//unit DecodeBase64Unit;

//interface

//implementation

function DecodeBase64_Char2Byte(org:Char):Byte;
begin
    Result := 255;
    case org of
        'A':    Result := 0;
        'B':    Result := 1;
        'C':    Result := 2;
        'D':    Result := 3;
        'E':    Result := 4;
        'F':    Result := 5;
        'G':    Result := 6;
        'H':    Result := 7;
        'I':    Result := 8;
        'J':    Result := 9;
        'K':    Result := 10;
        'L':    Result := 11;
        'M':    Result := 12;
        'N':    Result := 13;
        'O':    Result := 14;
        'P':    Result := 15;
        'Q':    Result := 16;
        'R':    Result := 17;
        'S':    Result := 18;
        'T':    Result := 19;
        'U':    Result := 20;
        'V':    Result := 21;
        'W':    Result := 22;
        'X':    Result := 23;
        'Y':    Result := 24;
        'Z':    Result := 25;

        'a':    Result := 26;
        'b':    Result := 27;
        'c':    Result := 28;
        'd':    Result := 29;
        'e':    Result := 30;
        'f':    Result := 31;
        'g':    Result := 32;
        'h':    Result := 33;
        'i':    Result := 34;
        'j':    Result := 35;
        'k':    Result := 36;
        'l':    Result := 37;
        'm':    Result := 38;
        'n':    Result := 39;
        'o':    Result := 40;
        'p':    Result := 41;
        'q':    Result := 42;
        'r':    Result := 43;
        's':    Result := 44;
        't':    Result := 45;
        'u':    Result := 46;
        'v':    Result := 47;
        'w':    Result := 48;
        'x':    Result := 49;
        'y':    Result := 50;
        'z':    Result := 51;

        '0':    Result := 52;
        '1':    Result := 53;
        '2':    Result := 54;
        '3':    Result := 55;
        '4':    Result := 56;
        '5':    Result := 57;
        '6':    Result := 58;
        '7':    Result := 59;
        '8':    Result := 60;
        '9':    Result := 61;

        '+':    Result := 62;
        '/':    Result := 63;
        '=':    Result := 64;
    end;
end;

function DecodeBase64_Sub(substr:string):string;
var
    te :Byte;
    re :array[0..2] of Byte;
    co :array[0..3] of Byte;
    i:integer;
    rec:string;
begin
    rec := '   ';
    for i := 1 to 4 do
    begin
        co[i-1] := DecodeBase64_Char2Byte(substr[i]);
    end;

    re[0] := co[0] shl 2;
    te := Byte(co[1] and $30);
    re[0] := Byte(re[0] or (te shr 4));

    re[1] := co[1] shl 4;
    te := Byte(co[2] and $3c);
    re[1] := Byte(re[1] or (te shr 2));

    re[2] := co[2] shl 6;
    re[2] := Byte(re[2] or (co[3] and $3f));

    for i:= 0 to 2 do
    begin
        rec[i+1] := Char(re[i]);
    end;
    Result := string(rec);
end;

function DecodeBase64(org:string):string;
var
    ter:string;
    i:integer;
    len:integer;
    substr:string;
    size:integer;
begin
    ter := '';
    substr := '    ';
    len := Length(org);
    size := 1;
    for i := 1 to len do
    begin
        substr[size] := org[i];
        Inc(size);
        if size > 4 then
        begin
            ter := ter + DecodeBase64_Sub(substr);
            size := 1;
        end;
    end;
    Result := ter;
end;

//end.