Create a file named substitution.cpp$.$ In this file, write a program that uses a substitution cipher to encrypt or decrypt its input.

Your program should take two command line arguments: the first argument must be either $-$e $($for encryption$)$ or $-$d $($for decryption$).$ The second argument must be a twenty$-$six$-$character string containing each letter of the alphabet exactly once. If you run the program with the wrong number of arguments, or if any argument is invalid, it should print a usage message and exit with exit code $1$:

$[$julius@gallia lab$4]$$ $./$substitution

USAGE: substitution $[-$ed$][$key$]$

If the command line arguments are valid, your program should read its input one line at a time, encrypting or decrypting each line and then printing the result. When encrypting, the letter A should be replaced with the first letter of the key, the letter B with the second, C with the third, and so on$.$

$[$julius@gallia lab$4]$$ $./$substitution $-$e egjhvakrzxpsynbtudmqiclfow

Outside of a dog, a book is man's best friend.

Biqmzhv ba e hbk$,$ e gbbp zm yen'm gvmq adzvnh.

Inside of a dog, it's too dark to read.

Znmzhv ba e hbk$,$ zq$\text{'}$m qbb hedp qb dveh.

To make it easier to see what happened in the example above, you can line up the key with the unmodified alphabet. Each letter in the alphabet maps to the letter in the key at the same index:

abcdefghijklmnopqrstuvwxyz

egjhvakrzxpsynbtudmqiclfow

When decrypting, the first letter of the key should be replaced with A$,$ the second letter with B$,$ the third with C$,$ and so on$.$

$[$julius@gallia lab$4]$$ $./$substitution $-$d rptxnfhewgzdkicmbujsavloqy

Leq jecadx ln ervn r jsrixwih rukq?

Why should we have a standing army?

Pntrajn sers lrq ln jrvn kcinq ci terwuj.

Because that way we save money on chairs.

The letters of the key can be either upper or lower case $($or mixed$),$ as long as every letter of the alphabet is present. When encrypting and decrypting, match the case of the input. The key abcdefghijklmnopqrstuvwxyz will leave the input unchanged.