Advent of code 2024 part 6a (in bash

aoc-2024-6a.sh

#!/bin/bash

function initialize() {

    # Make sure the guard is oriented to the right
    #
    # I happen to know that the guard is oriented up for my input
    # So the commands below do a clock-wise rotation
    # (this would be easy enough to generalize)

    tac $state > tmp
    state="tmp"
    transpose | sed 's/\^/>/' > states/t0000
    state="states/t0000"
    rm tmp
}

function transpose() {

    # Transpose the map (rows become columns, columns become rows)
    N=$(wc -L $state | awk '{print $1}')
    for i in $(seq 1 $N); do
        cut -c$i $state | tr '\n' 'M' | sed 's/M//g'; echo ""
    done
}

function checkTurn() {

    # Returns a non-empty output if the guard turns
    grep '>#' $state
}

function checkDone() {

    # Returns a non-empty output if the guard is about to exit the map
    grep '>$' $state
}

function turn() {

    # Make the guard turn right (by rotating the map counter clock wise)
    # A counter clock wise rotation is the same a a transposition followed by a horizontal flip 
    transpose | tac >> $nextState
}

function step() {

    # Make the guard take one step forward
    sed -e 's/>./X>/g' $state > $nextState
}

function finish() {

    # Last step, remove the guard from the map 
    sed -e 's/>/X/g' $state > $nextState
}

function count() {

    # Count the visited locations (X's) 
    n=$(grep -on 'X' $state | wc -l)
}


state=$1    # current state
nt=$2       # max number of time steps

initialize

# Current and next time points
t=0
tn=$((t+1))


n=0
for t in $(seq 0 $nt);
do

    echo "t: $t"
    tn=$((t+1))
    state=$(printf states/t%04d $t)
    nextState=$(printf states/t%04d $tn)

    if [[ ! -z $(checkDone) ]]; then
        finish
        echo "  DONE"
        break
    elif [[ ! -z $(checkTurn) ]]; then
        turn
    else
        step
    fi

done

state=$nextState
count
echo ""
echo "FINAL COUNT: $n"