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2020

Day 1: Report Repair​

Part 1: Find Pair​

Find two entries that sum to 2020 and multiply them. Using itertools for efficient pair combinations.

import itertools

expenses = [int(line.strip()) for line in data]

for pair in itertools.combinations(expenses, 2):
    if pair[0] + pair[1] == 2020:
        print(pair[0] * pair[1])

Part 2: Find Triplet​

Same task but with three numbers instead of two.

for pair in itertools.combinations(expenses, 3):
    if pair[0] + pair[1] + pair[2] == 2020:
        print(pair[0] * pair[1] * pair[2])

Useful Tools

  • itertools.combinations for unique pairs/triplets
  • List comprehension for data loading
  • Simple arithmetic checks

Day 2: Password Philosophy​

Part 1: Count Valid Passwords​

Check if number of occurrences of a character in password falls within range.

for line in data:
    password = line.strip().split(":")[1]
    character = line.strip().split(":")[0].split()[1]
    upper = int(line.strip().split(":")[0].split()[0].split("-")[1])
    lower = int(line.strip().split(":")[0].split()[0].split("-")[0])
    
    if lower <= password.count(character) <= upper:
        result += 1

Part 2: Position Validation​

Character must appear at exactly one of two positions (XOR).

if (password[lower] == character) ^ (password[upper] == character):
    result += 1

Useful Tools

  • String splitting for parsing input
  • XOR operator for exact matches
  • String count method

Day 3: Toboggan Trajectory​

Part 1: Count Trees​

Navigate slope moving right 3, down 1, checking for trees (#). Pattern repeats horizontally.

for i in range(0, len(slope)-1):
    down += 1
    across += 3
    if slope[down][across % len(slope[down])] == '#':
        count += 1

Part 2: Multiple Slopes​

Check multiple slopes and multiply results.

def checkslope(downnum, acrossnum, slope):
    down, across = 0, 0
    count = 0
    for i in range(0, int((len(slope)-1)/downnum)):
        down += downnum
        across += acrossnum
        if slope[down][across % len(slope[down])] == '#':
            count += 1
    return count

result = checkslope(1,1,slope) * checkslope(1,3,slope) * \
         checkslope(1,5,slope) * checkslope(1,7,slope) * \
         checkslope(2,1,slope)

Useful Tools

  • Modulo for wrapping pattern
  • Functions for reusable slope checking
  • Integer division for step counting

Day 4: Passport Processing​

Part 1: Required Fields​

Check passports for presence of all required fields (excluding cid).

mandatory = ["hcl","iyr","eyr","ecl","pid","byr","hgt"]

# Parse input into dictionaries
entry = {}
for line in data:
    if line.strip() != "":
        for pair in line.strip().split():
            key, value = pair.split(":")
            entry[key] = value
    else:
        parsed.append(entry)
        entry = {}

# Count entries with all mandatory fields
for entry in parsed:
    fieldcount = 0
    for field in mandatory:
        if field in entry:
            fieldcount += 1
    if fieldcount == 7:
        count += 1

Part 2: Field Validation​

Add specific validation rules for each field using regex.

def validate(key, value):
    if key == "byr":
        if re.search("^[0-9]{4}$", value):
            if 1920 <= int(value) <= 2002:
                return 1
    elif key == "hgt":
        if match := re.search("^([0-9]{2,3})((in)|(cm))$", value):
            if match[2] == 'cm':
                if 150 <= int(match[1]) <= 193:
                    return 1
            elif match[2] == 'in':
                if 59 <= int(match[1]) <= 76:
                    return 1
    elif key == "hcl":
        if re.search("^#([0-9]|[a-f]){6}$", value):
            return 1
    elif key == "ecl":
        if value in {'amb','blu','brn','gry','grn','hzl','oth'}:
            return 1
    return 0

Useful Tools

  • Regular expressions for validation
  • Dictionary for field storage
  • Set for valid value checking
  • String splitting for parsing

Day 5: Binary Boarding​

Part 1: Decode Seat IDs​

Convert boarding pass codes to seat IDs using binary conversion. F/L = 0, B/R = 1.

def read_boarding(input):
    rowdata = input[0:7].replace('F','0').replace('B','1')
    coldata = input[7:].replace('L','0').replace('R','1')
    
    rowdec = int(rowdata, 2)
    coldec = int(coldata, 2)
    
    return rowdec * 8 + coldec

# Find highest seat ID
passes = [read_boarding(line) for line in data]
print(max(passes))

Part 2: Find Missing Seat​

Find the missing seat ID that has occupied seats on either side.

for seat in range(0, (128*8)):
    if (seat not in passes and 
        seat-1 in passes and 
        seat+1 in passes):
        print(seat)

Useful Tools

  • String replace for binary conversion
  • int(x, 2) for binary to decimal
  • Range to check all possible seats
  • List membership testing

Day 6: Custom Customs​

Part 1: Any Yes​

Count unique questions answered yes in each group.

result = 0
for group in data.split('\n\n'):
    # Remove newlines and count unique characters
    result += len(set(group.replace("\n", "")))

Part 2: All Yes​

Count questions everyone in group answered yes to.

result = 0
for group in data.split('\n\n'):
    questions = 'abcdefghijklmnopqrstuvwxyz'
    for item in group.split('\n'):
        # Find common letters between current and previous answers
        questions = ''.join(set(questions).intersection(set(item)))
    result += len(questions)

Useful Tools

  • Set operations (intersection)
  • String splitting on double newlines
  • join() to recombine sets
  • String replace for cleanup

Day 7: Handy Haversacks​

Part 1: Containing Bags​

Parse bag rules and count how many bags can eventually contain a shiny gold bag.

# Parse rules with regex
for line in lines:
    contentlist = {}
    bag = line.split(' bags contain')
    for contents in bag[1].split(","):
        r = re.findall('(\d|no) (.*) bag', contents.strip('.').strip())
        if r[0][0] != 'no':
            contentlist[r[0][1]] = r[0][0]
    input.append((bag[0], contentlist))

def part1(input):
    bags = nx.DiGraph()
    for bag in input:
        for contents in bag[1]:
            bags.add_edge(bag[0], contents)
    return len(nx.ancestors(bags, 'shiny gold'))

Part 2: Required Bags​

Count total bags required inside a shiny gold bag using weighted edges.

def part2(input):
    bags = nx.DiGraph()
    for bag in input:
        for contents in bag[1]:
            bags.add_edge(bag[0], contents, weight=int(bag[1][contents]))
            
    def dfs_cost(node):
        cost = 0
        for n, weight in bags[node].items():
            cost += weight['weight'] * (1 + dfs_cost(n))
        return cost
        
    return dfs_cost('shiny gold')

Useful Tools

  • NetworkX for graph representation
  • Regular expressions for parsing rules
  • DFS for recursive counting
  • Weighted edges for bag quantities

Day 8: Handheld Halting​

Part 1: Find Loop​

Execute instructions until one repeats, return accumulator value. Similar to 2019 Day 2: 1202 Program Alarm but simpler.

def execute_command(command, index, result):
    if command[0] == 'nop':
        index += 1
    elif command[0] == 'acc':
        index += 1
        result += int(command[1])
    elif command[0] == 'jmp':
        index += int(command[1])
    return index, result

def part1(input):
    visited = []
    index = 0
    result = 0
    while index not in visited:
        visited.append(index)
        index, result = execute_command(input[index], index, result)
    return result

Part 2: Fix Program​

Try flipping each nop/jmp instruction until program completes.

def flip_instruction(this_input, flip):
    output = deepcopy(this_input)
    if output[flip][0] == 'nop':
        output[flip][0] = 'jmp'
    elif output[flip][0] == 'jmp':
        output[flip][0] = 'nop'
    return output

def part2(input):
    for flip in range(0, len(input)):
        flipped_input = flip_instruction(input, flip)
        visited = []
        index = 0
        result = 0
        while index not in visited:
            visited.append(index)
            if index >= len(input):
                return result
            index, result = execute_command(flipped_input[index], 
                                         index, result)

Useful Tools

  • deepcopy for program modification
  • Lists for tracking visited instructions
  • Simple state machine implementation
  • Index/accumulator tracking