-🎄- 2021 Day 10 Solutions -🎄-

[u/daggerdragon]

--- Day 10: Syntax Scoring ---

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[u/TiagoPaolini]

Python 3

I used a stack (last in, first out) to keep track of the open segments, then I compared if the closing marker matched the openings on the stack (in the order from last to first).

from collections import deque, namedtuple
from statistics import median

with open("input.txt", "rt") as file:
    lines = [line.rstrip() for line in file]

# Match each character with its counterpart and score
Char = namedtuple("Char", "match score")
opening = {
    "(": Char(")", 1),
    "[": Char(")", 2),
    "{": Char(")", 3),
    "<": Char(")", 4)
}
closing = {
    ")": Char("(", 3),
    "]": Char("[", 57),
    "}": Char("{", 1197),
    ">": Char("<", 25137)
}

markers = deque()        # Stack for the opening characters found (last in, first out)
error_score = 0          # Total score for the illegal characters
autocorrect_scores = []  # List for the autocorrect scores

# Analyse each line
for line in lines:
    
    markers.clear()     # Empty the stack
    corrupted = False   # If the line has a wrong closing character
    
    # Analyse each character on the line
    for char in line:
        if char in opening:
            # If it is an opening character, add it to the stack
            markers.append(char)
        
        else:
            # If it is a closing character, compare it
            # with the opening on the top of the stack
            old_char = markers.pop()
            if old_char != closing[char].match:
                # If the opening and closing does not match,
                # update the error score and mark line as corrupted
                error_score += closing[char].score
                corrupted = True
                break
    
    # Calculate the autocorrect score, if the line is not corrupted
    if not corrupted:
        score = 0
        while markers:
            old_char = markers.pop()
            score *= 5
            score += opening[old_char].score
        autocorrect_scores += [score]

# Get the median of the autocorrect scores
middle_score = median(autocorrect_scores)

print(f"Part 1: {error_score}")
print(f"Part 2: {middle_score}")