subtree(users/wpcarro): docking briefcase at '24f5a642' r/3226

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c09813172c79820bcf526bb10cf4208
git-subtree-split: 24f5a642af3aa1627bbff977f0a101907a02c69f
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

1 files changed, 34 insertions, 0 deletions

diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py b/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py
new file mode 100644
index 0000000000..bdfbad2426
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py
@@ -0,0 +1,34 @@
+import random
+
+
+def get_random(floor, ceiling):
+    return random.randrange(floor, ceiling + 1)
+
+
+# shuffle_in_place :: [a] -> IO ()
+def shuffle_in_place(xs):
+    """Fisher-Yates algorithm. Notice that shuffling here is the same as
+    selecting a random permutation of the input set, `xs`."""
+    n = len(xs) - 1
+    for i in range(len(xs)):
+        r = get_random(i, n)
+        xs[i], xs[r] = xs[r], xs[i]
+    return xs
+
+
+# shuffle :: [a] -> [a]
+def shuffle_not_in_place(xs):
+    result = []
+
+    while xs:
+        i = get_random(0, len(xs) - 1)
+        x = xs.pop(i)
+        result.append(x)
+
+    return result
+
+
+xs = [x for x in range(9)]
+print(xs)
+# print(shuffle_not_in_place(xs))
+print(shuffle_in_place(xs[:]))