AoC 2022 D10: Cathode-Ray Tube

Haskell | Problem statement | Source code | Tags: VMManual inspection

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Part 1

I simulate the process of executing the instructions, keeping track of the current cycle and the value of the register X, putting all values of X at each cycle in a list. Then I can just sum the values at the specified cycles.

When parsing the instructions, I represent each instruction as (Int, Int), containing the execution time and the value to add to X (0 for noop).

parseCycles :: [Text] -> [(Int, Int)]
parseCycles [] = []
parseCycles (line : xs)
  | line == T.pack "noop" = (1, 0) : parseCycles xs
  | otherwise = (2, readT $ T.words line !! 1) : parseCycles xs
hs

Then I fold over the list of the instructions. The state tracks three things: current cycle, value of X before the instruction, and the list of X values.

updateRegister :: ((Int, Int), [Int]) -> (Int, Int) -> ((Int, Int), [Int])
updateRegister ((time, val), history) (dur, delta) = ((time', val'), history')
  where
    time' = time + dur
    val' = val + delta
    history' = replicate dur val ++ history
hs

With the list of X values, finding the sum of the specified cycles is straightforward.

Part 2

With the full list of X values, I can just iterate through them and print the appropriate character for each cycle (turns out that rendering a CRT is the same as printing to the console). For each X value, I check if the pixel corresponding to the cycle is within the sprite.

draw :: Int -> Int -> Char
draw time spritePos = if isDrawn then '#' else ' '
  where
    pixel = time `mod` 40 - 1
    isDrawn = pixel >= spritePos - 1 && pixel <= spritePos + 1
hs

Once again, I appreciate the ASCII art produced:

###  #### #  # ####  ##    ##  ##  ###
#  #    # #  # #    #  #    # #  # #  #
#  #   #  #### ###  #       # #    ###
###   #   #  # #    # ##    # #    #  ##
# #  #    #  # #    #  # #  # #  # #  ##
#  # #### #  # #     ###  ##   ##  ###

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