Detecting loops in polygons and multipolygons in geojson.

find_geojson_loops.js

var fs = require("fs");
var geojson = JSON.parse(fs.readFileSync(process.argv[2], "utf8"));
for (var n=0;n<geojson.features.length;n++) {
  var feature = geojson.features[n];
  var geom = feature.geometry.coordinates;
  if (feature.geometry.type=='Polygon') 
    geom=[geom];
  var list=[];
  for (var i=0;i<geom.length;i++)
    for (var j=0;j<geom[i].length;j++)
      for (var k=0;k<geom[i][j].length-1;k++)
        for (var m=k+1;m<geom[i][j].length;m++)
          if ((k!=0 || m!=geom[i][j].length-1) && geom[i][j][k][0]==geom[i][j][m][0] && geom[i][j][k][1]==geom[i][j][m][1])
            list.push(geom[i][j][k]);
  if (list.length>0)
    console.log(feature.properties.id+" "+list.join(" "));
}