Solve InterviewCake's cafe-order-checker problem

Write a predicate that tests if a given list of integers, zs, is a possible
interleaving of two other lists, xs and ys.

scratch/deepmind/part_two/cafe-order-checker.py

@@ -0,0 +1,64 @@
+import unittest
+
+
+# Solution
+def is_first_come_first_served(xs, ys, zs):
+    i, j = 0, 0
+    for z in zs:
+        if i < len(xs) and z == xs[i]:
+            i += 1
+        elif j < len(ys) and z == ys[j]:
+            j += 1
+        else:
+            return False
+    return i == len(xs) and j == len(ys)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_both_registers_have_same_number_of_orders(self):
+        result = is_first_come_first_served([1, 4, 5], [2, 3, 6],
+                                            [1, 2, 3, 4, 5, 6])
+        self.assertTrue(result)
+
+    def test_registers_have_different_lengths(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 2, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_one_register_is_empty(self):
+        result = is_first_come_first_served([], [2, 3, 6], [2, 3, 6])
+        self.assertTrue(result)
+
+    def test_served_orders_is_missing_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_served_orders_has_extra_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6],
+                                            [1, 2, 3, 5, 6, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_extra_orders(self):
+        result = is_first_come_first_served([1, 9], [7, 8], [1, 7, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_unserved_orders(self):
+        result = is_first_come_first_served([55, 9], [7, 8], [1, 7, 8, 9])
+        self.assertFalse(result)
+
+    # Bonus
+    def test_handles_repeats(self):
+        actual = is_first_come_first_served([1, 2, 1], [3, 4, 5, 5],
+                                            [3, 4, 1, 5, 5, 2, 1])
+        self.assertTrue(actual)
+
+    def test_kitchen_didnt_serve(self):
+        actual = is_first_come_first_served([1, 2], [3, 4], [1, 3, 4])
+        self.assertFalse(actual)
+
+    def test_customer_didnt_pay(self):
+        actual = is_first_come_first_served([2], [3, 4], [1, 3, 4])
+        self.assertFalse(actual)
+
+
+unittest.main(verbosity=2)