See See http://saml.rilspace.org/node/248

gc_leledumbo.pas

program gc;
 
uses
    Sysutils;
 
var
    FastaFile: TextFile;
    CurrentLine: String;
    GCCount: LongInt;
    ATCount: LongInt;
    TotalBaseCount: LongInt;
    c: Char;
    GCFraction: Single;
    PC,PCEnd: PChar;
 
begin
    GCCount := 0;
    TotalBaseCount := 0;
 
    Assign(FastaFile, 'Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa'); 
    Reset(FastaFile);
    while not EOF(FastaFile) do begin
        Readln(FastaFile, CurrentLine); 
        PC := @CurrentLine[1];
        PCEnd := @CurrentLine[Length(CurrentLine)];
        while PC <= PCEnd do
        begin
            c := PC^;
            if c in ['G','C']  then
                Inc(GCCount)
            else if c in ['A','T'] then
                Inc(ATCount);
            Inc(PC);
        end;
    end;
    Close(FastaFile);
    TotalBaseCount := GCCount + ATCount;
    GCFraction := GCCount / TotalBaseCount;
    Writeln(FormatFloat('00.0000', GCFraction * 100));
end.

Compile flags used:

fpc -Ur -O3 -Xs- -OWall -FWgc_leledumbo -XX -CX -ogc_fpc_leledumbo gc_leledumbo.pas
fpc -Ur -O3 -Xs- -Owall -Fwgc_leledumbo -XX -CX -ogc_fpc_leledumbo gc_leledumbo.pas