feat: Add swap external function to AMM contract

[2PykeETH]

Summary

This PR introduces the swap external function to the TestAMMContract, enabling users to swap between token pairs within a prediction market pool. The function is built on the constant product formula (x * y = k) to preserve AMM invariants and includes slippage protection via a minimum output amount check.

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Changes

• New Function: swap

  • Parameters:

    • _marketId — unique identifier of the prediction market pool.
    • _amountIn — amount of input tokens provided by the user.
    • _amountOutMinimum — minimum acceptable output amount (slippage protection).
    • _zeroForOne — swap direction flag (true = tokenA → tokenB, false = tokenB → tokenA).

  • Core Logic:

    • Determines input/output tokens and reserves based on _zeroForOne.
    • Validates pool is initialized and has sufficient reserves.
    • Uses constant product formula:

      amountOut = (reserveOut * amountIn) / (reserveIn + amountIn)
      

      Ensures the invariant (reserveIn + amountIn) * (reserveOut - amountOut) = reserveIn * reserveOut.
    • Transfers input tokens from the user to the pool.
    • Updates pool reserves (reserveA and reserveB).
    • Transfers output tokens back to the user.
    • Emits TokensSwapped event with details of the swap.

• Event Emission:

  • Reuses the TokensSwapped event for transparency and analytics.

• Formatting:

  • Applied forge fmt for style consistency.

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Motivation

• Core AMM Functionality: Provides the essential swap feature, enabling token exchanges within market-specific liquidity pools.
• User Protection: Introduces _amountOutMinimum check to mitigate slippage attacks and guarantee minimum expected returns.
• Protocol Growth: With addLiquidity, removeLiquidity, and now swap, the AMM contract supports the full lifecycle of pool interactions.

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Next Steps

• Add unit/integration tests for swap:
  • Successful swap with sufficient liquidity.
  • Failure when amountOut < _amountOutMinimum.
  • Failure when pool reserves are insufficient.
  • Edge cases (e.g., very small input amounts, large swaps relative to reserves).
• Future optimization: Consider adding a swap fee mechanism to incentivize liquidity providers.