Skip to content

perf(escrow,milestones): O(n log n) largest-remainder dust distribution #61

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
chonilius merged 2 commits into from
Aug 17, 2026
Merged

perf(escrow,milestones): O(n log n) largest-remainder dust distribution #61

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
0819d46
perf(escrow,milestones): O(n log n) largest-remainder dust distribution
aristotle224 Aug 16, 2026
28df4f3
Merge branch 'main' into perf/compute-split-ologn
chonilius Aug 17, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
110 changes: 86 additions & 24 deletions contracts/escrow/src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -409,45 +409,107 @@ pub(crate) fn compute_split(
let distributable = total - fee;

let mut shares: Vec<(Address, i128)> = Vec::new(env);
let mut remainders: Vec<i128> = Vec::new(env);
let mut order: Vec<(u32, i128, Address)> = Vec::new(env);
let mut allocated: i128 = 0;

for (recipient, bps) in recipients.iter() {
let numerator = distributable * (bps as i128);
let share = numerator / BPS_DENOMINATOR;
let remainder = numerator % BPS_DENOMINATOR;
allocated += share;
shares.push_back((recipient, share));
remainders.push_back(remainder);
shares.push_back((recipient.clone(), share));
order.push_back((order.len(), remainder, recipient));
}

let mut dust = distributable - allocated;
while dust > 0 {
let mut best_index: u32 = 0;
let mut best_remainder: i128 = -1;
for (i, remainder) in remainders.iter().enumerate() {
if remainder > best_remainder {
best_index = i as u32;
best_remainder = remainder;
} else if remainder == best_remainder && remainder != -1 {
let current_addr = shares.get(i as u32).unwrap().0;
let best_addr = shares.get(best_index).unwrap().0;
if current_addr < best_addr {
best_index = i as u32;
best_remainder = remainder;
}
}
// Distribute the rounding dust by largest remainder (with the existing
// address-based tie-break) in O(n log n): sort the (index, remainder,
// address) records once, then award one unit to each of the first `dust`
// entries. This is equivalent to the previous repeated-linear-scan loop,
// because each award only consumes the selected entry and never changes
// any other entry's remainder. `dust` is at most `recipients.len() - 1`,
// so the first `dust` sorted entries always exist.
let dust = distributable - allocated;
if dust > 0 {
sort_remainders_desc(&mut order);
for k in 0..dust as u32 {
let (index, _, _) = order.get(k).unwrap();
let (recipient, share) = shares.get(index).unwrap();
shares.set(index, (recipient, share + 1));
}

let (recipient, share) = shares.get(best_index).unwrap();
shares.set(best_index, (recipient, share + 1));
remainders.set(best_index, -1);
dust -= 1;
}

Ok(Payouts { fee, shares })
}

/// True if `a` sorts before `b` in largest-remainder order: remainder
/// descending, then address ascending, then original index ascending (which
/// reproduces the address-based tie-break of the previous O(n²) loop).
fn remainder_order_less(a: &(u32, i128, Address), b: &(u32, i128, Address)) -> bool {
b.1.cmp(&a.1)
.then_with(|| a.2.cmp(&b.2))
.then_with(|| a.0.cmp(&b.0))
== core::cmp::Ordering::Less
}

/// Sifts the element at `start` down a max-heap occupying `[start, end)`,
/// ordering elements by [`remainder_order_less`].
fn sift_down_remainder_order(order: &mut Vec<(u32, i128, Address)>, start: u32, end: u32) {
let mut root = start;
loop {
let mut child = 2 * root + 1;
if child >= end {
break;
}
if child + 1 < end
&& remainder_order_less(&order.get(child).unwrap(), &order.get(child + 1).unwrap())
{
child += 1;
}
if remainder_order_less(&order.get(root).unwrap(), &order.get(child).unwrap()) {
let a = order.get(root).unwrap();
let b = order.get(child).unwrap();
order.set(root, b);
order.set(child, a);
root = child;
} else {
break;
}
}
}

/// In-place heapsort of `(index, remainder, address)` records into
/// largest-remainder order. O(n log n) worst case, with no recursion and no
/// heap allocation, so it is safe under `#![no_std]` and only mutates the
/// host-backed `order` through `get`/`set`.
fn sort_remainders_desc(order: &mut Vec<(u32, i128, Address)>) {
let n = order.len();
if n < 2 {
return;
}

// Build a max-heap over the whole array.
let mut start = n / 2;
loop {
start -= 1;
sift_down_remainder_order(order, start, n);
if start == 0 {
break;
}
}

// Repeatedly move the largest remaining element to the end of the array,
// shrinking the heap until the array is sorted ascending by `less`.
let mut end = n;
while end > 1 {
end -= 1;
let a = order.get(0).unwrap();
let b = order.get(end).unwrap();
order.set(0, b);
order.set(end, a);
sift_down_remainder_order(order, 0, end);
}
}

pub(crate) fn require_admin(env: &Env) -> Result<Address, Error> {
mergefi_common::require_admin::<DataKey>(env).ok_or(Error::NotInitialized)
}
Expand Down
85 changes: 85 additions & 0 deletions contracts/escrow/src/test.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -302,6 +302,91 @@ fn test_adversarial_ordering_resistance() {
);
}

#[test]
fn test_large_split_distributes_dust_by_largest_remainder() {
let env = Env::default();
env.mock_all_auths();

let admin = Address::generate(&env);
let treasury = Address::generate(&env);
let contract_id = env.register(crate::EscrowContract, ());
let client = crate::EscrowContractClient::new(&env, &contract_id);
// 0% fee so the whole total is distributable.
client.initialize(&admin, &treasury, &0u32);

// 60 recipients: 59 with alternating 160/170 bps, the last one receiving
// the leftover of 10000. All 170-bps recipients share an identical
// remainder, so most of the dust has to be resolved by the address-based
// tie-break, exercising both the O(n log n) sort and the tie-break at
// scale.
let mut recipients = Vec::new(&env);
let mut total_bps: u32 = 0;
for i in 0..59u32 {
let bps = if i % 5 == 0 { 160 } else { 170 };
recipients.push_back((Address::generate(&env), bps));
total_bps += bps;
}
let last_bps = BPS_DENOMINATOR as u32 - total_bps;
recipients.push_back((Address::generate(&env), last_bps));

// Chosen so that integer division leaves exactly 40 dust units to
// distribute.
let total: i128 = 123_457;
let payouts = env.as_contract(&contract_id, || {
crate::compute_split(&env, total, &recipients).unwrap()
});

// Reference result computed with the previous O(n²) repeated
// largest-remainder scan; the new implementation must match it exactly.
let mut expected: Vec<i128> = Vec::new(&env);
let mut remainders: Vec<i128> = Vec::new(&env);
let mut allocated: i128 = 0;
for (_, bps) in recipients.iter() {
let numerator = total * (bps as i128);
let share = numerator / BPS_DENOMINATOR;
let remainder = numerator % BPS_DENOMINATOR;
allocated += share;
expected.push_back(share);
remainders.push_back(remainder);
}
let mut dust = total - allocated;
assert!(
dust >= 2,
"test must exercise multiple dust units, got {dust}"
);
while dust > 0 {
let mut best_index: u32 = 0;
let mut best_remainder: i128 = -1;
for (i, remainder) in remainders.iter().enumerate() {
if remainder > best_remainder {
best_index = i as u32;
best_remainder = remainder;
} else if remainder == best_remainder && remainder != -1 {
let current_addr = recipients.get(i as u32).unwrap().0;
let best_addr = recipients.get(best_index).unwrap().0;
if current_addr < best_addr {
best_index = i as u32;
best_remainder = remainder;
}
}
}
expected.set(best_index, expected.get(best_index).unwrap() + 1);
remainders.set(best_index, -1);
dust -= 1;
}

let mut total_paid: i128 = 0;
for (i, _) in recipients.iter().enumerate() {
let (_, share) = payouts.shares.get(i as u32).unwrap();
assert_eq!(share, expected.get(i as u32).unwrap(), "recipient {i}");
total_paid += share;
}
assert_eq!(
total_paid, total,
"all distributable funds must be paid out"
);
}

// ---------------------------------------------------------------------------
// Access-control boundary matrix (#30)
// ---------------------------------------------------------------------------
Expand Down
108 changes: 84 additions & 24 deletions contracts/milestones/src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -369,45 +369,105 @@ fn compute_split(
let distributable = total - fee;

let mut shares: Vec<(Address, i128)> = Vec::new(env);
let mut remainders: Vec<i128> = Vec::new(env);
let mut order: Vec<(u32, i128, Address)> = Vec::new(env);
let mut allocated: i128 = 0;

for (recipient, bps) in recipients.iter() {
let numerator = distributable * (bps as i128);
let share = numerator / BPS_DENOMINATOR;
let remainder = numerator % BPS_DENOMINATOR;
allocated += share;
shares.push_back((recipient, share));
remainders.push_back(remainder);
shares.push_back((recipient.clone(), share));
order.push_back((order.len(), remainder, recipient));
}

let mut dust = distributable - allocated;
while dust > 0 {
let mut best_index: u32 = 0;
let mut best_remainder: i128 = -1;
for (i, remainder) in remainders.iter().enumerate() {
if remainder > best_remainder {
best_index = i as u32;
best_remainder = remainder;
} else if remainder == best_remainder && remainder != -1 {
let current_addr = shares.get(i as u32).unwrap().0;
let best_addr = shares.get(best_index).unwrap().0;
if current_addr < best_addr {
best_index = i as u32;
best_remainder = remainder;
}
}
// Distribute the rounding dust by largest remainder (with the existing
// address-based tie-break) in O(n log n): sort the (index, remainder,
// address) records once, then award one unit to each of the first `dust`
// entries. This is equivalent to the previous repeated-linear-scan loop,
// because each award only consumes the selected entry and never changes
// any other entry's remainder. `dust` is at most `recipients.len() - 1`,
// so the first `dust` sorted entries always exist.
let dust = distributable - allocated;
if dust > 0 {
sort_remainders_desc(&mut order);
for k in 0..dust as u32 {
let (index, _, _) = order.get(k).unwrap();
let (recipient, share) = shares.get(index).unwrap();
shares.set(index, (recipient, share + 1));
}

let (recipient, share) = shares.get(best_index).unwrap();
shares.set(best_index, (recipient, share + 1));
remainders.set(best_index, -1);
dust -= 1;
}

Ok(Payouts { fee, shares })
}

/// True if `a` sorts before `b` in largest-remainder order: remainder
/// descending, then address ascending, then original index ascending (which
/// reproduces the address-based tie-break of the previous O(n²) loop).
fn remainder_order_less(a: &(u32, i128, Address), b: &(u32, i128, Address)) -> bool {
b.1.cmp(&a.1)
.then_with(|| a.2.cmp(&b.2))
.then_with(|| a.0.cmp(&b.0))
== core::cmp::Ordering::Less
}

/// Sifts the element at `start` down a max-heap occupying `[start, end)`,
/// ordering elements by [`remainder_order_less`].
fn sift_down_remainder_order(order: &mut Vec<(u32, i128, Address)>, start: u32, end: u32) {
let mut root = start;
loop {
let mut child = 2 * root + 1;
if child >= end {
break;
}
if child + 1 < end
&& remainder_order_less(&order.get(child).unwrap(), &order.get(child + 1).unwrap())
{
child += 1;
}
if remainder_order_less(&order.get(root).unwrap(), &order.get(child).unwrap()) {
let a = order.get(root).unwrap();
let b = order.get(child).unwrap();
order.set(root, b);
order.set(child, a);
root = child;
} else {
break;
}
}
}

/// In-place heapsort of `(index, remainder, address)` records into
/// largest-remainder order. O(n log n) worst case, with no recursion and no
/// heap allocation, so it is safe under `#![no_std]` and only mutates the
/// host-backed `order` through `get`/`set`.
fn sort_remainders_desc(order: &mut Vec<(u32, i128, Address)>) {
let n = order.len();
if n < 2 {
return;
}

// Build a max-heap over the whole array.
let mut start = n / 2;
loop {
start -= 1;
sift_down_remainder_order(order, start, n);
if start == 0 {
break;
}
}

// Repeatedly move the largest remaining element to the end of the array,
// shrinking the heap until the array is sorted ascending by `less`.
let mut end = n;
while end > 1 {
end -= 1;
let a = order.get(0).unwrap();
let b = order.get(end).unwrap();
order.set(0, b);
order.set(end, a);
sift_down_remainder_order(order, 0, end);
}
/// Pays each contributor their share of `milestone.remaining_budget` (the
/// unallocated remainder of the pool), computed as
/// `remaining_budget * contribution.amount / total_budget` — i.e. in
Expand Down
Loading
Loading