Function adjust_market_buy_amount 

pub fn adjust_market_buy_amount(
    amount: Decimal,
    user_pusd_balance: Decimal,
    price: Decimal,
    fee_rate: Decimal,
    fee_exponent: Decimal,
    builder_taker_fee_rate: Decimal,
) -> Result<Decimal>

Adjusts a market-BUY pUSD amount to fit within the user’s pUSD balance once platform and builder taker fees are deducted. Mirrors adjust_market_buy_amount in polymarket-rs-clob-client-v2’s clob/utilities.rs.

Returns amount unchanged when the balance already covers amount + fees. Otherwise solves for the principal that, with fees, exactly consumes the balance, then truncates to USDC_DECIMALS (the on-chain pUSD scale).

The fee-curve step (p * (1 - p))^exponent is the only computation that crosses into f64, matching the reference SDK so we agree with the venue’s authoritative match-time fee calculation regardless of whether Polymarket ships a fractional exponent in the future.

price must be strictly inside (0, 1). The SDK relies on its order-builder pipeline to enforce this. Because adjust_market_buy_amount is public, it repeats the precondition here.

Errors

Returns an error if price is outside the open (0, 1) interval, or if amount or balance is non-positive, a fee input is negative, arithmetic overflows, or the adjusted amount truncates to zero.