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feat: deterministic split rounding fairness #391

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menawar wants to merge 2 commits into from
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feat: deterministic split rounding fairness #391

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d372326
feat: improve deterministic rounding fairness in split math
menawar Mar 25, 2026
99d5495
Merge branch 'main' into feature/remit-split-remainder-fairness
menawar Mar 29, 2026
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122 changes: 78 additions & 44 deletions remittance_split/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -653,33 +653,11 @@ impl RemittanceSplit {
env: Env,
total_amount: i128,
) -> Result<Vec<i128>, RemittanceSplitError> {
if total_amount <= 0 {
return Err(RemittanceSplitError::InvalidAmount);
}

let split = Self::get_split(&env);
let s0 = split.get(0).unwrap() as i128;
let s1 = split.get(1).unwrap() as i128;
let s2 = split.get(2).unwrap() as i128;

let spending = total_amount
.checked_mul(s0)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
let savings = total_amount
.checked_mul(s1)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
let bills = total_amount
.checked_mul(s2)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
// Insurance gets the remainder to handle rounding
let insurance = total_amount
.checked_sub(spending)
.and_then(|n| n.checked_sub(savings))
.and_then(|n| n.checked_sub(bills))
.ok_or(RemittanceSplitError::Overflow)?;
let amounts = Self::compute_split_amounts(&env, total_amount)?;
let spending = amounts[0];
let savings = amounts[1];
let bills = amounts[2];
let insurance = amounts[3];

// Emit SplitCalculated event

Expand Down Expand Up @@ -1348,6 +1326,38 @@ impl RemittanceSplit {
env: &Env,
total_amount: i128,
emit_events: bool,
) -> Result<[i128; 4], RemittanceSplitError> {
let amounts = Self::compute_split_amounts(env, total_amount)?;
let spending = amounts[0];
let savings = amounts[1];
let bills = amounts[2];
let insurance = amounts[3];

if emit_events {
let event = SplitCalculatedEvent {
total_amount,
spending_amount: spending,
savings_amount: savings,
bills_amount: bills,
insurance_amount: insurance,
timestamp: env.ledger().timestamp(),
};
env.events().publish((SPLIT_CALCULATED,), event);
env.events().publish(
(symbol_short!("split"), SplitEvent::Calculated),
total_amount,
);
}

Ok([spending, savings, bills, insurance])
}

/// @notice Compute deterministic split amounts with fair remainder distribution.
/// @dev Uses the largest-remainder method with a fixed tie-break order:
/// spending, savings, bills, then insurance. Ensures sum == total_amount.
fn compute_split_amounts(
env: &Env,
total_amount: i128,
) -> Result<[i128; 4], RemittanceSplitError> {
if total_amount <= 0 {
return Err(RemittanceSplitError::InvalidAmount);
Expand All @@ -1366,24 +1376,39 @@ impl RemittanceSplit {
Some(v) => v as i128,
None => return Err(RemittanceSplitError::Overflow),
};
let s3 = match split.get(3) {
Some(v) => v as i128,
None => return Err(RemittanceSplitError::Overflow),
};

let spending = total_amount
.checked_mul(s0)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
let savings = total_amount
.checked_mul(s1)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
let bills = total_amount
.checked_mul(s2)
.and_then(|n| n.checked_div(100))
.ok_or(RemittanceSplitError::Overflow)?;
let insurance = total_amount
.checked_sub(spending)
.and_then(|n| n.checked_sub(savings))
.and_then(|n| n.checked_sub(bills))
let percents = [s0, s1, s2, s3];
let mut base = [0i128; 4];
let mut remainders = [0i128; 4];
let mut base_sum: i128 = 0;

for i in 0..4 {
let raw = total_amount
.checked_mul(percents[i])
.ok_or(RemittanceSplitError::Overflow)?;
let portion = raw
.checked_div(100)
.ok_or(RemittanceSplitError::Overflow)?;
let remainder = raw
.checked_rem(100)
.ok_or(RemittanceSplitError::Overflow)?;
base[i] = portion;
remainders[i] = remainder;
base_sum = base_sum
.checked_add(portion)
.ok_or(RemittanceSplitError::Overflow)?;
}

let mut remainder_units = total_amount
.checked_sub(base_sum)
.ok_or(RemittanceSplitError::Overflow)?;
if remainder_units < 0 || remainder_units > 4 {
return Err(RemittanceSplitError::Overflow);
}

if emit_events {
let event = SplitCalculatedEvent {
Expand All @@ -1410,7 +1435,16 @@ impl RemittanceSplit {
);
}

Ok([spending, savings, bills, insurance])
let total = out[0]
.checked_add(out[1])
.and_then(|n| n.checked_add(out[2]))
.and_then(|n| n.checked_add(out[3]))
.ok_or(RemittanceSplitError::Overflow)?;
if total != total_amount {
return Err(RemittanceSplitError::Overflow);
}

Ok(out)
}

/// Extend the TTL of instance storage
Expand Down
27 changes: 16 additions & 11 deletions remittance_split/src/test.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -378,10 +378,12 @@ fn test_calculate_split_rounding() {
let token_admin = Address::generate(&env);
let token_id = setup_token(&env, &token_admin, &owner, 0);

client.initialize_split(&owner, &0, &token_id, &33, &33, &33, &1);
let amounts = client.calculate_split(&100);
let sum: i128 = amounts.iter().sum();
assert_eq!(sum, 100);
client.initialize_split(&owner, &0, &token_id, &50, &30, &15, &5);
let amounts = client.calculate_split(&101);
assert_eq!(amounts.get(0).unwrap(), 51);
assert_eq!(amounts.get(1).unwrap(), 30);
assert_eq!(amounts.get(2).unwrap(), 15);
assert_eq!(amounts.get(3).unwrap(), 5);
}

#[test]
Expand Down Expand Up @@ -808,27 +810,30 @@ fn test_distribute_usdc_split_math_100_0_0_0() {
}

#[test]
fn test_distribute_usdc_rounding_remainder_goes_to_insurance() {
fn test_distribute_usdc_rounding_remainder_deterministic() {
let env = Env::default();
env.mock_all_auths();
let contract_id = env.register_contract(None, RemittanceSplit);
let client = RemittanceSplitClient::new(&env, &contract_id);
let owner = Address::generate(&env);
let token_admin = Address::generate(&env);
// 33/33/33/1 with amount=100: 33+33+33=99, insurance gets remainder=1
let token_id = setup_token(&env, &token_admin, &owner, 100);
// 50/30/15/5 with amount=101: spending gets remainder by highest fractional part
let token_id = setup_token(&env, &token_admin, &owner, 101);

client.initialize_split(&owner, &0, &token_id, &33, &33, &33, &1);
client.initialize_split(&owner, &0, &token_id, &50, &30, &15, &5);
let accounts = make_accounts(&env);
client.distribute_usdc(&token_id, &owner, &1, &accounts, &100);
client.distribute_usdc(&token_id, &owner, &1, &accounts, &101);

let token = TokenClient::new(&env, &token_id);
let total = token.balance(&accounts.spending)
+ token.balance(&accounts.savings)
+ token.balance(&accounts.bills)
+ token.balance(&accounts.insurance);
assert_eq!(total, 100, "all funds must be distributed");
assert_eq!(token.balance(&accounts.insurance), 1);
assert_eq!(total, 101, "all funds must be distributed");
assert_eq!(token.balance(&accounts.spending), 51);
assert_eq!(token.balance(&accounts.savings), 30);
assert_eq!(token.balance(&accounts.bills), 15);
assert_eq!(token.balance(&accounts.insurance), 5);
}

// ---------------------------------------------------------------------------
Expand Down
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