perf(escrow,milestones): O(n log n) largest-remainder dust distribution - #61
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Replace the O(n²) repeated-linear-scan dust loop in compute_split with a single heapsort by (remainder desc, address asc, index asc) followed by a linear pass awarding one unit to each of the first `dust` entries. The two approaches are equivalent: each award only consumes the chosen entry, so repeated largest-remainder selection is identical to taking the top-dust sorted entries. Heapsort is in-place, non-recursive, and allocation-free, so it is safe under #![no_std]. Adds test_large_split_distributes_dust_by_largest_remainder to both contracts: 60 recipients with 40 dust units and 47 tied remainders, verified against a reference copy of the old O(n²) scan. Closes MergeFi#55.
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Description
Closes #55 — makes
compute_split's dust-distribution step O(n log n) instead of O(n²).Before
The dust loop (
contracts/escrow/src/lib.rs:295-317, byte-identical incontracts/milestones/src/lib.rs:279-301) did a full linear scan over allnrecipients per dust unit to find the largest remaining remainder. With up ton-1dust units worst case, that's ~n²/2comparisons on every split.After
Build an
(index, remainder, address)record per recipient, heapsort once by (remainder desc, address asc, index asc), then award one unit to each of the firstdustentries in a linear pass.dustof a single sort".dust ≤ n-1always, so the firstdustsorted entries always exist.#.Motivation
Compounds with #8 (unbounded recipients): the quadratic cost scaled with both recipient count and dust. This flattens the curve so recipient-count growth (the #8 axis) no longer carries a per-dust O(n) penalty.
Tests
test_large_split_distributes_dust_by_largest_remainder(escrow + milestones): 60 recipients, 40 dust units, 47 tied remainders exercising the address tie-break; every share matched against a reference copy of the old O(n²) approach, total payout = total distributable.test_adversarial_ordering_resistanceand both*_rounding_dust_by_largest_remaindertests.cargo test --workspace,cargo clippy --workspace --all-targets, andcargo fmt --checkall clean.