inverse formula

__tests__/bancorx.test.ts

@@ -36,19 +36,19 @@ test("bancorx.bancorFormula - EOS/BNT", () => {
     });
 });
 
-test.skip("bancorx.bancorInverseFormula - EOS/BNT", () => {
-  const balanceFrom = split(`77814.0638 EOS`);
-  const balanceTo = split(`429519.5539120331 BNT`);
-  const amountDesired = split(`1.0000000000 BNT`);
-  expect(balanceFrom.symbol.precision).toBe(4);
-  expect(balanceTo.symbol.precision).toBe(10);
-  expect(amountDesired.symbol.precision).toBe(10);
-
-  expect(
-    bancorx
-      .bancorInverseFormula(balanceFrom, balanceTo, amountDesired)
-      .toNumber()
-  ).toBe(0.1811657798);
+test("bancorx.bancorInverseFormula - EOS/BNT", () => {
+  trades
+    .map(([amount, bluBalance, redBalance, reward]) => [
+      split(amount),
+      split(bluBalance),
+      split(redBalance),
+      split(reward)
+    ])
+    .forEach(([amount, blueBalance, redBalance, reward]) => {
+      expect(
+        bancorx.bancorInverseFormula(blueBalance, redBalance, reward)
+      ).toEqual(amount);
+    });
 });
 
 test("bancorx.composeMemo", () => {

package.json

@@ -1,6 +1,6 @@
 {
     "name": "bancorx",
-    "version": "0.2.65",
+    "version": "0.2.66",
     "description": "BancorX Utility",
     "main": "build/index.js",
     "types": "index.d.ts",

src/index.ts

@@ -83,10 +83,21 @@ export function bancorInverseFormula(
 ) {
   if (!balanceTo.symbol.isEqual(amountDesired.symbol))
     throw new Error("From symbol does not match amount symbol");
-  const reward =
-    balanceFrom.amount / (1.0 - amountDesired.amount / balanceTo.amount) -
-    balanceFrom.amount;
-  return new Asset(reward, amountDesired.symbol);
+  const balanceFromNumber = balanceFrom.toDecimal()
+  const balanceToNumber = balanceTo.toDecimal()
+  const amountNumber = amountDesired.toDecimal()
+  const oneNumber = new Decimal(1);
+
+  const reward = balanceFromNumber
+    .div(oneNumber.minus(amountNumber.div(balanceToNumber)))
+    .minus(balanceFromNumber)
+    .toFixed(0, Decimal.ROUND_UP);
+
+  const formatted = new Decimal(
+    Number(reward) * Math.pow(10, balanceTo.symbol.precision)
+  ).toFixed(0, Decimal.ROUND_DOWN);
+
+  return new Asset(Number(formatted), balanceFrom.symbol);
 }
 
 /**