'use strict';

/**
 * Converts Celsius to Fahrenheit.
 * @param {number} tempC
 * @return {number} The temperature in Fahrenheit.
 */
function cToF(tempC) {
    return tempC * 9 / 5 + 32;
}

const MAGNUS_B = 17.62;
const MAGNUS_C = 243.12;

/**
 * The gamma function to calculate dew point.
 *
 * @param {number} tempC The temperature, in degrees Celsius.
 * @param {number} rhPct The relative humidity, in percent.
 * @return {number} The value of the gamma function.
 */
function gammaFn(tempC, rhPct) {
    return Math.log(rhPct / 100) + MAGNUS_B * tempC / (MAGNUS_C + tempC);
}

/**
 * Calculates the dew point.
 *
 * @param {number} tempC The temperature, in degrees Celsius.
 * @param {number} rhPct The relative humidity, in percent.
 * @return {number} The dew point, in degrees Celsius.
 */
function dewPointC(tempC, rhPct) {
    const gamma = gammaFn(tempC, rhPct);
    return MAGNUS_C * gamma / (MAGNUS_B - gamma);
}

/** The height of the chart in degrees. */
const CHART_HEIGHT = 15;

/**
 * Charts some data.
 *
 * @param {CanvasRenderingContext2D} ctx The context to draw in.
 * @param {[number, number][]} data The data to chart, as `[x, y]` pairs.
 * @param {[number, number]} xRange The bounds of the X axis to draw.
 * @param {[number, number]} size The `[width, height]` of the context.
 */
function drawChart(ctx, data, xRange, size) {
    const yRange = calculateYRange(data.map(d => d[1]));

    ctx.lineWidth = 1.5;

    ctx.beginPath();
    const first = project(data[0], size, xRange, yRange);
    ctx.moveTo(...first);
    for (const pt of data) {
        const projected = project(pt, size, xRange, yRange);
        ctx.lineTo(...projected);
    }
    ctx.stroke();
}

/**
 * Determines what the Y range of the chart should be.
 * @param {number[]} ys The Y values of the chart.
 * @return {[number, number]} The lowest and highest values of the range.
 */
function calculateYRange(ys) {
    const yMax = Math.max(...ys);
    const yMin = Math.min(...ys);
    const yMid = (yMin + yMax) / 2;
    const lastY = ys[ys.length - 1];

    // We want the last value to be, at most, at the top or bottom 1/4 line
    // of the chart.

    // If the middle of the range is already close enough, just use that.
    if (CHART_HEIGHT / 4 <= Math.abs(yMid - lastY)) {
        return [yMid - CHART_HEIGHT / 2, yMid + CHART_HEIGHT / 2];
    }
    // Otherwise, clamp the chart range.
    if (lastY < yMid) {
        return [lastY - CHART_HEIGHT / 4, lastY + 3 * CHART_HEIGHT / 4];
    }
    return [lastY - 3 * CHART_HEIGHT / 4, lastY + CHART_HEIGHT / 4];
}

/**
 * Projects a Cartesian coordinate into Canvas space.
 *
 * @param {[number, number]} coord The `[x, y]` coordinate to project.
 * @param {[number, number]} size The `[width, height]` of the context.
 * @param {[number, number]} xRange The range of X values in the context.
 * @param {[number, number]} yRange The range of Y values in the context.
 * @return {[number, number]} The `[x, y]` coordinate in Canvas space.
 */
function project(coord, size, xRange, yRange) {
    const [x, y] = coord;
    const [xMin, xMax] = xRange;
    const xSpan = xMax - xMin;
    const [yMin, yMax] = yRange;
    const ySpan = yMax - yMin;
    const [xSize, ySize] = size;
    return [
        (x - xMin) / xSpan * xSize,
        (yMax- y) / ySpan * ySize,
    ]
}