Bugfix/throw error on divergence (#83)

* throw an error if iterate rates does not converge

* update package version

* removing unary ++

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@notional-finance/sdk-v2",
-  "version": "0.5.0-beta.32",
+  "version": "0.5.1",
   "description": "Notional Finance SDK V2",
   "author": "Jeff Wu <jeff@notional.finance>",
   "homepage": "https://notional.finance",

src/system/Market.ts

@@ -519,10 +519,13 @@ export default class Market {
 
     // Calculate derivative
     let delta = BigNumber.from(0);
+    let numLoops = 0;
     do {
       const exchangeRate = this.getExchangeRate(totalCashUnderlying, rateScalar, rateAnchor, fCashGuess);
       delta = this.calculateDelta(cashAmount, rateScalar, fCashGuess, totalCashUnderlying, exchangeRate, feeRate);
       fCashGuess = TypedBigNumber.from(fCashGuess.n.sub(delta), fCashGuess.type, fCashGuess.symbol);
+      if (numLoops > 200) throw Error('Rate calculation did not converge');
+      numLoops += 1;
     } while (delta.abs().gt(0));
 
     return fCashGuess;