type LogHasher interface {
	// EmptyRoot supports returning a special case for the root of an empty tree.
	EmptyRoot() []byte
	// HashLeaf computes the hash of a leaf that exists.
	HashLeaf(leaf []byte) ([]byte, error)
	// HashChildren computes interior nodes.
	HashChildren(l, r []byte) []byte
	// Size is the number of bytes in the underlying hash function.
	// TODO(gbelvin): Replace Size() with BitLength().
	Size() int
}

type MapHasher interface {
	// HashEmpty returns the hash of an empty branch at a given depth.
	// A height of 0 indicates an empty leaf. The maximum height is Size*8.
	// TODO(gbelvin) fully define index.
	HashEmpty(treeID int64, index []byte, height int) []byte
	// HashLeaf computes the hash of a leaf that exists.
	HashLeaf(treeID int64, index []byte, leaf []byte) ([]byte, error)
	// HashChildren computes interior nodes.
	HashChildren(l, r []byte) []byte
	// Size is the number of bytes in the underlying hash function.
	// TODO(gbelvin): Replace Size() with BitLength().
	Size() int
	// BitLen returns the number of bits in the underlying hash function.
	// It is also the height of the merkle tree.
	BitLen() int
}