#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "f.h"

int main (int argc, const char **argv)
{
    FILE *in;
    double x0,*x,*y,res;
    int n,i;
    clock_t time;

    if(argc!=3)
    {
        fprintf(stderr,"Usage %s n input_file x0\n",argv[0]);
        return ERROR_READ;
    }
    if(sscanf(argv[1], "%d" ,&n)!=1 || n<2)
    {
        fprintf(stderr,"Can not read n!\n");
        return ERROR_READ;
    }
    if(!(x = (double *)malloc(n*sizeof(double))))
    {
        fprintf(stderr,"Can not allocate memory x[n]\n");
        return ERROR_MEMORY;
    }
    if(!(y = (double *)malloc(n*sizeof(double))))
    {
        fprintf(stderr,"Can nor allocate memory y[n]\n");
        return ERROR_MEMORY;
    }
    if(!(in = fopen(argv[2],"r")))
    {
        fprintf(stderr,"Can nor read %s\n",argv[2]);
        free(x);
        free(y);
        return ERROR_OPEN;
    }
    for(i = 0; i<(n);i++)
    {
        if(fscanf(in,"%lf",x + i)!=1)
        {
            fprintf(stderr,"Can nor read elments from %s\n",argv[2]);
                        free(x);
                        free(y);
                        return ERROR_READ;
        }
        if(fscanf(in,"%lf",y + i)!=1)
        {
            fprintf(stderr,"Can nor read elments from %s\n",argv[2]);
            free(x);
            free(y);
            return ERROR_READ;
        }
    }
    fclose(in);
    printf("x0 = ");
    if( scanf("%lf", &x0)!=1 )
    {
        fprintf(stderr, "Error! Can not read x0!\n");
        free(x);
        free(y);
        return ERROR_READ;
    }
    /*if(sscanf(argv[4], "%lf", &x0)!=1)
    {
        fprintf(stderr,"Can not read %s\n",argv[4]);
        free(x);
        free(y);
        return ERROR_READ;
    }*/
    time = clock();
    res = fun(n,x0,x,y);
    printf("RESULT: %lf\n  TIME: %lf\n",res,((double)(clock() - time) )/CLOCKS_PER_SEC);
    free(x);
    free(y);
    return 0;
}
#include<stdio.h>
#include<stdlib.h>
#include "f.h"

double fun(int n,double x0,double*x,double*y)
{
    int i,j;
    double res = y[0],k = 1;
    for(i=0;i<n-1;i++)
    {
        k *=(x0 - x[i]);
        for(j=0;j<(n-1-i);j++)
            y[j] = (y[j + 1] - y[j])/(x[j + i + 1] - x[j]);
        res += y[0]*k;

    }
    return res;
}
#ifndef F_H
#define F_H

double fun(int n,double x0, double *x, double *y);

#define ERROR_READ 1
#define ERROR_OPEN 2
#define ERROR_MEMORY 3
#endif