# Solve Grid-Tangram

The Tangram is a dissection puzzle made from seven shapes: Five differently sized triangles, a parallelogram and a square. Given a shape, the goal is recreating the shape using all the pieces and without overlapping. There are obviously infinitely many ways to arrange this set of pieces in the plane. An interesting subset are the

### Grid Tangrams

We can draw the "standard" Tangram square into a bigger square which is subdivided by a grid into 16 smaller squares. Grid tangrams are just shapes made up of the tangram pieces, such that all vertices of the pieces are on the grid points. These are the kinds of Tangram puzzles we want to consider in this challenge, as they are probably easier to handle than the more general ones.

As a side note: The Chinese mathematicians Chuan-Chin Hsiung and Fu Traing Wang proved in 1942 that there are only 13 convex tangrams. They first showed that the problem can be reduced to grid tangrams, and then used some combinatorial and geometric arguments. These are all of those 13: ### Challenge

Given a solvable grid tangram, output a dissection of the grid tangram into the seven tangram pieces.

### IO

A tangram is given as a black and white image (the shape is in black, background in white), with both sides multiples of 50px. The grid has a width of exactly 50px. The grid lines are parallel to the sides of the image.

EDIT: The image can be accepted as input and returned as output in any convenient raster image format like PNG,TIFF,PBM etc. but a representation as a binary 2d array or string or matrix is acceptable.

The output should again have the same size and should have again the same shape, but with each piece a different colour, or alternatively with white lines separating all the pieces. It is worth noting that the non rectangular quadrangle can be flipped.

The pixels on the border of the pieces do not exactly have to match the one on the shape, also if there are aliasing effects or other fuzz this is still ok.

Example Input and Output: Examples:         Possible solutions: • image processing is an entirely unnecessary hurdle in this challenge. I'd find it much more appealing if you just specified the input as a small binary array. – Sparr Sep 10 '16 at 22:00
• As I said, that was discussed when it was in the sandbox. But I doubt that it does add a lot of bytes, as the task itself is a lot more difficult. – flawr Sep 13 '16 at 15:56
• I was one of the people who recommended that the input and output be the way they are, and I made that recommendation because this is, in my opinion, the most natural and fitting way to present a Tangram challenge. Any form of input/output would take a significant number of bytes, so I don't think that's really an issue here. – El'endia Starman Sep 13 '16 at 16:20
• I agree with Elendia. The only problem with graphical I/O is that it could limit languages that don't have graphics facilities. That said, PBM and PGM are so close to ASCII art that there is no real problem, IF it is the case that people are aware of such formats. en.wikipedia.org/wiki/Netpbm_format – Level River St Sep 14 '16 at 0:14
• @LevelRiverSt That is a good point, I think it would be totally accepptable to use those formats or even e.g. a 2d array/string of zeros and ones. – flawr Sep 14 '16 at 8:19

# BBC BASIC, 570 514 490 bytes ASCII

435 bytes tokenised

Full program displays an input from L.bmp on the screen, then modifies it to find a solution.

*DISPLAY L
t=PI/8q=FNa(1)
DEFFNa(n)IFn=7END
LOCALz,j,p,i,c,s,x,y,m,u,v
F.z=0TO99u=z MOD10*100v=z DIV10*100ORIGINu,v
F.j=0TO12S.4p=0F.i=j+3TOj+9S.2c=9*COS(i*t)s=9*SIN(i*t)p=p*4-(POINT(c,s)<>0)*2-(POINT(9*c,9*s)<>0)N.
m=n:IFn=5A.(43A.p)=0p=0m=7
IF(ASCM."??O|(C",n)-64A.p)=0THEN
F.i=-1TO0GCOL0,-i*n:c=99*COS(j*t)s=99*SIN(j*t)y=402/3^m MOD3-1MOVE-c-s*y,c*y-s:x=n<3MOVEc*x-s*x,s*x+c*x:x=2778/3^m MOD3-1y=5775/3^m MOD3-1PLOT85-32*(n MOD6>3),c*x-s*y,s*x+c*y:IFi q=FNa(n+1)ORIGINu,v
N.
ENDIF
N.N.=0


Explanation

Note that in BBC basic a distance of 1 pixel = 2 units, so the 50x50 pixel grid becomes a 100x100 grid.

We use a recursive function to place the 2 large triangles, medium triangle, square and parallelogram into the shape. The earlier shape in the list is drawn before the next recursive call is made. if a recursive call returns without finding a solution, the earlier shape is overdrawn in black and a new position of the earlier shape is tried.

Once these five shapes are drawn, placing the two small triangles is just a formality. It is necessary to draw one of them though, in order to distiguish them if they share a common edge. We only colour one of the two small triangles. The other is left in natural black.

Placing of each shape is attempted at different x,y coordinates and in 4 different rotations. To test if there is free space to draw a shape we use the template below, with 45 degree angles. The rotations are made about the * and the 8 pixels tested are in 2 semicircles of radius 9 and 81 units and fall on radiating lines at odd multiples of 22.5 degrees to the x and y axes.

For a large triangle all 8 spaces are required to be clear. For other shapes only some of the cells must be clear so a mask is applied.

+----+----   Shape             Mask HGFEDCBA Mask decimal
|\ E/|\G /
| \/F|H\/    1,2. Large triangle    11111111    -1
|C/\ | /     3. Med triangle        00001111    15
|/ D\|/      4. Square              00111100    60
+----*       5. Parallelogram       11101000   -24
|\ B/        6. Small triangle      00000011     3
|A\/         7. Parallogr reversed  00101011    43
| /          Note: reversed parallelogram is checked/drawn at recursion depth n=5
|/           with a special check, but the coordinates are encoded as m=7.


Once it is established that a shape will fit, it must be drawn. If it is a triangle it is plotted with PLOT 85, if it is a parallelogram, the number is 32 higher (note that for PLOT purposes we consider a square a special parallelogram). In either case 3 consecutive vertices must be given. The second vertex is the origin of the shape (marked * in the above table) except in the case of the large triangle, where (prior to rotation) it is -1,-1. The other 2 vertices can have x and y coordinates of -1,0 or 1 which are extracted from base 3 encoded numbers, then scaled by 99 and rotated as necessary by transformation with c and s.

Ungolfed code

  *DISPLAY L
t=PI/8                                          :REM Constant 22.5 degrees.
q=FNa(1)                                        :REM Call function, return dummy value to q
END                                             :REM End the program gracefully if no solution. Absent in golfed version.

DEFFNa(n)                                       :REM Recursive function to place shapes.
IFn=7END                                        :REM If n=7 solution found, end program.
LOCALk,z,j,p,i,c,s,x,y,m,u,v                    :REM declare local variables for function.
k=ASCMID\$("??O|(C",n)-64                        :REM Bitmasks for big tri, big tri, med tri, sq, normal paralellogram, small tri.
FORz=0TO99                                      :REM For each point on the grid
u=z MOD10*100:v=z DIV10*100                   :REM calculate its x and y coordinates relative to bottom left of screen
ORIGINu,v                                     :REM and set the origin to this point.
FORj=0TO12STEP4                               :REM For each rotation 0,90,180,270deg
p=0                                         :REM assume no non-black pixels found
FORi=j+3TOj+9STEP2                          :REM test angles of 3,5,7,9 times 22.5 deg anticlockwise from right x axis.
c=9*COS(i*t)                             :REM Coords of test points at radius ll
s=9*SIN(i*t)
p*=4                                      :REM Leftshift any existing data in p
p-=(POINT(c,s)<>0)*2+(POINT(9*c,9*s)<>0)  :REM and check pixels at radius 11 and 99.
NEXT
m=n                                         :REM The index of the shape to plot normally corresponds with recursion depth n.
IF n=5 AND (43ANDp)=0 p=0:m=7               :REM If n=5 check if a reverse parallelogram is possible (mask 43). If so, clear p and change m to 7.
REM                                         :REM Check p against mask k, if the shape fits then...
IF (k ANDp)=0 THEN
FOR i=-1 TO 0                               :REM draw the shape in colour, and if deeper recursions prove unsuccesful, redraw it in black.
GCOL0,-i*n                                :REM Colour is equal to n.
c=99*COS(j*t)                             :REM Set parameters c and s for scaling by 99
s=99*SIN(j*t)                             :REM and rotation by 0,90,180 or 270 as appropriate.
x=-1                                      :REM For vertex 1, x=-1 always.
y=402/3^m MOD3-1                          :REM Lookup y value for vertex 1.
MOVEc*x-s*y,s*x+c*y                       :REM Use c and s to transform the vertex and move to it.
x=n<3                                     :REM For vertex 2, coords are 0,0 except for large triangle where they are -1,-1
y=x                                       :REM in BBC BASIC, TRUE=-1
MOVEc*x-s*y,s*x+c*y                       :REM Use c and s to transform the vertex and move to it.
x=2778/3^m MOD3-1                         :REM Lookup x and y value for vertex 3.
y=5775/3^m MOD3-1                         :REM PLOT85 uses last 2 points + specified point to make triangle, PLOT85+32 makes paralelogram (or square.)
PLOT85-32*(n MOD6>3),c*x-s*y,s*x+c*y      :REM Use c and s to transform the vertex and draw shape.
IFi q=FNa(n+1):ORIGINu,v                  :REM If i=-1 recurse to next level. If it fails, reset the origin before replotting this level's shape in black.
NEXT
ENDIF
NEXT
NEXT
=0                                                :REM Dummy value to return from function


Output

This is a montage of the solutions found by the program for the test cases. Use of 99 instead of 100 for golfing reasons leaves some small black gaps. As the shapes are redrawn during the searches, it can take a few seconds to run for some cases, and is quite fascinating to watch. • Wow, I'm impressed. Now I'd love to see a video of it in action! =) – flawr Sep 25 '16 at 23:27