Alright, code adventurers! Ready to dive into Day 1 Part 2 of Advent of Code 2023? This challenge throws a little twist our way, making it more interesting than just simple digit extraction. We're not only looking for numerical digits but also have to consider spelled-out numbers like 'one', 'two', 'three', and so on. This means our code needs to be a bit smarter to handle both types of number representations. Let's break down the problem, explore a solution, and walk through the code step by step.

Understanding the Challenge

The core of the challenge remains the same: We need to process a series of lines, each containing a mix of text and numbers. Our goal is to find the first and last digit in each line to form a two-digit number, and then sum up these numbers across all lines. The catch in Part 2 is that the digits can be represented either as numerals (1, 2, 3) or as spelled-out numbers (one, two, three). This introduces a string-matching element that wasn't present in Part 1.

For example, consider the following lines:

• two1nine
• eightwothree
• abcone2threexyz
• xtwone3four
• 4nineeightseven2
• zoneight234
• 7pqrstsixteen

In the first line, two1nine, the first digit is '2' (from 'two') and the last digit is '9' (from 'nine'), resulting in the number 29.

In the second line, eightwothree, the first digit is '8' (from 'eight') and the last digit is '3' (from 'three'), resulting in the number 83.

Your mission, should you choose to accept it, is to write a program that can correctly identify these spelled-out numbers alongside the numerical digits and compute the final sum.

Crafting a Solution

To tackle this, we can outline a plan that involves identifying and extracting the first and last digits, considering both numerical and spelled-out forms. Here’s a breakdown of the approach:

1. Define a mapping: Create a dictionary or map that associates spelled-out numbers with their numerical equivalents. For example, "one" maps to 1, "two" maps to 2, and so on.
2. Search for the first digit: Iterate through the input string from the beginning, checking each character to see if it's a numerical digit. Simultaneously, check if the string starts with any of the spelled-out numbers. If you find a numerical digit or a spelled-out number, store that as the first digit.
3. Search for the last digit: Iterate through the input string from the end, using a similar approach to find the last digit. Again, check for both numerical digits and spelled-out numbers.
4. Combine the digits: Once you have the first and last digits, combine them to form a two-digit number. Be sure to handle cases where there's only one digit present in the input string.
5. Sum the results: Accumulate the two-digit numbers from each line to compute the final sum.

Implementing the Code (Python)

Here's a Python implementation that embodies this approach:

import re

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

    total_sum = 0

    with open('input.txt', 'r') as file:
        for line in file:
            line = line.strip()
            digits = []

            for i, char in enumerate(line):
                if char.isdigit():
                    digits.append(char)
                else:
                    for word, digit in number_words.items():
                        if line[i:].startswith(word):
                            digits.append(digit)
                            break

            if digits:
                total_sum += int(digits[0] + digits[-1])

    print(total_sum)

solve()

Code Explanation

1. Number Words Dictionary: This dictionary, number_words, maps the string representations of numbers ("one", "two", etc.) to their corresponding digit characters ("1", "2", etc.). It's crucial for converting the spelled-out numbers into a format we can use for calculation.
2. Iterating Through Lines: The code reads each line from the input file (input.txt), stripping any leading or trailing whitespace.
3. Finding Digits: For each character in the line, the code first checks if it's a digit using char.isdigit(). If it is, the digit is added to the digits list. If the character is not a digit, the code then iterates through the number_words dictionary. For each word in the dictionary, it checks if the line, starting from the current character, begins with that word using line[i:].startswith(word). If a match is found, the corresponding digit from the dictionary is added to the digits list.
4. Combining and Summing Digits: After processing each line, the code checks if any digits were found. If the digits list is not empty, the first and last elements are combined to form a two-digit number, which is then added to total_sum. Finally, the total sum is printed.

Optimizations and Considerations

• Overlapping Words: Consider cases where spelled-out numbers might overlap, like "twone". The provided solution correctly identifies both "two" and "one" due to the iterative approach.
• Performance: For very large inputs, you might want to explore regular expressions or more advanced string searching algorithms to optimize performance.
• Error Handling: Consider adding error handling to deal with unexpected input formats or missing files.

Tips and Tricks

• Test Cases: Prepare a comprehensive set of test cases, including edge cases and examples with overlapping numbers, to ensure your solution is robust.
• Debugging: Use print statements or a debugger to trace the execution of your code and identify any issues.
• Modularize: Break your code into smaller, more manageable functions to improve readability and maintainability.

Alternative Approaches

While the above solution works well, here are some alternative approaches you could consider:

• Regular Expressions: Use regular expressions to find all occurrences of digits and spelled-out numbers in a single pass.
• String Searching Algorithms: Explore algorithms like the Aho-Corasick algorithm for efficient multi-pattern string matching.

Conclusion

Day 1 Part 2 of Advent of Code 2023 presents an engaging challenge that requires a combination of string manipulation and numerical processing. By breaking down the problem into smaller steps and carefully considering edge cases, you can craft a robust solution that accurately identifies and extracts the necessary digits. Remember to test your code thoroughly and explore alternative approaches to optimize performance and improve your problem-solving skills. Keep coding, and happy adventuring!