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#W  sparselist.gd               GAP library                      Steve Linton
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#Y  Copyright (C)  1996,  Lehrstuhl D für Mathematik,  RWTH Aachen,  Germany
#Y  (C) 1998 School Math and Comp. Sci., University of St Andrews, Scotland
#Y  Copyright (C) 2002 The GAP Group
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##  This file declares the operations for sparse lists
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#O  SparseStructureOfList( <list> )
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##  This operation returns a sparse structure of <list>
##  The return value is a length three list, of which the first
##  position may or may not be bound. If bound, it is the default
##  value, if not bound, the default is to be unbound. The default value
##  must by immutable
##
##  The second entry is a dense duplicate-free list of integers
##  between  1 and Length(<list>) inclusive, representing the
##  positions where non-default entries may appear. 
##
##  The third entry is a list not longer than the second entry, giving 
##  the values that appear in the positions given in the second
##  entry. Holes in this list represent holes in the original list.
##
##  The second and third entries may be mutable (to avoid copying in
##  time-critical code) but should not be changed.
##

DeclareOperation( "SparseStructureOfList", [IsList]);

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#F  IsSparseList( <l> )
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##  A sparse list is a list for which the sparse structure and corresponding
##  sparse methods should be used.
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##  The filter is ranked up so that sparse list methods will beat
##  competing methods for finite, homogenous, etc. lists, which is
##  usually right
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DeclareFilter( "IsSparseList", IsList, 20 );

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#F  SparseListBySortedList  ( <poss>, <vals>, <len>, <default> )
#F  SparseListBySortedListNC( <poss>, <vals>, <len>, <default> )
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##  These two functions can be used to create homogeneous sparse
##  lists, in a representation where the positions are stored as a
##  sorted list and the values as a corresponding list. These lists
##  must be dense.
##
##  The NC version refrains from both checking and copying its arguments
##  The non-NC version makes a shallow copy of <poss> and <vals>
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DeclareGlobalFunction( "SparseListBySortedList");
DeclareGlobalFunction( "SparseListBySortedListNC");

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#P IsSparseRowVector( <sl> ) 
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##  A sparse list is a sparse row vector if it is dense and its default value 
##  is a common Zero of all its non-default values
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DeclareProperty( "IsSparseRowVector", IsSparseList);
InstallTrueMethod( IsRowVector, IsSparseRowVector );
InstallTrueMethod( IsHomogeneousList, IsSparseRowVector );
InstallTrueMethod( IsCollection, IsSparseRowVector );

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#F  SparseVectorBySortedList  ( <poss>, <vals>, <len>[, <zero>] )
#F  SparseVectorBySortedListNC( <poss>, <vals>, <len>[, <zero>])
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##  These two functions can be used to create sparse row vectors using
##  the SparseListBySortedList representation, but knowing a priori
##  that they are sparse row vectors. The optional <zero> argument is
##  needed only when <poss> and <vals> are empty.
##
##  The NC version refrains from both checking and copying its arguments
##  The non-NC version makes a shallow copy of <poss> and <vals>
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DeclareGlobalFunction( "SparseVectorBySortedList");
DeclareGlobalFunction( "SparseVectorBySortedListNC");

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#E