look up table /w linear interpolation

test.c

/*
 look up table /w linear interpolation

 $ gcc -std=c99 test.c && a.out
 $ gnuplot
 gnuplot> plot "k.txt" notitle
 */

#include <stdio.h>
#include <stdlib.h>

typedef unsigned char  u1;

#define X(x) (u1)((x)/(100.0/255)+0.5)     /* unit: % */
#define Y(y) (u1)((y)/(100.0/255)+0.5)     /* unit: % */
const u1 table[] = {
    11,
    X(0), X(10), X(20), X(30), X(40), X(50), X(60), X(70), X(80), X(90), X(100),
    Y(0), Y(37), Y(55), Y(67), Y(74), Y(81), Y(85), Y(90), Y(94), Y(98), Y(100)
};

u1 lookup_table(const u1 x, const u1 tbl[])
{
    u1 n;
    u1 *ex, *ey;

    n = tbl[0];                 /* number of element */
    ex = (u1 *)&tbl[1];
    ey = (u1 *)&tbl[1+n];

    if(x <= ex[0])   return ey[0];
    if(x >= ex[n-1]) return ey[n-1];

    for(u1 i=n-2;; i--) {
        if(x <  ex[i]) continue;
        if(x == ex[i]) return ey[i];
        return ((ey[i+1] - ey[i]) * (x - ex[i])) / (ex[i+1] - ex[i]) + ey[i];
    }
}

int main(void)
{
    FILE *fp;

    fp=fopen("k.txt", "w");

    for(int x=0; x<256; x++) {
        int y = lookup_table(x, table);
        fprintf(fp, "%d %d\n", x, y);
    }

    fclose(fp);
    return 0;
}