\subsubsection{OInt}

\index{OInt@\te{OInt} (type)}
\index{OInt@\te{OInt} (package)}


{\bf Package}

\begin{verbatim}
import OInt :: * ;
\end{verbatim}


{\bf Description}


The {\mbox{\te{OInt\#(n)}}} type is an abstract type that can store
a number in the range \qbs{0..n-1}.
The representation of a \te{OInt\#(n)} takes up $n$ bits, where
exactly one bit is a set to one, and the others are zero, i.e., it is
a \emph{one-hot} decoded version of the number.
The reason to use a \te{OInt} number is that the \te{select}
operation is more efficient than for a binary-encoded  number;
the code generated for select takes advantage of the fact that
only one of the bits may be set at a time.


{\bf Types and type classes}

Definition of \te{OInt}

\begin{libverbatim}
typedef ...  OInt #(numeric type n) ... ;
\end{libverbatim}

\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|}
\hline
\multicolumn{10}{|c|}{Type Classes used by \te{OInt}}\\
\hline
\hline
&\te{Bits}&\te{Eq}&\te{Literal}&\te{Arith}&\te{Ord}&\te{Bounded}&\te{Bit}&\te{Bit}&\te{Bit}\\
&&&&&&&\te{wise}&\te{Reduction}&\te{Extend}\\
\hline
\te{OInt}&$\surd$  &$\surd$&$\surd$& & &$\surd$ &&&\\
\hline
\end{tabular}
\end{center}

% \te{Oint} belongs to the following type classes:
% \begin{libverbatim}
% instance Literal #(OInt#(n));
% instance Eq #(OInt#(n));
% instance Bounded #(OInt#(n));
% \end{libverbatim}
 
{\bf Functions}

A binary-encoded number can be converted to an \te{OInt}.

\begin{center}
\begin{tabular}{|p{1 in}|p {4 in}|}
\hline
\te{toOInt}&Converts from a bit-vector in unsigned binary format  to an \te{OInt}. An 
out-of-range   number gives an unspecified result.\\
\cline{2-2}
&\begin{libverbatim}
function OInt#(n) toOInt(Bit#(k) k)
  provisos( Log#(n,k)) ;
\end{libverbatim}
\\
\hline
\end{tabular}
\end{center}

An \te{OInt} can be converted to a binary-encoded number.
\begin{center}
\begin{tabular}{|p{1 in}|p {4 in}|}
\hline
\te{fromOInt}&Converts an \te{OInt} to a bit-vector in unsigned binary
format.\\
\cline{2-2}
&\begin{libverbatim}
function Bit#(k) fromOInt(OInt#(n) o)
  provisos( Log#(n,k)) ;
\end{libverbatim}
\\
\hline
\end{tabular}
\end{center}

An \te{OInt} can be used to select an element from a
Vector in an efficient way.


\begin{center}
\begin{tabular}{|p{1 in}|p {4 in}|}
\hline
\te{select}&The Vector \te{select} function, where the type of the
index is an \te{OInt}.\\
\cline{2-2}
&\begin{libverbatim}
function a_type select(Vector#(vsize, a_type) vecta, 
                       OInt#(vsize) index)
  provisos (Bits#(a_type, sizea));
\end{libverbatim}
\\
\hline
\end{tabular}
\end{center}