add changes, i know not a good commit

src/Market.php

@@ -1,18 +1,79 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 final class Market
 {
+    /**
+     * @var PriceList
+     */
+    private $milkPriceList;
+
+    public function __construct(PriceListBuilder $priceListBuilder)
+    {
+        $this->milkPriceList = $priceListBuilder->milkPrices();
+    }
+
     public function priceFor(Good $good): Pound
     {
-        return new Pound(5);
+        if ($good->isMilk()) {
+            return $this->milkPriceList->current();
+        }
+
+        $this->throwInvalidGoodException($good);
+    }
+
+    public function buyTo(Offer $offer)
+    {
+        $pound = new Pound(0);
+
+        $pound->calc($offer, $this->priceFor($offer->good()));
+
+        $this->increasePrice($offer->good());
+
+        return $pound;
+    }
+
+    private function reducePrice(Good $good): void
+    {
+        if ($good->isMilk()) {
+            $this->milkPriceList->down();
+
+            return;
+        }
+
+        $this->throwInvalidGoodException($good);
+    }
+
+    private function increasePrice(Good $good): void
+    {
+        if ($good->isMilk()) {
+            $this->milkPriceList->up();
+
+            return;
+        }
+
+        $this->throwInvalidGoodException($good);
     }
 
     public function sellTo(Offer $offer): Pound
     {
-        return new Pound(
-            $offer->amount()->amount() *
-            $this->priceFor($offer->good())->amount()
+        $pound = new Pound(0);
+
+        $pound->calc($offer, $this->priceFor($offer->good()));
+
+        $this->reducePrice($offer->good());
+
+        return $pound;
+    }
+
+    private function throwInvalidGoodException(Good $good)
+    {
+        throw new UnknownGoodException(
+            sprintf(
+                'Good with name: "%s" is unkown.',
+                get_class($good)
+            )
         );
     }
 }

src/Offer.php

@@ -1,4 +1,5 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 final class Offer
@@ -15,8 +16,8 @@ final class Offer
 
     public function __construct(Quantity $unit, Good $good)
     {
-        $this->amount = $unit;
-        $this->good   = $good;
+        $this->amount       = $unit;
+        $this->good         = $good;
     }
 
     public function amount(): Quantity

src/Pound.php

@@ -1,4 +1,5 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 final class Pound
@@ -17,4 +18,9 @@ public function amount(): int
     {
         return $this->amount;
     }
+
+    public function calc(Offer $offer, Pound $courrentPrice): void
+    {
+        $this->amount += $courrentPrice->amount() * $offer->amount()->amount();
+    }
 }

src/PriceList.php

@@ -1,4 +1,5 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 final class PriceList
@@ -30,4 +31,18 @@ public function current(): Pound
     {
         return $this->prices[$this->position];
     }
+
+    public function up()
+    {
+        if (\count($this->prices) !== $this->position + 1) {
+            $this->position++;
+        }
+    }
+
+    public function down()
+    {
+        if (-1 !== $this->position - 1) {
+            $this->position--;
+        }
+    }
 }

src/UnknownGoodException.php

@@ -0,0 +1,6 @@
+<?php declare(strict_types=1);
+namespace ClansOfCaledonia;
+
+final class UnknownGoodException extends \InvalidArgumentException implements Exception
+{
+}

tests/MarketTest.php

@@ -1,28 +1,33 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 use PHPUnit\Framework\TestCase;
 
 /**
  * @covers \ClansOfCaledonia\Market
  *
- * @uses \ClansOfCaledonia\Pound
- * @uses \ClansOfCaledonia\Good
- * @uses \ClansOfCaledonia\Offer
- * @uses \ClansOfCaledonia\Quantity
+ * @uses   \ClansOfCaledonia\Pound
+ * @uses   \ClansOfCaledonia\Good
+ * @uses   \ClansOfCaledonia\Milk
+ * @uses   \ClansOfCaledonia\Offer
+ * @uses   \ClansOfCaledonia\Quantity
+ * @uses   \ClansOfCaledonia\PriceListBuilder
+ * @uses   \ClansOfCaledonia\PriceList
  */
 final class MarketTest extends TestCase
 {
     public function testMilkCosts5PoundsInitially(): void
     {
-        $market = new Market;
+        $market = new Market(new PriceListBuilder());
 
         $this->assertEquals(new Pound(5), $market->priceFor(Good::milk()));
     }
 
     public function testMilkCanBeSoldToTheMarket(): Market
     {
-        $market = new Market;
+        $market = new Market(new PriceListBuilder());
+        $this->assertEquals(new Pound(5), $market->priceFor(Good::milk()));
 
         $payment = $market->sellTo(
             new Offer(
@@ -41,6 +46,39 @@ public function testMilkCanBeSoldToTheMarket(): Market
      */
     public function testSellingMilkToTheMarketReducesMilkPrice(Market $market): void
     {
+        //simulate a second sell because the current price is 5 and equels the default price
+        $market->sellTo(
+            new Offer(
+                new Quantity(2),
+                Good::milk()
+            )
+        );
+
         $this->assertEquals(new Pound(4), $market->priceFor(Good::milk()));
     }
+
+    public function testBuyGoods(): Market
+    {
+        $market = new Market(new PriceListBuilder());
+        $this->assertEquals(new Pound(5), $market->priceFor(Good::milk()));
+
+        $pound = $market->buyTo(
+            new Offer(
+                new Quantity(2),
+                Good::milk()
+            )
+        );
+
+        $this->assertEquals(new Pound(10), $pound);
+
+        return $market;
+    }
+
+    /**
+     * @depends testBuyGoods
+     */
+    public function testBuyGoodsIncreasePrice(Market $market)
+    {
+        $this->assertEquals(new Pound(6), $market->priceFor(Good::milk()));
+    }
 }

tests/OfferTest.php

@@ -1,13 +1,14 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 use PHPUnit\Framework\TestCase;
 
 /**
  * @covers \ClansOfCaledonia\Offer
  *
- * @uses \ClansOfCaledonia\Good
- * @uses \ClansOfCaledonia\Quantity
+ * @uses   \ClansOfCaledonia\Good
+ * @uses   \ClansOfCaledonia\Quantity
  */
 final class OfferTest extends TestCase
 {
@@ -17,4 +18,12 @@ public function testHasAmount(): void
 
         $this->assertEquals(new Quantity(1), $offer->amount());
     }
+
+    public function testHasGood(): void
+    {
+        $offer = new Offer(new Quantity(1), Good::milk());
+
+        $this->assertEquals(new Milk(), $offer->good());
+    }
+
 }

tests/PoundTest.php

@@ -1,10 +1,16 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 use PHPUnit\Framework\TestCase;
 
 /**
  * @covers \ClansOfCaledonia\Pound
+ *
+ * @uses   \ClansOfCaledonia\Milk
+ * @uses   \ClansOfCaledonia\Offer
+ * @uses   \ClansOfCaledonia\Quantity
+ * @uses   \ClansOfCaledonia\Good
  */
 final class PoundTest extends TestCase
 {
@@ -16,4 +22,19 @@ public function testHasAmount(): void
 
         $this->assertSame($amount, $p->amount());
     }
+
+    public function testCalcIncreaseThePoundAndThePrice(): void
+    {
+        $pound = new Pound(100);
+
+        $pound->calc(
+            new Offer(
+                new Quantity(2),
+                Good::milk()
+            ),
+            new Pound(2)
+        );
+
+        $this->assertSame(104, $pound->amount());
+    }
 }

tests/PriceListTest.php

@@ -1,16 +1,17 @@
 <?php declare(strict_types=1);
+
 namespace ClansOfCaledonia;
 
 use PHPUnit\Framework\TestCase;
 
 /**
  * @covers \ClansOfCaledonia\PriceList
  *
- * @uses \ClansOfCaledonia\Pound
+ * @uses   \ClansOfCaledonia\Pound
  */
 final class PriceListTest extends TestCase
 {
-    public function testHasInitialPrice(): void
+    public function testHasInitialPrice(): PriceList
     {
         $prices = PriceList::fromList(
             new Pound(1),
@@ -26,5 +27,56 @@ public function testHasInitialPrice(): void
         );
 
         $this->assertEquals(new Pound(4), $prices->current());
+
+        return $prices;
+    }
+
+    /**
+     * @depends testHasInitialPrice
+     */
+    public function testCountUp(PriceList $prices): PriceList
+    {
+        $prices->up();
+
+        $this->assertEquals(new Pound(5), $prices->current());
+
+        return $prices;
+    }
+
+    /**
+     * @depends testCountUp
+     */
+    public function testCountUpAndRunsNotOutOfIndex(PriceList $prices): PriceList
+    {
+        for ($x = 0; $x < 10; $x++) {
+            $prices->up();
+        }
+
+        $this->assertEquals(new Pound(10), $prices->current());
+
+        return $prices;
+    }
+
+    /**
+     * @depends testCountUpAndRunsNotOutOfIndex
+     */
+    public function testCountDown(PriceList $prices): PriceList
+    {
+        $prices->down();
+
+        $this->assertEquals(new Pound(9), $prices->current());
+
+        return $prices;
+    }
+    /**
+     * @depends testCountUpAndRunsNotOutOfIndex
+     */
+    public function testCountDownRunsNotOutOfIndex(PriceList $prices): void
+    {
+       for ($x = 0; $x < 20; $x++) {
+            $prices->down();
+        }
+
+        $this->assertEquals(new Pound(1), $prices->current());
     }
 }