make computePairAddress available to users (#59)

* make computePairAddress available to users

* move sort check to computePairAddress

src/entities/pair.test.ts

 import { ChainId, Token, TokenAmount, WETH9, Price } from '@uniswap/sdk-core'
 import { InsufficientInputAmountError } from '../errors'
-import { Pair } from './pair'
+import { computePairAddress, Pair } from './pair'
+
+describe('computePairAddress', () => {
+  it('should correctly compute the pool address', () => {
+    const tokenA = new Token(ChainId.MAINNET, '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48', 18, 'USDC', 'USD Coin')
+    const tokenB = new Token(ChainId.MAINNET, '0x6B175474E89094C44Da98b954EedeAC495271d0F', 18, 'DAI', 'DAI Stablecoin')
+    const result = computePairAddress({
+      factoryAddress: '0x1111111111111111111111111111111111111111',
+      tokenA,
+      tokenB
+    })
+
+    expect(result).toEqual('0xb50b5182D6a47EC53a469395AF44e371d7C76ed4')
+  })
+  it('should give same result regardless of token order', () => {
+    const USDC = new Token(ChainId.MAINNET, '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48', 18, 'USDC', 'USD Coin')
+    const DAI = new Token(ChainId.MAINNET, '0x6B175474E89094C44Da98b954EedeAC495271d0F', 18, 'DAI', 'DAI Stablecoin')
+    let tokenA = USDC
+    let tokenB = DAI
+    const resultA = computePairAddress({
+      factoryAddress: '0x1111111111111111111111111111111111111111',
+      tokenA,
+      tokenB
+    })
+
+    tokenA = DAI
+    tokenB = USDC
+    const resultB = computePairAddress({
+      factoryAddress: '0x1111111111111111111111111111111111111111',
+      tokenA,
+      tokenB
+    })
+
+    expect(resultA).toEqual(resultB)
+  })
+})
 
 describe('Pair', () => {
   const USDC = new Token(ChainId.MAINNET, '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48', 18, 'USDC', 'USD Coin')

src/entities/pair.ts

@@ -8,18 +8,28 @@ import babylonianSqrt from '../utils/babylonianSqrt'
 import { FACTORY_ADDRESS, INIT_CODE_HASH, MINIMUM_LIQUIDITY, FIVE, _997, _1000, ONE, ZERO } from '../constants'
 import { InsufficientReservesError, InsufficientInputAmountError } from '../errors'
 
+export const computePairAddress = ({
+  factoryAddress,
+  tokenA,
+  tokenB
+}: {
+  factoryAddress: string
+  tokenA: Token
+  tokenB: Token
+}): string => {
+  const [token0, token1] = tokenA.sortsBefore(tokenB) ? [tokenA, tokenB] : [tokenB, tokenA] // does safety checks
+  return getCreate2Address(
+    factoryAddress,
+    keccak256(['bytes'], [pack(['address', 'address'], [token0.address, token1.address])]),
+    INIT_CODE_HASH
+  )
+}
 export class Pair {
   public readonly liquidityToken: Token
   private readonly tokenAmounts: [TokenAmount, TokenAmount]
 
   public static getAddress(tokenA: Token, tokenB: Token): string {
-    const tokens = tokenA.sortsBefore(tokenB) ? [tokenA, tokenB] : [tokenB, tokenA] // does safety checks
-
-    return getCreate2Address(
-      FACTORY_ADDRESS,
-      keccak256(['bytes'], [pack(['address', 'address'], [tokens[0].address, tokens[1].address])]),
-      INIT_CODE_HASH
-    )
+    return computePairAddress({ factoryAddress: FACTORY_ADDRESS, tokenA, tokenB })
   }
 
   public constructor(tokenAmountA: TokenAmount, tokenAmountB: TokenAmount) {