# -*- prolog -*-

# smallest
# the smallest running program doing nothing
main.

# hello world
# print a simple string on stdout
main :- write('Hello World'), nl.

# argv
# access command line parameters (no segmentation fault accepted, nor silent exception, so some languages must explicitly check the presence of the argument)
main :- current_prolog_flag(argv, CmdLine),
        append(_, [--, Arg1|_], CmdLine),
        write(Arg1), nl.


# env
# access environment variable
main :- getenv('HOME', Home), write(Home), nl.

# test file exists
# return exit code error (non zero) if a file does not exist
main :- exists_file('/etc/mtab') -> halt(0) ; halt(1).

# test file readable
# return exit code error (non zero) if a file is not readable
main :- access_file('/etc/mtab', read) -> halt(0) ; halt(1).

# formatting
# print integers in a simple formatted string
main :- A is 1, B is 2, Result is A + B, 
        format('~d + ~d = ~d\n', [A, B, Result]).

# system
# call an external program and check the return value
main :- shell('false', Status),
        (Status \= 0 ->
              current_stream(2, _, StdErr),
              format(StdErr, 'false failed~n', [])
           ;
              true),
        shell('echo done', _).


# sed in place
# remove #-comments from a file (modifying the file, i.e. in place)
main :-
  getUserArg(1, File),
  atom_concat(File, '.tmp', TmpFile),
  strip_comments_from_file(File, TmpFile),
  delete_file(File),
  rename_file(TmpFile, File).

strip_comments_from_file(FileIn, FileOut) :-
  open(FileIn, read, In, [type(binary)]), 
  open(FileOut, write, Out, [type(binary)]),
  process_lines(In, Out),
  close(In), close(Out).

process_lines(In, _) :- at_end_of_stream(In), !.

process_lines(In, Out) :-
  read_line_to_codes(In, Line),
  partition(Line, 35, Prefix, _),
  format(Out, '~s~n', [Prefix]),
  !, process_lines(In, Out).

partition(ListIn, Element, Prefix, Suffix) :-
  (member(Element, ListIn) ->
    append(Prefix, [Element|Suffix], ListIn)
  ;
    Prefix = ListIn, Suffix = []).

getUserArg(N, Arg) :-
  current_prolog_flag(argv, Cmdline),
  append(_, [--|Args], Cmdline),
  nth1(N, Args, Arg).


# compile what must be
# find and compile .c files into .o when the .o is old or absent
main :-
  expand_file_name('*.c', CFiles),
  forall(member(CFile, CFiles),
	(sub_atom(CFile, 0, _, 2, BaseName),
	 atom_concat(BaseName, '.o', ObjFile),
	 (should_compile(CFile, ObjFile) 
            -> compile(CFile, ObjFile) ; true))).

should_compile(SrcFile, ObjFile) :-
  (exists_file(ObjFile) ->
    time_file(SrcFile, SrcTime), 
    time_file(ObjFile, ObjTime), 
    ObjTime < SrcTime
  ;
    true).

compile(SrcFile, ObjFile) :-
  concat_atom(['gcc -c "', SrcFile, '" -o "', ObjFile, '"'], '', Cmd),
  format('compiling ~w to ~w\n', [SrcFile, ObjFile]),
  trap(Cmd, _).

trap(Cmd, Output) :-
  open(pipe(Cmd), read, Input),
  read_line_to_codes(Input, Output),
  close(Input).