Go: package main

import (
        "fmt"
        "math/big"
)

func main() {
        var x1, x2, x3, x4, y1, y2, y3, y4 big.Float
        var n int
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                fmt.Scanf("%f %f %f %f %f %f %f %f",
                        &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4)
                var lx, ly, mx, my, mul1, mul2, sub, zz big.Float
                lx.Sub(&x2, &x1)
                ly.Sub(&y2, &y1)
                mx.Sub(&x4, &x3)
                my.Sub(&y4, &y3)
                mul1.Mul(&lx, &my)
                mul2.Mul(&ly, &mx)
                sub.Sub(&mul1, &mul2)
                zz.SetFloat64(0.0)

                if sub.Cmp(&zz) == 0 {
                        fmt.Println("YES")
                } else {
                        fmt.Println("NO")
                }
        }
}

C++: #include <stdio.h>

int
main(void)
{
	int n;

	scanf("%d", &n);

	double x1, y1, x2, y2, x3, y3, x4, y4;

	while (n--) {
		scanf("%lf %lf %lf %lf %lf %lf %lf %lf",
				   &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4);

		if ((y2 - y1)/(x2 - x1) == (y4 - y3)/(x4 - x3)) {
			printf("YES\n");
		} else {
			printf("NO\n");
		}
	}

	return 0;
}

C++: #include<cstdio>
#include<cstdlib>
#include<cmath>

using namespace std;
int main(void)
{
	int n;
	double x[4],y[4];
	double k[2];
	scanf("%d",&n);
	for(int i = 0;i < n;i++)
	{
		for(int j = 0;j < 4;j++) scanf("%lf%lf",&x[j],&y[j]);
		k[0] = (y[0]-y[1])/(x[0]-x[1]);
		k[1] = (y[2]-y[3])/(x[2]-x[3]);
		if(k[0] == k[1]) printf("YES\n");
		else printf("NO\n");
	}
	return 0;
}

C++: #include<iostream>
#include<complex>
#include<cmath>
#include<string>
#include<queue>
using namespace std;
typedef complex<double> xy_t;
int main()
{
  int n;
  double x,y;
  queue<string> qs;
  cin >> n;
  for( int i = 0; i < n; ++i){
    xy_t p[4];
    for( int j = 0; j < 4; ++j){
      cin >> x >> y;
      p[j] = xy_t(x,y);
    }
    double S =(conj(p[0] - p[1])*(p[2] - p[3])).imag();
    if( S == 0) qs.push("YES");
    else qs.push("NO");
  }
  while(!qs.empty()){
    cout << qs.front() << endl;
    qs.pop();
  }
}

C++: #include <iostream>
#include <sstream>
#include <string>
#include <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cassert>

using namespace std;

#define FOR(i,k,n) for(int i=(k); i<(int)(n); ++i)
#define REP(i,n) FOR(i,0,n)
#define FORIT(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();++i)

template<class T> void debug(T begin, T end){ for(T i = begin; i != end; ++i) cerr<<*i<<" "; cerr<<endl; }
inline bool valid(int x, int y, int W, int H){ return (x >= 0 && y >= 0 && x < W && y < H); }

typedef long long ll;
const int INF = 100000000;
const double EPS = 1e-8;
const int MOD = 1000000007;
int dx[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy[8] = {0, 1, 0, -1, 1, 1, -1, -1};

#include <complex>
typedef complex<double> Point;
double cross(Point a, Point b){
  return imag(conj(a) * b);
}
struct Line : public vector<Point> {
  Line(Point a, Point b){
    push_back(a);
    push_back(b);
  }
  Point vector(){
    return back() - front();
  }
};
bool parallel(Line l, Line m){
  if(abs(cross(l.vector(), m.vector())) < EPS) return true;
  else return false;
}
Point input(){
  Point p;
  cin >> p.real() >> p.imag();
  return p;
}

int main(){
  int N;
  cin >> N;
  while(N--){
    Point a[4];
    REP(i, 4) a[i] = input();
    if(parallel(Line(a[0], a[1]), Line(a[2], a[3]))){
      cout << "YES" << endl;
    }else{
      cout << "NO" << endl;
    }
  }
  return 0;
}

C++: #include<stdio.h>
#include<math.h>
 
int main(void)
{
    double x1, x2, x3, x4, y1, y2, y3, y4;
    int i = 0;
    int n;
    double s;
    double eps = 0.0000000001;
     
    scanf("%d", &n);
     
    for(i = 0; i < n; i++) {
        scanf("%lf %lf %lf %lf %lf %lf %lf %lf", &x1, &x2, &x3, &x4, &y1, &y2, &y3, &y4);
         
        x2 -= x1;
        y2 -= y1;
        x4 -= x3;
        y4 -= y3;
         
        s = (x2 * y4) - (y2 * x4);
         
        s = abs(s);
         
        if (s < eps) {
            s = 0;
        }
         
        if (s == 0) {
            printf("YES\n");
        }
         
        else {
            printf("NO\n");
        }
    }
     
    return 0;
}

C++: #include <iostream>
using namespace std;
int main(void)
{
	double x[4];
	double y[4];
	int n;
	cin >> n;
	while(n--){
		for(int i=0; i<4; i++){
			cin >> x[i] >> y[i];
		}
		double d1,d2;
		d1 = (y[1]-y[0])/(x[1]-x[0]);
		d2 = (y[3]-y[2])/(x[3]-x[2]);
		if(d1==d2) cout << "YES" << endl;
		else cout << "NO" << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>

using namespace std;

void swap(double &x, double &y, double &xx, double &yy) 
{
  double tmp_x = x;
  double tmp_y = y;
  x = xx;
  y = yy;
  xx = tmp_x;
  yy = tmp_y;
}

bool isZero(double x) 
{
  return x < 0.00001;
}

void compute(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) 
{
  if (x1 > x2) swap(x1, y1, x2, y2);
  if (x3 > x4) swap(x3, y3, x4, y4);

  double diff_x12 = x2 - x1;
  double diff_y12 = y2 - y1;
  double diff_x34 = x4 - x3;
  double diff_y34 = y4 - y3;

  // if (isZero(diff_x12) && isZero(diff_y12))
  //   cout << "NO" << endl;
  // else if (isZero(diff_x34) && isZero(diff_y34))
  //   cout << "NO" << endl;  
  if (!isZero(diff_x12) && !isZero(diff_x34)) {
    double ab = diff_y12 / diff_x12;
    double cd = diff_y34 / diff_x34;
    if (isZero(abs(ab - cd)))
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  }
  else if (isZero(diff_x12) && isZero(diff_x34)) 
    cout << "YES" << endl;  
  else
    cout << "NO" << endl;  
}

int main(void) 
{
  int n;
  cin >> n;
  for (int i=0; i < n; i++) {
    double x1, y1, x2, y2, x3, y3, x4, y4;
    cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
    compute(x1, y1, x2, y2, x3, y3, x4, y4);
  }
  
  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
using ld = long double;

int main()
{
	int n;
	ld x1, x2, x3, x4, y1, y2, y3, y4;
	cin >> n;
	while (n--) {
		cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
		ld dx1 = x2 - x1, dy1 = y2 - y1;
		ld dx2 = x4 - x3, dy2 = y4 - y3;
		if (abs(dx1 * dy2 - dy1 * dx2) < 1e11 && (dx1 * dx2 >= 1e11 || (dx1 == 0 && dy2 == 0 && dy1 * dy2 > 0))) {
			puts("YES");
		}
		else {
			puts("NO");
		}
	}
	return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<algorithm>
#include<vector>
#include<set>
#include<map>
#include<string>
#include<functional>
#include<queue>
#include<stack>
#include<math.h>
#define INF 999999
#define EPS 1.0e-6

using namespace std;

typedef long long ll;

int n;
double x[5];
double y[5];

int main()
{
	cin >> n;
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < 4; j++) {
			cin >> x[j] >> y[j];
		}

		double k = (x[1] - x[0])*(x[3] - x[2]) + (y[1] - y[0])*(y[3] - y[2]);
		double j = sqrt((x[1] - x[0])*(x[1] - x[0]) + (y[1] - y[0])*(y[1] - y[0]))*sqrt((x[3] - x[2])*(x[3] - x[2]) + (y[3] - y[2])*(y[3] - y[2]));
		if (fabs(k - j) < EPS || fabs(k + j) < EPS)cout << "YES" << endl;
		else cout << "NO" << endl;
	}

	
	return 0;
}

C++: #include<iostream>
#define DFF 1e-10
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    double p[4][2],ans;
    for(int i=0; i<4; i++) for(int j=0; j<2; j++) cin >>p[i][j];
    for(int i=0; i<2; i++) for(int j=0; j<2; j++) p[i*2+1][j]-=p[i*2][j];
    ans = p[1][0]*p[3][1]-p[1][1]*p[3][0];
    if(ans>-DFF && ans<DFF) cout<<"YES"<<endl;
    else cout<<"NO"<<endl;
  }
  return 0;
}