Router: Swapping

Key takeaway: Uniswap V2's Router exposes six swap variants (exact-in vs exact-out, token vs ETH on either side), all built on a single internal _swap() that loops through a path of pairs. The key gas optimization: intermediate hops send their output token directly to the next pair in the path, so the Router never touches intermediate tokens. This saves roughly 10-15k gas per hop versus routing through the Router twice. amountOutMin (or amountInMax for exact-out) provides slippage protection against MEV and stale-mempool execution.

What You Are Building

This is the heart of the Router. Every token trade on Uniswap V2 flows through these functions. There are six external swap variants covering every combination of exact input vs. exact output, and token vs. ETH on either side. All of them rely on a single internal _swap() function that handles the core routing logic. Understanding _swap() deeply is the key to understanding every swap variant.