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#W  wpobj.gi                     GAP library                 Steve Linton
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##
#Y  Copyright (C)  1997,  
#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 contains the implementations for weak pointer objects
##

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##
#M  <wp> [<pos>]  access function for weak pointer objects   
##
##  We cannot use the kernel function directly, as it returns fail for unbound
##  see comments in wpobj.gd for the reason.
##

InstallMethod(\[\], 
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject, IsPosInt ],
        0,
        function( wp, pos)
    local elm;
    elm := ElmWPObj(wp, pos);
    if elm <> fail or IsBoundElmWPObj(wp,pos) then
        return elm;
    else
        Error("<wpobj>[<pos>] must have a value.");
    fi;
end);

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##
#M <wp> [<pos>] := <obj>  weak pointer object member assignment
##

InstallMethod(\[\]\:\=, 
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject and IsMutable, IsPosInt, IsObject ],
        0,
        SetElmWPObj);
        
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##
#M  Length( <wp> ) note that the answer may not stay valid
##

InstallMethod(Length, 
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject ],
        0,
        LengthWPObj);

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##
#M  IsBound(<wp>[<pos>]) note that the answer may not stay valid
##

InstallMethod(IsBound\[\], 
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject, IsPosInt ],
        0,
        IsBoundElmWPObj);


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##
#M  Unbind(<wp>[<pos>]) 
##

InstallMethod(Unbind\[\], 
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject and IsMutable, IsPosInt ],
        0,
        UnbindElmWPObj);


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##
#M  Print method, ~ is not supported, so self-referential weak pointer
##  objects cannot be printed
##

InstallMethod(PrintObj,   
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject ],
        0,
        function(wp)
    local i,l,x;
    Print("WeakPointerObj( [ ");
    l := Length(wp);
    if l <> 0 then
        x := ElmWPObj(wp,1);
        if x <> fail or IsBoundElmWPObj(wp,1) then
            PrintObj(x);
        fi;
        for i in [2..l] do
            Print(", ");
            x := ElmWPObj(wp,i);
            if x <> fail or IsBoundElmWPObj(wp,i) then
                PrintObj(x);
            fi;
        od;
    fi;
    Print(" ] )");
end);

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##
#M  View method, ~ is not supported, so self-referential weak pointer
##  objects cannot be printed
##

InstallMethod(ViewObj,   
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject ],
        0,
        function(wp)
    local i,l,x;
    Print("WeakPointerObj( [ ");
    l := Length(wp);
    if l <> 0 then
        x := ElmWPObj(wp,1);
        if x <> fail or IsBoundElmWPObj(wp,1) then
            ViewObj(x);
        fi;
        for i in [2..l] do
            Print(", ");
            x := ElmWPObj(wp,i);
            if x <> fail or IsBoundElmWPObj(wp,i) then
                ViewObj(x);
            fi;
        od;
    fi;
    Print(" ] )");
end);


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##
#M  ShallowCopy(<wp>)  
##
##  Note that we do not use wp[i] access (see wpobj.gd for explanation)
##

InstallMethod(ShallowCopy,
        "method for a weak pointer object",
        true,
        [ IsWeakPointerObject ],
        0,
        function(wp)
    local w,i,l,x;
    w := WeakPointerObj([]);
    l := Length(wp);
    for i in [1..l] do
        x := ElmWPObj(wp,i);
        if x <> fail or IsBound(wp[i]) then
            w[i] := x;
        fi;
    od;
    return w;
end);


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#E