another coding evening

07/solve01.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python
+
+import re
+
+RE_LINE = re.compile('^Step (.*) must be finished before step (.*) can begin.')
+tree = dict()
+
+def read_input():
+    with open('input.txt', 'r') as f:
+        for line in f:
+            yield line
+
+def add_dependency(a, b):
+    if a not in tree:
+        tree[a] = {
+            'node': a,
+            'parent': '',
+            'child': '',
+        } 
+    if b not in tree:
+        tree[b] = {
+            'node': b,
+            'parent': '',
+            'child': '',
+        } 
+    tree[a]['child'] += b
+    tree[b]['parent'] += a
+
+for l in read_input():
+    m = RE_LINE.match(l)
+    if m is None:
+        raise Exception('unpaseable line')
+    add_dependency(m.group(1), m.group(2))
+
+result = []
+while len(tree):
+   root = min([ n['node'] for n in tree.values() if len(n['parent']) == 0])
+   for c in tree[root]['child']:
+       tree[c]['parent'] = tree[c]['parent'].replace(root, "")
+   del tree[root]
+   result.append(root)
+
+
+print(''.join(result))