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#include <algorithm>
#include <filesystem>
#include <fstream>
#include <iostream>
#include <map>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
namespace {
vector<string>
read_line(const filesystem::path& filename)
{
ifstream file{ filename };
vector<string> result;
for ( string line; getline(file, line, ','); ) {
result.emplace_back(line);
}
return result;
}
vector<string>
split(const string& line, char sep = '/')
{
stringstream strm{ line };
vector<string> result;
for ( string line; getline(strm, line, sep); ) {
result.emplace_back(line);
}
return result;
}
void
spin(string& line, size_t n)
{
ranges::reverse(line);
reverse(line.begin(), line.begin() + static_cast<string::difference_type>(n));
reverse(line.begin() + static_cast<string::difference_type>(n), line.end());
}
void
exchange(string& line, size_t a, size_t b)
{
swap(line.at(a), line.at(b));
}
void
partner(string& line, char a, char b)
{
auto iter_a = ranges::find(line, a);
auto iter_b = ranges::find(line, b);
iter_swap(iter_a, iter_b);
}
void
dance(const vector<string>& instructions, string& line)
{
for ( const auto& instr: instructions ) {
auto values = split(instr.substr(1));
switch ( instr.at(0) ) {
case 's':
spin(line, stoul(values.at(0)));
break;
case 'x':
exchange(line, stoul(values.at(0)), stoul(values.at(1)));
break;
case 'p':
partner(line, values.at(0).at(0), values.at(1).at(0));
break;
default:
throw runtime_error("inknown command");
}
}
}
void
part1(const vector<string>& instructions, string line)
{
dance(instructions, line);
cout << "Part1: " << line << '\n';
}
#if defined(WASTE_TIME)
void
part2_bf(const vector<string>& instructions, string line)
{
for ( long round = 0; round != 1'000'000'000; ++round ) {
dance(instructions, line);
}
cout << "Part2-BF: " << line << '\n';
}
#endif
void
part2(const vector<string>& instructions, string line)
{
map<string, size_t> seen;
vector<string> history;
size_t cycle_start = 0;
size_t cycle_length = 0;
for ( size_t round = 0;; ++round ) {
if ( seen.contains(line) ) {
cycle_start = seen.at(line);
cycle_length = round - cycle_start;
break;
}
seen[line] = round;
history.emplace_back(line);
dance(instructions, line);
}
const size_t total_rounds = 1'000'000'000L;
const size_t remaining = (total_rounds - cycle_start) % cycle_length;
cout << "Part2: " << history.at(cycle_start + remaining) << '\n';
}
void
part2_alt(const vector<string>& instructions, const string& line)
{
auto slow = line;
dance(instructions, slow);
auto fast = line;
dance(instructions, fast);
dance(instructions, fast);
// find cycle
while ( slow != fast ) {
dance(instructions, slow);
dance(instructions, fast);
dance(instructions, fast);
}
// find cycle length
size_t cycle_length = 1;
dance(instructions, slow);
while ( slow != fast ) {
dance(instructions, slow);
++cycle_length;
}
// process remaining rounds
const size_t total_rounds = 1'000'000'000L;
for ( auto remaining = total_rounds % cycle_length; remaining-- > 0; ) {
dance(instructions, slow);
}
cout << "Part2-Alt: " << slow << '\n';
}
} // namespace
int
main()
{
auto instructions = read_line("data/day16.txt");
auto line = "abcdefghijklmnop"s;
part1(instructions, line);
part2(instructions, line);
part2_alt(instructions, line);
}
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