Set brightness based on hour

Brightness.swift

/* Brightness.swift -- Set brightness based on hour
 This file is part of brightness.

 Brightness is free software: you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation, either version 3 of the License, or
 (at your option) any later version.

 Brightness is distributed in the hope that it will be useful,
 but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 GNU General Public License for more details.

 You should have received a copy of the GNU General Public License
 along with Brightness. If not, see <http://www.gnu.org/licenses/>.
 Copyright (c) 2013 Jakub Tymejczyk <tymmej@gmail.com>
 */

import Swift

let TRANSITION_LOW = -6.0
let TRANSITION_HIGH = 3.0

/*also from Redshift*/
func calculate_interpolated_value(elevation: Double, _ day: Float, _ night: Float) -> Float {
    var result: Float = 0
    if elevation < TRANSITION_LOW {
        result = night
    } else if elevation < TRANSITION_HIGH {
        /* Transition period: interpolate */
        let a = Float((TRANSITION_LOW - elevation) / (TRANSITION_LOW - TRANSITION_HIGH))
        result = (1.0 - a) * night + a * day
    } else {
        result = day
    }
    
    return result
}