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OpenBit

function OpenBit: Integer;

Example​

procedure ScriptEvent(var Value: variant);
var
  Flags: TBits;
  Index: Integer;
begin
  Flags := TBits.Create;
  try
    Flags.Size := 3;
    Flags.Bits[0] := True;
    Flags.Bits[1] := True;

    Index := Flags.OpenBit;
    if Index < Flags.Size then
    begin
      Flags.Bits[Index] := True;  // reserve it explicitly
      Value := Index;
    end
    else
      Value := -1;
  finally
    Flags.Free;
  end;
end;

Usage​

OpenBit returns the first false bit index, or Size when no logical bit is open, without reserving or growing the set.

Additional Technical Info​

OpenBit scans from bit zero and returns the first logical position whose value is false. The search reads packed Integer words, skips words that are entirely true and then scans individual bits in the first non-full word.

The method is observational: it does not set the returned bit, change Size or reserve anything. Another mutation between the search and the write can invalidate the result. In a single-threaded script, explicitly set Bits[Index] after checking the sentinel, as the example does.

If every bit in the logical range is true, the current Delphi implementation returns Size. It also clamps an unused false padding bit in the last allocated word to Size. For an empty set, Size=0 and the method returns 0; that value is the full/empty sentinel and is not a readable bit index.

This differs materially from Free Pascal, whose official TBits.OpenBit documentation specifies -1 when no open bit exists. Velox executes Delphi System.Classes, so test Index < Flags.Size; do not test only for -1 and do not copy the Free Pascal sentinel into a Velox flow.

The scan is O(Size) in the worst case with word-level skipping. It allocates nothing and normally raises no exception for a valid object, but calling through a nil or stale reference remains invalid. The set is not thread-safe.

The source-reviewed example converts the Delphi sentinel to -1 for its own result and explicitly reserves a found bit. It was not executed by the documentation workflow.

External references

Created 2026-07-15