AIOJ

Prerequisite knowledge: General

This page briefly introduces the concept and usage of validators.

Overview

A validator is used to verify the validity of generated test data. When the data for a problem has been generated but the problem setter is concerned that it may be invalid (that it does not satisfy the problem's constraints: overflow, a disconnected graph, not being a tree, ...), the setter usually uses a validator to check it.¹

Because Codeforces supports hacking, every problem on Codeforces must have a validator. UOJ is the same. Polygon has built-in support for validators.

Usage

Simply enter ./val on the command line. The data is supplied through stdin. If you want to read from a file, use ./val < a.in.

If there is no problem with the data, nothing will be output and 0 will be returned; otherwise, an error message will be output and a non-zero value will be returned.

Notes

When writing a validator, do not make any assumptions about the data being validated, because it may contain anything. Therefore, the problem setter must check all kinds of invalid cases (Testlib greatly simplifies this process).
- For example, given a tree with $n$ vertices as input, the main tasks are to determine whether $n$ is within the range and whether the input is a tree. However, after checking the range of $n$ , do not fail to check that the endpoints of each subsequently input edge are within the valid range, otherwise the validator may RE.
- Even if it will not RE, it should still be checked; otherwise, your error report will be incorrect. In the example above, if this is not checked, the error might be "not a tree", but the correct error should be "the edge's start/endpoints are not in $[1,n]$ ".
Do not make any assumptions about how contestants read the input. Therefore, you must ensure that the data passed by the validator completely conforms to the input format.
- For example, a contestant may read digits character by character, reading only one space between numbers. Therefore, when writing a validator, every whitespace character in the data must be explicitly read by the validator (such as spaces and newlines).
Do not forget inf.readEof() at the end.
If the problem is open to hacks (or if the validator's error messages will be visible to others), make the error messages as user-friendly as possible.
- Use "item aliases" when reading variables.
- When the expression used in a check is not very easy to understand, use ensuref rather than ensure.

Example

The following is the validator for CF Gym 100541A - Stock Market:

#include "testlib.h"

int main(int argc, char* argv[]) {
  registerValidation(argc, argv);
  int testCount = inf.readInt(1, 10, "testCount");
  inf.readEoln();

  for (int i = 0; i < testCount; i++) {
    int n = inf.readInt(1, 100, "n");
    inf.readSpace();
    inf.readInt(1, 1000000, "w");
    inf.readEoln();

    for (int i = 0; i < n; ++i) {
      inf.readInt(1, 1000, "p_i");
      if (i < n - 1) inf.readSpace();
    }
    inf.readEoln();
  }

  inf.readEof();
}

External links

References and notes

Footnotes

Validators with testlib.h - Codeforces ↩

Validator