From 10ae7a42bed7726f4186c30bb0cf6b01bb42db21 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Sun, 3 Dec 2023 02:51:45 -0500
Subject: [PATCH] Day 3: Gear Ratios

---
 README.md          |  2 +-
 py/aoc2023/day3.py | 56 ++++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml  |  1 +
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2023/day3.py

diff --git a/README.md b/README.md
index 94ce467e..a347b072 100644
--- a/README.md
+++ b/README.md
@@ -7,4 +7,4 @@ Development occurs in language-specific directories:
 |--:|--:|--:|--:|
 |[Day1.hs](hs/src/Day1.hs)|[Day1.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day1.kt)|[day1.py](py/aoc2023/day1.py)|[day1.rs](rs/src/day1.rs)|
 |[Day2.hs](hs/src/Day2.hs)|[Day2.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day2.kt)|[day2.py](py/aoc2023/day2.py)|[day2.rs](rs/src/day2.rs)|
-|[Day3.hs](hs/src/Day3.hs)|[Day3.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day3.kt)|||
+|[Day3.hs](hs/src/Day3.hs)|[Day3.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day3.kt)|[day3.py](py/aoc2023/day3.py)||
diff --git a/py/aoc2023/day3.py b/py/aoc2023/day3.py
new file mode 100644
index 00000000..ab0b550a
--- /dev/null
+++ b/py/aoc2023/day3.py
@@ -0,0 +1,56 @@
+"""
+Day 3: Gear Ratios
+"""
+
+import re
+from collections import defaultdict
+from math import prod
+
+SAMPLE_INPUT = """
+467..114..
+...*......
+..35..633.
+......#...
+617*......
+.....+.58.
+..592.....
+......755.
+...$.*....
+.664.598..
+"""
+
+NUMBER_RE = re.compile(r"\d+")
+SYMBOL_RE = re.compile(r"[^\s\d.]")
+
+
+def _parse(data):
+    lines = data.splitlines()
+    for y, line in enumerate(lines):
+        for match in NUMBER_RE.finditer(line):
+            number = int(match.group(0))
+            x0, x1 = match.span()
+            for yy in range(max(y - 1, 0), min(y + 2, len(lines))):
+                for match in SYMBOL_RE.finditer(lines[yy], x0 - 1, x1 + 1):
+                    yield match.start(), yy, number
+
+
+def part1(data):
+    """
+    >>> part1(SAMPLE_INPUT)
+    4361
+    """
+    return sum(number for _, _, number in _parse(data))
+
+
+def part2(data):
+    """
+    >>> part2(SAMPLE_INPUT)
+    467835
+    """
+    gears = defaultdict(list)
+    for x, y, number in _parse(data):
+        gears[x, y].append(number)
+    return sum(prod(numbers) for numbers in gears.values() if len(numbers) == 2)
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index b94cddbc..778e9112 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -24,6 +24,7 @@ aoc2023 = "aoc2023.main:main"
 [tool.poetry.plugins."aoc2023.days"]
 day1 = "aoc2023.day1:parts"
 day2 = "aoc2023.day2:parts"
+day3 = "aoc2023.day3:parts"
 
 [tool.black]
 target_version = ["py312"]