import type {Asset} from './types.ts';

/** Repository file listings can be truncated. Only a failed image request,
 * not absence from that listing, means a thumbnail is unavailable.
 * Also upgrades catalogues generated by earlier app versions.
 */
export function previewSources(asset:Pick<Asset,'image'|'imageFallback'|'preview'>):string[]{
  const prepared=asset.preview?[asset.preview]:[];
  const url=new URL(asset.image);
  if(!url.pathname.endsWith('.png'))return [...new Set([...prepared,asset.image,asset.imageFallback])];
  const base=url.pathname.replace(/(?:_(?:transparent|light))?\.png$/,'');
  const variants=['_transparent.png','_light.png','.png'].map(suffix=>{
    const candidate=new URL(url);candidate.pathname=base+suffix;return candidate.href;
  });
  return [...new Set([...prepared,...variants,asset.imageFallback,asset.image])];
}

/** Read native alpha bounds without altering colors or transparency.
 * Opaque previews retain their background; color-keying them can destroy
 * dark details and leave baked shadows behind. Antialiased edges stay intact.
 */
export function alphaBounds(data:Uint8ClampedArray,width:number,height:number){
  let left=width,right=-1,top=height,bottom=-1;
  for(let y=0;y<height;y++)for(let x=0;x<width;x++)if(data[(y*width+x)*4+3]>0){
    left=Math.min(left,x);right=Math.max(right,x);top=Math.min(top,y);bottom=Math.max(bottom,y);
  }
  return right<left||bottom<top?null:{left,top,width:right-left+1,height:bottom-top+1};
}
export function normalizePreview(image:HTMLImageElement):string|null{
  try{
    const canvas=document.createElement('canvas');canvas.width=image.naturalWidth;canvas.height=image.naturalHeight;
    if(canvas.width>2048||canvas.height>2048)return null;
    const context=canvas.getContext('2d',{willReadFrequently:true});if(!context)return null;context.drawImage(image,0,0);
    const pixels=context.getImageData(0,0,canvas.width,canvas.height);
    const bounds=alphaBounds(pixels.data,canvas.width,canvas.height);if(!bounds)return null;
    const output=document.createElement('canvas');output.width=output.height=320;const out=output.getContext('2d');if(!out)return null;
    out.imageSmoothingEnabled=true;out.imageSmoothingQuality='high';
    const {left,top,width:w,height:h}=bounds,scale=292/Math.max(w,h);
    out.drawImage(canvas,left,top,w,h,(320-w*scale)/2,(320-h*scale)/2,w*scale,h*scale);
    return output.toDataURL('image/png');
  }catch{return null;}
}