add qa for lt hash

accounts-db/src/accounts_hash.rs

@@ -1248,6 +1248,27 @@ pub struct AccountHash(pub Hash);
 // This also ensures there are no padding bytes, which is required to safely implement Pod
 const _: () = assert!(std::mem::size_of::<AccountHash>() == std::mem::size_of::<Hash>());
 
+/// # Account Lattice Hash Q/A
+///
+/// ## What's account Lattice hash?
+/// It is a new way to compute accounts hash based on the Algebra struct Lattice
+/// defined on hash field. The field elements are 2048 bytes of hash of
+/// individual accounts. This 2048 bytes of hash is computed from blake3 hash.
+/// There are two operator defined on the field, add/sub, which are u16 x 1024
+/// wrapping add and subtract.
+///
+/// ## How is the account Lattice hash used?
+/// It is used to represent the account state for each slot. Each slot will
+/// "add" in the new state of changed accounts and "sub" the old state of the
+/// account. This way the accumulated hash of the slot is equal to sum all the
+/// accounts state at that slot. The end goal is to replace the current merkle
+/// tree hash.
+///
+/// ## How to handle account deletion and account creation?
+/// That's where the identity element in the lattice play it role. All accounts
+/// with zero lamport are treated to have the "identity" hash. add/sub such
+/// element don't impact the accumulated hash.
+///
 /// Lattice hash
 pub const LT_HASH_BYTES: usize = 2048;
 pub const LT_HASH_ELEMENT: usize = 1024;