// Package btcwire implements the Bitcoin P2P wire format and the message
// payloads needed by a BIP157 compact-filter light client.
//
// The package is a pure codec: it performs no I/O of its own beyond reading
// from and writing to the io.Reader and []byte values supplied by the caller.
// It deliberately avoids the net and os packages so that it can be compiled
// for constrained targets such as wasm.
//
// # Framing
//
// Every message is framed as a 24-byte header followed by the payload:
//
//	magic        uint32, big endian
//	command      12 bytes, ASCII, NUL padded
//	length       uint32, little endian
//	checksum     4 bytes, first 4 bytes of SHA256d(payload)
//	payload      length bytes
//
// EncodeMessage produces one such frame; DecodeMessage and Reader consume
// them.
//
// # Byte order
//
// Unless documented otherwise, integers use Bitcoin's little-endian wire
// encoding. Hashes are chainhash.Hash values in internal byte order: the
// bytes as they appear on the wire, which is the reverse of the order used
// for display.
//
// The frame magic is big endian, and network ports are big endian as specified
// by the version message. Both are exceptions: a magic is a four-byte string
// that Bitcoin writes "f9beb4d9" and sends as those bytes, so the uint32
// constants hold that string read big endian and the codec writes it back out
// the same way. Writing a magic little endian reverses it on the wire, and a
// node reads the reversed bytes as an unknown network and drops the peer.
//
// # Untrusted input
//
// Decoders never panic: truncated, oversized, or otherwise invalid input is
// reported as an error. Truncation errors wrap io.ErrUnexpectedEOF, so
// callers can test for them with errors.Is. Compact size integers must be
// minimally encoded.
//
// Byte slices returned by DecodePayload (and by ReadVarBytes) alias the input
// buffer they were decoded from. Callers that keep such a slice after the
// payload buffer is reused must copy it.
package btcwire