Add day 3 solution

03/example

@@ -0,0 +1,6 @@
+vJrwpWtwJgWrhcsFMMfFFhFp
+jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL
+PmmdzqPrVvPwwTWBwg
+wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn
+ttgJtRGJQctTZtZT
+CrZsJsPPZsGzwwsLwLmpwMDw

03/rucksacks.py

@@ -0,0 +1,46 @@
+
+import fileinput
+
+from typing import Iterable
+
+def letter_to_priority(i: str) -> int:
+    i = ord(i)
+    return (i & 0b11111) + (~((i & 0b00100000) >> 5) & 1) * 26
+
+def compartment(items: str) -> int:
+    field = 0
+    for item in items:
+        field |= 1 << (letter_to_priority(item) - 1)
+    return field
+
+def bitmap_to_priority(field: int) -> int:
+    for idx in range(52):
+        if field & 0b1:
+            return idx + 1
+        else:
+            field >>= 1
+
+def solve(lines: Iterable[str]) -> int:
+    score = 0
+    for line in map(lambda x: x.strip(), lines):
+        items = len(line)
+        lh = compartment(line[:items//2])
+        rh = compartment(line[items//2:])
+        score += bitmap_to_priority(lh & rh)
+    return score
+
+def solveb(lines: Iterable[str]) -> int:
+    score = 0
+    iterator = map(lambda x: x.strip(), lines.__iter__())
+    for first in iterator:
+        second = next(iterator)
+        third = next(iterator)
+        score += bitmap_to_priority(compartment(first) & compartment(second) & compartment(third))
+    return score
+
+if __name__ == "__main__":
+    print(solve(fileinput.input()))
+    print(solveb(fileinput.input()))
+
+        
+