1 files changed, 91 insertions, 0 deletions
diff --git a/universe/data_structures_and_algorithms/cafe-order-checker.py b/universe/data_structures_and_algorithms/cafe-order-checker.py
new file mode 100644
index 000000000000..e34a2b136ab6
--- /dev/null
+++ b/universe/data_structures_and_algorithms/cafe-order-checker.py
@@ -0,0 +1,91 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+def is_first_come_first_served(to, di, xs):
+    # All the guards, assertions we should need.
+    if to == di == xs == []:
+        return True
+    elif to == di == []:
+        return False
+    elif to == []:
+        return di == xs
+    elif to == []:
+        return di == xs
+    elif di == []:
+        return to == xs
+    elif xs == []:
+        return False
+    elif len(xs) != (len(to) + len(di)):
+        return False
+
+    fst, snd = to, di
+
+    if xs[0] == to[0]:
+        fst, snd = to, di
+    elif xs[0] == di[0]:
+        fst, snd = di, to
+    else:
+        return False
+
+    fst_done, snd_done = False, False
+    fi, si = 1, 0
+
+    for i in range(1, len(xs)):
+        # Short-circuit and avoid index-out-of-bounds without introducing overly
+        # defensive, sloppy code.
+        if fst_done:
+            return snd[si:] == xs[i:]
+        elif snd_done:
+            return fst[fi:] == xs[i:]
+
+        if fst[fi] == xs[i]:
+            fi += 1
+        elif snd[si] == xs[i]:
+            si += 1
+        else:
+            return False
+
+        fst_done, snd_done = fi == len(fst), si == len(snd)
+
+    return True
+
+
+################################################################################
+# 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)
+
+
+unittest.main(verbosity=2)