Try this test:

-10 -1 1
-1 8 1
-7 2 1
3

Answer: 14.38

I know that the task was rejudged.
I added the condition for new test, where A, B and C collinear: cos(ACB) - 1.0 < eps, where eps = 1e-8, then answer = AB, but I've still get WA 48. Give me a hint, please.
Thanks in advance.

You should add some test.
So my AC programm give wrong answer on this test
20 0 0
-20 0 0
0 0 0
10
---
Write answer is 51.42;
and my answer is 45.11
Please do something!!!

this may be true: 1.000 > 1.
so add this func:
    double acos1(double x)
    {
        if(x > 1)
          x = 1;
        if(x < -1)
          x = -1;
        return acos(x);
    }

New tests have been added to the problem which exploit a common mistake when using acos function. Hint: try to calculate acos(sqrt(3.0) * sqrt(3.0) - 4.0) in your programming language. 649 authors (60%) with accepted solutions have been challenged.

Submission ID 7846927 (not my source, found online) gets Accepted but on this test it prints nan:

2 4 0
3 6 0
0 0 0
3

I'm pretty sure the test is correct (I plotted it). It's a sphere with center in origin and two points co-linear with the center which lie outside the sphere.

Submission ID 7846930 (also found online) also gets Accepted despite printing nan on the test above.

Edited by author 12.04.2018 03:03

Edited by author 12.04.2018 03:08

If you already have solved (simpler!) "1285. Thread in a Hyperspace", just replace N=8 with N=3.

That's how I get AC after so much time.

Edited by author 06.09.2013 22:11

#include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
struct point { double x;
               double y;
               double z; };
double pi=3.14159265358979323846;
int main () {
    point a,b,c;
    double tanga,tangb,distac,distbc,ang,arc,r,angpt;
    cin >> a.x >> a.y >> a.z >> b.x >> b.y >> b.z >> c.x >> c.y >> c.z >> r ;
    distac=sqrt((a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y)+(a.z-c.z)*(a.z-c.z));
    distbc=sqrt((b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y)+(b.z-c.z)*(b.z-c.z));
    tanga=sqrt(distac*distac-r*r);
    tangb=sqrt(distbc*distbc-r*r);
    angpt=acos(((a.x-c.x)*(b.x-c.x)+(a.y-c.y)*(b.y-c.y)+(a.z-c.z)*(b.z-c.z))/
                (sqrt((a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y)+(a.z-c.z)*(a.z-c.z))*
                 sqrt((b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y)+(b.z-c.z)*(b.z-c.z))))*(180/pi); //angle between the two vectors AC and BC
    ang=angpt-(((atan2(tanga,distac)+atan2(tangb,distbc))*(180/pi)));
    ang/=360;
    arc=ang*2*pi*r;
    printf("%.2lf",tanga+tangb+arc);
    cout << endl ;
    return 0;
}

I know that I have to check if there could be a straight line but I think that the sample test should be right for my program.
My program writes 19.78 not 19.71. Thanks in advance.

First, Anatoly's tip helped me (see "WA#4 - I don't know why" thread).

Another one tip for those receiving WA with "swear correct algo": I've multiplyed whole input data to some koef (tried 500, 800) and divided result by this koef before printing — got AC while without it got WA on 8-th test. It's all about error of calculations.

And, of course, I've used double, not float.

AB, AC, BC - Vectors.
r - Radius.
Arctangent(y, x) - Coprocessor function FPATAN, calculates an angle between vectors A(x, y) and X(1, 0)(using y and x coordinates of the A point).

My algorithm:

d = Abs(VectorMultiply(AC, BC)) / AB
if (d >= r) then Write(Length(AB))
else begin
    s1 = Sqrt(Length(AC) * Length(AC) - r * r)
    angle1 = Arctangent(Sqrt(Length(AC) * Length(AC) -
             d * d), d) - Arctangent(s1, r);
    s2 = Sqrt(Length(BC) * Length(BC) - r * r)
    angle2 = Arctangent(Sqrt(Length(BC) * Length(BC) -
             d * d), d) - Arctangent(s2, r);
    s = r * (angle1 + angle2)
    Write(s1 + s + s2)
end

-----

Am I wrong?

I got AC as I changed all ints to longs. I thought ints would be sufficient to do the sum. Still can't figure out why longs are needed.

What is wrong with my code? I can't figure out by myself:(

Some people talking about some tricky case. What is it?

Try to increase your EPS, I got AC with EPS = 1e-7

1 1 1
1 1 1
0 0 0
0

ans=0

I had WA28. After two hints I get AC.
1) check if A=B
2) check if sphere has intersections with segment using long long integers, not doubles

Edited by author 06.02.2011 08:52

Edited by author 24.04.2012 15:50

I had WA 11 and this test helped me:
0 0 0
0 0 10
0 1 168
7
Correct answer is 10.00. In short the problem is that the distance between C and the line between A and B is less then R but still the thread doesn't pass through the ball.