Add helper to get the swap price (#216)

packages/stableswap-sdk/src/calculator/index.ts

@@ -1,2 +1,3 @@
 export * from "./amounts";
 export * from "./curve";
+export * from "./price";

packages/stableswap-sdk/src/calculator/price.ts

@@ -0,0 +1,44 @@
+import { Price, TokenAmount } from "@saberhq/token-utils";
+import BN from "bn.js";
+
+import type { IExchangeInfo } from "..";
+import { calculateEstimatedSwapOutputAmount } from "..";
+
+/**
+ * Gets the price of the second token in the swap, i.e. "Token 1", with respect to "Token 0".
+ *
+ * To get the price of "Token 0", use `.invert()` on the result of this function.
+ * @returns
+ */
+export const calculateSwapPrice = (exchangeInfo: IExchangeInfo): Price => {
+  const reserve0 = exchangeInfo.reserves[0].amount;
+  const reserve1 = exchangeInfo.reserves[1].amount;
+
+  // We try to get at least 4 decimal points of precision here
+  // Otherwise, we attempt to swap 1% of total supply of the pool
+  // or at most, $1
+  const inputAmountNum = Math.max(
+    10_000,
+    Math.min(
+      10 ** reserve0.token.decimals,
+      Math.floor(parseInt(reserve0.toU64().div(new BN(100)).toString()))
+    )
+  );
+
+  const inputAmount = new TokenAmount(reserve0.token, inputAmountNum);
+  const outputAmount = calculateEstimatedSwapOutputAmount(
+    exchangeInfo,
+    inputAmount
+  );
+
+  const frac = outputAmount.outputAmountBeforeFees.asFraction.divide(
+    inputAmount.asFraction
+  );
+
+  return new Price(
+    reserve0.token,
+    reserve1.token,
+    frac.denominator,
+    frac.numerator
+  );
+};