# `Lightstreamer.Protocol.Update`
[🔗](https://github.com/ktec/lightstreamer/blob/v0.1.0/lib/lightstreamer/protocol/update.ex#L1)

Pure decoder for the field payload of a `U` notification.

Updates are deltas against the previous per-item field state: the payload is
a pipe-separated token list where an empty token means *unchanged*, `#`
means null, `$` means empty string, `^n` skips `n` unchanged fields at once,
and anything else is percent-encoded content (`#`, `$` and `^` arrive
escaped when literal at position 0). `^P`/`^T` diff tokens are rejected
explicitly — this library disables them via `LS_supported_diffs=` at session
creation, so seeing one is an error, never a silently wrong value.

# `apply`

```elixir
@spec apply(binary(), [String.t() | nil]) ::
  {:ok, [String.t() | nil], [pos_integer()]}
  | {:error, {:unsupported_diff, binary()}}
```

Applies one raw `U` field payload to the previous field values.

`previous_values` is the positional per-item state (use all-`nil` before the
first update — a snapshot carries every field). Returns the merged values
plus the ascending 1-based indexes of the fields this payload transmitted.

Payloads that do not cover the schema exactly violate a protocol invariant
and raise `ArgumentError` — the session process is expected to crash.

## Examples

    iex> Lightstreamer.Protocol.Update.apply("21:14:00|3.05|^2|$", [nil, nil, "2.41", "3.67", "old"])
    {:ok, ["21:14:00", "3.05", "2.41", "3.67", ""], [1, 2, 5]}

    iex> Lightstreamer.Protocol.Update.apply("^Pnot-json-patch", [nil])
    {:error, {:unsupported_diff, "P"}}

---

*Consult [api-reference.md](api-reference.md) for complete listing*