:- lib(xpath).
:- lib(pack_errors).  % type/3

/**  svg_tag_coords( +Svg, +Tag, -Coords ).
     svg_tag_coords( +Svg, +Tag, +AsPos, -Coords ).

Find the locations (X-Y, Coords) of all tags of type Tag in an Svg file.<br>
Predicate simply looks for x=, y= into Atts of matching tags.<br>
AsPos is a boolean (false when missing) that when set translates each X and Y to it positive value.

==
?- svg_tag_coords( pack('svg/examples/rea_smp-e10_fclr.svg'), text, Coords ), maplist( writeln, Coords ), fail.
197- -302.3
197- -230.3
446- -374.3
...
344- -14.3
197- -86.3
false.

?- svg_tag_coords( pack('svg/examples/rea_smp-e10_fclr.svg'), text, true, Coords ), maplist( writeln, Coords ), fail.
197-706.3
197-634.3
446-778.3
...
344-418.3
197-490.3
false.

?- svg_size( pack('svg/examples/rea_smp-e10_fclr.svg'), W, H ).
W = 497,
H = 404.
==

@author nicos angelopoulos
@version  0.1 2018/2/21
*/
svg_tag_coords( SvgIn, Tag, Coords ) :-
    svg_tag_coords( SvgIn, Tag, false, Coords ).

svg_tag_coords( SvgIn, Tag, AsPos, Coords ) :-
    type( boolean, AsPos, [pack(svg),pred(svg_tag_coords(arg3)/4)] ),
    svg( SvgIn, Svg ),
    Svg = svg(SvgAtts,Dom),
    svg_tag_coords_iff( AsPos, SvgAtts, W, H ),
    findall( X-Y, (  
                        xpath( Dom, //(Tag), element(Tag,Atts,_Elems) ),
                        memberchk( x=Xatm, Atts ),
                        memberchk( y=Yatm, Atts ),
                        atom_number( Xatm, Xnak ),
                        atom_number( Yatm, Ynak ),
                        svg_tag_coord_to_positive( AsPos, W, Xnak, X ),
                        svg_tag_coord_to_positive( AsPos, H, Ynak, Y )
                   ),
                        Coords ).

svg_tag_coords_iff( true, Atts, W, H ) :-
    memberchk( width=Wat, Atts ),
    memberchk( height=Hat, Atts ),
    maplist( svg_size_dim, [Wat,Hat], [W,H] ).
svg_tag_coords_iff( false, _Atts, t, t ).

svg_tag_coord_to_positive( true, Size, Val, Ret ) :-
    ( Val < 0 -> Ret is Size + Val; Ret is Val ).  % fixme: + Val - 1 ???
svg_tag_coord_to_positive( false, _Size, Val, Val ).