transpose () {

    transpose_file=$(mktemp)

    while read line; do
        transpose_col=$( echo $line | sed 's/./& /g' | xargs -n 1 )
        echo "$(echo "$transpose_col" | paste $transpose_file - -d '')" > $transpose_file
    done <<< $(cat $1)
    
    echo "$(cat $transpose_file)"

}

lower_triangular_diagonals () {
    # Takes a file containing a matrix of letters and prints 
    # the diagonals of the lower triangle, starting with the major diagonal,
    # each on a new line.

    diagonal_file=$(mktemp)

    # Use awk because wc -l undercounts by 1 if the file does not end with a blank line.
    local i=$(awk 'END { print NR }' < $1)

    while [ $i -gt 0 ]; do
        tail -n $i $1 | awk -F '' '{++f; print $(f + 0)}' | xargs | tr -d ' ' >> $diagonal_file
        i=$((i-1))
    done

    cat $diagonal_file

}

word_search () {
    input=$1
    tupni=$(echo $1 | sed 's/./&\n/g' | tac | xargs | sed 's/ //g')
    
    # Transpose the grid, both for searching top-to-bottom, and so 
    # the lower_triangular_diagonals function can get the upper triangular diagonals
    # via the transpose.
    transposed=$(mktemp)
    transpose $2 > $transposed

    # Reverse the grid to get the lower-right triangle, and transpose the reverse
    # to get the upper-right triangle.
    reversed=$(mktemp)
    rev $2 > $reversed 
    transposed_reversed=$(mktemp)
    transpose $reversed > $transposed_reversed  

    diagonals=$(mktemp)

    # Lower-left.
    lower_triangular_diagonals $2 > $diagonals

    # Upper-left; Use tail -n +2 to avoid saving the major diagonal again.
    lower_triangular_diagonals $transposed | tail -n +2 >> $diagonals

    # Lower-right.
    lower_triangular_diagonals $reversed >> $diagonals

    # Upper-right; Use tail -n +2 to avoid saving the major diagonal again.
    lower_triangular_diagonals $transposed_reversed | tail -n +2 >> $diagonals

    # Search for the word both forward and backward along all orthogonal
    # and diagonal slices.
    forward=$(cat $2 | grep -o -e "$input" | wc -l)
    backward=$(cat $2 | grep -o -e "$tupni" | wc -l)
    down=$(cat $transposed | grep -o -e "$input" | wc -l)
    up=$(cat $transposed | grep -o -e "$tupni" | wc -l)
    criss=$(cat $diagonals | grep -o -e "$input" | wc -l)
    cross=$(cat $diagonals | grep -o -e "$tupni" | wc -l)
    

    echo $((forward + backward + down + up + criss + cross))

}


word_search XMAS input.txt