Collapse an array of arrays of numeric pairs by consecutiveness, or whatever

array_consecutive_collapser.js

// given an array of arrays of paired numbers (say, years)
// collapse consecutive into an aggregate human-readable string
// so like
// [[2001, 2002], [2002, 2003], [2003, 2004], [2015, 2016]]
// becomes "2001-2004, 2015-2016"

function makeHumanReadableYears(arr) {

  var out = [];
  var breakpoints = [];
  var len = arr.length;

  for (var i=0; i<len; i++) {
    if (i > 0) {
      if (arr[i][0] !== arr[i-1][1]) {
        breakpoints.push(i);
      }
    } else {
      breakpoints.push(0);
    }
  }

  var breakpoints_len = breakpoints.length;

  for (var x=0; x<breakpoints_len; x++) {
    out.push(arr.slice(breakpoints[x], breakpoints[x+1]));
  }

  var str = [];

  out.forEach(function(x, i) {
    var flattened = [].concat.apply([], x);

    var uniq = flattened.filter(function(x, i, a) {
      return a.indexOf(x) === i;
    }).sort();

    var min = Math.min.apply(null, uniq);
    var max = Math.max.apply(null, uniq);
    str.push(min + "-" + max);
  });

  return str.join(", ");

}