Solution to problem 3 in Python

src/Year_2023/Day03.py

@@ -0,0 +1,141 @@
+# --- Day 3: Gear Ratios ---
+
+# You and the Elf eventually reach a gondola lift station; he says the gondola
+# lift will take you up to the water source, but this is as far as he can bring
+# you. You go inside.
+
+# It doesn't take long to find the gondolas, but there seems to be a problem:
+# they're not moving.
+
+# "Aaah!"
+
+# You turn around to see a slightly-greasy Elf with a wrench and a look of
+# surprise. "Sorry, I wasn't expecting anyone! The gondola lift isn't working
+# right now; it'll still be a while before I can fix it." You offer to help.
+
+# The engineer explains that an engine part seems to be missing from the engine,
+# but nobody can figure out which one. If you can add up all the part numbers in
+# the engine schematic, it should be easy to work out which part is missing.
+
+# The engine schematic (your puzzle input) consists of a visual representation
+# of the engine. There are lots of numbers and symbols you don't really
+# understand, but apparently any number adjacent to a symbol, even diagonally,
+# is a "part number" and should be included in your sum. (Periods (.) do not
+# count as a symbol.)
+
+# Here is an example engine schematic:
+
+# 467..114..
+# ...*......
+# ..35..633.
+# ......#...
+# 617*......
+# .....+.58.
+# ..592.....
+# ......755.
+# ...$.*....
+# .664.598..
+
+# In this schematic, two numbers are not part numbers because they are not
+# adjacent to a symbol: 114 (top right) and 58 (middle right). Every other
+# number is adjacent to a symbol and so is a part number; their sum is 4361.
+
+# Of course, the actual engine schematic is much larger. What is the sum of all
+# of the part numbers in the engine schematic?
+
+from re import finditer
+
+with open("files/P3.txt") as f:
+    grid = [line for line in f.read().strip().split()]
+
+chars = {
+    (row, col)
+    for row, _ in enumerate(grid[0])
+    for col, _ in enumerate(grid)
+    if grid[row][col] not in "01234566789."
+}
+
+
+def part1():
+    total = 0
+    for idx_r, row in enumerate(grid):
+        for num in finditer(r"\d+", row):
+            surroundings = {
+                (idx_r + di, idx_c + dj)
+                for di in (-1, 0, 1)
+                for dj in (-1, 0, 1)
+                for idx_c in range(num.start(), num.end())
+            }
+            if chars & surroundings:
+                total += int(num[0])
+
+    print(f"The sum of all of the part number is {total}")
+
+
+# --- Part Two ---
+
+# The engineer finds the missing part and installs it in the engine! As the
+# engine springs to life, you jump in the closest gondola, finally ready to
+# ascend to the water source.
+
+# You don't seem to be going very fast, though. Maybe something is still wrong?
+# Fortunately, the gondola has a phone labeled "help", so you pick it up and the
+# engineer answers.
+
+# Before you can explain the situation, she suggests that you look out the
+# window. There stands the engineer, holding a phone in one hand and waving with
+# the other. You're going so slowly that you haven't even left the station. You
+# exit the gondola.
+
+# The missing part wasn't the only issue - one of the gears in the engine is
+# wrong. A gear is any * symbol that is adjacent to exactly two part numbers.
+# Its gear ratio is the result of multiplying those two numbers together.
+
+# This time, you need to find the gear ratio of every gear and add them all up
+# so that the engineer can figure out which gear needs to be replaced.
+
+# Consider the same engine schematic again:
+
+# 467..114..
+# ...*......
+# ..35..633.
+# ......#...
+# 617*......
+# .....+.58.
+# ..592.....
+# ......755.
+# ...$.*....
+# .664.598..
+
+# In this schematic, there are two gears. The first is in the top left; it has
+# part numbers 467 and 35, so its gear ratio is 16345. The second gear is in the
+# lower right; its gear ratio is 451490. (The * adjacent to 617 is not a gear
+# because it is only adjacent to one part number.) Adding up all of the gear
+# ratios produces 467835.
+
+# What is the sum of all of the gear ratios in your engine schematic?
+
+from collections import defaultdict
+from math import prod
+
+
+def part2():
+    gears = defaultdict(list)
+    for idx_r, row in enumerate(grid):
+        for num in finditer(r"\d+", row):
+            surroundings = {
+                (idx_r + di, idx_c + dj)
+                for di in (-1, 0, 1)
+                for dj in (-1, 0, 1)
+                for idx_c in range(num.start(), num.end())
+            }
+            for c in surroundings & chars:
+                gears[c].append(int(num[0]))
+
+    total = sum((prod(val) for val in gears.values() if len(val) == 2))
+    print(f"The sum of all of the gear ratios if {total}")
+
+
+if __name__ == "__main__":
+    part1()
+    part2()