder<T>(input:DepthFace<T>[]):DepthFace<T>[] {
 const visit=(faces:DepthFace<T>[]):DepthFace<T>[]=>{
  if(!faces.length)return [];
  const seed=faces[0],a=seed.vertices[0];let normal=new Vector3();
  for(let i=1;i<seed.vertices.length-1&&normal.lengthSq()<1e-16;i++)normal=seed.vertices[i].clone().sub(a).cross(seed.vertices[i+1].clone().sub(a));
  if(normal.lengthSq()<1e-16)return visit(faces.slice(1));
  normal.normalize();const offset=normal.dot(a),front:DepthFace<T>[]=[],back:DepthFace<T>[]=[],same:DepthFace<T>[]=[seed];
  for(const face of faces.slice(1)){
   const ds=face.vertices.map(p=>normal.dot(p)-offset),positive=ds.some(d=>d>1e-5),negative=ds.some(d=>d< -1e-5);
   if(!positive&&!negative){same.push(face);continue;}
   if(!negative){front.push(face);continue;}if(!positive){back.push(face);continue;}
   for(const [sign,list] of [[1,front],[-1,back]] as const){
    const vertices:Vector3[]=[],incoming:boolean[]=[];
    for(let i=0;i<face.vertices.length;i++){
     const j=(i+1)%face.vertices.length,p=face.vertices[i],q=face.vertices[j],inside=ds[i]*sign>=-1e-5,next=ds[j]*sign>=-1e-5;
     if(inside&&next){vertices.push(q);incoming.push(face.edges[i]);}
     else if(inside&&!next){vertices.push(p.clone().lerp(q,ds[i]/(ds[i]-ds[j])));incoming.push(face.edges[i]);}
     else if(!inside&&next){vertices.push(p.clone().lerp(q,ds[i]/(ds[i]-ds[j])),q);incoming.push(false,face.edges[i]);}
    }
    if(vertices.length>=3)list.push({vertices,edges:incoming.map((_,i)=>incoming[(i+1)%incoming.length]),data:face.data});
   }
  }
  return normal.z<0?[...visit(back),...same,...visit(front)]:[...visit(front),...same,...visit(back)];
 };
 return visit(input.flatMap(face=>Array.from({length:face.vertices.length-2},(_,j)=>{const i=j+1;return {vertices:[face.vertices[0],face.vertices[i],face.vertices[i+1]],edges:[i===1&&face.edges[0],face.edges[i],i===face.vertices.length-2&&face.edges[i+1]],data:face.data};})));
}