From d40296e9834fffce72e8312a8ac625d742cb94d8 Mon Sep 17 00:00:00 2001
From: Daniel Lin <ephemient@gmail.com>
Date: Mon, 4 Dec 2023 01:47:40 -0500
Subject: [PATCH] Day 4: Scratchcards

---
 README.md          |  2 +-
 py/aoc2023/day4.py | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 py/pyproject.toml  |  1 +
 3 files changed, 49 insertions(+), 1 deletion(-)
 create mode 100644 py/aoc2023/day4.py

diff --git a/README.md b/README.md
index d937725f..57c69bc3 100644
--- a/README.md
+++ b/README.md
@@ -8,4 +8,4 @@ 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.py](py/aoc2023/day3.py)|[day3.rs](rs/src/day3.rs)|
-|[Day4.hs](hs/src/Day4.hs)|[Day4.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day4.kt)|||
+|[Day4.hs](hs/src/Day4.hs)|[Day4.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day4.kt)|[day4.py](py/aoc2023/day4.py)||
diff --git a/py/aoc2023/day4.py b/py/aoc2023/day4.py
new file mode 100644
index 00000000..3b933a22
--- /dev/null
+++ b/py/aoc2023/day4.py
@@ -0,0 +1,47 @@
+"""
+Day 4: Scratchcards
+"""
+
+SAMPLE_INPUT = """
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+"""
+
+
+def _parse(data):
+    for line in data.splitlines():
+        if not line:
+            continue
+        line = line[line.index(":") + 1 :]
+        left, right = line.split("|", maxsplit=1)
+        left = set(left.split())
+        right = set(right.split())
+        yield len(left & right)
+
+
+def part1(data):
+    """
+    >>> part1(SAMPLE_INPUT)
+    13
+    """
+    return sum(1 << card >> 1 for card in _parse(data))
+
+
+def part2(data):
+    """
+    >>> part2(SAMPLE_INPUT)
+    30
+    """
+    cards = list(_parse(data))
+    counts = [1 for _ in cards]
+    for i, card in enumerate(cards):
+        for j in range(card):
+            counts[i + j + 1] += counts[i]
+    return sum(counts)
+
+
+parts = (part1, part2)
diff --git a/py/pyproject.toml b/py/pyproject.toml
index 778e9112..267ccf64 100644
--- a/py/pyproject.toml
+++ b/py/pyproject.toml
@@ -25,6 +25,7 @@ aoc2023 = "aoc2023.main:main"
 day1 = "aoc2023.day1:parts"
 day2 = "aoc2023.day2:parts"
 day3 = "aoc2023.day3:parts"
+day4 = "aoc2023.day4:parts"
 
 [tool.black]
 target_version = ["py312"]