Implement day 25, in python

2017/day-25/input.txt

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+Begin in state A.
+Perform a diagnostic checksum after 12399302 steps.
+
+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state C.
+
+In state B:
+  If the current value is 0:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the right.
+    - Continue with state D.
+
+In state C:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state D.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state E.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state D.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state F.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state B.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state E.

2017/day-25/solution.py

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+#!/usr/bin/python
+import fileinput
+import re
+
+def read_action(it):
+    to_write = 1 if "1" in next(it) else 0
+    offset = 1 if "right" in next(it) else -1
+    goto = next(it).strip()[-2]
+
+    return (to_write, offset, goto)
+
+def read_input():
+    in_iter = fileinput.input()
+
+    initial = next(in_iter).strip()[-2]
+    runs = int(next(in_iter).split(" ")[-2])
+    states = {}
+
+    for line in in_iter:
+        line = line.strip()
+
+        if not line:
+            continue
+
+        state = line[-2]
+        next(in_iter)
+
+        if_0 = read_action(in_iter)
+        next(in_iter)
+
+        if_1 = read_action(in_iter)
+
+        states[state] = (if_0, if_1)
+
+    return initial, runs, states
+
+state, steps, states = read_input()
+
+ones = set()
+pos = 0
+
+for _ in range(steps):
+    if pos in ones:
+        instr = states[state][1]
+    else:
+        instr = states[state][0]
+
+    if instr[0] == 1:
+        ones.add(pos)
+    else:
+        ones.discard(pos)
+
+    pos += instr[1]
+    state = instr[2]
+
+print(len(ones))