Challenge 3: A DiffUtil.Callback — Solution

class ProductDiffCallback(
    private val oldList: List<Product>,
    private val newList: List<Product>
) : DiffUtil.Callback() {

    override fun getOldListSize() = oldList.size
    override fun getNewListSize() = newList.size

    override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition].name == newList[newItemPosition].name
    }

    override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        return oldList[oldItemPosition] == newList[newItemPosition]
    }
}

// If two lists differ by one product's price changing (same name, e.g.
// "Keyboard" going from 49.99 to 39.99 in a sale):
//   - areItemsTheSame returns TRUE for that product (same name = same
//     identity), so DiffUtil treats it as the SAME item, not a
//     remove-then-add.
//   - areContentsTheSame returns FALSE for that product (data class
//     equals() compares both name AND price, and price differs), so
//     DiffUtil marks it as CHANGED.
//   - The net effect: RecyclerView plays a "changed" animation (a subtle
//     flash/update) on just that one row, re-running onBindViewHolder
//     for it specifically — every other row is left completely
//     untouched, with no animation and no rebinding at all.

Notes:
- Using product name as the identity key in areItemsTheSame is a
  simplification for this challenge — in a real app, a genuinely stable
  unique ID (like a database primary key) is the correct choice, since
  two different products could coincidentally share a name.
- The distinction between areItemsTheSame (identity) and
  areContentsTheSame (value equality, via the Product data class's
  auto-generated equals() from Kotlin Fundamentals Chapter 4) is what
  lets DiffUtil tell "this is a different item entirely" apart from
  "this is the same item, but something about it changed."