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refactor(amm): extract swap-output formula into amm_core

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Lift the constant-product SwapExactInput math out of the guest's swap_logic
into amm_core::swap_exact_in_amounts(amount_in, reserve_in, reserve_out,
fee_bps) -> (effective_in, amount_out). swap_logic now calls it and keeps its
nonzero input/output asserts. Behavior-preserving (the two panic-message
tests still pass); the only change is that the impossible-overflow expects
become saturating.

Makes the on-chain pricing one reusable function so the off-chain swap quote
can produce byte-identical expected-output figures instead of re-deriving the
formula.
This commit is contained in:
r4bbit
2026-08-05 22:54:37 +02:00
parent 6a951f3cad
commit c3dc9dd94f
2 changed files with 59 additions and 21 deletions
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programs/amm/src/swap.rs
+13 -21
View File
@@ -1,7 +1,7 @@
use amm_core::{
assert_supported_fee_tier, compute_config_pda, compute_pool_pda_seed, mul_div_ceil,
mul_div_floor, read_vault_fungible_balances, spot_price_q64_64, AmmConfig, FEE_BPS_DENOMINATOR,
MINIMUM_LIQUIDITY,
read_vault_fungible_balances, spot_price_q64_64, swap_exact_in_amounts, AmmConfig,
FEE_BPS_DENOMINATOR, MINIMUM_LIQUIDITY,
};
pub use amm_core::{compute_liquidity_token_pda_seed, compute_vault_pda_seed, PoolDefinition};
use clock_core::CLOCK_01_PROGRAM_ACCOUNT_ID;
@@ -291,29 +291,21 @@ fn swap_logic(
reserve_withdraw_vault_amount: u128,
pool_id: AccountId,
) -> (Vec<ChainedCall>, u128, u128) {
let fee_multiplier = FEE_BPS_DENOMINATOR
.checked_sub(fee_bps)
.expect("fee_bps exceeds fee denominator");
// floor(swap_amount_in * fee_multiplier / FEE_BPS_DENOMINATOR), product widened to U256.
let effective_amount_in = mul_div_floor(swap_amount_in, fee_multiplier, FEE_BPS_DENOMINATOR);
// Fee-adjust the input and price via constant product. Shared with the
// off-chain swap quote (`amm_core::swap_exact_in_amounts`) so the preview and
// the executed trade agree exactly. The recorded pool reserves are updated
// later with the full `swap_amount_in`, so LP fees accrue inside `reserve_*`
// via invariant growth rather than as a vault-balance surplus over `reserve_*`.
let (effective_amount_in, withdraw_amount) = swap_exact_in_amounts(
swap_amount_in,
reserve_deposit_vault_amount,
reserve_withdraw_vault_amount,
fee_bps,
);
assert!(
effective_amount_in != 0,
"Effective swap amount should be nonzero"
);
// Compute the withdraw amount using the fee-adjusted input for pricing.
// The recorded pool reserves are updated later with the full
// `swap_amount_in`, so LP fees accrue inside `reserve_*` via invariant
// growth rather than as a separate vault balance surplus over `reserve_*`.
// The denominator sum stays u128 (overflows only near u128::MAX, an unstorable reserve);
// only the `reserve * effective` product is widened to U256.
let reserve_plus_effective = reserve_deposit_vault_amount
.checked_add(effective_amount_in)
.expect("reserve + effective_amount_in overflows u128");
let withdraw_amount = mul_div_floor(
reserve_withdraw_vault_amount,
effective_amount_in,
reserve_plus_effective,
);
// Slippage check
assert!(