2023: d19: ex2: add solution

2023/d19/ex2/ex2.py

@@ -0,0 +1,103 @@
+#!/usr/bin/env python
+
+import math
+import sys
+from collections.abc import Iterator
+from enum import StrEnum
+from typing import NamedTuple, Optional
+
+
+class Attribute(StrEnum):
+    COOL = "x"
+    MUSIC = "m"
+    AERODYNAMIC = "a"
+    SHINY = "s"
+
+
+Extant = tuple[int, int]
+Extants = dict[Attribute, Extant]
+
+
+class Rule(NamedTuple):
+    attr: Attribute
+    cmp: str
+    n: int
+    success: str
+    failure: str
+
+    def apply(self, extants: Extants) -> Iterator[tuple[str, Extants]]:
+        min, max = extants[self.attr]
+        win: Optional[Extant] = None
+        lose: Optional[Extant] = None
+        match self.cmp:
+            case "<":
+                if min < self.n:
+                    win = min, self.n - 1
+                if self.n <= max:
+                    lose = self.n, max
+            case ">":
+                if min <= self.n:
+                    lose = min, self.n
+                if self.n < max:
+                    win = self.n + 1, max
+        for attr, extant in (
+            (self.success, win),
+            (self.failure, lose),
+        ):
+            if extant is None:
+                continue
+            yield attr, extants | {self.attr: extant}
+
+
+Workflow = dict[str, Rule]
+
+
+def solve(input: str) -> int:
+    def parse_rules(rules: list[str]) -> Workflow:
+        def parse_line(line: str) -> Workflow:
+            name, rules = line.split("{")
+            rules = rules[:-1]  # Remove trailing '}'
+            # I translate one rule into a succession of pass/fail transitions
+            res: Workflow = {}
+            raw = rules.split(",")
+
+            for i, rule in enumerate(raw[:-1]):
+                test, success = rule.split(":")
+                attr = Attribute(test[0])
+                cmp = test[1]
+                n = int(test[2:])
+                failure = raw[-1] if (i == len(raw) - 2) else f"{name}_{i + 1}"
+                rule_name = name if i == 0 else f"{name}_{i}"
+                res[rule_name] = Rule(attr, cmp, n, success, failure)
+
+            return res
+
+        return {
+            name: rule for line in map(parse_line, rules) for name, rule in line.items()
+        }
+
+    def explore_workflow(rules: Workflow) -> int:
+        def recurse(state: str, extants: Extants) -> int:
+            if state == "R":
+                return 0
+            if state == "A":
+                return math.prod((max - min + 1) for min, max in extants.values())
+            return sum(
+                recurse(new_state, new_extant)
+                for new_state, new_extant in rules[state].apply(extants)
+            )
+
+        return recurse("in", {attr: (1, 4000) for attr in Attribute})
+
+    paragraphs = input.split("\n\n")
+    rules = parse_rules(paragraphs[0].splitlines())
+    return explore_workflow(rules)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()