add sierpinski challenge

Author: Kristian Rother

challenges/sierpinski.rst

@@ -0,0 +1,74 @@
+Sierpinski Triangle
+===================
+
+**🎯 Construct a Sierpinski Triangle with a Cellular Automaton.**
+
+.. figure:: ../images/sierpinski.png
+
+Consider the following rules:
+
+.. code:: python3
+
+   rules = {
+       '   ': ' ',
+       '  #': '#',
+       ' # ': '#',
+       ' ##': ' ',
+       '#  ': '#',
+       '# #': ' ',
+       '## ': ' ',
+       '###': ' ',
+   }
+
+Now when you start with a string consisting of a single hash flanked by spaces:
+
+::
+
+   #
+
+The rules define how the next line looks like.
+For the new line you look up character triplets from the original string.
+The three triplets `"  #", " # ", "#  "` involving the original hash result in a new hash each.
+So the second line is:
+
+::
+
+   ###
+
+If you propagate that line once again, the triplets with two hashes reult in a space:
+
+::
+
+   #   #
+
+If you propagate once again, the first four lines look like:
+
+::
+
+      #
+     ###
+    #   #
+   ### ###
+
+Write a program that propagates the following line 32 times:
+
+.. code:: python3
+   
+   line = " " * 32 + "#" + " " * 32
+
+Add an empty space at the beginning and end of each new line so that the line length stays the same.
+
+.. hint::
+
+   Experiment with the rule set defined by the dictionary by changing its values.
+
+What you can practise in this coding challenge
+----------------------------------------------
+
+-  loops
+-  string operations
+-  looking up things in dictionaries
+
+.. seealso::
+
+   `Sierpinski Triangle on Wikipedia <https://en.wikipedia.org/wiki/Sierpi%C5%84ski_triangle>`__

images/sierpinski.png

@@ -56,7 +56,6 @@ Coding Challenges
    :maxdepth: 1
 
    challenges/ctree.rst
-   challenges/pyramid.rst
    challenges/fizzbuzz.rst
    challenges/quiz.rst
    challenges/checker.rst
@@ -70,6 +69,7 @@ Coding Challenges
    challenges/querprodukt.rst
    challenges/birthdays.rst
    challenges/misty_mountains.rst
+   challenges/sierpinski.rst
    challenges/count_words.rst
    challenges/spiral.rst
 

index.rst

@@ -0,0 +1,21 @@
+
+code = {
+    '   ': ' ',
+    '  #': '#',
+    ' # ': '#',
+    ' ##': ' ',
+    '#  ': '#',
+    '# #': ' ',
+    '## ': ' ',
+    '###': ' ',
+}
+
+N = 32
+line = " " * N + "#" + " " * N
+for _ in range(N):
+    print(line)
+    new = " "
+    for start in range(len(line) - 2):
+        sub = line[start: start + 3]
+        new += code[sub]
+    line = new + " "