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fix(amm): BigInt-safe ratio guard for estimateSwapOutput #1018

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fix(amm): BigInt-safe ratio guard for estimateSwapOutput #1018

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33 changes: 30 additions & 3 deletions src/ammCalculator.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -83,9 +83,11 @@ export function estimateSwapOutput(
};
}

// Check against max ratio threshold
const ratio = Number(amountIn) / Number(reserveIn);
if (ratio > maxRatio) {
// Issue #1007 — Check against max ratio threshold without coercing
// arbitrary-size BigInts to Number. Both operands can exceed
// Number.MAX_VALUE, where Infinity / Infinity would otherwise become NaN
// and silently bypass this guard.
if (ratioExceedsLimit(amountIn, reserveIn, maxRatio)) {
throw new InsufficientLiquidityError(
`Input amount exceeds ${(maxRatio * 100).toFixed(0)}% of pool reserves`,
inputReserve.amount,
Expand Down Expand Up @@ -409,6 +411,31 @@ function formatScaled(value: bigint, decimals: number): string {
return `${intPart}.${fracPart.toString().padStart(decimals, "0")}`;
}

// Issue #1007 — cross-multiply BigInts against the limit ratio so that
// arbitrary-size inputs (e.g. 10^400) are not coerced to Infinity and then
// NaN by Number(). This replaces the prior `Number(amountIn) / Number(reserveIn) > maxRatio`.
function ratioExceedsLimit(amount: bigint, reserve: bigint, limit: number): boolean {
if (!Number.isFinite(limit)) {
// Preserve the prior comparison semantics for non-finite caller values:
// NaN/+Infinity never reject, while -Infinity rejects every positive ratio.
return limit === -Infinity;
}

const [coefficient, exponentText] = limit.toString().toLowerCase().split("e");
const [integerPart, fractionalPart = ""] = coefficient!.split(".");
let numerator = BigInt(`${integerPart}${fractionalPart}`);
let denominator = 10n ** BigInt(fractionalPart.length);
const exponent = Number(exponentText ?? "0");

if (exponent > 0) {
numerator *= 10n ** BigInt(exponent);
} else if (exponent < 0) {
denominator *= 10n ** BigInt(-exponent);
}

return amount * denominator > reserve * numerator;
}

function computeSpotPrice(reserveIn: bigint, reserveOut: bigint): string {
// spotPrice = reserveOut / reserveIn as a decimal string
if (reserveIn === 0n) return "0";
Expand Down