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On bit-truncation - libzahl - big integer library |
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git clone git://git.suckless.org/libzahl |
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--- |
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commit a9388ee62cd0553aa62f7305956b37d677d23c97 |
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parent 4ff3671b42f011862ab7a8c6b8ddf66780a2054b |
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Author: Mattias Andrée <[email protected]> |
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Date: Sat, 14 May 2016 17:01:55 +0200 |
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On bit-truncation |
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Signed-off-by: Mattias Andrée <[email protected]> |
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Diffstat: |
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M doc/bit-operations.tex | 25 ++++++++++++++++++++++++- |
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1 file changed, 24 insertions(+), 1 deletion(-) |
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diff --git a/doc/bit-operations.tex b/doc/bit-operations.tex |
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@@ -88,7 +88,30 @@ whenever possible. One such wrapper could be |
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\section{Truncation} |
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\label{sec:Truncation} |
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-TODO % ztrunc |
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+In \secref{sec:Shift} we have seen how bit-shift |
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+operations can be used to multiply or divide by a |
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+power of two. There is also a bit-truncation |
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+operation: {\tt ztrunc}, which is used to keep |
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+only the lowest bits, or equivalently, calculate |
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+the remainder of a division by a power of two. |
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+ |
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+\begin{alltt} |
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+ void ztrunc(z_t r, z_t a, size_t b); |
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+\end{alltt} |
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+ |
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+\noindent |
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+is consistent with {\tt zmod}; like {\tt zlsh} and |
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+{\tt zrsh}, {\tt a}'s sign is preserved into {\tt r} |
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+assuming the result is non-zero. |
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+ |
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+{\tt ztrunc(r, a, b)} stores only the lowest {\tt b} |
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+bits in {\tt a} into {\tt r}, or equivalently, |
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+calculates $r \gets a \mod 2^b$. For example, if |
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+ |
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+$a = 100011000_2$ then |
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+ |
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+$r = \phantom{10001}1000_2$ after calling |
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+{\tt ztrunc(r, a, 4)}. |
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\newpage |
