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/*
* Curve interpolation
*
* Copyright (c) 2018-2020 Michael Buesch <m@bues.ch>
*
* 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.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#include "compat.h"
#include "debug.h"
#include "curve.h"
#include "util.h"
/* Calculate and interpolate a curve Y value from an X value.
*
* y
* ^ *
* | *
* -|- *
* | *
* |*
* +------|---->x
*/
uint16_t curve_interpolate(const struct curve_point __flash *curve,
uint8_t curve_size,
uint16_t x)
{
const struct curve_point __flash *rhp, *lhp;
uint8_t i;
uint16_t lx, ly;
uint16_t rx, ry;
uint16_t y;
uint32_t tmp;
/* Find the curve points
* left handed and right handed to the x value. */
lhp = &curve[0];
rhp = lhp;
for (i = 0u; i < curve_size; i++) {
rhp = &curve[i];
if (rhp->x >= x)
break;
lhp = rhp;
}
lx = lhp->x;
ly = lhp->y;
rx = rhp->x;
ry = rhp->y;
/* Linear interpolation between lhp and rhp:
* ((x - lx) * ((ry - ly) / (rx - lx))) + ly */
if (rx - lx == 0u) {
y = ly;
} else {
tmp = x - lx;
tmp *= ry - ly;
tmp /= rx - lx;
tmp += ly;
y = lim_u16(tmp);
}
return y;
}
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