// Implements a horizon layout, which is a variation of a single-series

// area chart where the area is folded into multiple bands. Color is used to

// encode band, allowing the size of the chart to be reduced significantly

// without impeding readability. This layout algorithm is based on the work of

// J. Heer, N. Kong and M. Agrawala in "Sizing the Horizon: The Effects of Chart

// Size and Layering on the Graphical Perception of Time Series Visualizations",

// CHI 2009. http://hci.stanford.edu/publications/2009/heer-horizon-chi09.pdf

d3.chart.horizon = function() {

  var bands = 1, // between 1 and 5, typically

      mode = "offset", // or mirror

      interpolate = "linear", // or basis, monotone, step-before, etc.

      x = d3_chart_horizonX,

      y = d3_chart_horizonY,

      w = 960,

      h = 40,

      duration = 0;

  var color = d3.scale.linear()

      .domain([-1, 0, 1])

      .range(["#d62728", "#fff", "#1f77b4"]);

  // For each small multiple…

  function horizon(g) {

    g.each(function(d, i) {

      var g = d3.select(this),

          n = 2 * bands + 1,

          xMin = Infinity,

          xMax = -Infinity,

          yMax = -Infinity,

          x0, // old x-scale

          y0, // old y-scale

          id; // unique id for paths

      // Compute x- and y-values along with extents.

      var data = d.map(function(d, i) {

        var xv = x.call(this, d, i),

            yv = y.call(this, d, i);

        if (xv < xMin) xMin = xv;

        if (xv > xMax) xMax = xv;

        if (-yv > yMax) yMax = -yv;

        if (yv > yMax) yMax = yv;

        return [xv, yv];

      });

      // Compute the new x- and y-scales.

      var x1 = d3.scale.linear().domain([xMin, xMax]).range([0, w]),

          y1 = d3.scale.linear().domain([0, yMax]).range([0, h * bands]);

      // Retrieve the old scales, if this is an update.

      if (this.__chart__) {

        x0 = this.__chart__.x;

        y0 = this.__chart__.y;

        id = this.__chart__.id;

      } else {

        x0 = d3.scale.linear().domain([0, Infinity]).range(x1.range());

        y0 = d3.scale.linear().domain([0, Infinity]).range(y1.range());

        id = ++d3_chart_horizonId;

      }

      // We'll use a defs to store the area path and the clip path.

      var defs = g.selectAll("defs")

          .data([data]);

      var defsEnter = defs.enter().append("svg:defs");

      // The clip path is a simple rect.

      defsEnter.append("svg:clipPath")

          .attr("id", "d3_chart_horizon_clip" + id)

        .append("svg:rect")

          .attr("width", w)

          .attr("height", h);

      defs.select("rect").transition()

          .duration(duration)

          .attr("width", w)

          .attr("height", h);

      // The area path is rendered with our resuable d3.svg.area.

      defsEnter.append("svg:path")

          .attr("id", "d3_chart_horizon_path" + id)

          .attr("d", d3_chart_horizonArea

          .interpolate(interpolate)

          .x(function(d) { return x0(d[0]); })

          .y0(h * bands)

          .y1(function(d) { return h * bands - y0(d[1]); }))

        .transition()

          .duration(duration)

          .attr("d", d3_chart_horizonArea

          .x(function(d) { return x1(d[0]); })

          .y1(function(d) { return h * bands - y1(d[1]); }));

      defs.select("path").transition()

          .duration(duration)

          .attr("d", d3_chart_horizonArea);

      // We'll use a container to clip all horizon layers at once.

      g.selectAll("g")

          .data([null])

        .enter().append("svg:g")

          .attr("clip-path", "url(#d3_chart_horizon_clip" + id + ")");

      // Define the transform function based on the mode.

      var transform = mode == "offset"

          ? function(d) { return "translate(0," + (d + (d < 0) - bands) * h + ")"; }

          : function(d) { return (d < 0 ? "scale(1,-1)" : "") + "translate(0," + (d - bands) * h + ")"; };

      // Instantiate each copy of the path with different transforms.

      var u = g.select("g").selectAll("use")

          .data(d3.range(-1, -bands - 1, -1).concat(d3.range(1, bands + 1)), Number);

      // TODO don't fudge the enter transition

      u.enter().append("svg:use")

          .attr("xlink:href", "#d3_chart_horizon_path" + id)

          .attr("transform", function(d) { return transform(d + (d > 0 ? 1 : -1)); })

          .style("fill", color)

        .transition()

          .duration(duration)

          .attr("transform", transform);

      u.transition()

          .duration(duration)

          .attr("transform", transform)

          .style("fill", color);

      u.exit().transition()

          .duration(duration)

          .attr("transform", transform)

          .remove();

      // Stash the new scales.

      this.__chart__ = {x: x1, y: y1, id: id};

    });

    d3.timer.flush();

  }

  horizon.duration = function(x) {

    if (!arguments.length) return duration;

    duration = +x;

    return horizon;

  };

  horizon.bands = function(x) {

    if (!arguments.length) return bands;

    bands = +x;

    color.domain([-bands, 0, bands]);

    return horizon;

  };

  horizon.mode = function(x) {

    if (!arguments.length) return mode;

    mode = x + "";

    return horizon;

  };

  horizon.colors = function(x) {

    if (!arguments.length) return color.range();

    color.range(x);

    return horizon;

  };

  horizon.interpolate = function(x) {

    if (!arguments.length) return interpolate;

    interpolate = x + "";

    return horizon;

  };

  horizon.x = function(z) {

    if (!arguments.length) return x;

    x = z;

    return horizon;

  };

  horizon.y = function(z) {

    if (!arguments.length) return y;

    y = z;

    return horizon;

  };

  horizon.width = function(x) {

    if (!arguments.length) return w;

    w = +x;

    return horizon;

  };

  horizon.height = function(x) {

    if (!arguments.length) return h;

    h = +x;

    return horizon;

  };

  return horizon;

};

var d3_chart_horizonArea = d3.svg.area(),

    d3_chart_horizonId = 0;

function d3_chart_horizonX(d) {

  return d[0];

}

function d3_chart_horizonY(d) {

  return d[1];

}