feat(settlement): implement round-half-up fee computation to prevent …

contracts/settlement/Cargo.toml

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+[package]
+name = "settlement"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["lib", "cdylib"]
+doctest = false
+
+[dependencies]
+soroban-sdk = { workspace = true }
+
+[dev-dependencies]
+soroban-sdk = { workspace = true, features = ["testutils"] }

contracts/settlement/src/constants.rs

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+/// Default protocol fee rate in basis points (1% = 100 bps)
+#[allow(dead_code)]
+pub const FEE_RATE_BPS: u32 = 100;
+
+/// Minimum fee rate in basis points (0.01%)
+#[allow(dead_code)]
+pub const MIN_FEE_RATE_BPS: u32 = 1;
+
+/// Maximum fee rate in basis points (10%)
+pub const MAX_FEE_RATE_BPS: u32 = 1000;
+
+/// Maximum settlement amount with 7-decimal precision
+#[allow(dead_code)]
+pub const MAX_SETTLEMENT: i128 = 1_000_000_000_000_000_000; // 1e18
+
+/// Denominator for basis points calculations
+pub const BPS_DENOMINATOR: u32 = 10000;

contracts/settlement/src/fees.rs

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+use crate::constants::BPS_DENOMINATOR;
+
+/// Compute protocol fee using round-half-up (commercial rounding).
+///
+/// fee = floor((amount * rate_bps + 5000) / 10000)
+///
+/// This prevents systematic value extraction via micro-settlements
+/// where plain truncation would round the fee to zero for every
+/// dust-amount transaction.
+///
+/// # Rounding invariants (round-half-up)
+/// - fee * 10000 <= amount * rate_bps + 5000  (max 0.5 unit over-collection)
+/// - fee * 10000 >= amount * rate_bps - 4999  (max 0.5 unit under-collection)
+/// - |fee * 10000 - amount * rate_bps| <= 5000
+pub fn compute_fee(amount: i128, rate_bps: u32) -> i128 {
+    ((amount * rate_bps as i128) + 5000) / BPS_DENOMINATOR as i128
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::constants::{BPS_DENOMINATOR, MAX_FEE_RATE_BPS, MAX_SETTLEMENT, MIN_FEE_RATE_BPS};
+
+    /// Round-half-up invariant: error magnitude ≤ 0.5 units of the smallest denomination.
+    /// |fee * 10000 - amount * rate_bps| <= 5000
+    fn invariant_error_bounded(amount: i128, rate_bps: u32) -> bool {
+        let fee = compute_fee(amount, rate_bps);
+        let scaled_fee = fee * BPS_DENOMINATOR as i128;
+        let exact = amount * rate_bps as i128;
+        let diff = (scaled_fee - exact).abs();
+        diff <= 5000
+    }
+
+    #[test]
+    fn test_compute_fee_round_half_up() {
+        // 0.5 rounds up
+        assert_eq!(compute_fee(1, 5000), 1);
+        // 0.5001 rounds up
+        assert_eq!(compute_fee(1, 5001), 1);
+        // 0.4999 rounds down
+        assert_eq!(compute_fee(1, 4999), 0);
+        // exact
+        assert_eq!(compute_fee(2, 5000), 1);
+        assert_eq!(compute_fee(10000, 100), 100);
+    }
+
+    #[test]
+    fn test_fee_precision_scenarios() {
+        // 1 token * 10% = 0.1 tokens
+        assert_eq!(compute_fee(10_000_000, 1000), 1_000_000);
+
+        // minimum non-zero (1e-7 tokens) * 1000 bps (10%)
+        // = 1 * 1000 / 10000 = 0.1 → rounds to 0
+        assert_eq!(compute_fee(1, 1000), 0);
+
+        // 0.0000001 * 5001 bps ≈ 0.5001 → rounds up to 1
+        assert_eq!(compute_fee(1, 5001), 1);
+
+        // 0.0000001 * 4999 bps ≈ 0.4999 → rounds down to 0
+        assert_eq!(compute_fee(1, 4999), 0);
+
+        // exact half at 5000 rounds up
+        assert_eq!(compute_fee(1, 5000), 1);
+    }
+
+    #[test]
+    fn test_invariant_error_bounded_edge_cases() {
+        let rates = [MIN_FEE_RATE_BPS, 50, 100, 500, MAX_FEE_RATE_BPS];
+        let amounts = [1, 10_000_000, 100_000_000, MAX_SETTLEMENT];
+
+        for &amount in &amounts {
+            for &rate_bps in &rates {
+                assert!(
+                    invariant_error_bounded(amount, rate_bps),
+                    "Invariant violated: amount={}, rate_bps={}, fee={}",
+                    amount,
+                    rate_bps,
+                    compute_fee(amount, rate_bps)
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn test_micro_settlement_cumulative_fairness() {
+        let rate_bps = 100;
+        let micro_amount: i128 = 1;
+        let num_transactions: i128 = 1_000;
+
+        // Each micro-transaction yields fee = (1*100 + 5000) / 10000 = 0
+        // Total collected = 0
+        let mut total_fee_collected: i128 = 0;
+        for _ in 0..num_transactions {
+            total_fee_collected += compute_fee(micro_amount, rate_bps);
+        }
+        assert_eq!(total_fee_collected, 0);
+
+        // A single lump-sum of same total amount
+        let lump_fee = compute_fee(num_transactions * micro_amount, rate_bps);
+
+        // The difference is due to rounding — each micro-txn loses ~0.01 units
+        let diff = (total_fee_collected - lump_fee).abs();
+        // With per-txn error ≤ 5000, total error ≤ N * 5000
+        let max_per_txn_error_exact = 5000i128;
+        let max_cumulative_error = num_transactions * max_per_txn_error_exact;
+        assert!(
+            diff <= max_cumulative_error,
+            "Cumulative error {} exceeds max {} (N={}, err/txn={})",
+            diff,
+            max_cumulative_error,
+            num_transactions,
+            max_per_txn_error_exact
+        );
+    }
+
+    #[test]
+    fn test_cumulative_fee_equivalence() {
+        // With amounts where each individual fee rounds the same way,
+        // cumulative and lump-sum should agree within 1 unit.
+        let rate_bps = 100;
+        let num_transactions: i128 = 100;
+        let per_txn_amount: i128 = 10000; // each yields fee = (10000*100+5000)/10000 = 100
+
+        let mut total_fee_collected: i128 = 0;
+        for _ in 0..num_transactions {
+            total_fee_collected += compute_fee(per_txn_amount, rate_bps);
+        }
+
+        let lump_fee = compute_fee(per_txn_amount * num_transactions, rate_bps);
+
+        let diff = (total_fee_collected - lump_fee).abs();
+        assert!(
+            diff <= 1,
+            "Cumulative vs lump-sum fee mismatch: {} vs {} (diff={})",
+            total_fee_collected,
+            lump_fee,
+            diff
+        );
+    }
+
+    #[test]
+    fn test_fee_monotonicity() {
+        let rate_bps = 100;
+        let mut prev_fee: i128 = 0;
+        for i in 0..1000 {
+            let fee = compute_fee(i, rate_bps);
+            assert!(
+                fee >= prev_fee,
+                "Fee decreased: amount={}, prev_fee={}, fee={}",
+                i,
+                prev_fee,
+                fee
+            );
+            prev_fee = fee;
+        }
+    }
+
+    #[test]
+    fn test_zero_rate_returns_zero() {
+        assert_eq!(compute_fee(1_000_000, 0), 0);
+        assert_eq!(compute_fee(0, 100), 0);
+        assert_eq!(compute_fee(0, 0), 0);
+    }
+
+    #[test]
+    fn test_large_amount_no_overflow() {
+        let amount: i128 = 1_000_000_000_000_000_000;
+        let rate_bps = 1000;
+        let fee = compute_fee(amount, rate_bps);
+        assert_eq!(fee, 100_000_000_000_000_000);
+    }
+}

contracts/settlement/src/lib.rs

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+#![no_std]
+
+mod constants;
+mod fees;
+mod token_utils;
+
+use soroban_sdk::{contract, contractimpl, contracterror, panic_with_error, Address, Env};
+
+#[cfg(test)]
+mod test;
+
+use crate::constants::MAX_FEE_RATE_BPS;
+use crate::fees::compute_fee;
+use crate::token_utils::collect_fee;
+
+#[contracterror]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
+#[repr(u32)]
+pub enum SettlementError {
+    InvalidFeeRate = 1,
+    InsufficientBalance = 2,
+}
+
+#[contract]
+pub struct SettlementContract;
+
+#[contractimpl]
+impl SettlementContract {
+    /// Settle a payment and collect the protocol fee.
+    ///
+    /// # Arguments
+    /// * `env` - Contract environment
+    /// * `token` - Token contract address
+    /// * `payer` - Address paying the settlement
+    /// * `payee` - Address receiving the net settlement
+    /// * `fee_collector` - Address collecting the protocol fee
+    /// * `amount` - Gross settlement amount
+    /// * `rate_bps` - Fee rate in basis points
+    ///
+    /// # Returns
+    /// (net_amount, fee_amount)
+    pub fn settle(
+        env: Env,
+        token: Address,
+        payer: Address,
+        payee: Address,
+        fee_collector: Address,
+        amount: i128,
+        rate_bps: u32,
+    ) -> (i128, i128) {
+        if rate_bps > MAX_FEE_RATE_BPS {
+            panic_with_error!(&env, SettlementError::InvalidFeeRate);
+        }
+
+        if amount <= 0 {
+            return (0, 0);
+        }
+
+        payer.require_auth();
+
+        let fee = collect_fee(&env, &token, &payer, &fee_collector, amount, rate_bps);
+        let net_amount = amount.saturating_sub(fee);
+
+        // Transfer net amount to payee
+        if net_amount > 0 {
+            let token_client = soroban_sdk::token::Client::new(&env, &token);
+            token_client.transfer(&payer, &payee, &net_amount);
+        }
+
+        (net_amount, fee)
+    }
+
+    /// Compute the fee for a given amount and rate (pure, no side effects).
+    pub fn calculate_fee(_env: Env, amount: i128, rate_bps: u32) -> i128 {
+        compute_fee(amount, rate_bps)
+    }
+}