CompareValue for booleans

CompareValue function is incredibly practical when you are writing comparers (functions that determine how some data structure is ordered). System.Math and System.StrUtils define a bunch of functions that can be used to compare integers, doubles, strings … There’s, however, no CompareValue for booleans.

A CompareValue function compares two parameters, traditionally named left and right, and returns 0 if they are the same, –1 if left is smaller and 1 if right is smaller.

If we use the usual ordering of false < true, we can write the missing function as follows:

function CompareValue(left, right: boolean): integer; overload;
begin
  if left < right then
    Result := -1
  else if left > right then
    Result := 1
  else
    Result := 0;
end;

Your task for today – if you choose to accept it – is: Write this function without any if statements.

One possible solution:

function CompareValue(left, right: boolean): integer; overload;
begin
  Result := Ord(left and not right) - Ord(right and not left);
end;

A proof – in a form of an Excel spreadsheet, nonetheless:

left	right	left and not right	right and not left	Ord(l & ^r)	Ord(r & ^l)	Ord(l & ^r) - Ord(r & ^l)
FALSE	FALSE	FALSE	FALSE	0	0	0
FALSE	TRUE	FALSE	TRUE	0	1	-1
TRUE	FALSE	TRUE	FALSE	1	0	1
TRUE	TRUE	FALSE	FALSE	0	0	0