#!/usr/bin/env bash

#!/usr/bin/env bash

# Strip all spaces from input
input=$(echo $1 | tr -d ' ')

# Check input length
if [ ${#input} -le 1 ]
then
  echo "false"
  exit 0
fi

# Check for non-digit characters
if [[ $input =~ [^[:digit:]] ]]
then
  echo "false"
  exit 0
fi

# Double every second digit from the right
doubled_input=""
for (( i=${#input}-1; i>=0; i-- ))
do
  # Checks if the current digit is a second digit
  if [ $(((${#input}-i) % 2)) -eq 0 ]
  then
    doubled_digit=$(( ${input:$i:1} * 2 ))
    if [ $doubled_digit -gt 9 ]
    then
      doubled_digit=$(( doubled_digit - 9 ))
    fi
    # Adds the new digit to the left of "doubled_input"
    doubled_input="${doubled_digit}${doubled_input}"
  # If is not a second digit, copy it to the new string.
  else
    doubled_input="${input:$i:1}${doubled_input}"
  fi
done

# Calculate the sum of all digits
sum=0
for (( i=0; i<${#doubled_input}; i++ ))
do
  sum=$(( sum + ${doubled_input:$i:1} ))
done

# Check if sum is divisible by 10
if [ $(( sum % 10 )) -eq 0 ]
then
  echo "true"
else
  echo "false"
fi
