#include <stdio.h>
#include <math.h>
#define NMAX 8

float x[NMAX];

int set_original(float *x, int len){
    for(int i = 0; i < len; i++){
        x[i] = i*i;
        printf("%f\n",x[i]);
    }
}


float lag(float *X, int len, float target){
    int i, j;
    int begin = (int)target; > 2 ? nearest - 2: 0;
    int end = begin + 4 < len ? begin + 4 : len - 1;
    float ret_y = 0;
    for(i = begin; i < end; i++){
        float upper = 1.0f, lower = 1.0f;
        for(j = begin; j < end; j++){
            if(i != j){
                upper *= target - j;
                lower *= i - j;
            }
        }
        ret_y += (upper / lower) * X[i];
    }
    return ret_y;
}


void main(void){
    set_original(x, NMAX);
    for(int i = 2; i < 6; i++){
        float tgt = i * 0.5f;
        printf("%f:%f\n",tgt, lag(x,NMAX, tgt));
    }
}
4点補間
