consensus: Update Majority and Supermajority definitions

Remove the definition of the two constants as multipliers and
define majority and supermajority as functions.

This eliminates the risk of computing the supermajority of 2/3
with the `0.67` multiplier, which could yield a wrong result
of 2/3+1.

consensus/src/config.rs

@@ -9,19 +9,10 @@ use std::time::Duration;
 /// Maximum number of iterations Consensus runs per a single round.
 pub const CONSENSUS_MAX_ITER: u8 = 50;
 
-/// Percentage number that determines quorums.
-pub const SUPERMAJORITY_THRESHOLD: f64 = 0.67;
-pub const MAJORITY_THRESHOLD: f64 = 0.5;
-
 /// Total credits of steps committees
 pub const PROPOSAL_COMMITTEE_CREDITS: usize = 1;
 pub const VALIDATION_COMMITTEE_CREDITS: usize = 64;
-pub const VALIDATION_COMMITTEE_QUORUM: f64 =
-    VALIDATION_COMMITTEE_CREDITS as f64 * SUPERMAJORITY_THRESHOLD;
-
 pub const RATIFICATION_COMMITTEE_CREDITS: usize = 64;
-pub const RATIFICATION_COMMITTEE_QUORUM: f64 =
-    RATIFICATION_COMMITTEE_CREDITS as f64 * SUPERMAJORITY_THRESHOLD;
 
 pub const DEFAULT_BLOCK_GAS_LIMIT: u64 = 5 * 1_000_000_000;
 
@@ -35,32 +26,45 @@ pub const MAX_STEP_TIMEOUT: Duration = Duration::from_secs(40);
 pub const TIMEOUT_INCREASE: Duration = Duration::from_secs(2);
 pub const MINIMUM_BLOCK_TIME: u64 = 10;
 
-/// Returns delta between full quorum and super_majority
-pub fn validation_extra() -> usize {
-    VALIDATION_COMMITTEE_CREDITS - validation_committee_quorum()
+// Returns `floor(value/2) + 1`
+pub fn majority(value: usize) -> usize {
+    value / 2 + 1
 }
 
-pub fn ratification_extra() -> usize {
-    RATIFICATION_COMMITTEE_CREDITS - ratification_committee_quorum()
+// Returns `ceil( value/3*2 )`
+pub fn supermajority(value: usize) -> usize {
+    let sm = value as f32 / 3.0 * 2.0;
+    sm.ceil() as usize
+}
+
+/// Returns the quorum of a Ratification committee
+pub fn ratification_quorum() -> usize {
+    supermajority(RATIFICATION_COMMITTEE_CREDITS)
 }
 
-/// Returns ceil of RATIFICATION_COMMITTEE_QUORUM
-pub fn ratification_committee_quorum() -> usize {
-    RATIFICATION_COMMITTEE_QUORUM.ceil() as usize
+/// Returns the quorum of a Validation committee
+pub fn validation_quorum() -> usize {
+    supermajority(VALIDATION_COMMITTEE_CREDITS)
 }
 
-/// Returns ceil of VALIDATION_COMMITTEE_QUORUM
-pub fn validation_committee_quorum() -> usize {
-    VALIDATION_COMMITTEE_QUORUM.ceil() as usize
+/// Returns the number of credits beyond the quorum for a Validation committee
+pub fn validation_extra() -> usize {
+    VALIDATION_COMMITTEE_CREDITS - validation_quorum()
+}
+
+/// Returns the number of credits beyond the quorum for a Ratification committee
+pub fn ratification_extra() -> usize {
+    RATIFICATION_COMMITTEE_CREDITS - ratification_quorum()
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
     #[test]
-    fn test_quorum_consts() {
-        assert_eq!(validation_committee_quorum(), 43);
-        assert_eq!(ratification_committee_quorum(), 43);
+    fn test_quorums() {
+        assert_eq!(majority(VALIDATION_COMMITTEE_CREDITS), 33);
+        assert_eq!(validation_quorum(), 43);
+        assert_eq!(ratification_quorum(), 43);
         assert_eq!(validation_extra(), 21);
         assert_eq!(ratification_extra(), 21);
     }

consensus/src/user/committee.rs

@@ -8,7 +8,7 @@ use crate::user::provisioners::Provisioners;
 use crate::user::sortition;
 
 use super::cluster::Cluster;
-use crate::config;
+use crate::config::{majority, supermajority};
 use node_data::bls::{PublicKey, PublicKeyBytes};
 use std::collections::{BTreeMap, HashMap};
 use std::fmt;
@@ -36,13 +36,10 @@ impl Committee {
     pub fn new(provisioners: &Provisioners, cfg: &sortition::Config) -> Self {
         // Generate committee using deterministic sortition.
         let extracted = provisioners.create_committee(cfg);
-        let committee_credits = cfg.committee_credits() as f64;
+        let committee_credits = cfg.committee_credits();
 
-        let super_majority = (committee_credits
-            * config::SUPERMAJORITY_THRESHOLD)
-            .ceil() as usize;
-        let majority =
-            (committee_credits * config::MAJORITY_THRESHOLD) as usize + 1;
+        let majority = majority(committee_credits);
+        let super_majority = supermajority(committee_credits);
 
         // Turn the raw vector into a hashmap where we map a pubkey to its
         // occurrences.