advent_of_code_2020_day3_part2

  #!/bin/bash
  
  function ride_slope(){
    pos=1
    line_num=0
    trees=0
    slope_right=$1
    slope_down=$2
    while read line
    do
      len=$(echo $line | wc -c)
  
  
      # Don't count the first line
      if [ $line_num -gt 0 ]
      then
        if [ $(($line_num % slope_down)) -eq 0 ]
       then
           # Calculate next pos
          pos=$(($pos + slope_right))
          if [ $pos -ge $len ]
          then
            pos=$((($pos % $len) + 1))
          fi
          char_at_pos=$(echo $line | cut -c $pos)
          if [ $char_at_pos != "." ]
          then
            trees=$(($trees + 1))
          fi
        fi
      fi
      line_num=$(($line_num + 1))
    done <<< $(< $input)
    echo $trees
  }
  
  input=$1
  prod=$(ride_slope 1 1)
  prod=$(($prod * $(ride_slope 3 1)))
  prod=$(($prod * $(ride_slope 5 1)))
  prod=$(($prod * $(ride_slope 7 1)))
  prod=$(($prod * $(ride_slope 1 2)))
  echo $prod