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Optimize part 2.

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Allocate only the size of the buffer that's necessary. Also calculate the partial sums backwards so there's no switch needed later.
This commit is contained in:
Bert Peters
2019-12-16 23:21:09 +01:00
parent 77b1728397
commit 73fe093450
1 changed files with 36 additions and 19 deletions
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55
2019/src/day16.cpp
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@@ -54,6 +54,36 @@ namespace {
std::copy(numbers.begin(), numbers.begin() + 8, std::ostream_iterator<int>(output));
output << std::endl;
}
int get_offset(const std::vector<int> &numbers) {
int offset = 0;
for (int i = 0; i < 7; ++i) {
offset *= 10;
offset += numbers[i];
}
return offset;
}
std::vector<int> numbers_from_offset(const std::vector<int> &numbers, unsigned int offset) {
constexpr auto repetitions = 10000;
const auto desired_length = repetitions * numbers.size() - offset;
std::vector<int> numbers_after;
numbers_after.reserve(desired_length);
numbers_after.insert(numbers_after.end(), numbers.begin() + (offset % numbers.size()), numbers.end());
while (numbers_after.size() < desired_length) {
auto remaining = desired_length - numbers_after.size();
if (remaining >= numbers.size()) {
numbers_after.insert(numbers_after.end(), numbers.begin(), numbers.end());
} else {
numbers_after.insert(numbers_after.end(), numbers.begin(), numbers.end() + remaining);
}
}
return numbers_after;
}
}
void aoc2019::day16_part1(std::istream &input, std::ostream &output) {
@@ -64,33 +94,20 @@ void aoc2019::day16_part1(std::istream &input, std::ostream &output) {
void aoc2019::day16_part2(std::istream &input, std::ostream &output) {
auto numbers = read_input(input);
const auto initial_size = numbers.size();
constexpr auto repetitions = 10000;
const int offset = get_offset(numbers);
numbers.reserve(repetitions * numbers.size());
numbers = numbers_from_offset(numbers, offset);
int offset = 0;
for (int i = 0; i < 7; ++i) {
offset *= 10;
offset += numbers[i];
}
for (int i = 1; i < repetitions; ++i) {
std::copy(numbers.begin(), numbers.begin() + initial_size, std::back_inserter(numbers));
}
numbers = std::vector(numbers.begin() + offset, numbers.end());
std::vector<int> new_numbers(numbers.size());
std::vector<int> partial_sums(numbers.size());
for (int i = 0; i < 100; ++i) {
std::partial_sum(numbers.begin(), numbers.end(), partial_sums.begin());
std::partial_sum(numbers.rbegin(), numbers.rend(), partial_sums.rbegin());
for (int j = 0; j < numbers.size(); ++j) {
int n = partial_sums.back() - partial_sums[j] + numbers[j];
new_numbers[j] = std::abs(n % 10);
}
std::transform(partial_sums.begin(), partial_sums.end(), new_numbers.begin(), [](int x) {
return x % 10;
});
std::swap(numbers, new_numbers);
}