/**
 * Copyright © 2016  Mattias Andrée <maandree@member.fsf.org>
 * 
 * This program 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.
 * 
 * This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
 */
#include "libred.h"
#include "macros.h"
#include <math.h>
#include <errno.h>
#include <fcntl.h>



/**
 * The file descriptor to the colour lookup table.
 */
static int fd = -1;



/**
 * This function must be called, once,
 * before calling `libred_get_colour`.
 * 
 * @return  0 on success, -1 on error.
 * 
 * @throws  Any error specified for `open(3)`.
 */
int libred_init_colour(void)
{
  libred_term_colour();
  fd = open(SYSDEPRESDIR "/" PACKAGE "/10deg", O_RDONLY);
  return fd < 0 ? -1 : 0;
}


/**
 * Call this when the process will not
 * longer make calls to `libred_get_colour`.
 */
void libred_term_colour(void)
{
  if (fd >= 0)
    close(fd), fd = -1;
}


/**
 * Convert from CIE xyY to [0, 1] sRGB.
 * 
 * @param  x  The 'x' component.
 * @param  y  The 'y' component.
 * @param  Y  The 'Y' component.
 * @param  r  Output parameter for the “red” value.
 *            (Seriously, sRGB red is orange, just look at it fullscreen.)
 * @param  g  Output parameter for the green value.
 * @param  b  Output parameter for the blue value.
 */
static void ciexyy_to_srgb(double x, double y, double Y, double *r, double *g, double *b)
{
#define SRGB(C)  (((C) <= 0.0031308) ? (12.92 * (C)) : ((1.0 + 0.055) * pow((C), 1.0 / 2.4) - 0.055))
  double X, Z;
  
  /* Convert CIE xyY to CIE XYZ. */
  X = Y * (y == 0.0 ? 0.0 : (x / y));
  Z = Y * (y == 0.0 ? 0.0 : ((1.0 - x - y) / y));
  
  /* Convert CIE XYZ to [0, 1] linear RGB. (ciexyz_to_linear) */
  *r = ( 3.240450 * X) + (-1.537140 * Y) + (-0.4985320 * Z);
  *g = (-0.969266 * X) + ( 1.876010 * Y) + ( 0.0415561 * Z);
  *b = (0.0556434 * X) + (-0.204026 * Y) + ( 1.0572300 * Z);
  
  /* Convert [0, 1] linear RGB to [0, 1] sRGB. */
  SRGB(*r), SRGB(*g), SRGB(*b);
}


/**
 * Perform linear interpolation (considered very good)
 * between the CIE xyY values for two colour temperatures
 * and convert the result to sRGB. The two colours should
 * be the closest below the desired colour temperature,
 * and the closest above the desired colour temperature.
 * 
 * @param  x1    The 'x' component for the low colour.
 * @param  y1    The 'y' component for the low colour.
 * @param  x2    The 'x' component for the high colour.
 * @param  y2    The 'y' component for the high colour.
 * @param  temp  The desired colour temperature.
 * @param  r     Output parameter for the “red” value.
 * @param  g     Output parameter for the green value.
 * @param  b     Output parameter for the blue value.
 */
static void interpolate(double x1, double y1, double x2, double y2, double temp, double *r, double *g, double *b)
{
  double weight = fmod(temp, (double)LIBRED_DELTA_TEMPERATURE) / (double)LIBRED_DELTA_TEMPERATURE;
  double x = x1 * (1 - weight) + x2 * weight;
  double y = y1 * (1 - weight) + y2 * weight;
  ciexyy_to_srgb(x, y, 1.0, r, g, b);
}


/**
 * Get the [0, 1] sRGB values of a colour temperature.
 * 
 * @param   temp  The desired colour temperature.
 * @param   r     Output parameter for the “red” value.
 * @param   g     Output parameter for the green value.
 * @param   b     Output parameter for the blue value.
 * @return        0 on succeess, -1 on error.
 * 
 * @throws  0     The file did not have the expected size.
 * @throws  EDOM  The selected temperature is below 1000 K.
 * @throws        Any error specified for pread(3).
 */
int libred_get_colour(long int temp, double *r, double *g, double *b)
{
  double values[10]; /* low:x,y,r,g,b + high:x,y,r,g,b */
  off_t offset;
  double max;
  
  /* We do not have any values for above 40 000 K, but
   * the differences will be unnoticeable, perhaps even
   * unencodeable. */
  if (temp > LIBRED_HIGHEST_TEMPERATURE)  temp = LIBRED_HIGHEST_TEMPERATURE;
  /* Things do not glow below 1000 K. Yes, fire is hot! */
  if (temp < LIBRED_LOWEST_TEMPERATURE)   t ((errno = EDOM));
  
  /* Read table. */
  offset = ((off_t)temp - LIBRED_LOWEST_TEMPERATURE) / LIBRED_DELTA_TEMPERATURE;
  offset *= (off_t)(sizeof(values) / 2);
  errno = 0; xpread(fd, values, sizeof(values), offset);
  
  /* Get colour. */
  if (temp % LIBRED_DELTA_TEMPERATURE)
    interpolate(values[0], values[1], values[6], values[7], (double)temp, r, g, b);
  else
    *r = values[2], *g = values[3], *b = values[4];
  
  /* Adjust colours for use. */
  max = fmax(fmax(fabs(*r), fabs(*g)), fabs(*b));
  if (max != 0)  *r /= max, *g /= max, *b /= max;
  *r = *r > 0.0 ? *r : 0.0;
  *g = *g > 0.0 ? *g : 0.0;
  *b = *b > 0.0 ? *b : 0.0;
  
  return 0;
 fail:
  return -1;
}