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##
#W  print.gi                  GAP4 package `Utils'              Chris Wensley
##
#Y  Copyright (C) 2015-2025, The GAP Group

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##  these operations have not been transferred from any other package
##
#M  PrintSelection( <args> )
#M  PrintSelectionFromList( <list>, <first>, <step>, <last> )
#M  PrintSelectionFromIterator( <iterator>, <first>, <step>, <last> )
##
BindGlobal( "PrintSelection",

function( arg )

    local  nargs, obj, first, step, last, L, ok;

    nargs := Length( arg );
    obj := arg[1];
    if ( ( nargs = 1 ) or ( nargs > 4 ) ) then
        Error( "expecting 2, 3 or 4 arguments" );
    elif ( nargs = 2 ) then
        L := arg[2];
        if IsList( L ) then
            ok := ForAll( L, IsPosInt );
            if not ok then
                Error( "L must only contain positive integers" );
            fi;
            if IsList( obj ) then
                PrintSelectionFromListByList( obj, L );
            elif IsIterator( obj ) then
                PrintSelectionFromIteratorByList( obj, L );
            else
                PrintSelectionFromIteratorByList( Iterator( obj ), L );
            fi;
        else
            Error( "second argument not a list" );
        fi;
    else
        first := arg[2];
        step := arg[3];
        if ( nargs = 3 ) then
            if IsList( obj ) then
                last := Length( obj );
            else
                last := infinity;
            fi;
        else
            last := arg[4];
        fi;
        if IsList( obj ) then
            PrintSelectionFromList( obj, first, step, last );
        elif IsIterator( obj ) then
            PrintSelectionFromIterator( obj, first, step, last );
        else
            PrintSelectionFromIterator( Iterator( obj ), first, step, last );
        fi;
    fi;
end );

InstallMethod( PrintSelectionFromList, "generic method for lists",
    true, [ IsList, IsPosInt, IsPosInt, IsPosInt ], 0,
function( L, first, step, last )

    local  len, i;

    len := Minimum( Length( L ), last );
    if ( first <= len ) then
        Print( first, " : ", L[first], "\n" );
        len := len-first;
        i := first;
        while ( len >= step ) do
            i := i+step;
            Print( i, " : ", L[i], "\n" );
            len := len-step;
        od;
    fi;
end );

InstallOtherMethod( PrintSelectionFromIterator, "generic method for iterators",
    true, [ IsIterator, IsPosInt, IsPosInt, IsAdditiveElement ], 0,
function( iter0, first, step, last )

    local  iter, pos, i, x;

    if ( last < first ) then
        Error( "expecting first <= last" );
    fi;
    iter := ShallowCopy( iter0 );
    i := 0;
    while( ( i < first ) and not IsDoneIterator( iter ) ) do
        i := i+1;
        x := NextIterator( iter );
    od;
    if ( i = first ) then
        pos := first;
        Print( pos, " : ", x, "\n" );
    fi;
    while ( not IsDoneIterator( iter ) and ( pos < last ) ) do
        while ( not IsDoneIterator( iter ) and ( pos < last ) ) do
            i := 0;
            while( ( i<step ) and ( pos+i < last )
                   and not IsDoneIterator( iter ) ) do
                i := i+1;
                x := NextIterator( iter );
            od;
            pos := pos + i;
            if ( i = step ) then
                Print( pos, " : ", x, "\n" );
            fi;
        od;
    od;
end );

InstallMethod( PrintSelectionFromListByList, "generic method for lists",
    true, [ IsList, IsList ], 0,
function( L, posL )

    local  len, i;

    if not ForAll( posL, IsPosInt ) then
        Error( "posL should be a list of positive integers" );
    fi;
    len := Length( L );
    for i in [1..len] do
        if ( i in posL ) then
            Print( i, " : ", L[i], "\n" );
        fi;
    od;
end );

InstallMethod( PrintSelectionFromIteratorByList, "generic method for iterators",
    true, [ IsIterator, IsList ], 0,
function( iter0, posL )

    local  iter, max, i, x;

    if not ForAll( posL, IsPosInt ) then
        Error( "posL should be a list of positive integers" );
    fi;
    iter := ShallowCopy( iter0 );
    max := Maximum( posL );
    i := 0;
    while ( ( i < max ) and not IsDoneIterator( iter ) ) do
        x := NextIterator( iter );
        i := i+1;
        if ( i in posL ) then
            Print( i, " : ", x, "\n" );
        fi;
    od;
end );

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#E  print.gi  . . . . . . . . . . . . . . . . . . . . . . . . . . . ends here

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