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# Problem 225: Tribonacci non-divisors

The sequence 1, 1, 1, 3, 5, 9, 17, 31, 57, 105, 193, 355, 653, 1201 ...
is defined by T_1 = T_2 = T_3 = 1 and T_n = T_{n-1} + T_{n-2} + T_{n-3}.

It can be shown that 27 does not divide any terms of this sequence.
In fact, 27 is the first odd number with this property.

Find the 124th odd number that does not divide any terms of the above sequence.

# My Algorithm

Computing a new element of the Tribonacci sequence requires the last three elements.
They are called tri1, tri2 and tri3 in my code, the next element is triNext:
unsigned int triNext = tri1 + tri2 + tri3;

My divisibility check computes a number's modulo and aborts if it becomes zero (current is the current divisor, e.g. 27):
triNext %= current;
if (triNext == 0)
 break;

Unfortunately, Tribonacci numbers get massive after just a few steps. Luckily:
(x + y) mod m = (x mod m + y mod m) mod m

Hence my variables tri1, tri2, tri3 and triNext don't have to store true Tribonacci numbers but only the remainder modulo the current divisor.

To find out for sure that a number is a non-divisor you have to find a cycle, that means that the remainders repeat themselves.
There are a few smart ways to do that in a 100% safe way - but I decided to have a fixed number of steps and assume a number is a non-divisor if it passed all tests.
The only optimization in my code is to detect whether tri1 == tri2 == tri3 == 1 because then it returned to the starting point and there is a loop.
(side note: that simple optimization is sufficient to reliably find the 124th number, though, and I'm not entirely sure why).
My prototype found that 1577 is the divisor where my program needed 22710 steps to prove that it divides a Tribonacci number.
That's why maxSteps = 22710.

## Note

It seems divisor^2 is a hard limit for MaxSteps. My empirically found value is much smaller for most Tribonacci numbers and therefore faster.

The modulo operation triNext %= current; can be slow.
tri1, tri2 and tri3 are each smaller than current, therefore their sum triNext must be smaller than 3*current, too.
The following code is about twice as fast as the modulo operation (which I kept in my solution):
while (triNext >= current)
 triNext -= current;

I thought that a fully unrolled loop could be even faster, but it isn't:
if (triNext >= current)
{
 triNext -= current;
 if (triNext >= current)
 triNext -= current;
}

# Interactive test

You can submit your own input to my program and it will be instantly processed at my server:

Input data (separated by spaces or newlines):

This is equivalent to
echo 10 | ./225

Output:

Note: the original problem's input 124 cannot be entered
because just copying results is a soft skill reserved for idiots.

(this interactive test is still under development, computations will be aborted after one second)

# My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too.

       #include <iostream>

int main()
{
// look for 124th number
unsigned int index = 124;
std::cin >> index;

// 22710 steps until 1577 is a divisor (worst number I observed until the result is found)
const unsigned int MaxSteps = 22710;

// current divisor
unsigned int current  = 1; // yes, I could start at 29, too

// found non-divisors
unsigned int numFound = 0;
while (numFound < index)
{
// check next divisor
current += 2;

// first three numbers of the sequence
unsigned int tri1 = 1;
unsigned int tri2 = 1;
unsigned int tri3 = 1;

bool isDivisor = false;
for (unsigned int step = 0; step <= MaxSteps; step++)
{
// next element in sequence
unsigned int triNext = tri1 + tri2 + tri3;
// I need only its remainder
triNext %= current;

// no remainder ? => divisor
if (triNext == 0)
{
isDivisor = true;
break;
}

// update last three elements of the sequence
tri1 = tri2;
tri2 = tri3;
tri3 = triNext;

// returned to original state ? must be a loop
if (tri1 == 1 && tri2 == 1 && tri3 == 1)
break;
}

// found a new non-divisor
if (!isDivisor)
numFound++;
}

// display last non-divisor
std::cout << current << std::endl;
return 0;
}


This solution contains 11 empty lines, 13 comments and 1 preprocessor command.

# Benchmark

The correct solution to the original Project Euler problem was found in 0.02 seconds on an Intel® Core™ i7-2600K CPU @ 3.40GHz.
(compiled for x86_64 / Linux, GCC flags: -O3 -march=native -fno-exceptions -fno-rtti -std=gnu++11 -DORIGINAL)

See here for a comparison of all solutions.

Note: interactive tests run on a weaker (=slower) computer. Some interactive tests are compiled without -DORIGINAL.

# Changelog

June 23, 2017 submitted solution

# Difficulty

Project Euler ranks this problem at 45% (out of 100%).

# Heatmap

Please click on a problem's number to open my solution to that problem:

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The 289 solved problems (level 11) had an average difficulty of 32.1% at Project Euler and
I scored 13,486 points (out of 15700 possible points, top rank was 17 out of ≈60000 in August 2017) at Hackerrank's Project Euler+.

My username at Project Euler is stephanbrumme while it's stbrumme at Hackerrank.

Look at my progress and performance pages to get more details.

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