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

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Lift the inverse constant-product SwapExactOutput math out of the guest's
exact_output_swap_logic into amm_core::swap_exact_out_amounts(amount_out,
reserve_in, reserve_out, fee_bps) -> Option<(effective_in, required_in)>:
ceil(reserve_in * amount_out / (reserve_out - amount_out)) lifted through the
fee via mul_div_ceil. exact_output_swap_logic now calls it and keeps its
nonzero/exceeds-reserve asserts. Behavior-preserving (the panic-message tests
still pass); None (out >= reserve or zero fee multiplier) surfaces as the
existing expect.

Makes the on-chain exact-output pricing one reusable function so the
off-chain quote can produce byte-identical required-input figures instead of
re-deriving the inverse formula. Mirrors swap_exact_in_amounts.
This commit is contained in:
r4bbit
2026-08-06 14:31:49 +02:00
parent abc6d27a9f
commit cd8a84374b
2 changed files with 75 additions and 25 deletions
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programs/amm/src/swap.rs
+12 -25
View File
@@ -1,7 +1,7 @@
use amm_core::{
assert_supported_fee_tier, compute_config_pda, compute_pool_pda_seed, mul_div_ceil,
read_vault_fungible_balances, spot_price_q64_64, swap_exact_in_amounts, AmmConfig,
FEE_BPS_DENOMINATOR, MINIMUM_LIQUIDITY,
assert_supported_fee_tier, compute_config_pda, compute_pool_pda_seed,
read_vault_fungible_balances, spot_price_q64_64, swap_exact_in_amounts, swap_exact_out_amounts,
AmmConfig, MINIMUM_LIQUIDITY,
};
pub use amm_core::{compute_liquidity_token_pda_seed, compute_vault_pda_seed, PoolDefinition};
use clock_core::CLOCK_01_PROGRAM_ACCOUNT_ID;
@@ -514,29 +514,16 @@ fn exact_output_swap_logic(
"Exact amount out exceeds reserve"
);
// Compute the minimum effective input required to achieve exact_amount_out
// using the same floor-rounded fee application as swap_exact_input.
//
// Solve constant product for effective_in (fee already removed):
// effective_in >= ceil(reserve_in * amount_out / (reserve_out - amount_out))
// ceil(reserve_in * amount_out / (reserve_out - amount_out)). The `reserve_in * amount_out`
// product is widened to U256; the denominator is a subtraction that stays u128.
let effective_in_denominator = reserve_withdraw_vault_amount
.checked_sub(exact_amount_out)
.expect("reserve_out - amount_out underflows");
let effective_in_min = mul_div_ceil(
reserve_deposit_vault_amount,
// Required gross input via the shared amm_core::swap_exact_out_amounts (same
// pricing as the off-chain exact-output quote). The `amount_out < reserve`
// guard above means it always resolves.
let (_, deposit_amount) = swap_exact_out_amounts(
exact_amount_out,
effective_in_denominator,
);
// Lift back to gross input so that
// floor(gross_in * (FEE_DENOM - fee) / FEE_DENOM) >= effective_in_min
// ceil(effective_in_min * FEE_BPS_DENOMINATOR / fee_multiplier), product widened to U256.
let fee_multiplier = FEE_BPS_DENOMINATOR
.checked_sub(fee_bps)
.expect("fee_bps exceeds fee denominator");
let deposit_amount = mul_div_ceil(effective_in_min, FEE_BPS_DENOMINATOR, fee_multiplier);
reserve_deposit_vault_amount,
reserve_withdraw_vault_amount,
fee_bps,
)
.expect("swap exact output: reserves and fee must yield a valid input");
// Slippage check
assert!(