/* Inspired by Lee Byron's test data generator. */

function stream_layers(n, m, o) {

  if (arguments.length < 3) o = 0;

  function bump(a) {

    var x = 1 / (.1 + Math.random()),

        y = 2 * Math.random() - .5,

        z = 10 / (.1 + Math.random());

    for (var i = 0; i < m; i++) {

      var w = (i / m - y) * z;

      a[i] += x * Math.exp(-w * w);

    }

  }

  return d3.range(n).map(function() {

      var a = [], i;

      for (i = 0; i < m; i++) a[i] = o + o * Math.random();

      for (i = 0; i < 5; i++) bump(a);

      return a.map(stream_index);

    });

}

/* Another layer generator using gamma distributions. */

function stream_waves(n, m) {

  return d3.range(n).map(function(i) {

    return d3.range(m).map(function(j) {

        var x = 20 * j / m - i / 3;

        return 2 * x * Math.exp(-.5 * x);

      }).map(stream_index);

    });

}

function stream_index(d, i) {

  return {x: i, y: Math.max(0, d)};

}