Add drb functions

crates/hotshot/src/traits/election.rs

@@ -6,6 +6,8 @@
 
 //! elections used for consensus
 
+/// Dynamic leader election with epochs.
+pub mod dynamic;
 /// leader completely randomized every view
 pub mod randomized_committee;
 /// static (round robin) committee election

crates/hotshot/src/traits/election/dynamic.rs

@@ -0,0 +1,68 @@
+// Copyright (c) 2021-2024 Espresso Systems (espressosys.com)
+// This file is part of the HotShot repository.
+
+// You should have received a copy of the MIT License
+// along with the HotShot repository. If not, see <https://mit-license.org/>.
+
+use anyhow::{Context, Result};
+use hotshot_types::traits::{node_implementation::NodeType, signature_key::SignatureKey};
+use sha2::{Digest, Sha256};
+
+/// Time a DRB calculation will take, in terms of number of views.
+const DRB_CALCULATION_NUM_VIEW: u64 = 300;
+
+// TODO: Replace `hashes_per_second` parameter with a const once we benchmark the hash time.
+// <https://github.com/EspressoSystems/HotShot/issues/3880>
+/// Difficulty level of the DRB calculation.
+///
+/// Represents the number of times the hash function will be repeatedly called.
+///
+/// # Arguments
+/// * `hashes_per_second` - Number of hashes that can be completed in a second.
+/// * `view_timeout` - Timeout duration for a view, in seconds.
+#[must_use]
+pub fn difficulty_level(hashes_per_second: u64, view_timeout: u64) -> u64 {
+    hashes_per_second * view_timeout * DRB_CALCULATION_NUM_VIEW
+}
+
+// TODO: Replace `hashes_per_second` parameter with a const once we benchmark the hash time.
+// <https://github.com/EspressoSystems/HotShot/issues/3880>
+/// Compute the DRB result for the leader rotation.
+///
+/// This is to be started two epochs in advance.
+///
+/// # Arguments
+/// * `qc_signature` - Aggregated signature in the current quorum proposal.
+/// * `view_timeout` - Timeout duration for a view, in seconds.
+///
+/// # Errors
+/// * If failed to serialize the QC signature.
+pub fn compute_drb<TYPES: NodeType>(
+    qc_signature: &<TYPES::SignatureKey as SignatureKey>::QcType,
+    hashes_per_second: u64,
+    view_timeout: u64,
+) -> Result<usize> {
+    // Hash the QC signature.
+    let mut hash = bincode::serialize(&qc_signature)
+        .with_context(|| "Failed to serialize the QC signature.")?;
+    let difficulty_level = difficulty_level(hashes_per_second, view_timeout);
+    for _iter in 0..difficulty_level {
+        hash = Sha256::digest(hash).to_vec();
+    }
+
+    // Convert the hash to a seed as the DRB result.
+    let mut seed = [0u8; 8];
+    seed.copy_from_slice(&hash[..size_of::<usize>()]);
+    Ok(usize::from_le_bytes(seed))
+}
+
+/// Use the DRB result to get the leader.
+#[must_use]
+pub fn leader<TYPES: NodeType>(
+    stake_table: &[<TYPES::SignatureKey as SignatureKey>::StakeTableEntry],
+    drb: usize,
+) -> TYPES::SignatureKey {
+    let index = drb % stake_table.len();
+    let res = stake_table[index].clone();
+    TYPES::SignatureKey::public_key(&res)
+}