Hey guys! Let's dive into another fun coding challenge from Advent of Code 2023! Today, we're tackling Day 1, Part 2. This one's a bit trickier than Part 1 because we need to consider spelled-out numbers in addition to numerical digits. So, buckle up, and let’s get started!

    Understanding the Challenge

    In this Advent of Code challenge, the goal is to process a series of lines, each containing a mix of numbers and spelled-out digits (e.g., "one," "two," "three," etc.). Your mission, should you choose to accept it, is to find the first and last digit in each line (whether it's a numerical digit or a spelled-out one) and combine them to form a two-digit number. Finally, sum up all these two-digit numbers to get the final answer.

    For example, if a line reads twone, the first digit is "two" (2), and the last digit is "one" (1). Combining these gives us 21. If a line reads abcone2threexyz, the first digit is "one" (1), and the last digit is "three" (3), resulting in 13. If a line includes a number like 1abc2, the first digit is 1, and the last digit is 2, giving us 12.

    Why This Matters: This challenge tests your string manipulation skills, your ability to handle different data types, and your problem-solving logic. Plus, it’s a great way to sharpen your coding skills during the holiday season!

    Breaking Down the Solution

    To effectively solve this puzzle, we can break it down into several manageable steps. This approach not only simplifies the problem but also makes it easier to debug and optimize our code.

    1. Reading the Input:

      • Start by reading the input file line by line. Each line will be a string that we need to process. You can use standard file reading techniques in your chosen programming language.

    2. Identifying Digits (Numerical and Spelled-Out):

      • This is the core of the challenge. We need to find both numerical digits (0-9) and spelled-out digits ("one", "two", ..., "nine") in each line.
      • Create a dictionary or a mapping that associates each spelled-out digit with its numerical value. For example: {"one": 1, "two": 2, ..., "nine": 9}.
      • Iterate through the string, checking each character to see if it's a numerical digit. If not, check if the substring starting at that character matches any of the spelled-out digits in your dictionary.

    3. Extracting the First and Last Digits:

      • As you iterate through the string, keep track of the first digit you find. This could be either a numerical digit or a spelled-out digit.
      • Continue iterating through the entire string to find the last digit. Again, this could be either a numerical digit or a spelled-out digit.

    4. Combining the Digits:

      • Once you have the first and last digits, combine them to form a two-digit number. Since these digits are likely stored as integers, you can multiply the first digit by 10 and add the last digit to it.

    5. Summing Up the Numbers:

      • Keep a running total of all the two-digit numbers you've calculated. After processing all the lines in the input file, the running total will be your final answer.

    Example Implementation (Python)

    Here’s how you might implement this solution in Python. This example provides a clear and concise way to tackle the problem.

    import re

def solve():
    digit_map = {
        'one': '1', 'two': '2', 'three': '3', 'four': '4',
        'five': '5', 'six': '6', 'seven': '7', 'eight': '8', 'nine': '9'
    }

    total = 0
    with open('input.txt', 'r') as file:
        for line in file:
            digits = []
            for i, char in enumerate(line):
                if char.isdigit():
                    digits.append(char)
                else:
                    for word, digit in digit_map.items():
                        if line[i:].startswith(word):
                            digits.append(digit)
                            break

            if digits:
                first_digit = digits[0]
                last_digit = digits[-1]
                total += int(first_digit + last_digit)

    return total

print(solve())

    Explanation:

    • We start by defining a digit_map dictionary to map spelled-out digits to numerical digits. This makes it easy to convert "one" to 1, "two" to 2, and so on.
    • The solve() function reads each line from the input file, iterates through the characters, and checks if each character is a digit or the start of a spelled-out digit. If it finds a digit (either numerical or spelled-out), it appends it to the digits list.
    • After processing each line, it extracts the first and last digits from the digits list and combines them to form a two-digit number. It then adds this number to the total.
    • Finally, it returns the total, which is the sum of all the two-digit numbers.

    Optimizing Your Code

    While the above solution works, there are several ways to optimize it for better performance and readability.

    • Regular Expressions: You can use regular expressions to find all the digits (numerical and spelled-out) in a single pass. This can significantly reduce the number of iterations required.
    • Pre-compile Regular Expressions: If you're using regular expressions, pre-compiling them can improve performance, especially if you're processing a large number of lines.
    • Use Generators: If you're dealing with a very large input file, consider using generators to process the lines in chunks. This can reduce memory usage.
    import re

def solve_optimized():
    digit_map = {
        'one': '1', 'two': '2', 'three': '3', 'four': '4',
        'five': '5', 'six': '6', 'seven': '7', 'eight': '8', 'nine': '9'
    }
    
    # Create a regex pattern that matches either a digit or a word digit
    pattern = r'(\d|' + '|'.join(digit_map.keys()) + r')'
    compiled_pattern = re.compile(pattern)

    total = 0
    with open('input.txt', 'r') as file:
        for line in file:
            matches = compiled_pattern.findall(line)
            if matches:
                first_digit = matches[0]
                last_digit = matches[-1]

                # Convert word digits to numerical digits using the digit_map
                first_digit = digit_map.get(first_digit, first_digit)
                last_digit = digit_map.get(last_digit, last_digit)

                total += int(first_digit + last_digit)

    return total

print(solve_optimized())

    Explanation of Optimizations:

    • Regular Expression: A regular expression (\d|one|two|three|four|five|six|seven|eight|nine) that searches for either a numerical digit (\d) or any of the spelled-out digits is constructed.
    • Pre-compilation: The regular expression is pre-compiled using re.compile() to improve performance.
    • Mapping and Conversion: The first and last matches, which could be either numerical or spelled-out digits, are converted to numerical digits using the digit_map.

    This optimized approach reduces the number of manual checks and iterations, resulting in faster processing times, especially for larger input files.

    Common Pitfalls and How to Avoid Them

    • Overlapping Digits: Be careful when dealing with overlapping digits, such as "twone." Make sure your code correctly identifies both "two" and "one." The provided solutions handle this by checking for spelled-out digits at each character index.
    • Incorrect Mapping: Double-check your digit mapping to ensure that each spelled-out digit is correctly associated with its numerical value. A simple typo can lead to incorrect results.
    • Edge Cases: Always test your code with edge cases, such as empty lines, lines with only one digit, and lines with multiple overlapping digits. This will help you identify and fix any potential bugs.

    Testing Your Solution

    To ensure that your solution is correct, it’s essential to test it with a variety of inputs.

    • Sample Inputs: Use the sample inputs provided in the challenge description to verify that your code produces the correct results.
    • Custom Inputs: Create your own custom inputs to test different scenarios, such as lines with overlapping digits, lines with only one digit, and lines with no digits.
    • Large Inputs: If you’re concerned about performance, test your code with large inputs to see how it handles them. This will help you identify any potential bottlenecks.

    Conclusion

    Day 1, Part 2 of Advent of Code 2023 is a fun and challenging problem that tests your string manipulation skills and your ability to handle different data types. By breaking down the problem into smaller steps, using appropriate data structures, and optimizing your code, you can solve it efficiently and effectively.

    Keep practicing and happy coding, guys! And remember, the key to success is to keep learning and keep pushing yourself to improve.

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