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commit 12c7344ec6770c692094456bc81e7ed4322552aa
parent bc8390c7615cf9a466420aa05130322488d54d7b
Author: Mattias Andrée <>
Date:   Sat, 14 May 2016 20:51:30 +0200

On bit-splitting

Signed-off-by: Mattias Andrée <>

doc/bit-operations.tex | 46+++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/doc/bit-operations.tex b/doc/bit-operations.tex @@ -110,7 +110,51 @@ $r = \phantom{10001}1000_2$ after calling \section{Split} \label{sec:Split} -TODO % zsplit +In \secref{sec:Shift} and \secref{sec:Truncation} +we have seen how bit operations can be used to +calculate division by a power of two and +modulus a power of two efficiently using +bit-shift and bit-truncation operations. libzahl +also has a bit-split operation that can be used +to efficently calculate both division and +modulus a power of two efficiently in the same +operation, or equivalently, storing low bits +in one integer and high bits in another integer. +This function is + +\begin{alltt} + void zsplit(z_t high, z_t low, z_t a, size_t b); +\end{alltt} + +\noindent +Unlike {\tt zdivmod}, it is not more efficient +than calling {\tt zrsh} and {\tt ztrunc}, but +it is more convenient. {\tt zsplit} requires +that {\tt high} and {\tt low} are from each +other distinct references. + +Calling {\tt zsplit(high, low, a, b)} is +equivalent to + +\begin{alltt} + ztrunc(low, a, delim); + zrsh(high, a, delim); +\end{alltt} + +\noindent +assuming {\tt a} and {\tt low} are not the +same reference (reverse the order of the +functions if they are the same reference.) + +{\tt zsplit} copies the lowest {\tt b} bits +of {\tt a} to {\tt low}, and the rest of the +bits to {\tt high}, with the lowest {\tt b} +removesd. For example, if $a = 1010101111_2$, +then $high = 101010_2$ and $low = 1111_2$ +after calling {\tt zsplit(high, low, a, 4)}. + +{\tt zsplit} is especially useful in +divide-and-conquer algorithms. \newpage