Bug: turf/difference fails on a shared edge

[PizzaBrandon]

It appears difference is failing due to a bug in a supporting library: w8r/martinez#35

Here is a runnable test that exhibits the behavior

const difference = require('@turf/difference');
tape('Difference failure', (t) => {
  const polygon1 = {
    'type': 'Polygon',
    'coordinates': [
      [[ 10, 10 ], [ 80, 10 ], [ 80, 80 ], [ 10, 80 ], [ 10, 10 ]]
    ]
  };

  const polygon2 = {
    'type': 'Polygon',
    'coordinates': [
      [[ 20, 10 ], [ 30, 10 ], [ 30, 80 ], [ 20, 80 ], [ 20, 10 ]]
    ]
  };

  const result = difference(polygon1, polygon2);
});
// Expected result to be a multi-polygon
// Actual fails with exception `Each LinearRing of a Polygon must have 4 or more Positions.`

This failure happens since martinez returns a malformed polygon when two edges overlap. It returns a "good" difference and as well as a two- or three-point polygon. Could this be detected and handled prior to handing off to helper.polygon, which is where the exception occurs?

(Hoping creating a tracking issue between the two libraries helps find a common solution faster.)

[PizzaBrandon]

Additional note, there is a larger bug in martinez where it returns an incorrect result for the above test, but at least need to make sure difference doesn't exception.

[w8r]

linking the issue here w8r/martinez#81

[plepe]

Happens to me as well, here even shorter code:

const difference = require('@turf/difference')

let result = turf.difference(
  {"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[16.2,48.0],[16.6,48.0],[16.6,48.1],[16.2,48.1],[16.2,48.0]]]}},
  {"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[16.4,48.0],[16.6,48.0],[16.6,48.1],[16.4,48.1],[16.4,48.0]]]}}
)
console.log(JSON.stringify(result))

[HansonYip]

I also found a similar problem about shared edge.

var p1 = [[[0,0],[2,2],[4,2],[2,0],[0,0]]];
var p2 = [[[1,0],[2,1],[3,1],[2,0],[1,0]]];
var differenced = difference(p1, p2);

But the result is

[[[[0,0],[1,0],[2,0],[2,1],[3,1],[4,2],[2,2],[0,0]]]]

[rowanwins]

Resolved in v6.2.0 onwards