/*!
 * chartjs-plugin-trendline.js
 * Version: 0.1.3
 *
 * Copyright 2017 Marcus Alsterfjord
 * Released under the MIT license
 * https://github.com/Makanz/chartjs-plugin-trendline/blob/master/README.md
 *
 * Mod by: vesal: accept also xy-data so works with scatter
 */
var pluginTrendlineLinear = {
    beforeDraw: function(chartInstance) {
        var yScale;
        var xScale;
        for (var axis in chartInstance.scales) {
            if ( axis[0] == 'x')
                xScale = chartInstance.scales[axis];
            else
                yScale = chartInstance.scales[axis];
            if ( xScale && yScale ) break;
        }
        var ctx = chartInstance.chart.ctx;

        chartInstance.data.datasets.forEach(function(dataset, index) {
            if (dataset.trendlineLinear && chartInstance.isDatasetVisible(index)) {
                var datasetMeta = chartInstance.getDatasetMeta(index);
                addFitter(datasetMeta, ctx, dataset, xScale, yScale);
            }
        });

        ctx.setLineDash([]);
    }
};

function addFitter(datasetMeta, ctx, dataset, xScale, yScale) {
    if(datasetMeta.data == [] || datasetMeta.data == null || datasetMeta.data.length == 0) return;
    var style = dataset.trendlineLinear.style || dataset.borderColor;
    var lineWidth = dataset.trendlineLinear.width || dataset.borderWidth;
    var lineStyle = dataset.trendlineLinear.lineStyle || "solid";

    style = (style !== undefined) ? style : "rgba(169,169,169, .6)";
    lineWidth = (lineWidth !== undefined) ? lineWidth : 3;

    var fitter = new LineFitter();
    var lastIndex = dataset.data.length - 1;
    var startPos = datasetMeta.data[0]._model.x;
    var endPos = datasetMeta.data[lastIndex]._model.x;

    var xy = false;
    if ( dataset.data && typeof dataset.data[0] === 'object') xy = true;

    dataset.data.forEach(function(data, index) {
        if(data == null)
            return;
        if ( xy ) fitter.add(data.x, data.y);
        else fitter.add(index, data);
    });

    var x1 = xScale.getPixelForValue(fitter.minx);
    var x2 = xScale.getPixelForValue(fitter.maxx);
    var y1 = yScale.getPixelForValue(fitter.f(fitter.minx));
    var y2 = yScale.getPixelForValue(fitter.f(fitter.maxx));
    if ( !xy ) { x1 = startPos; x2 = endPos; }
    
    var drawBottom = datasetMeta.controller.chart.chartArea.bottom;
    var chartWidth = datasetMeta.controller.chart.width;

    if(y1 > drawBottom) { // Left side is below zero
        var diff = y1 - drawBottom;
        var lineHeight = y1 - y2;
        var overlapPercentage = diff / lineHeight;
        var addition = chartWidth * overlapPercentage;

        y1 = drawBottom;
        x1 = (x1 + addition);
    } else if(y2 > drawBottom) { // right side is below zero
        var diff = y2 - drawBottom;
        var lineHeight = y2 - y1;
        var overlapPercentage = diff / lineHeight;
        var subtraction = chartWidth - (chartWidth * overlapPercentage);

        y2 = drawBottom;
        x2 = chartWidth - (x2 - subtraction);
    }

    ctx.lineWidth = lineWidth;
    if (lineStyle === "dotted") { ctx.setLineDash([2, 3]); }
    ctx.beginPath();
    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);
    ctx.strokeStyle = style;
    ctx.stroke();
}

Chart.plugins.register(pluginTrendlineLinear);

function LineFitter() {
    this.count = 0;
    this.sumX = 0;
    this.sumX2 = 0;
    this.sumXY = 0;
    this.sumY = 0;
    this.minx = 1e100;
    this.maxx = -1e100;
}

LineFitter.prototype = {
    'add': function (x, y) {
        this.count++;
        this.sumX += x;
        this.sumX2 += x * x;
        this.sumXY += x * y;
        this.sumY += y;
        if ( x < this.minx ) this.minx = x;
        if ( x > this.maxx ) this.maxx = x;
    },
    'f': function (x) {
        var det = this.count * this.sumX2 - this.sumX * this.sumX;
        var offset = (this.sumX2 * this.sumY - this.sumX * this.sumXY) / det;
        var scale = (this.count * this.sumXY - this.sumX * this.sumY) / det;
        return offset + x * scale;
    }
};