From 22ef615c91f2a259e30210e433c51063172a18d5 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Mon, 2 Dec 2024 01:34:04 -0500
Subject: [PATCH] Day 2: Red-Nosed Reports

---
 README.md          |  2 +-
 py/aoc2024/day2.py | 68 ++++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml  |  1 +
 3 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2024/day2.py

diff --git a/README.md b/README.md
index 9eb7522..3719efd 100644
--- a/README.md
+++ b/README.md
@@ -6,4 +6,4 @@ Development occurs in language-specific directories:
 |[Haskell](hs) ![Haskell CI](https://github.com/ephemient/aoc2024/workflows/Haskell%20CI/badge.svg)|[Kotlin](kt) ![Kotlin CI](https://github.com/ephemient/aoc2024/workflows/Kotlin%20CI/badge.svg)|[Python](py) ![Python CI](https://github.com/ephemient/aoc2024/workflows/Python%20CI/badge.svg)|[Rust](rs) ![Rust CI](https://github.com/ephemient/aoc2024/workflows/Rust%20CI/badge.svg)|
 |--:|--:|--:|--:|
 |[Day1.hs](hs/src/Day1.hs)|[Day1.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day1.kt)|[day1.py](py/aoc2024/day1.py)|[day1.rs](rs/src/day1.rs)|
-|[Day2.hs](hs/src/Day2.hs)|[Day2.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day2.kt)|||
+|[Day2.hs](hs/src/Day2.hs)|[Day2.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day2.kt)|[day2.py](py/aoc2024/day2.py)||
diff --git a/py/aoc2024/day2.py b/py/aoc2024/day2.py
new file mode 100644
index 0000000..0846946
--- /dev/null
+++ b/py/aoc2024/day2.py
@@ -0,0 +1,68 @@
+"""
+Day 2: Red-Nosed Reports
+"""
+
+from typing import Generator
+
+SAMPLE_INPUT = """
+7 6 4 2 1
+1 2 7 8 9
+9 7 6 2 1
+1 3 2 4 5
+8 6 4 4 1
+1 3 6 7 9
+"""
+
+
+def _parse(data: str) -> Generator[list[int]]:
+    for line in data.splitlines():
+        line = line.strip()
+        if line:
+            yield [int(level) for level in line.split()]
+
+
+def _issafe1(report: list[int]) -> bool:
+    decreasing, increasing = False, False
+    for x, y in zip(report, report[1:]):
+        delta = x - y
+        if -3 <= delta <= -1:
+            decreasing = True
+            if increasing:
+                break
+        elif 1 <= delta <= 3:
+            increasing = True
+            if decreasing:
+                break
+        else:
+            break
+    else:
+        return True
+    return False
+
+
+def _issafe2(report: list[int]) -> bool:
+    report2 = report[:-1].copy()
+    for i in range(len(report2), 0, -1):
+        if _issafe1(report2):
+            return True
+        report2[i - 1] = report[i]
+    return _issafe1(report2)
+
+
+def part1(data: str) -> int:
+    """
+    >>> part1(SAMPLE_INPUT)
+    2
+    """
+    return sum(map(_issafe1, _parse(data)))
+
+
+def part2(data) -> int:
+    """
+    >>> part2(SAMPLE_INPUT)
+    4
+    """
+    return sum(map(_issafe2, _parse(data)))
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index a121a04..0515cf6 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -25,6 +25,7 @@ aoc2024 = "aoc2024.main:main"
 
 [tool.poetry.plugins."aoc2024.days"]
 day1 = "aoc2024.day1:parts"
+day2 = "aoc2024.day2:parts"
 
 [build-system]
 requires = ["poetry-core"]