Day 9

src/s9a.d

@@ -0,0 +1,49 @@
+module s9a;
+import util;
+
+struct Vec2 {
+    int x;
+    int y;
+}
+
+void main() {
+    string[] commands = readText("input/9.txt").strip.splitter("\n").array;
+    Vec2 head;
+    Vec2 tail;
+    Set!Vec2 visited = Set!Vec2([]);
+    foreach (command; commands) {
+        char direction;
+        int amount;
+        formattedRead(command, "%c %d", direction, amount);
+        Vec2 dirVec;
+        if (direction == 'U') {
+            dirVec = Vec2(0, 1);
+        } else if (direction == 'D') {
+            dirVec = Vec2(0, -1);
+        } else if (direction == 'R') {
+            dirVec = Vec2(1, 0);
+        } else {
+            dirVec = Vec2(-1, 0);
+        }
+        for (int i = 0; i < amount; i++) {
+            Vec2 prevHeadPos = head;
+            head.x += dirVec.x;
+            head.y += dirVec.y;
+            int distX = abs(head.x - tail.x);
+            int distY = abs(head.y - tail.y);
+            if (distX > 1 || distY > 1) {
+                if (distX == 0 || distY == 0) {
+                    if (head.x > tail.x) tail.x++;
+                    if (head.x < tail.x) tail.x--;
+                    if (head.y > tail.y) tail.y++;
+                    if (head.y < tail.y) tail.y--;
+                } else {
+                    tail.x = prevHeadPos.x;
+                    tail.y = prevHeadPos.y;
+                }
+            }
+            visited.add(tail);
+        }
+    }
+    writeln(visited.items.length);
+}

src/s9b.d

@@ -0,0 +1,51 @@
+module s9b;
+import util;
+
+struct Vec2 {
+    int x;
+    int y;
+}
+
+void main() {
+    string[] commands = readText("input/9.txt").strip.splitter("\n").array;
+    Set!Vec2 visited = Set!Vec2([]);
+    Vec2[10] rope;
+    foreach (command; commands) {
+        char direction;
+        int amount;
+        formattedRead(command, "%c %d", direction, amount);
+        Vec2 dirVec;
+        if (direction == 'U') {
+            dirVec = Vec2(0, 1);
+        } else if (direction == 'D') {
+            dirVec = Vec2(0, -1);
+        } else if (direction == 'R') {
+            dirVec = Vec2(1, 0);
+        } else {
+            dirVec = Vec2(-1, 0);
+        }
+        for (int i = 0; i < amount; i++) {
+            rope[0].x += dirVec.x;
+            rope[0].y += dirVec.y;
+            for (int r = 0; r + 1 < rope.length; r++) {
+                Vec2* head = &rope[r];
+                Vec2* tail = &rope[r + 1];
+                int distX = head.x - tail.x;
+                int distY = head.y - tail.y;
+                if (abs(distX) > 1 || abs(distY) > 1) {
+                    if (distX == 0 || distY == 0) {
+                        if (head.x > tail.x) tail.x++;
+                        if (head.x < tail.x) tail.x--;
+                        if (head.y > tail.y) tail.y++;
+                        if (head.y < tail.y) tail.y--;
+                    } else {
+                        tail.x += distX > 0 ? 1 : -1;
+                        tail.y += distY > 0 ? 1 : -1;
+                    }
+                }
+                if (r + 2 == rope.length) visited.add(*tail);
+            }
+        }
+    }
+    writeln(visited.items.length);
+}

src/util.d

@@ -12,6 +12,7 @@ public import std.path;
 public import std.uni;
 public import std.array;
 public import std.format;
+public import std.math;
 
 /** 
  * Simple set implementation since the stdlib doesn't really have a good one.
@@ -38,7 +39,7 @@ struct Set(T) {
 
     bool remove(T element) {
         size_t[] indexes;
-        foreach (e, i; this.items) {
+        foreach (i, e; this.items) {
             if (e == element) indexes ~= i;
         }
         this.items = this.items.remove(indexes);