<< problem 371 - Licence plates (prime-k) factorial - problem 381 >>

# Problem 375: Minimum of subsequences

Let S_n be an integer sequence produced with the following pseudo-random number generator:
S_0 = 290797
S_{n+1} = S^2_n mod 50515093

Let A(i, j) be the minimum of the numbers S_i, S_{i+1}, ... , S_j for i <= j.
Let M(N) = sum{A(i, j)} for 1 <= i <= j <= N.
We can verify that M(10) = 432256955 and M(10 000) = 3264567774119.

Find M(2 000 000 000).

# My Algorithm

I wrote a bruteForce() algorithm in a few minutes which - to my surprise - even solves the M(10000) case in less than a second.
After precomputing the first 10000 pseudo-random values it performs the following tasks for each position:

• set minimum to the current value data[i]
• go backwards until reaching the first position, compare minimum to data[j] and update it according
• add minimum to the result
This O(n^2) algorithm repeatedly processes the same values.
It's more efficient to keep track of the positions where data[j] was smaller than minimum.
For example, if the next minimum is 5 positions away, then I can easily add 5 times the current minimum to result.

The search() function implements this idea:
• it has a stack best which contains those numbers smaller than the current number ("previous ''minimum's") and their position in the stream of pseudo-random numbers
• this stack is initialized with 0, a number smaller than anything generated by pseudoRandom so that the stack is never empty (prevents a few corner-cases)
• whenever a new pseudo-random number called current is generated then I reduce the stack until the top-most element is smaller than current
• current and its position i are pushed to the stack
• for all elements of the stack I add their value and their distance to the next element of the stack to result

## Alternative Approaches

My program needs just under a minute to solve the problem.
I didn't realize that pseudoRandom() has a short cycle:

• there can be at most 50515093 different outputs (50515093 is the modulo)
• each number depends only on the previous number
• whenever the internal state becomes a number I have already seen before then a new cycle starts
• if you enable FIND_CYCLE then my code will almost instantly display 6308947.
Pretty much everyone on the Project Euler forum spotted the cycle - I didn't ...
... but it can dramatically reduce execution time:
• process the first cycle the same way I did
• remember the value of result
• process the second cycle and determine how much result changed (let's call it delta)
• skip 6308947 iterations of the loop and just add delta to the result
• the last cycle will not be complete, process it the same way I did
You will process less than 2 * 10^6 pseudo-random values instead of 2 * 10^9 and will find the result in less than a second (remember, my code takes a minute).
A major drawback is that much more code is required for this faster algorithm.

## Note

The stack best remains quite small. It contains less than 40 values at any time.

# 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 10000 | ./375

Output:

Note: the original problem's input 2000000000 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>
#include <vector>
#include <algorithm>

// produce S1, S2, etc.
unsigned int pseudoRandom()
{
static unsigned long long state = 290797;
state *= state;
state %= 50515093;
return (unsigned int)state;
}

// basic O(n^2) algorithm, surprisingly fast for size = 10000
unsigned long long bruteForce(unsigned int size)
{
// precompute all values
std::vector<unsigned int> data = { 0 }; // dummy element to have S1,S2,... at positions 1,2,...
for (unsigned int i = 1; i <= size; i++)
data.push_back(pseudoRandom());

// just a dumb scan over all intervals
unsigned long long result = 0;
for (unsigned int i = 1; i <= size; i++)
{
auto minimum = data[i];
//for (unsigned int j = i; j <= size; j++)
for (unsigned int j = i; j >= 1; j--) // same concept as above but going backwards
{
minimum = std::min(minimum, data[j]);
result += minimum;
}
}

return result;
}

// "number" is the smallest number from its "position" to the current position
struct Minimum
{
unsigned int number;
unsigned int position;
};

// almost O(n), about a minute for size = 2*10^9
unsigned long long search(unsigned int size)
{
// basically a stack of all previous minimums
std::vector<Minimum> best;
// add an unused dummy element so that the stack never becomes empty
best.push_back({ 0, 0 });

unsigned long long result = 0;
for (unsigned int i = 1; i <= size; i++)
{
// produce next pseudo-random number
auto current = pseudoRandom();

// remove all elements larger than the current one
auto validUntil = i;
while (best.back().number >= current)
{
validUntil = best.back().position;
best.pop_back();
}

// the current value is at least the smallest value for a sequence of length 1 starting at i
best.push_back({ current, validUntil });

auto last = i + 1;
for (unsigned int backwards = best.size() - 1; backwards > 0; backwards--)
{
// its the same minimum number for all those positions
unsigned long long distance = last - best[backwards].position;
result += distance * best[backwards].number;

// update position
last = best[backwards].position;
}
}

return result;
}

int main()
{
//#define FIND_CYCLE
#ifdef  FIND_CYCLE
auto first = pseudoRandom();
unsigned int cycleLength = 0;
while (pseudoRandom() != first)
cycleLength++;
std::cout << "cycle length: " << cycleLength << std::endl;
#else
unsigned int limit = 2000000000;
std::cin >> limit;
//std::cout << bruteForce(limit) << std::endl;
std::cout << search(limit) << std::endl;
#endif

return 0;
}


This solution contains 14 empty lines, 17 comments and 6 preprocessor commands.

# Benchmark

The correct solution to the original Project Euler problem was found in 56.6 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

December 6, 2017 submitted solution

# Difficulty

Project Euler ranks this problem at 40% (out of 100%).

# Heatmap

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

 green solutions solve the original Project Euler problem and have a perfect score of 100% at Hackerrank, too yellow solutions score less than 100% at Hackerrank (but still solve the original problem easily) gray problems are already solved but I haven't published my solution yet blue solutions are relevant for Project Euler only: there wasn't a Hackerrank version of it (at the time I solved it) or it differed too much orange problems are solved but exceed the time limit of one minute or the memory limit of 256 MByte red problems are not solved yet but I wrote a simulation to approximate the result or verified at least the given example - usually I sketched a few ideas, too black problems are solved but access to the solution is blocked for a few days until the next problem is published [new] the flashing problem is the one I solved most recently
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200
 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400
 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500
 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600
 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633
The 310 solved problems (that's level 12) had an average difficulty of 32.6% at Project Euler and
I scored 13526 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.

 << problem 371 - Licence plates (prime-k) factorial - problem 381 >>
more about me can be found on my homepage, especially in my coding blog.
some names mentioned on this site may be trademarks of their respective owners.
thanks to the KaTeX team for their great typesetting library !