/**
 * Binary decoding utilities for territory snapshot data from Rust backend.
 *
 * The TerritorySnapshot uses a sparse binary format to minimize data transfer:
 * - Only claimed tiles (owner_id != 0) are transmitted
 * - All other tiles default to unclaimed (owner_id = 0)
 * - Binary format: [count:4][tile_changes...]
 *   where each change is [index:4][owner:2] (6 bytes per claimed tile)
 */

export interface TileOwnership {
  index: number;
  owner_id: number;
}

/**
 * Decode sparse binary territory snapshot from Rust backend.
 *
 * @param data Binary data array (serialized from Vec<u8> in Rust)
 * @returns Array of claimed tiles with indices and owner IDs, or null if invalid
 */
export function decodeTerritorySnapshot(data: number[] | Uint8Array): TileOwnership[] | null {
  // Convert to Uint8Array if needed for faster access
  const bytes = data instanceof Uint8Array ? data : new Uint8Array(data);

  if (bytes.length < 4) {
    console.error("Invalid territory snapshot: not enough data for count");
    return null;
  }

  // Use DataView for faster little-endian reads
  const view = new DataView(bytes.buffer, bytes.byteOffset, bytes.byteLength);

  // Read count (4 bytes, little-endian u32)
  const count = view.getUint32(0, true);

  const expectedSize = 4 + count * 6;
  if (bytes.length !== expectedSize) {
    console.error(
      `Invalid territory snapshot: expected ${expectedSize} bytes, got ${bytes.length}`
    );
    return null;
  }

  const tiles: TileOwnership[] = new Array(count);
  for (let i = 0; i < count; i++) {
    const offset = 4 + i * 6;

    // Read index (4 bytes, little-endian u32)
    const index = view.getUint32(offset, true);

    // Read owner_id (2 bytes, little-endian u16)
    const owner_id = view.getUint16(offset + 4, true);

    tiles[i] = { index, owner_id };
  }

  return tiles;
}