/**
 * Minimal Gmsh MSH 2.2 ASCII writer for a triangle surface mesh — plain
 * string building, no dependencies. Vertices not referenced by any facet
 * (interior points of the cloud) are dropped, and indices are remapped to
 * the dense 1-based node ids the format expects.
 */
export function trianglesToMsh(points: number[][], facets: number[][]): string {
  const used = new Map<number, number>() // original index -> 1-based node id
  for (const f of facets) {
    for (const idx of f) {
      if (!used.has(idx)) used.set(idx, used.size + 1)
    }
  }

  const lines = ['$MeshFormat', '2.2 0 8', '$EndMeshFormat', '$Nodes', String(used.size)]
  for (const [idx, id] of used) {
    const [x, y, z] = points[idx]
    lines.push(`${id} ${x} ${y} ${z}`)
  }
  lines.push('$EndNodes', '$Elements', String(facets.length))
  facets.forEach((f, i) => {
    // element type 2 = 3-node triangle, two tags
    lines.push(`${i + 1} 2 2 1 1 ${used.get(f[0])} ${used.get(f[1])} ${used.get(f[2])}`)
  })
  lines.push('$EndElements')
  return lines.join('\n') + '\n'
}

export function downloadText(filename: string, text: string) {
  const blob = new Blob([text], { type: 'application/octet-stream' })
  const a = document.createElement('a')
  a.href = URL.createObjectURL(blob)
  a.download = filename
  a.click()
  URL.revokeObjectURL(a.href)
}