Description

1. General matter
1.1. Aims. The purpose of the assignment is to:
• design and implement an interface based on the desired behaviour of an application program; • practice the use of Python syntax;
• develop problem solving skills.
1.3. Assessment. The assignment is worth 13 marks. It is going to be tested against a number of input files. For each test, the automarking script will let your program run for 30 seconds.
The outputs of your programs should be exactly as indicated.
1
2. Background
The game of tangram consists in creating shapes out of pieces. We assume that each piece has its own colour, different to the colour of any other piece in the set we are working with. Just for reference, here is the list of colours that are available to us (you will not make use of this list):
https://www.w3.org/TR/2011/REC-SVG11-20110816/types.html#ColorKeywords A representation of the pieces will be stored in an .xml file thanks to a simple, fixed syntax.
2.1. Pieces. Here is an example of the contents of the file pieces_A.xml, typical of the contents of any file of this kind (so only the number of pieces, the colour names, and the various coordinates can differ from one such file to another—we do not bother with allowing for variations, in the use of space in particular).
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 50 50 L 50 90 L 90 90 z” fill=”red”/>
<path d=”M 160 170 L 160 130 L 120 130 z” fill=”green”/>
<path d=”M 200 30 L 180 30 L 180 50 L 220 50 z” fill=”blue”/>
<path d=”M 40 100 L 40 140 L 60 140 L 60 120 z” fill=”yellow”/>
<path d=”M 210 70 L 230 90 L 270 90 L 270 50 L 230 50 z” fill=”purple”/>
<path d=”M 180 130 L 180 170 L 220 210 L 240 190 z” fill=”olive”/>
<path d=”M 100 200 L 120 180 L 80 140 L 80 180 z” fill=”magenta”/>
</svg>
Opened in a browser, pieces_A.xml displays as follows:

Note that the coordinates are nonnegative integer√s. This means that the sets of pieces we consider rule out those of the traditional game of tangram, where 2 is involved everywhere…
We require every piece to be a convex polygon. An .xml file should represent a piece with n sides (n≥3) by an enumeration of n pairs of coordinates, those of consecutive vertices, the first vertex being arbitrary, and the enumeration being either clockwise or anticlockwise.
The pieces can be flipped over horizontally, flipped over vertically, and rotated by 90 degrees. For instance, the file pieces_AA.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 90 90 L 90 110 L 110 110 L 110 70 z” fill=”yellow”/>
<path d=”M 130 80 L 170 120 L 170 80 z” fill=”red”/>
<path d=”M 290 150 L 270 170 L 210 110 L 250 110 z” fill=”olive”/>
<path d=”M 190 120 L 190 160 L 210 180 L 230 160 z” fill=”magenta”/>
<path d=”M 320 80 L 300 100 L 280 100 L 280 80 z” fill=”blue”/>
<path d=”M 320 150 L 360 150 L 320 190 z” fill=”green”/>
<path d=”M 230 30 L 230 70 L 210 90 L 190 70 L 190 30 z” fill=”purple”/>
</svg> and which displays as

The pieces can overlap, but that does not concern us. In practice, we will just use representations where the pieces do not overlap as that allows us to visualise the pieces properly when we open the corresponding .xml file, but it is just for convenience and irrelevant to the tasks we tackle.
2.2. Shapes. A representation of a shape is provided thanks to an .xml file with the same structure, storing the coordinates of the vertices of just one polygon.
The file shape_A_1.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 30 20 L 110 20 L 110 120 L 30 120 z” fill=”grey”/>
</svg> and which displays as

is such an example. The file shape_A_2.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 50 10 L 90 10 L 90 50 L 130 50 L 130 90 L 90 90 L 90 130…
…L 50 130 L 50 90 L 10 90 L 10 50 L 50 50 z” fill=”brown”/>
</svg> and which displays as

is another such example.
Contrary to pieces, shapes are not assumed to be convex polygons. Still they are assumed to be simple polygons (the boundary of a simple polygon does not cross itself; in particular, it cannot consist of at least 2 polygons that are connected by letting two of them just “touch” each other at one of their vertices—e.g., two rectangles such that the upper right corner of one rectangle is the lower left corner of the other rectangle; that is not allowed).
Whereas you will have to check that the representation of the pieces in an .xml file satisfies our constraints, you will not have to do so for the representation of a shape; you can assume that any shape we will be dealing with satisfies our constraints.
2.3. Tangrams. The first shape can be built from our set of pieces, in many ways. Here is one, given by the file tangram_A_1_a.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”> <path d=”M 30 60 L 30 20 L 70 20 z” fill=”green”/>
<path d=”M 50 120 L 50 80 L 70 60 L 90 80 L 90 120 z” fill=”purple”/>
<path d=”M 30 100 L 90 40 L 70 20 L 30 60 z” fill=”olive”/>
<path d=”M 70 60 L 110 100 L 110 60 L 90 40 z” fill=”magenta”/>
<path d=”M 50 120 L 30 120 L 30 100 L 50 80 z” fill=”blue”/>
<path d=”M 110 20 L 70 20 L 110 60 z” fill=”red”/>
<path d=”M 110 100 L 110 120 L 90 120 L 90 80 z” fill=”yellow”/>
</svg> and which displays as follows.

Here is another one, given by the file tangram_A_1_b.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 30 20 L 50 20 L 50 60 L 30 40 z” fill=”yellow”/>
<path d=”M 50 60 L 50 20 L 90 20 L 90 60 L 70 80 z” fill=”purple”/>
<path d=”M 70 120 L 110 80 L 110 120 z” fill=”green”/>
<path d=”M 90 20 L 110 20 L 110 40 L 90 60 z” fill=”blue”/>
<path d=”M 30 120 L 30 80 L 70 120 z” fill=”red”/>
<path d=”M 70 120 L 30 80 L 30 40 L 90 100 z” fill=”olive”/>
<path d=”M 70 80 L 110 40 L 110 80 L 90 100 z” fill=”magenta”/>
</svg> and which displays as follows.

The second shape can also be built from our set of pieces, in many ways. Here is one, given by the file tangram_A_2_a.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”>
<path d=”M 50 10 L 70 10 L 70 30 L 50 50 z” fill=”yellow”/>
<path d=”M 70 10 L 90 10 L 90 50 L 70 30 z” fill=”blue”/>
<path d=”M 10 50 L 50 50 L 10 90 z” fill=”green”/>
<path d=”M 90 50 L 130 50 L 130 90 z” fill=”red”/>
<path d=”M 10 90 L 70 30 L 90 50 L 50 90 z” fill=”olive”/>
<path d=”M 90 50 L 130 90 L 90 90 L 70 70 z” fill=”magenta”/>
<path d=”M 70 70 L 90 90 L 90 130 L 50 130 L 50 90 z” fill=”purple”/>
</svg> and which displays as follows.

Here is another one, given by the file tangram_A_2_b.xml whose contents is
<svg version=”1.1″ xmlns=”http://www.w3.org/2000/svg”> <path d=”M 90 10 L 50 50 L 50 10 z” fill=”red”/>
<path d=”M 130 50 L 130 90 L 90 90 z” fill=”green”/>
<path d=”M 50 50 L 90 10 L 90 50 L 70 70 z” fill=”magenta”/>
<path d=”M 10 90 L 10 50 L 50 50 L 70 70 L 50 90 z” fill=”purple”/>
<path d=”M 70 110 L 70 130 L 50 130 L 50 90 z” fill=”yellow”/>
<path d=”M 90 50 L 130 50 L 70 110 L 50 90 z” fill=”olive”/>
<path d=”M 90 90 L 90 130 L 70 130 L 70 110 z” fill=”blue”/>
</svg> and which displays as follows.

3. First task (4 marks)
You have to check that the pieces represented in an .xml file satisfy our constraints. So you have to check that each piece is convex, and if it represents a polygon with n sides (n≥3) then the representation consists of an enumeration of the n vertices, either clockwise or anticlockwise. Here is the expected behaviour of your program.
$ python3 …
>>> from tangram import *
>>> TangramPieces(‘pieces_A.xml’)
<tangram.TangramPieces object at …>
>>> TangramPieces(‘pieces_AA.xml’)
<tangram.TangramPieces object at …> >>> TangramPieces(‘incorrect_pieces_1.xml’) …
tangram.TangramPiecesError: At least one piece is invalid >>> TangramPieces(‘incorrect_pieces_2.xml’) …
tangram.TangramPiecesError: At least one piece is invalid >>> TangramPieces(‘incorrect_pieces_3.xml’) …
tangram.TangramPiecesError: At least one piece is invalid >>> TangramPieces(‘incorrect_pieces_4.xml’) …
tangram.TangramPiecesError: At least one piece is invalid
4. Second task (4 marks)
You have to check whether the sets of pieces represented in two .xml files are identical, allowing for pieces to be flipped over horizontally, flipped over vertically, and rotated by 90 degrees. Here is the expected behaviour of your program.
$ python3 …
>>> from tangram import *
>>> pieces_1 = TangramPieces(‘pieces_A.xml’)
>>> pieces_2 = TangramPieces(‘pieces_AA.xml’)
>>> pieces_1.are_identical_to(pieces_2)
True
>>> # Next line is OK as the shape in shape_A_1.xml, being convex, is a valid piece
>>> pieces_3 = TangramPieces(‘shape_A_1.xml’)
>>> pieces_1.are_identical_to(pieces_3) False
Note that the method are_identical_to() does not print out True or False, but returns True or False.
5. Third task (5 marks)
You have to check whether the pieces represented in an .xml file are a solution to a tangram puzzle represented in another .xml file. Here is the expected behaviour of your program.
$ python3 …
>>> from tangram import *
>>> shape = TangramShape(‘shape_A_1.xml’)
>>> pieces = TangramPieces(‘tangram_A_1_a.xml’)
>>> shape.has_as_solution(pieces)
True
>>> pieces = TangramPieces(‘tangram_A_1_b.xml’)
>>> shape.has_as_solution(pieces)
True
>>> pieces = TangramPieces(‘tangram_A_2_a.xml’)
>>> shape.has_as_solution(pieces)
False
>>> shape = TangramShape(‘shape_A_2.xml’)
>>> pieces = TangramPieces(‘tangram_A_2_a.xml’)
>>> shape.has_as_solution(pieces)
True
>>> pieces = TangramPieces(‘tangram_A_2_b.xml’)
>>> shape.has_as_solution(pieces)
True
>>> pieces = TangramPieces(‘tangram_A_1_a.xml’)
>>> shape.has_as_solution(pieces) False
Note that the method shape.has_as_solution() does not print out True or False, but returns True or False.