色相が均等な間隔になるよう、色を割り当てる。Picks a color on hue circle with a constant interval.

util.js

/**
 * util.js
 * @auther kyama http://sukimaprogrammer.blogspot.jp/
 * @since 0.0.1
 * @version 0.0.3
 * 
 * This source is distributed under the MIT License.
 * 
 * Copyright (c) 2013 lazykyama http://sukimaprogrammer.blogspot.jp/
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

kyama = kyama || {};
kyama.util = kyama.util || {};
kyama.util.version = '0.0.3';

kyama.util.constant = kyama.constant || {};
kyama.util.constant.TWO_PI_DEGREE = 360.0;

/**
 * pick a rgb color on circle.
 * @param splitSize the number of intervals on circle.
 * @param index the index of intervals on circle.
 * @param offset the offset on Hue circle, [0, 359].
 * @return rgbObj the object represents RGB color.
 *         rgbObj.r is red channel value in [0, 1].
 *         rgbObj.g is green channel value in [0, 1].
 *         rgbObj.b is blue channel value in [0, 1].
 */
kyama.util.pickRGBColorOnCircle = function(splitSize, index, offset) {
    if (splitSize <= 0) {
        throw new Error('negative splitSize[' + splitSize + '] is invalid.');
    }
    if (index < 0) {
        throw new Error('negative index[' + index + '] is invalid.');
    }
    if (splitSize <= index) {
        throw new Error('too large index[' + index + '] is invalid, splitSize = '
			+ splitSize);
    }
    if (offset < 0) {
        throw new Error('negative offset[' + offset + '] is invalid.');
    }
    if (kyama.util.constant.TWO_PI_DEGREE < offset) {
        throw new Error('too large offset[' + offset + '] is invalid.');
    }

    var interval = kyama.util.constant.TWO_PI_DEGREE / splitSize;
    var hue = ((index * interval) + offset) % kyama.util.constant.TWO_PI_DEGREE;
    var sat = 1.0;
    var val = 1.0;

    // this code written in reference to the following site and Wikipedia.
    // http://www.technotype.net/tutorial/tutorial.php?fileId=%7BImage%20processing%7D&sectionId=%7B-converting-between-rgb-and-hsv-color-space%7D
    var hi = Math.floor(hue / 60.0) % 6;
    var f  = (hue / 60.0) - hi;
    var p  = val * (1.0 - sat);
    var q  = val * (1.0 - sat * f);
    var t  = val * (1.0 - sat * (1.0 - f));

    switch(hi) {
    case 0:
        return {
            r: val,
            g: t,
            b: p
        };
    case 1:
        return {
            r: q,
            g: val,
            b: p
        };
    case 2:
        return {
            r: p,
            g: val,
            b: t
        };
    case 3:
        return {
            r: p,
            g: q,
            b: val
        };
    case 4:
        return {
            r: t,
            g: p,
            b: val
        };
    case 5:
        return {
            r: val,
            g: p,
            b: q
        };
    default:
        throw new Error('unexpected error: args = ' + splitSize + ', ' + index);
    }
};