Day 6 fast

src/days/06/Day06.cpp

@@ -1,12 +1,14 @@
 #include "Day06.h"
+#include <cmath>
 
 Result Day06::Task1() {
 	uint64 product = 1;
 
 	for (auto &[time, distance] : parseRecords()){
-		uint64 wins = 0;
+		auto [x1, x2] = calculateEdges(double(time), double(distance));
-		for (uint64 holdTime = 1; holdTime < time; holdTime++)
+
-			if ((time - holdTime) * holdTime > distance) wins++;
+		uint64 wins = uint64(ceil(x2 - 1)) - uint64(x1);
+
 		product *= wins;
 	}
 
@@ -16,9 +18,9 @@ Result Day06::Task1() {
 Result Day06::Task2() {
 	auto [time, distance] = parseRecord();
 
-	uint64 wins = 0;
+	auto [x1, x2] = calculateEdges(double(time), double(distance));
-	for (uint64 holdTime = 1; holdTime < time; holdTime++)
+
-		if ((time - holdTime) * holdTime > distance) wins++;
+	uint64 wins = uint64(ceil(x2 - 1)) - uint64(x1);
 
 	return to_string(wins);
 }
@@ -44,3 +46,10 @@ Day06::Record Day06::parseRecord() const {
 	uint64 distance = std::stoul(rawDistance);
 	return {time, distance};
 }
+
+pair<double, double> Day06::calculateEdges(double t, double d) {
+	// parabula f(x) = -x² + tx | set it equal to d, calculate x1, x2
+	double x1 = t / 2 - sqrt(t * t / 4 - d);
+	double x2 = t / 2 + sqrt(t * t / 4 - d);
+	return {x1, x2};
+}

src/days/06/Day06.h

@@ -11,4 +11,5 @@ protected:
 
 	vector<Record> parseRecords() const;
 	Record parseRecord() const;
+	static pair<double, double> calculateEdges(double t, double d);
 };