You will make a chat-like app that can run in command-line.
All input / output will be made by command-line.
You may use input() function for getting input,
and use output() for showing output.
As input, next commands will be used.
add_user <id(string)>talk <from_id> <to_id> <content(string)>show_log <id_1> <id_2>
Register a new user into a system.
If there is an id dupplication with an old (already registered) user, output ERROR: ID already used!
If successfully registered, show <ID> registered!
ex1)
input: add_user adam
OUTPUT: adam registered!
input: add_user adam
OUTPUT: ERROR: ID already used!
Memorize the talk conversation in the system.
If there are none of the given user ID registered, show Error: no user ID
Else, don't output anything.
ex2) Only adam, eve registered
input: talk adam eve hi
input: talk eve adam I hate you
input: talk adam gogami nyugaku
OUTPUT: Error: no user ID
Show conversation between 2 users.
If there are none of the given user ID registered, show Error: no user ID
Else, output with this format.
<from_ID> -> <to_ID>: <content>
ex3) after ex2
input: show_log adam eve
OUTPUT: adam -> eve: hi
eve -> adam: I hate you
input: show_log adam gogami
OUTPUT: ERROR: no user ID