The Hashiwokakero Puzzle is a logical puzzle where one has to build **bridges** between islands.
The puzzle is also known under the name **Ai-Ki-Ai**.
The puzzles can also be played online.

The requirements for this puzzle are as follows:

- the goal is to build bridges between islands so as to generate a
**connected**graph - every island has a number on it, indicating exactly how many bridges should be linked with the island
- there is an upper bound (MAXBRIDGES) on the number of bridges that can be built-between two islands
- bridges cannot cross each other

A B model for this puzzle can be found below. The constants and sets of the model are as follows:

- N are the nodes (islands); we have added a constant ignore where one can stipulate which islands should be ignored in this puzzle
- nl (number of links) stipulates for each island how many bridges it should be linked with
- xc, yc are the x- and y-coordinates for every island

A simple puzzle with four islands would be defined as follows, assuming the basic set N is defined as `N = {a,b,c,d,e,f,g,h,i,j,k,l,m,n}`:

xc(a)=0 & xc(b)=1 & xc(c)=0 & xc(d) = 1 & yc(a)=0 & yc(b)=0 & yc(c)=1 & yc(d) = 1 & nl = {a|->2, b|->2, c|->2, d|->2} & ignore = {e,f,g,h,i,j,k,l,m,n}

Below we will use a more complicated puzzle to illustrate the B model.

The model then contains the following derived constants:

- plx,ply: the possible links between islands on the x- and y-axis respectively
- pl: the possible links both on the x- and y-axis combined
- cs: the conflict set of links which overlap, i.e., one cannot build bridges on both links (a,b) when the pair (a,b) is in cs
- connected: the set of links on which at least one bridge was built

The model also sets up the goal constant `sol` which maps every link in `pl` to a number indicating how many bridges are built on it.
The model also stipulates that the graph set up by connected generates a fully connected graph.

Here is the full model:

MACHINE Bridges DEFINITIONS MAXBRIDGES==2; LINKS == 1..(MAXBRIDGES*4); COORD == 0..10; p1 == prj1(nodes,nodes); p2 == prj2(nodes,nodes); p1i == prj1(nodes,INTEGER) SETS N = {a,b,c,d,e,f,g,h,i,j,k,l,m,n} CONSTANTS nodes, ignore, nl, xc,yc, plx,ply,pl, cs, sol, connected PROPERTIES nodes = N \ ignore & // target number of links per node: nl : nodes --> LINKS & /* number of links */ // coordinates of nodes xc: nodes --> COORD & yc: nodes --> COORD & // possible links: pl : nodes <-> nodes & plx : nodes <-> nodes & ply : nodes <-> nodes & plx = {n1,n2 | xc(n1)=xc(n2) & n1 /= n2 & yc(n2)>yc(n1) & !n3.(xc(n3)=xc(n1) => yc(n3) /: yc(n1)+1..yc(n2)-1) } & ply = {n1,n2 | yc(n1)=yc(n2) & n1 /= n2 & xc(n2)>xc(n1) & !n3.(yc(n3)=yc(n1) => xc(n3) /: xc(n1)+1..xc(n2)-1)} & pl = plx \/ ply & // compute conflict set (assumes xc,yc coordinates ordered in plx,ply) cs = {pl1,pl2 | pl1:plx & pl2:ply & xc(p1(pl1)): xc(p1(pl2))+1..xc(p2(pl2))-1 & yc(p1(pl2)): yc(p1(pl1))+1..yc(p2(pl1))-1} & sol : pl --> 0..MAXBRIDGES & !nn.(nn:nodes => SIGMA(l).(l:pl & (p1(l)=nn or p2(l)=nn)|sol(l))=nl(nn)) & !(pl1,pl2).( (pl1,pl2):cs => sol(pl1)=0 or sol(pl2)=0) // no conflicts & // check graph connected connected = {pl|sol(pl)>0} & closure1(connected \/ connected~)[{a}] = {nn|nn:nodes & nl(nn)>0} // encoding of puzzle & // A puzzle from bridges.png xc(a)=1 & yc(a)=1 & nl(a)=4 & xc(b)=1 & yc(b)=4 & nl(b)=6 & xc(c)=1 & yc(c)=6 & nl(c)=3 & xc(d)=2 & yc(d)=2 & nl(d)=1 & xc(e)=2 & yc(e)=5 & nl(e)=2 & xc(f)=3 & yc(f)=2 & nl(f)=4 & xc(g)=3 & yc(g)=4 & nl(g)=6 & xc(h)=3 & yc(h)=5 & nl(h)=4 & xc(i)=4 & yc(i)=3 & nl(i)=3 & xc(j)=4 & yc(j)=6 & nl(j)=3 & xc(k)=5 & yc(k)=2 & nl(k)=1 & xc(l)=6 & yc(l)=1 & nl(l)=4 & xc(m)=6 & yc(m)=3 & nl(m)=5 & xc(n)=6 & yc(n)=5 & nl(n)=2 & ignore = {} END

The puzzle encode above can be visualized as follows:

A solution for this puzzle is found by ProB in 0.08 seconds (on a MacBook Air 2.2GHz i7).
The conflict set is `{((d|->e),(b|->g)), ((i|->j),(h|->n))}` and the value for sol is

{((a|->b)|->2),((a|->l)|->2),((b|->c)|->2),((b|->g)|->2),((c|->j)|->1), ((d|->e)|->0),((d|->f)|->1),((e|->h)|->2),((f|->g)|->2),((f|->k)|->1), ((g|->h)|->2),((h|->n)|->0),((i|->j)|->2),((i|->m)|->1),((l|->m)|->2),((m|->n)|->2)}

To show the solution graphically, we can add the following custom graph to the `DEFINITIONS` clause in the model:

CUSTOM_GRAPH_NODES == {n,w,w2|(n|->w):nl & w=w2}; // %n1.(n1:nodes|nl(n1)); CUSTOM_GRAPH_EDGES == {n1,w,n2|n1:nl & n2:nl & (p1i(n1),p1i(n2),w):sol}

One can then load the model, perform the initialisation (double clicking on `INITIALISATION` in the operations pane) and the execute the command "Current State as Custom Graph" in the States sub-menu of the Visualize menu. This leads to the following picture:

One can load the Dot file generated by ProB into another tool (e.g., OmniGraffle) and then re-arrange the nodes to obtain the rectangular layout respecting the x- and y-coordinates: