/examples

# Golomb ruler

The following program finds Golomb rulers.

``````function main () {
array6<int> ruler;
int limit;

invariant ruler.distances.uniq?;
invariant ruler.ascending?;
invariant ruler.first.zero?;
invariant ruler.firstDist < ruler.lastDist;
invariant ruler.last < limit;

expose ruler, limit;
};

function distances (ruler) {
arr = [];

ruler.eachCombination(2, function^ (pair) {
arr = arr.push(pair.last - pair.first);
});

return arr;
};

function ascending? (ruler) {
asc = true;

ruler.eachCons(2, function^ (pair) {
asc &&= pair.first < pair.last;
});

return asc;
};

function firstDist (ruler) {
return ruler - ruler;
};

function lastDist (ruler) {
return ruler[ruler.length - 1] - ruler[ruler.length - 2];
};

main();
``````

By repeatedly adjusting the limit based on the result of the previous run, Sentient can find the optimal ruler, which is thought to be an NP-hard problem.

## How does it work?

We declare an array of integers for the ‘ruler’ and a single integer for the ‘limit’. We specify invariants for each property as described in the Wikipedia article. We use Sentient’s method syntax to simplify the high-level code in the ‘main’ function.

The ‘distances’ function iterates through pairs of marks on the ruler with ‘eachCombination’. It pushes the difference between marks onto an array. The ‘ascending?’ function iterates through consecutive elements to check if the element on the left is less than the right. The ‘&&=’ syntax is shorthand for ‘a = a && b’.

We then specify that rulers must begin at zero (a common convention) and eliminate translated solutions by enforcing the smaller of the first and last distances must be on the left. More generally, this technique is known as symmetry breaking. The final invariant lets us limit our search to rulers shorter than a given length.

## CLI example

Here is an example of running this program with the command-line interface:

``````sentient --run golomb-ruler.json --assign '{ limit: 19 }' --number 0

# standard output:
{"limit":19,"ruler":[0,1,8,11,13,17]}
{"limit":19,"ruler":[0,1,4,10,12,17]}
{"limit":19,"ruler":[0,1,3,8,12,18]}
{"limit":19,"ruler":[0,2,6,7,15,18]}
{"limit":19,"ruler":[0,1,5,7,15,18]}
{"limit":19,"ruler":[0,1,8,12,14,17]}
{"limit":19,"ruler":[0,1,3,8,14,18]}
{"limit":19,"ruler":[0,1,4,10,15,17]}
{}
``````

This finds all rulers with a length shorter than 19.