/*
 * Copyright © 2009 Keith Packard <keithp@keithp.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 2 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, write to the Free Software Foundation, Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
 */

#include "aoview.h"

static int16_t altitude_table[] = {
#include "altitude.h"
};

#define ALT_FRAC_SCALE	(1 << ALT_FRAC_BITS)
#define ALT_FRAC_MASK	(ALT_FRAC_SCALE - 1)

int16_t
aoview_pres_to_altitude(int16_t pres)
{
	uint8_t	o;
	int16_t	part;

	if (pres < 0)
		pres = 0;
	o = pres >> ALT_FRAC_BITS;
	part = pres & ALT_FRAC_MASK;

	return ((int32_t) altitude_table[o] * (ALT_FRAC_SCALE - part) +
		(int32_t) altitude_table[o+1] * part + (ALT_FRAC_SCALE >> 1)) >> ALT_FRAC_BITS;
}

int16_t
aoview_altitude_to_pres(int16_t alt)
{
	int16_t span, sub_span;
	uint8_t	l, h, m;
	int32_t pres;

	l = 0;
	h = NALT - 1;
	while ((h - l) != 1) {
		m = (l + h) >> 1;
		if (altitude_table[m] < alt)
			h = m;
		else
			l = m;
	}
	span = altitude_table[l] - altitude_table[h];
	sub_span = altitude_table[l] - alt;
	pres = ((((int32_t) l * (span - sub_span) + (int32_t) h * sub_span) << ALT_FRAC_BITS) + (span >> 1)) / span;
	if (pres > 32767)
		pres = 32767;
	if (pres < 0)
		pres = 0;
	return (int16_t) pres;
}