Add 2021-03 in Python

Signed-off-by: AKU <tom@tdpain.net>

challenges/2021/03-binaryDiagnostic/README.md

@@ -0,0 +1,2 @@
+# [Day 3: Binary Diagnostic](https://adventofcode.com/2021/day/3)
+

challenges/2021/03-binaryDiagnostic/info.json

@@ -0,0 +1,17 @@
+{
+    "inputFile": "input.txt",
+    "testCases": {
+        "one": [
+            {
+                "input": "00100\n11110\n10110\n10111\n10101\n01111\n00111\n11100\n10000\n11001\n00010\n01010\n",
+                "expected": "198"
+            }
+        ],
+        "two": [
+            {
+                "input": "00100\n11110\n10110\n10111\n10101\n01111\n00111\n11100\n10000\n11001\n00010\n01010\n",
+                "expected": "230"
+            }
+        ]
+    }
+}

challenges/2021/03-binaryDiagnostic/py/__init__.py

@@ -0,0 +1,98 @@
+from typing import List, Tuple
+from aocpy import BaseChallenge
+
+
+def parse(instr: str) -> Tuple[List[int], int]:
+    # parse returns a list of integers and the original length of the first of
+    # those integers in the binary representation in the input.
+
+    y = instr.strip().splitlines()
+    return [int(x, base=2) for x in y], len(y[0])
+
+
+def get_bit(number: int, n: int) -> int:
+    # get_bit returns a given bit from the binary representation of `number`.
+    #
+    # In the number 111001011110, n=4 would refer to this bit:
+    #                      ^
+    return (number >> n) & 0b1
+
+def analyse_bits(numbers: List[int], n: int) -> Tuple[int, int]:
+    # analyse_bits returns a tuple containing the most common bit (either 1 or
+    # 0) and the least common bit (either 1 or 0) in position `n` of each 
+    # number in `numbers`.
+    #
+    # If the most common bit and least common bit occur the same amount of 
+    # times, both return values are -1.
+    #
+    # Returns: most common bit, least common bit
+
+    zeros = 0
+    ones = 0
+
+    for number in numbers:
+        sel = get_bit(number, n)
+        if sel == 0:
+            zeros += 1
+        elif sel == 1:
+            ones += 1
+
+    if zeros > ones:
+        return 0, 1
+    elif ones > zeros:
+        return 1, 0
+    return -1, -1
+
+
+class Challenge(BaseChallenge):
+
+    @staticmethod
+    def one(instr: str) -> int:
+        numbers, bit_length = parse(instr)
+
+        gamma = ""
+        epsilon = ""
+
+        for bit_number in reversed(range(bit_length)):
+
+            most_common, least_common = analyse_bits(numbers, bit_number)
+            gamma += str(most_common)
+            epsilon += str(least_common)
+
+        return int(gamma, 2) * int(epsilon, 2)
+
+    @staticmethod
+    def two(instr: str) -> int:
+        numbers, bit_length = parse(instr)
+
+        def find(inp: List[int], use_most_common: bool, n: int = bit_length-1) -> int:
+            # find implements the bit criteria-based filtering as defined in 
+            # AoC 2021 day 3 part 2. If `use_most_common` is True, the bit
+            # criteria for the oxygen generator rating is used, else, the bit
+            # criteria for the CO2 scrubber rating is used.
+
+            if len(inp) == 1:
+                return inp[0]
+            
+            most_common, least_common = analyse_bits(inp, n)
+
+            target = None
+            if most_common == -1 or least_common == -1:
+                target = 1 if use_most_common else 0
+            else:
+                target = most_common if use_most_common else least_common
+
+            # oooo, accidental tail recursion!
+            return find(
+                list(filter(
+                    lambda x: get_bit(x, n) == target,
+                    inp,
+                )),
+                use_most_common,
+                n - 1,
+            )
+
+        o2_rating = find(numbers, True)
+        co2_rating = find(numbers, False)
+
+        return o2_rating * co2_rating

challenges/2021/README.md

@@ -8,9 +8,10 @@ Solutions to the [2021 Advent of Code](https://adventofcode.com/2021).
 
 <!-- PARSE START -->
 
-| Day                                 | Status             | Solutions                                                                    | Notes                |
-| ----------------------------------- | ------------------ | ---------------------------------------------------------------------------- | -------------------- |
-| 01 - Sonar Sweep                    | Complete           | [Python](01-sonarSweep/py), [Go](01-sonarSweep/go), [Nim](01-sonarSweep/nim) |                      |
-| 02 - Dive!                          | Complete           | [Python](02-dive/py), [Go](02-dive/go)                                       |                      |
+| Day                                 | Status             | Solutions                                                                    | Notes                        |
+| ----------------------------------- | ------------------ | ---------------------------------------------------------------------------- | ---------------------------- |
+| 01 - Sonar Sweep                    | Complete           | [Python](01-sonarSweep/py), [Go](01-sonarSweep/go), [Nim](01-sonarSweep/nim) |                              |
+| 02 - Dive!                          | Complete           | [Python](02-dive/py), [Go](02-dive/go)                                       |                              |
+| 02 - Binary Diagnostic              | Complete           | [Python](03-binaryDiagnostic/py)                                             | Bit twiddling, my favourite! |
 
 <!-- PARSE END -->