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Problem: Torusia

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Time Limit: 1 second
Memory Limit: 128 MB

Download science.h

The training site is not able to judge this task correctly. So instead of running alison() and bill() in separate invocations of your program, both will be run one after the other.

Do not share any global variables between the implementation of your two functions! There is nothing stopping you from cheating like this, but in the spirit of the problem, you shouldn't. Alison and Bill are counting on you!

Scientists Alison and Bill were on an important scientific expedition for science to Torusia, a newly discovered donut-shaped planet (yes, the cosmologists are still scratching their heads). During some standard experiments, the two scientists were separated and freak solar winds destroyed their communications equipment. They now have no idea how to find each other, however they each have a machine designed for scientific purposes which they can use to re-unite.

Alison and Bill both view Torusia as a 4096 x 4096 grid, with (0, 0) in the top-left corner. The grid "wraps around" at the edges so that horizontal lines are in fact horizontal circles and vertical lines are vertical circles. Hence, the point east of (4095, 34) is (0, 34) and the point north of (17, 0) is (17, 4095).

Alison perceives her own position as (0, 0) and Bill perceives his own position as (0, 0) however their absolute positions are different and hence their co-ordinate systems are not aligned. Note however that they have the same orientation (direction of north, south, east, west) so if Alison is xA metres east and yA metres south of Bill, then a point Alison refers to as (x, y) would be the point on Bill's grid labelled ((x + xA) 4096, (y + yA) 4096).

Alison has a machine that performs one function:

Bill has a machine that can perform two functions:

Each minute, Alison and Bill can use their respective machines to perform one function. Alison's machine is a bit faster to start up than Bill's, so each minute you can assume that her marker is placed before Bill performs his query.

You are able to send Alison and Bill a program to help Alison place markers and Bill make measurements so that Bill can determine Alison's position as fast as possible.


Your code file will interact with functions provided in the downloadable source file science.h. You must implement the following two functions:

Your code should not have a "main" function, this will be supplied by science.h, in addition to the following:

void mark(int x, int y); which can only be called (directly or indirectly) by Alison.
int numRow(int y); which can only be called (directly or indirectly) by Bill.
int numColumn(int x); which can only be called (directly or indirectly) by Bill.
void found(int x, int y); which can only be called (directly or indirectly) by Bill.

You may assume that all these functions run in constant time.


You must add #include "science.h" to the beginning of your code, and make sure the file science.h is in the same folder as your code.


Whilst Alison and Bill are in theory simultaneously performing their actions, we are able to simulate this by first running the program "as Alison", which creates a log file of the cell she marks each minute, then running the program "as Bill". The executable created will take one line of standard input that determines which scientist it runs.

You may wish to temporarily change the value of the constant SIZE at the top of science.h, to test your program on smaller grids.

Sample Session

The following sample session is based on this code:

#include "science.h"

void alison() {
    mark(1, 100);
    mark(2000, 100);

void bill() {
    int a = numRow(120);
    int b = numColumn(904);
    int c = numRow(120);
    found(3000, 20);

First, the program is run and the following input is provided to standard input (the screen):


The program will run alison(), which makes these function calls:

Function Call Explanation
mark(1, 100) In the first minute, Alison marks the cell 1m east and 100m south of her.
mark(2000, 100) In the second minute, Alison marks the cell 2000m east and 100m south of her.

The program exits successfully and creates a file mark_log. We now run the program again and provide the following input, which places Alison 3000m east and 20m south of Bill.

B 3000 20

The program will run bill(), which makes these function calls:

Function Call Explanation
numRow(120) returns 1, as in the first minute Alison marked a cell 100m south of her, and she is 20m south of Bill, hence there is one marked cell on the row 120m south of Bill.
numColumn(904) returns 1, as in the second minute Alison, 3000m east of Bill, marked a cell 2000m east, which is the cell only 904m east of Bill since the grid is 4096m wide.
numRow(120) returns 2, as there are now two markers on the row 120m south of Bill
found(3000, 20) Through some amazingly lucky guesswork, Bill correctly finds Alison's position after only 3 minutes.


Your program will be given a score based on the time it takes Bill to find Alison. Specifically, if found is called with the correct parameters after M minutes:

Here is a table showing ten example scores and corresponding success time required:

Score 11 20 30 40 50 60 70 80 90 100
M 10,000 1,300 650 433 325 260 216 185 162 144

There are no subtasks for this problem. Your total score will be the minimum score your program received over all test cases.


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