const ITEM_CLASS_FIELD = 68;
const ITEM_SUBCLASS_MASK_FIELD = 69;
const ITEM_INVENTORY_MASK_FIELD = 70;

/** Read equipped-item metadata from a client Spell.dbc using its fixed fields. */
export function spellDbcEquipmentMetadata(buffer) {
  if (!Buffer.isBuffer(buffer) || buffer.toString("ascii", 0, 4) !== "WDBC") {
    throw new Error("Spell.dbc is not WDBC data");
  }
  const recordCount = buffer.readUInt32LE(4);
  const fieldCount = buffer.readUInt32LE(8);
  const recordSize = buffer.readUInt32LE(12);
  if (fieldCount <= ITEM_INVENTORY_MASK_FIELD || recordSize < fieldCount * 4
    || buffer.length < 20 + recordCount * recordSize) {
    throw new Error("Spell.dbc has an unsupported equipped-item schema");
  }

  const rows = new Map();
  for (let index = 0; index < recordCount; index += 1) {
    const base = 20 + index * recordSize;
    const id = buffer.readUInt32LE(base);
    if (rows.has(id)) throw new Error(`Spell.dbc repeats spell ID ${id}`);
    rows.set(id, {
      equippedItemClass: buffer.readInt32LE(base + ITEM_CLASS_FIELD * 4),
      equippedItemSubclassMask: buffer.readInt32LE(base + ITEM_SUBCLASS_MASK_FIELD * 4),
      equippedItemInventoryMask: buffer.readUInt32LE(base + ITEM_INVENTORY_MASK_FIELD * 4),
    });
  }
  return rows;
}