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TIP-1005: Fix Ask Swap Rounding Loss

Status: Draft
Authors: Dan Robinson

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Abstract

This TIP fixes a rounding bug in the swapExactAmountIn function when filling ask orders. Due to double-rounding, the maker can receive slightly less quote tokens than the taker paid, causing tokens to be lost.

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Motivation

When a taker swaps quote tokens for base tokens against an ask order, the following calculation occurs:
  1. Convert taker’s amountIn (quote) to base: base_out = floor(amountIn / price)
  2. Credit maker with quote: makerReceives = ceil(base_out * price)
Due to the floor in step 1, makerReceives can be less than amountIn. For example:
  • Taker pays amountIn = 102001 quote at price 1.02 (tick 2000)
  • base_out = floor(102001 / 1.02) = 100000
  • makerReceives = ceil(100000 * 1.02) = 102000
  • 1 token is lost
This violates the zero-sum invariant: the taker pays more than the maker receives. It also means there is no canonical amount swapped—the trade for the maker is different from the trade for the taker.

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Specification

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Bug Location

The bug is in _fillOrdersExactIn when processing ask orders (the baseForQuote = false path). Specifically, when a partial fill occurs:
  1. fillAmount (base) is calculated by rounding down: baseOut = (remainingIn * PRICE_SCALE) / price
  2. _fillOrder is called with fillAmount
  3. Inside _fillOrder, the maker’s quote credit is re-derived: quoteAmount = ceil(fillAmount * price)
The re-derivation in step 3 loses the original remainingIn information.

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Fix

For partial fills in the ask path, pass the actual remainingIn (quote) to _fillOrder and use it directly for the maker’s credit, rather than re-deriving it from fillAmount. The fix requires:
  1. Modify _fillOrder to accept an optional quoteOverride parameter for ask orders
  2. In _fillOrdersExactIn, when partially filling an ask, pass remainingIn as the quote override
  3. When quoteOverride is provided, use it directly for the maker’s balance increment instead of computing ceil(fillAmount * price)

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Reference Implementation Changes

The fix requires changes to two functions in docs/specs/src/StablecoinDEX.sol:

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1. _fillOrder

Add an optional quoteOverride parameter. When non-zero and the order is an ask, use quoteOverride directly for the maker’s balance increment: 
// Before:
uint128 quoteAmount =
    uint128((uint256(fillAmount) * uint256(price) + PRICE_SCALE - 1) / PRICE_SCALE);
balances[order.maker][book.quote] += quoteAmount;

// After:
uint128 quoteAmount = quoteOverride > 0
    ? quoteOverride
    : uint128((uint256(fillAmount) * uint256(price) + PRICE_SCALE - 1) / PRICE_SCALE);
balances[order.maker][book.quote] += quoteAmount;

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2. _fillOrdersExactIn

In the partial fill branch for asks, pass remainingIn as the quote override: 
// Before:
orderId = _fillOrder(orderId, fillAmount);

// After (for partial fills where fillAmount == baseOut):
orderId = _fillOrder(orderId, fillAmount, remainingIn);

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Affected Code Paths

  • _fillOrdersExactIn with baseForQuote = false (ask path), partial fill case only
  • Full fills are not affected because the quote amount is derived from order.remaining, not remainingIn
  • Bid swaps are not affected because the taker pays base tokens directly

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Example: Before and After

Before (buggy): 
amountIn = 102001 quote
base_out = floor(102001 / 1.02) = 100000
makerReceives = ceil(100000 * 1.02) = 102000
Lost: 1 token
After (fixed): 
amountIn = 102001 quote
base_out = floor(102001 / 1.02) = 100000
makerReceives = 102001 (passed directly)
Lost: 0 tokens

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Invariants

  • Zero-sum: for any swap, takerPaid == makerReceived (within the same token)
  • Taker receives floor(amountIn / price) base tokens (rounds in favor of protocol)
  • Maker receives exactly what taker paid in quote tokens