add methods for getting mid price to the pair

src/entities/pair.ts

+import { Price } from './fractions/price'
 import { TokenAmount } from './fractions/tokenAmount'
 import invariant from 'tiny-invariant'
 import JSBI from 'jsbi'
@@ -68,6 +69,32 @@ export class Pair {
     return token.equals(this.token0) || token.equals(this.token1)
   }
 
+  /**
+   * Returns the current mid price of the pair in terms of token0, i.e. the ratio of reserve1 to reserve0
+   */
+  public get token0Price(): Price {
+    return new Price(this.token0, this.token1, this.tokenAmounts[0].raw, this.tokenAmounts[1].raw)
+  }
+
+  /**
+   * Returns the current mid price of the pair in terms of token1, i.e. the ratio of reserve0 to reserve1
+   */
+  public get token1Price(): Price {
+    return new Price(this.token1, this.token0, this.tokenAmounts[1].raw, this.tokenAmounts[0].raw)
+  }
+
+  /**
+   * Return the price of the given token in terms of the other token in the pair.
+   * @param token token to return price of
+   */
+  public priceOf(token: Token): Price {
+    invariant(this.involvesToken(token), 'TOKEN')
+    return token.equals(this.token0) ? this.token0Price : this.token1Price
+  }
+
+  /**
+   * Returns the chain ID of the tokens in the pair.
+   */
   public get chainId(): ChainId {
     return this.token0.chainId
   }

test/pair.test.ts

-import { Token, Pair, TokenAmount, WETH } from '../src/entities'
-import { ChainId } from '../src/constants'
+import { ChainId, Token, Pair, TokenAmount, WETH, Price } from '../src'
 
 describe('Pair', () => {
   const USDC = new Token(ChainId.MAINNET, '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48', 18, 'USDC', 'USD Coin')
@@ -52,6 +51,40 @@ describe('Pair', () => {
     })
   })
 
+  describe('#token0Price', () => {
+    it('returns price of token0 in terms of token1', () => {
+      expect(new Pair(new TokenAmount(USDC, '101'), new TokenAmount(DAI, '100')).token0Price).toEqual(
+        new Price(DAI, USDC, '100', '101')
+      )
+      expect(new Pair(new TokenAmount(DAI, '100'), new TokenAmount(USDC, '101')).token0Price).toEqual(
+        new Price(DAI, USDC, '100', '101')
+      )
+    })
+  })
+
+  describe('#token1Price', () => {
+    it('returns price of token1 in terms of token0', () => {
+      expect(new Pair(new TokenAmount(USDC, '101'), new TokenAmount(DAI, '100')).token1Price).toEqual(
+        new Price(USDC, DAI, '101', '100')
+      )
+      expect(new Pair(new TokenAmount(DAI, '100'), new TokenAmount(USDC, '101')).token1Price).toEqual(
+        new Price(USDC, DAI, '101', '100')
+      )
+    })
+  })
+
+  describe('#priceOf', () => {
+    const pair = new Pair(new TokenAmount(USDC, '101'), new TokenAmount(DAI, '100'))
+    it('returns price of token in terms of other token', () => {
+      expect(pair.priceOf(DAI)).toEqual(pair.token0Price)
+      expect(pair.priceOf(USDC)).toEqual(pair.token1Price)
+    })
+
+    it('throws if invalid token', () => {
+      expect(() => pair.priceOf(WETH[ChainId.MAINNET])).toThrow('TOKEN')
+    })
+  })
+
   describe('#reserveOf', () => {
     it('returns reserves of the given token', () => {
       expect(new Pair(new TokenAmount(USDC, '100'), new TokenAmount(DAI, '101')).reserveOf(USDC)).toEqual(