diff --git a/CMakeLists.txt b/CMakeLists.txt
index c5dcae0..c2e71ac 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -42,5 +42,6 @@ add_subdirectory(day5)
 add_subdirectory(day6)
 add_subdirectory(day7)
 add_subdirectory(day8)
+add_subdirectory(day9)
 
 
diff --git a/day8/main.cpp b/day8/main.cpp
index b57022d..d81e7c6 100644
--- a/day8/main.cpp
+++ b/day8/main.cpp
@@ -1,174 +1,150 @@
 #include <algorithm>
 #include <cmath>
-#include <cstddef>
 #include <expected>
-#include <format>
 #include <fstream>
-#include <functional>
 #include <iostream>
-#include <map>
+#include <numeric>
 #include <print>
 #include <ranges>
 #include <string>
-#include <unordered_map>
-#include <utility>
+#include <string_view>
 #include <vector>
 
-using Point = std::tuple<int, int, int>;
+// Use a simple struct for clarity
+struct Point {
+    int x, y, z;
+    // Default spaceship operator for easy comparisons if needed
+    auto operator<=>(const Point &) const = default;
+};
+
+struct Edge {
+    int u; // Index of first point
+    int v; // Index of second point
+    double dist;
+};
+
+// --- Modern Union-Find (DSU) Implementation ---
+struct DSU {
+    std::vector<int> parent;
+    int groups;
+
+    explicit DSU(size_t n) : parent(n), groups(static_cast<int>(n)) {
+        // Fill with 0, 1, 2, ..., n-1
+        std::iota(parent.begin(), parent.end(), 0);
+    }
+
+    // Find with Path Compression
+    // Returns the representative ID of the set containing i
+    auto find(int i) -> int {
+        if (parent[i] == i) {
+            return i;
+        }
+        return parent[i] = find(parent[i]);
+    }
+
+    // Unite two sets. Returns true if they were different sets.
+    auto unite(int i, int j) -> bool {
+        int root_i = find(i);
+        int root_j = find(j);
+
+        if (root_i != root_j) {
+            parent[root_i] = root_j; // Link roots
+            groups--;
+            return true;
+        }
+        return false;
+    }
+};
+
+// --- Helper Functions ---
 
 static auto getDistance(const Point &a, const Point &b) -> double {
-    return sqrt(pow(std::get<0>(a) - std::get<0>(b), 2) + pow(std::get<1>(a) - std::get<1>(b), 2) +
-                pow(std::get<2>(a) - std::get<2>(b), 2));
+    // std::hypot is cleaner (C++17) and avoids manual pow/sqrt
+    // For 3D: hypot(x, y, z) is C++17 specific overload
+    return std::hypot(a.x - b.x, a.y - b.y, a.z - b.z);
 }
 
-static auto parseInput(const std::string &filename) -> std::expected<std::vector<Point>, std::string> {
-    std::ifstream inputF{filename};
-
+static auto parseInput(std::string_view filename) -> std::expected<std::vector<Point>, std::string> {
+    std::ifstream inputF{std::string(filename)};
     if (!inputF) {
-        return std::unexpected{"Some file open error."};
+        return std::unexpected{"Could not open file."};
     }
 
-    std::vector<Point> pointVec{};
+    std::vector<Point> points;
+    std::string line;
 
-    std::string puzzleLine;
-    while (std::getline(inputF, puzzleLine)) {
-        if (!puzzleLine.empty()) {
-            auto subParts = std::ranges::to<std::vector<std::string>>(std::views::split(puzzleLine, ','));
-            pointVec.emplace_back(std::stoi(subParts[0]), std::stoi(subParts[1]), std::stoi(subParts[2]));
+    while (std::getline(inputF, line)) {
+        if (line.empty()) {
+            continue;
+        }
+
+        // Modern splitting and parsing
+        auto parts = line | std::views::split(',') | std::views::transform([](auto &&rng) -> auto {
+                         // Convert range to string for stoi (C++23 common_range fix)
+                         return std::string(rng.begin(), rng.end());
+                     }) |
+                     std::ranges::to<std::vector<std::string>>();
+
+        if (parts.size() >= 3) {
+            points.emplace_back(std::stoi(parts[0]), std::stoi(parts[1]), std::stoi(parts[2]));
         }
     }
-    // std::println("pointList: {}", pointVec);
-    return pointVec;
+    return points;
 }
 
-static auto getCombinations(const std::vector<Point> &pointVec) -> std::vector<std::pair<Point, Point>> {
-    std::vector<std::pair<Point, Point>> result;
+static auto generateSortedEdges(const std::vector<Point> &points) -> std::vector<Edge> {
+    std::vector<Edge> edges;
+    edges.reserve((points.size() * (points.size() - 1)) / 2);
 
-    result.reserve(pointVec.size() * pointVec.size());
-
-    for (size_t i = 0; i < pointVec.size(); ++i) {
-        for (size_t j = i + 1; j < pointVec.size(); ++j) {
-            result.emplace_back(pointVec[i], pointVec[j]);
+    // Standard triangular loop is still clearer/faster than range combinatorics here
+    for (size_t i = 0; i < points.size(); ++i) {
+        for (size_t j = i + 1; j < points.size(); ++j) {
+            edges.push_back(
+                {.u = static_cast<int>(i), .v = static_cast<int>(j), .dist = getDistance(points[i], points[j])});
         }
     }
-    return std::move(result);
+
+    // Modern Sort with Projections
+    // Sorts by the 'dist' member automatically
+    std::ranges::sort(edges, {}, &Edge::dist);
+
+    return edges;
 }
 
-static auto getDistances(const std::vector<Point> &pointVec) -> auto {
-    auto comparisonLamb = [](const std::pair<Point, Point> &pair1, const std::pair<Point, Point> &pair2) -> bool {
-        return getDistance(pair1.first, pair1.second) < getDistance(pair2.first, pair2.second);
-    };
-    std::map<std::pair<Point, Point>, double, decltype(comparisonLamb)> sortedMap(comparisonLamb);
+// --- Main Logic ---
 
-    auto combinations = getCombinations(pointVec);
-    for (const auto &combination : combinations) {
+static auto solvePartTwo(const std::vector<Point> &points) -> long long {
+    auto edges = generateSortedEdges(points);
+    DSU dsu(points.size());
 
-        // std::println("Combinations: {}:{}", combination.first, combination.second);
-        sortedMap[combination] = getDistance(combination.first, combination.second);
-    }
-
-    return sortedMap;
-}
-
-static auto doTheThing(const std::vector<Point> &pointVec, int numberToTake) -> long long {
-    auto distanceMap = getDistances(pointVec);
-    std::map<Point, int> pointToCircuitMap{};
-    std::unordered_map<int, std::vector<Point>> circuitMap;
-    int currentCircuit = 0;
-    for (const auto &[combination, distance] : distanceMap) {
-        const auto &[left, right] = combination;
-        // std::println("DistanceMap: {}:{} = {}", left, right, distance);
-
-        if (pointToCircuitMap.contains(left) && pointToCircuitMap.contains(right)) {
-            int leftCircuit = pointToCircuitMap[left];
-            int rightCircuit = pointToCircuitMap[right];
-            // std::println("Left circuit: {}. Right circuit: {}", leftCircuit, rightCircuit);
-            if (leftCircuit != rightCircuit) {
-                std::println("Merge: {} to {}", left, right);
-
-                for (const auto &rightCircuitPoint : circuitMap[rightCircuit]) {
-                    pointToCircuitMap[rightCircuitPoint] = leftCircuit;
-                }
-                circuitMap[leftCircuit].append_range(circuitMap[rightCircuit]);
-                circuitMap.erase(rightCircuit);
-
-                // std::println("mergesize: {}", circuitMap[leftCircuit].size());
-                if (circuitMap[leftCircuit].size() >= pointVec.size()) {
-
-                    return (long long)std::get<0>(left) * (long long)std::get<0>(right);
-                }
+    for (const auto &[u, v, dist] : edges) {
+        // Try to unite the two points
+        if (dsu.unite(u, v)) {
+            // If this connection reduced us to exactly 1 group, we are done!
+            if (dsu.groups == 1) {
+                std::println("Connected all points! Last edge: {} <-> {}", u, v);
+                return (long long)points[u].x * (long long)points[v].x;
             }
-        } else if (pointToCircuitMap.contains(left)) {
-            int leftCircuit = pointToCircuitMap[left];
-            // std::println("Only left exists: {} to {}", left, right);
-            pointToCircuitMap[right] = pointToCircuitMap[left];
-
-            circuitMap[leftCircuit].push_back(right);
-            // std::println("leftsize: {}", circuitMap[leftCircuit].size());
-            if (circuitMap[leftCircuit].size() >= pointVec.size()) {
-
-                return (long long)std::get<0>(left) * (long long)std::get<0>(right);
-            }
-        } else if (pointToCircuitMap.contains(right)) {
-            int rightCircuit = pointToCircuitMap[right];
-            // std::println("Only right exists: {} to {}", left, right);
-            pointToCircuitMap[left] = pointToCircuitMap[right];
-
-            circuitMap[rightCircuit].push_back(left);
-            // std::println("rightsize: {}", circuitMap[rightCircuit].size());
-            if (circuitMap[rightCircuit].size() >= pointVec.size()) {
-
-                return (long long)std::get<0>(left) * (long long)std::get<0>(right);
-            }
-        } else {
-            currentCircuit++;
-            pointToCircuitMap[left] = currentCircuit;
-            pointToCircuitMap[right] = currentCircuit;
-
-            circuitMap[currentCircuit].push_back(left);
-            circuitMap[currentCircuit].push_back(right);
         }
     }
-    // std::println("pointToCircuitMap: {}", pointToCircuitMap);
 
-    std::println("circuitMap");
-    for (const auto &[circuit, pointsInCircuit] : circuitMap) {
-
-        std::println("{} : {}", circuit, pointsInCircuit);
-    }
-
-    auto sizesOfLists = circuitMap | std::views::transform([](const auto &kvp) -> int { return kvp.second.size(); }) |
-                        std::ranges::to<std::vector<int>>();
-    std::ranges::sort(sizesOfLists, std::greater{});
-    auto threeLargest = std::ranges::fold_left(sizesOfLists | std::views::take(3), 1, std::multiplies{});
-
-    // std::println("{}", threeLargest);
-    return threeLargest;
+    return -1; // Should not happen if graph is connected
 }
 
 auto main() -> int {
-    auto testCase = parseInput("test_input");
-    if (testCase) {
+    // Usage of std::string_view literal
+    auto puzzle = parseInput("puzzle_input");
 
-        auto testResult = doTheThing(*testCase, 10);
-        std::println("P1 Testcase result: {}", testResult);
-
-        // auto testResultP2 = treePartTwo(*testCase);
-        // std::println("P2 Testcase result: {}", testResultP2);
-    } else {
-        std::print("{}\n", testCase.error());
+    if (!puzzle) {
+        std::println(stderr, "Error: {}", puzzle.error());
+        return 1;
     }
 
-    auto realPuzzle = parseInput("puzzle_input");
-    if (realPuzzle) {
-        // auto realResult = doTheThing(*realPuzzle, 1000);
-        // std::println("P1 Real result: {}", realResult);
+    std::println("Loaded {} points.", puzzle->size());
 
-        auto realResultP2 = doTheThing(*realPuzzle, 1000);
-        std::println("P2 Real result: {}", realResultP2);
-    } else {
-        std::print("{}\n", realPuzzle.error());
-    }
+    // Part 2 Logic
+    auto result = solvePartTwo(*puzzle);
+    std::println("Part 2 Result: {}", result);
 
     return 0;
-}
+}
\ No newline at end of file
diff --git a/day9/CMakeLists.txt b/day9/CMakeLists.txt
new file mode 100644
index 0000000..bee3402
--- /dev/null
+++ b/day9/CMakeLists.txt
@@ -0,0 +1,2 @@
+ # Use top-level helper to add the target and copy input files
+ aoc_add_day(day9 "${CMAKE_CURRENT_SOURCE_DIR}" main.cpp)
\ No newline at end of file
diff --git a/day9/main.cpp b/day9/main.cpp
new file mode 100644
index 0000000..d092c2c
--- /dev/null
+++ b/day9/main.cpp
@@ -0,0 +1,112 @@
+#include <algorithm>
+#include <array>
+#include <charconv>
+#include <cstddef>
+#include <expected>
+#include <format>
+#include <fstream>
+#include <functional>
+#include <iostream>
+#include <iterator>
+#include <list>
+#include <map>
+#include <mdspan>
+#include <numeric>
+#include <print>
+#include <ranges>
+#include <set>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+struct CoordinateHash {
+    auto operator()(const std::pair<long long, long long> &p) const -> std::size_t {
+
+        auto h1 = std::hash<long long>{}(p.first);
+        auto h2 = std::hash<long long>{}(p.second);
+
+        return h1 ^ (h2 + 0x9e3779b9 + (h1 << 6) + (h1 >> 2));
+    }
+};
+
+using Coordinate = std::pair<int, int>;
+
+static auto parseInput(std::string_view filename) -> std::expected<std::vector<Coordinate>, std::string> {
+    std::ifstream inputF{std::string(filename)};
+    if (!inputF) {
+        return std::unexpected{"Could not open file."};
+    }
+
+    std::vector<Coordinate> coordinates;
+    std::string line;
+
+    while (std::getline(inputF, line)) {
+        if (line.empty()) {
+            continue;
+        }
+
+        auto parts = line | std::views::split(',') |
+                     std::views::transform([](auto &&rng) -> auto { return std::string(rng.begin(), rng.end()); }) |
+                     std::ranges::to<std::vector<std::string>>();
+
+        if (parts.size() >= 2) {
+            coordinates.emplace_back(std::stoi(parts[0]), std::stoi(parts[1]));
+        }
+    }
+    return coordinates;
+}
+
+static auto getArea(const Coordinate &left, const Coordinate &right) -> long long {
+    long long leftPart = std::abs(left.first - right.first) + 1;
+    long long rightPart = std::abs(left.second - right.second) + 1;
+    // std::print("{:2} * {:2} ", leftPart, rightPart);
+    return leftPart * rightPart;
+}
+
+static auto solvePartOne(std::vector<Coordinate> &coordinates) -> long long {
+    long long maxArea{};
+    for (size_t i = 0; i < coordinates.size(); ++i) {
+        for (size_t j = i + 1; j < coordinates.size(); ++j) {
+            long long area = getArea(coordinates[i], coordinates[j]);
+            maxArea = std::max(area, maxArea);
+            // std::println("= {:2}; area of {} and {}", area, coordinates[i], coordinates[j]);
+        }
+    }
+
+    return maxArea;
+}
+
+auto main() -> int {
+    auto test = parseInput("test_input");
+
+    if (!test) {
+        std::println(stderr, "Error: {}", test.error());
+        return 1;
+    }
+
+    std::println("Test: Loaded {} points.", test->size());
+
+    auto testResult = solvePartOne(*test);
+    std::println("Test: Max Area: {}.", testResult);
+
+    auto puzzle = parseInput("puzzle_input");
+
+    if (!puzzle) {
+        std::println(stderr, "Error: {}", puzzle.error());
+        return 1;
+    }
+
+    std::println("Puzzle: Loaded {} points.", puzzle->size());
+    auto puzzleResult = solvePartOne(*puzzle);
+    std::println("Puzzle: Max Area: {}.", puzzleResult);
+
+    // // Part 2 Logic
+    // auto result = solvePartTwo(*puzzle);
+    // std::println("Part 2 Result: {}", result);
+
+    return 0;
+}
\ No newline at end of file
diff --git a/day9/puzzle_input b/day9/puzzle_input
new file mode 100644
index 0000000..8dcddfd
--- /dev/null
+++ b/day9/puzzle_input
@@ -0,0 +1,496 @@
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diff --git a/day9/test_input b/day9/test_input
new file mode 100644
index 0000000..7fb70de
--- /dev/null
+++ b/day9/test_input
@@ -0,0 +1,8 @@
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+7,3
\ No newline at end of file