fsharp-dotnet, 142 bytes

Result

AC

Duration

7543ms

Code [DL]

let rec f x=let c=stdin.Read()in let x=(match c with -1->exit 0|84->51|75->x-50|_->x+1)%51 in stdout.Write(if x<1 then 'x'else char c);f x
f 0

stdin

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stdout

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       xxxxxxxxxxxxxxxxxxxxxx                     

stderr

The template "Console Application" was created successfully.

Processing post-creation actions...
Running 'dotnet restore' on /tmp/Proj/Proj.fsproj...
  Restoring packages for /tmp/Proj/Proj.fsproj...
  Generating MSBuild file /tmp/Proj/obj/Proj.fsproj.nuget.g.props.
  Generating MSBuild file /tmp/Proj/obj/Proj.fsproj.nuget.g.targets.
  Restore completed in 220.46 ms for /tmp/Proj/Proj.fsproj.

Restore succeeded.

strace