Solution to Google Hashcode 2020 Qualification Round

Individual Scores:

A - 21
B - 5,822,900
C - 5,689,822
D - 5,037,435
E - 5,071,740
F - 5,340,296

Final Score: 26,962,214

Solution Approach:

We define a time-difference

td = days - library signup_time 

and calculate a library score by

while(book_id <  td * book_per_day of library  &&  book_id < number of books in library)
{
    score += score for book with book_id
    book_id++
}

and using the score function

score /= library signup_time

to get the library score. The library with the highest score is added to the solution and we erase each book that is scanned already due to the selected library from each other library which has not yet been added to the solution.

Our high scores have been achieved by varying the score function, e.g. to

score = score * score / library signup_time

and by switching score functions after a given number of days.