// Package filter implements BIP158 basic compact block filters: Golomb-coded
// sets (GCS) over the outputs of SipHash-2-4.
//
// A filter is built from an element set and the hash of the block it commits
// to. The first 16 bytes of the block hash, in the internal (little-endian)
// byte order used by the Bitcoin wire protocol, are used as the SipHash key.
// The basic filter type uses M = 784931 and P = 19, giving a false positive
// rate of about 1/784931 per queried element.
//
// The BIP158 basic element set consists of the previous output scripts spent
// by non-coinbase inputs, plus every output scriptPubKey in the block except
// OP_RETURN scripts. Empty ("nil") elements must be omitted and the element
// set is a set: exact duplicate scripts are included only once. Build does not
// enforce those rules; callers that supply the element set must. BuildFromScripts
// drops empty and duplicate scripts, but leaves OP_RETURN filtering to the
// caller.
//
// The serialization is a CompactSize element count followed by the bit-packed
// Golomb-Rice coded deltas of the sorted, hashed values, zero-padded to a byte
// boundary. A filter over the empty set is exactly the single byte 0x00.
package filter