fix calculateSlippageAmount (#1497)

src/utils/index.test.ts

@@ -50,6 +50,30 @@ describe('utils', () => {
         calculateSlippageAmount(tokenAmount, new Percent(10000, 10_000)).map((bound) => bound.toString())
       ).toEqual(['0', '200'])
     })
+    it('works for 18 decimals', () => {
+      const tokenAmount = new TokenAmount(new Token(ChainId.MAINNET, AddressZero, 18), '100')
+      expect(() => calculateSlippageAmount(tokenAmount, new Percent(-1, 10_000))).toThrow('Unexpected slippage')
+      expect(() => calculateSlippageAmount(tokenAmount, new Percent(10_001, 10_000))).toThrow('Unexpected slippage')
+      expect(calculateSlippageAmount(tokenAmount, new Percent(0, 10_000)).map((bound) => bound.toString())).toEqual([
+        '100',
+        '100',
+      ])
+      expect(calculateSlippageAmount(tokenAmount, new Percent(5, 100)).map((bound) => bound.toString())).toEqual([
+        '95',
+        '105',
+      ])
+      expect(calculateSlippageAmount(tokenAmount, new Percent(100, 10_000)).map((bound) => bound.toString())).toEqual([
+        '99',
+        '101',
+      ])
+      expect(calculateSlippageAmount(tokenAmount, new Percent(200, 10_000)).map((bound) => bound.toString())).toEqual([
+        '98',
+        '102',
+      ])
+      expect(
+        calculateSlippageAmount(tokenAmount, new Percent(10000, 10_000)).map((bound) => bound.toString())
+      ).toEqual(['0', '200'])
+    })
   })
 
   describe('#isAddress', () => {

src/utils/index.ts

@@ -66,7 +66,11 @@ export function calculateGasMargin(value: BigNumber): BigNumber {
 const ONE = new Fraction(1, 1)
 export function calculateSlippageAmount(value: CurrencyAmount, slippage: Percent): [JSBI, JSBI] {
   if (slippage.lessThan(0) || slippage.greaterThan(ONE)) throw new Error('Unexpected slippage')
-  return [value.multiply(ONE.subtract(slippage)).quotient, value.multiply(ONE.add(slippage)).quotient]
+  const decimalScaled = JSBI.exponentiate(JSBI.BigInt(10), JSBI.BigInt(value.currency.decimals))
+  return [
+    value.multiply(ONE.subtract(slippage)).multiply(decimalScaled).quotient,
+    value.multiply(ONE.add(slippage)).multiply(decimalScaled).quotient,
+  ]
 }
 
 // account is not optional