If you have written some programming problems, and have prepared test cases, you will probably experience the terrible feeling that some test cases may be invalid (meaning it does not agree with the constraints in problem statement): upper bound can be violated, your graph not satisfied connectivity requirements or is not at tree... It is reasonable to feel that way. Even experienced problem setters make mistakes sometimes (for example, in the prestigious ACM ICPC World final 2007).

It is strictly recommended to write a special program (called validator) to formally check each test to satisfy all requirements from problem statements. Validators are strictly required for problems on Codeforces. Polygon has built-in support of validators.

It is really easy to write a validator using testlib.h.

Example

Following is the validator was written for the problem 100541A - Stock Market:

#include "testlib.h"
#include <bits/stdc++.h>
using namespace std;

int main() {
    registerValidation();
    int testCount = inf.readInt(1, 10, "testCount");
    inf.readEoln();
    
    for (int i = 0; i < testCount; i++) {
        int n = inf.readInt(1, 100, "n");
        inf.readChar(' ');
        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.readChar(' ');
        }
        inf.readEoln();
    }

    inf.readEof();
}

The wonderful thing about this validator is that it is very simple and it is very difficult to write something incorrect.

More examples can be found at the Github repo

Available methods

The first line of your code should be registerValidation() which does some magic in the background, so that you can use the necessary methods. Most methods for validators start with prefix "read" and it does the same thing: moves input stream pointer to next suitable place after reading something. It also detect violations (input does not match what you are trying to read), and then throw error.

Notes:

• Validator is strict. It cares about correct placing of spaces. For example, when you're trying to read an integer and the next character is a space (and then an integer), the validator will throw error.
• Some method has "regex" feature. It is not a full-featured regex as you may have used in many programming languages. It is a very simple version, which supports the following:
  • Set of character, e.g: [a-z] is a lowercase latin letter, [^a-z] matches anything but a lowercase latin letter.
  • Range, e.g. [a-z]{1,5} is a string of length 1 to 5 consists of only lowercase latin letter.
  • Or operator, e.g. mike|john is either mike or john.
  • Optional character, e.g. -?[1-9][0-9]{0,3} will match non-zero integers from -9999 to 9999 (note the optional minus sign), note that it will match '-0'.
  • Repetition, e.g. [0-9]* will match sequences (empty or non-empty) of digits, and [0-9]+ will match non-empty sequences of digits.
• Also regarding regex, very simple greedy algorithm is used. For example, pattern [0-9]?1 will not match 1, because of greedy nature of matching.

Following is full list of methods available:

Parameter variableName

It is recommended to insert last string parameter to readInt/readInteger/readLong/readDouble/readWord/readToken/readString/readLine called variableName to make error message be human-readable. So it is preffered to use inf.readInt(1, 100, n) instead of inf.readInt(1, 100). The first statement will fail with human-readable message like FAIL Integer parameter [name=n] equals to 0, violates the range [1, 100].

Using ensure/ensuref

To check a requirement (like a graph doesn't contain loops, i.e. x_i ≠ y_i) use ensuref(x_i != y_i, "Graph can't contain loops"). It is allowed to use C-language format specifiers like ensuref(s.length() % 2 == 0, "String 's' should have even length, but s.length()=%d", int(s.length())). Also you can use simple form like ensure(x > y), it will print failed condition if it doesn't hold in form FAIL Condition failed: "x > y".

Reference: Github page of testlib.h