/**
 * Send a logical payload through a callback, splitting into fragment
 * packets if it exceeds the channel's single-packet capacity.
 *
 * Layout of each emitted fragment packet (before the Channel adds its
 * 8-byte header):
 *
 *   uint8   FRAGMENT_PACKET_TYPE
 *   uint16  message_id   (little-endian)
 *   uint8   chunk_index
 *   uint8   total_chunks
 *   bytes   chunk_payload
 *
 * The chunk_payload bytes are slices of the original logical payload,
 * concatenated in chunk_index order they reconstruct the original
 * payload byte-for-byte. The logical payload's own packet-type byte is
 * the first byte of chunk 0 and is preserved by the reassembly.
 *
 * Single-packet messages (payload <= MAX_CHANNEL_PAYLOAD_BYTES) are sent
 * as-is with NO fragmentation header — the receiver dispatches them
 * normally via the FIRST byte. Fragmentation overhead only applies when
 * the payload genuinely doesn't fit.
 *
 * @param {Uint8Array} payload
 * @param {number} payload_length
 * @param {number} message_id  sender-assigned, unique per logical message
 * @param {number} fragment_packet_type  packet-type byte for fragment packets
 * @param {Uint8Array} scratch  reusable scratch buffer; must be >= MAX_CHANNEL_PAYLOAD_BYTES
 * @param {function(Uint8Array, number): void} send_fn
 *   invoked once per emitted packet — single send for non-fragmented case,
 *   N sends for fragmented case
 */
export function send_fragmented(payload: Uint8Array, payload_length: number, message_id: number, fragment_packet_type: number, scratch: Uint8Array, send_fn: (arg0: Uint8Array, arg1: number) => void): void;
/**
 * Re-emit a single fragment chunk for a previously-sent fragmented
 * message. Used by the NACK retransmit path: the sender's
 * {@link FragmentRetention} keeps the source bytes so any chunk can be
 * deterministically rebuilt.
 *
 * The wire layout matches what {@link send_fragmented} would have
 * produced for the same `(message_id, chunk_index, total_chunks)` —
 * receivers can't tell a retransmit from an original.
 *
 * @param {Uint8Array} payload  retained source bytes
 * @param {number} payload_length
 * @param {number} message_id
 * @param {number} chunk_index  must be < derived total_chunks; silently ignored otherwise
 * @param {number} fragment_packet_type
 * @param {Uint8Array} scratch  reusable scratch; must be >= MAX_CHANNEL_PAYLOAD_BYTES
 * @param {function(Uint8Array, number): void} send_fn
 */
export function resend_fragment_chunk(payload: Uint8Array, payload_length: number, message_id: number, chunk_index: number, fragment_packet_type: number, scratch: Uint8Array, send_fn: (arg0: Uint8Array, arg1: number) => void): void;
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