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annotate weather_server/static/script.js @ 12:9e6289598d8c

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Add script to draw charts with initial functions.
author Paul Fisher <paul@pfish.zone>
date Sun, 06 Oct 2019 15:21:03 -0400
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children 4eaa9d69c4e2
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1 'use strict';
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2
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3 /**
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4 * Converts Celsius to Fahrenheit.
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5 * @param {number} tempC
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6 * @return {number} The temperature in Fahrenheit.
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7 */
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8 function cToF(tempC) {
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9 return tempC * 9 / 5 + 32;
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10 }
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11
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12 const MAGNUS_B = 17.62;
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13 const MAGNUS_C = 243.12;
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14
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15 /**
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16 * The gamma function to calculate dew point.
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17 *
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18 * @param {number} tempC The temperature, in degrees Celsius.
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19 * @param {number} rhPct The relative humidity, in percent.
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20 * @return {number} The value of the gamma function.
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21 */
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22 function gammaFn(tempC, rhPct) {
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23 return Math.log(rhPct / 100) + MAGNUS_B * tempC / (MAGNUS_C + tempC);
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24 }
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25
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26 /**
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27 * Calculates the dew point.
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28 *
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29 * @param {number} tempC The temperature, in degrees Celsius.
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30 * @param {number} rhPct The relative humidity, in percent.
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31 * @return {number} The dew point, in degrees Celsius.
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32 */
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33 function dewPointC(tempC, rhPct) {
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34 const gamma = gammaFn(tempC, rhPct);
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35 return MAGNUS_C * gamma / (MAGNUS_B - gamma);
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36 }
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37
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38 /** The height of the chart in degrees. */
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39 const CHART_HEIGHT = 15;
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40
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41 /**
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42 * Charts some data.
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43 *
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44 * @param {CanvasRenderingContext2D} ctx The context to draw in.
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45 * @param {[number, number][]} data The data to chart, as `[x, y]` pairs.
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46 * @param {[number, number]} xRange The bounds of the X axis to draw.
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47 * @param {[number, number]} size The `[width, height]` of the context.
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48 */
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49 function drawChart(ctx, data, xRange, size) {
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50 const yRange = calculateYRange(data.map(d => d[1]));
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51
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52 ctx.lineWidth = 1.5;
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53
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54 ctx.beginPath();
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55 const first = project(data[0], size, xRange, yRange);
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56 ctx.moveTo(...first);
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57 for (const pt of data) {
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58 const projected = project(pt, size, xRange, yRange);
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59 ctx.lineTo(...projected);
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60 }
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61 ctx.stroke();
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62 }
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63
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64 /**
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65 * Determines what the Y range of the chart should be.
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66 * @param {number[]} ys The Y values of the chart.
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67 * @return {[number, number]} The lowest and highest values of the range.
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68 */
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69 function calculateYRange(ys) {
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70 const yMax = Math.max(...ys);
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71 const yMin = Math.min(...ys);
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72 const yMid = (yMin + yMax) / 2;
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73 const lastY = ys[ys.length - 1];
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74
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75 // We want the last value to be, at most, at the top or bottom 1/4 line
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76 // of the chart.
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77
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78 // If the middle of the range is already close enough, just use that.
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79 if (CHART_HEIGHT / 4 <= Math.abs(yMid - lastY)) {
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80 return [yMid - CHART_HEIGHT / 2, yMid + CHART_HEIGHT / 2];
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81 }
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82 // Otherwise, clamp the chart range.
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83 if (lastY < yMid) {
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84 return [lastY - CHART_HEIGHT / 4, lastY + 3 * CHART_HEIGHT / 4];
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85 }
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86 return [lastY - 3 * CHART_HEIGHT / 4, lastY + CHART_HEIGHT / 4];
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87 }
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88
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89 /**
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90 * Projects a Cartesian coordinate into Canvas space.
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91 *
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92 * @param {[number, number]} coord The `[x, y]` coordinate to project.
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93 * @param {[number, number]} size The `[width, height]` of the context.
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94 * @param {[number, number]} xRange The range of X values in the context.
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95 * @param {[number, number]} yRange The range of Y values in the context.
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96 * @return {[number, number]} The `[x, y]` coordinate in Canvas space.
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97 */
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98 function project(coord, size, xRange, yRange) {
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99 const [x, y] = coord;
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100 const [xMin, xMax] = xRange;
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101 const xSpan = xMax - xMin;
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102 const [yMin, yMax] = yRange;
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103 const ySpan = yMax - yMin;
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104 const [xSize, ySize] = size;
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105 return [
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106 (x - xMin) / xSpan * xSize,
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107 (yMax- y) / ySpan * ySize,
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108 ]
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109 }