/*

  datecalc.c

  Written 1996/96 by Oliver Kraus
  Published by Heinz Heise Verlag 1997 (c't 15/97) 
  Completly rewritten and put under GPL 2011 by Oliver Kraus
  
  (c) 2011 by Oliver Kraus (olikraus@gmail.com)

  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/>.


  Development goals:
    - English version
    - Optimized for 8 bit microcontroller 
    
  Definitions:
    Short Name: 	y
    Long Name: 	year 
    Range:			2000...
    
    Short Name: 	ydn
    Long Name: 	year day number
    Range:			1..366
    
    Short Name: 	cdn
    Long Name: 	century day number
    Range:			1...65535
    
    Short Name: 	ymd
    Long Name: 	Year, Month, Day
    Range:			2000...65535, 1..12, 1..31
    
  Conversions
    ymd --> y, ydn
      get_year_day_number()
    y, ydn --> ymd
      get_month_by_year_day_number()
      get_day_by_year_day_number()
    y, ydn  --> cdn
      to_century_day_number();
    cdn --> y, ydn
      from_century_day_number();
      

*/

#include <stdint.h>

/*
  Prototype:
    uint8_t is_leap_year(uint16_t y)
  Description:
    Calculate leap year
  Arguments:
    y           year, e.g. 2011 for year 2011
  Result:
    0           not a leap year
    1           leap year
*/
static uint8_t is_leap_year(uint16_t y)
{
   if ( 
          ((y % 4 == 0) && (y % 100 != 0)) || 
          (y % 400 == 0) 
      )
      return 1;
   return 0;
}

/*
  Prototype:
    uint16_t get_year_day_number(uint16_t y, uint8_t m, uint8_t d)
  Description:
    Calculate the day number within a year. 1st of Jan has the number 1.
    "Robertson" Algorithm
  Arguments:
    y           year, e.g. 2011 for year 2011
    m           month with 1 = january to 12 = december
    d           day starting with 1
  Result:
    The "day number" within the year: 1 for the 1st of Jan.
  See also:
    get_month_by_day_number()

*/
uint16_t get_year_day_number(uint16_t y, uint8_t m, uint8_t d)
{
  uint8_t tmp1; 
  uint16_t tmp2;
  tmp1 = 0;
  if ( m >= 3 )
    tmp1++;
  tmp2 = m;
  tmp2 +=2;
  tmp2 *=611;
  tmp2 /= 20;
  tmp2 += d;
  tmp2 -= 91;
  tmp1 <<=1;
  tmp2 -= tmp1;
  if ( tmp1 != 0 )
    tmp2 += is_leap_year(y);
  return tmp2;
}

/*
  Prototype:
    uint8_t get_month_by_year_day_number(uint16_t y, uint16_t ydn)
  Description:
    Get the month from year and day number within a year.
    "R. A. Stone" Algorithm
  Arguments:
    y           year, e.g. 2011 for year 2011
    ydn	year day number (1st of Jan has the number 1)
  Result:
    The month within the year: 1 for January.
  See also:
    get_year_day_number()
*/

static uint16_t corrected_year_day_number(uint16_t y, uint16_t ydn)
{
   uint8_t a;
   a = is_leap_year(y);
   if ( ydn > 59+a )
   {
      ydn += 2;
      ydn -= a;
   }
   ydn += 91;
   return ydn;
}

uint8_t get_month_by_year_day_number(uint16_t y, uint16_t ydn)
{
  uint8_t a;
  ydn = corrected_year_day_number(y, ydn);
  ydn *= 20;
  ydn /= 611;
  a = ydn;
  a -= 2;
  return a;  
}

/*
  Prototype:
    uint8_t get_day_by_year_day_number(uint16_t y, uint16_t ydn)
  Description:
    Get the day within month from year and day number within a year.
    "R. A. Stone" Algorithm
  Arguments:
    y           year, e.g. 2011 for year 2011
    ydn	year day number (1st of Jan has the number 1)
  Result:
    The day within a month: 1 for the first day of a month.
  See also:
    get_year_day_number()
*/
uint8_t get_day_by_year_day_number(uint16_t y, uint16_t ydn)
{
  uint8_t m;
  uint16_t tmp;
  m = get_month_by_year_day_number(y, ydn);
  m += 2;
  ydn = corrected_year_day_number(y, ydn);
  tmp = 611;
  tmp *= m;
  tmp /= 20;
  ydn -= tmp;
  return ydn;
}

/*
  Prototype:
    uint8_t get_weekday_by_year_day_number(uint16_t y, uint16_t ydn)
  Description:
    Get the day within week from year and day number within a year.
    "Zeller" Algorithm
  Arguments:
    y           year, e.g. 2011 for year 2011
    ydn	year day number (1st of Jan has the number 1)
  Result:
    The day within a week: 0..6 with 0 = Sunday, 1 = Monday, ...
  See also:
    get_year_day_number()
*/
uint8_t get_weekday_by_year_day_number(uint16_t y, uint16_t ydn)
{
  uint8_t j, c, tmp8;
  uint16_t tmp16;
  y--;
  j = y % 100;
  c = y / 100;
  tmp16 = c;
  tmp16 *= 5;
  tmp16 += ydn;
  tmp8 = j;
  j >>= 2;
  c >>= 2;
  tmp8 += j;
  tmp8 += c;
  tmp8 += 28;
  tmp16 += tmp8;
  tmp16 %= 7;
  return tmp16;  
}

/*
  Prototype:
    uint16_t to_century_day_number(uint16_t y, uint16_t ydn)
  Description:
    Calculate days since January, 1st, 2000
  Arguments:
    y           year, e.g. 2011 for year 2011
    ydn	year day number (1st of Jan has the number 1)
*/
uint16_t to_century_day_number(uint16_t y, uint16_t ydn)
{
  uint16_t cdn;
  cdn = ydn;
  cdn--;
  while( y > 2000 )
  {
    y--;
    cdn += 365;
    cdn += is_leap_year(y);
  }
  return cdn;
}

void from_century_day_number(uint16_t cdn, uint16_t *year, uint16_t *ydn)
{
  uint16_t y, days_per_year;
  y = 2000;
  for(;;)
  {
    days_per_year = 365;
    days_per_year += is_leap_year(y);
    if ( cdn >= days_per_year )
    {
      cdn -= days_per_year;
      y++;
    }
    else
      break;
  }
  cdn++;
  *year = y;
  *ydn = cdn;
}

/*
  Calculate the seconds after 2000-01-01 00:00. The largest possible
  time is 2136-02-07 06:28:15
*/
uint32_t to_time(uint16_t cdn, uint8_t h, uint8_t m, uint8_t s)
{
  uint32_t t;
  t = cdn;
  t *= 24;
  t += h;
  t *= 60;
  t += m;
  t *= 60;
  t += s;
  return t;
}


void from_time(uint32_t t, uint16_t *cdn, uint8_t *h, uint8_t *m, uint8_t *s)
{
  *s = t % 60;
  t /= 60;
  *m = t % 60;
  t /= 60;
  *h = t % 24;
  t /= 24;
  *cdn = t;
}

