Solve 2016 day 17.

2016/d16.py

@@ -0,0 +1,42 @@
+def step(a: str) -> str:
+    b = "".join(["1" if c == "0" else "0" for c in reversed(a)])
+    return a + "0" + b
+
+
+def fill(a: str, n: int) -> str:
+    while len(a) < n:
+        a = step(a)
+    return a[:n]
+
+
+def checksum(a: str) -> str:
+    while len(a) % 2 == 0:
+        xs = []
+        for i in range(0, len(a), 2):
+            if a[i] == a[i + 1]:
+                xs.append("1")
+            else:
+                xs.append("0")
+        a = "".join(xs)
+    return a
+
+
+def solve():
+    assert step("1") == "100"
+    assert step("0") == "001"
+    assert step("11111") == "11111000000"
+
+    with open("i16.txt") as f:
+        d = f.read().strip()
+
+    a = fill(d, 272)
+    b = checksum(a)
+    print(b)
+
+    a = fill(d, 35651584)
+    b = checksum(a)
+    print(b)
+
+
+if __name__ == "__main__":
+    solve()

2016/d17.py

@@ -0,0 +1,40 @@
+import hashlib
+from collections import deque
+from lib import add2
+
+
+def solve():
+    part_1 = False
+    DIRS = [("U", (-1, 0)), ("D", (1, 0)), ("L", (0, -1)), ("R", (0, 1))]
+    with open("i17.txt") as f:
+        passcode = f.read().strip()
+
+    maxlen = 0
+    poss = deque([((0, 0), f"{passcode}")])
+    while poss:
+        if part_1:
+            pos, path = poss.popleft()
+        else:
+            pos, path = poss.pop()
+
+        if pos == (3, 3):
+            if part_1:
+                print(path[len(passcode):])
+                return
+            else:
+                l = len(path) - len(passcode)
+                if l > maxlen:
+                    maxlen = l
+        else:
+            status = hashlib.md5(f"{path}".encode()).hexdigest()
+            for (c, d), s in zip(DIRS, status[:4]):
+                if s in "bcdef":
+                    npos = add2(pos, d)
+                    npath = path + c
+                    if npos[0] >= 0 and npos[0] < 4 and npos[1] >= 0 and npos[1] < 4:
+                        poss.append((npos, npath))
+    print(maxlen)
+
+
+if __name__ == "__main__":
+    solve()

README.md

@@ -47,7 +47,10 @@ written in Python.
 - Day 12: 10:05 okay
 - Day 13: 9:43 okayish
 - Day 14: 120:00 struggled with this one (example incorrect?)
-- Day 15: 
+- Day 15: Trial and error. Should use CRT instead.
+- Day 16: 14:11
+- Day 17: 45:00 didn't follow instructions... focus!
+- Day 18:
 
 # 2017