[book-store] Simplify example.py (#2175)

The existing example.py is quite complex and not particulary efficient,
even for quite small lists of books. This example.py is much simpler,
but could be made considerably more efficient through a bit of a caching
and some sensible short-circuit returns. These are left to the student
to figure out, however.

Author: yawpitch

exercises/book-store/example.py

@@ -1,51 +1,26 @@
-from functools import reduce
-
-BASE_COST = 800
-discount = [1.0, 1.0, 0.95, 0.9, 0.8, 0.75]
-
-
-def group_cost(group):
-    return len(group) * discount[len(group)]
-
-
-class Grouping:
-    def __init__(self, groups=None):
-        self.groups = [set()] if groups is None else groups
-
-    def total(self):
-        return sum(map(group_cost, self.groups)) * BASE_COST
-
-    def dup(self):
-        return Grouping(list(map(set, self.groups)))
-
-    def add_to_valid(self, book):
-        """Returns all possible groupings from the
-        current grouping adding book
-        """
-        other = self.dup()
-        other.groups.sort(key=lambda g: len(g))
-        results = []
-        for index, group in enumerate(other.groups):
-            if book not in group:
-                other2 = other.dup()
-                other2.groups[index].add(book)
-                results.append(other2)
-        if not results:
-            other.groups.append(set([book]))
-            return [other]
-        return results
-
-    def __lt__(self, other):
-        return self.total() < other.total()
-
-
-def step(basket, book):
-    return [group for groupings in basket
-            for group in groupings.add_to_valid(book)]
-
-
-def total(basket):
-    if len(basket) == 0:
+from collections import Counter
+
+PER_BOOK = 800.00
+PER_GROUP = {
+    1: 1 * PER_BOOK * 1.00,
+    2: 2 * PER_BOOK * 0.95,
+    3: 3 * PER_BOOK * 0.90,
+    4: 4 * PER_BOOK * 0.80,
+    5: 5 * PER_BOOK * 0.75,
+}
+
+
+def _total(books):
+    volumes = Counter(books)
+    price = len(books) * PER_BOOK
+    for size in range(len(volumes), 1, -1):
+        group = volumes - Counter(k for k, _ in volumes.most_common(size))
+        group_books = sorted(group.elements())
+        price = min(price, PER_GROUP[size] + _total(group_books))
+    return price
+
+
+def total(books):
+    if not books:
         return 0
-    start = Grouping([{basket[0]}])
-    return round(min(reduce(step, basket[1:], [start])).total())
+    return _total(sorted(books))