properly validate getInputAmount against balances

src/entities/exchange.ts

@@ -104,6 +104,7 @@ export class Exchange {
     invariant(JSBI.greaterThan(outputAmount.raw, ZERO), 'ZERO')
     invariant(JSBI.greaterThan(this.reserve0.raw, ZERO), 'ZERO')
     invariant(JSBI.greaterThan(this.reserve1.raw, ZERO), 'ZERO')
+    invariant(JSBI.lessThan(outputAmount.raw, this.reserveOf(outputAmount.token).raw), 'INSUFFICIENT_RESERVE')
 
     const inputReserve = outputAmount.token.equals(this.reserve0.token) ? this.reserve1 : this.reserve0
     const outputReserve = outputAmount.token.equals(this.reserve0.token) ? this.reserve0 : this.reserve1

src/entities/trade.ts

@@ -44,9 +44,6 @@ export class Trade {
 
   constructor(route: Route, amount: TokenAmount, tradeType: TradeType) {
     invariant(amount.token.equals(tradeType === TradeType.EXACT_INPUT ? route.input : route.output), 'TOKEN')
-    const firstExchange = route.exchanges[tradeType === TradeType.EXACT_INPUT ? 0 : route.exchanges.length - 1]
-    // ensure that the amount is strictly less that the exchange's balance
-    invariant(JSBI.lessThan(amount.raw, firstExchange.reserveOf(amount.token).raw), 'RESERVE')
     const amounts: TokenAmount[] = new Array(route.path.length)
     const nextExchanges: Exchange[] = new Array(route.exchanges.length)
     if (tradeType === TradeType.EXACT_INPUT) {