Day 9

09/1.zig

@@ -0,0 +1,79 @@
+const std = @import("std");
+
+const v2d = [2]i32;
+
+pub fn main() void {
+    const input = @embedFile("input");
+    std.debug.print("Part 1: {}\n", .{nVisitedSnake(input, 2)});
+    std.debug.print("Part 2: {}\n", .{nVisitedSnake(input, 10)});
+}
+
+pub fn nVisitedSnake(input: []const u8, comptime len: usize) usize {
+    if (len < 2) @compileError("The length has to be greater than 1.");
+
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    var knots = [_]v2d{v2d{ 0, 0 }} ** len;
+    var visited = std.ArrayList(v2d).init(allocator);
+
+    var iter = std.mem.split(u8, input, "\n");
+    while (iter.next()) |line| {
+
+        // Line not valid
+        if (line.len < 2) continue;
+
+        // Get the number, continue on error
+        const num = std.fmt.parseInt(usize, line[2..], 10) catch continue;
+
+        // Direction of said movement
+        const dir: v2d = switch (line[0]) {
+            'U' => .{ 0, 1 },
+            'D' => .{ 0, -1 },
+            'R' => .{ 1, 0 },
+            'L' => .{ -1, 0 },
+            else => unreachable,
+        };
+
+        // Move the head one step at a time
+        var step: usize = 0;
+        while (step < num) : (step += 1) {
+            // Move the head
+            knots[0][0] += dir[0];
+            knots[0][1] += dir[1];
+
+            // Go through each pair of "head/tail" knots
+            var tailIdx: usize = 1;
+            while (tailIdx < knots.len) : (tailIdx += 1) {
+                const head = &knots[tailIdx - 1];
+                const tail = &knots[tailIdx];
+
+                var axis: u8 = 0;
+                while (axis < 2) : (axis += 1) {
+                    const diff = head[axis] - tail[axis];
+                    const dist = std.math.absInt(diff) catch unreachable;
+                    if (dist >= 2) {
+                        // Move it to where the head is -1
+                        tail[axis] = head[axis] - @divTrunc(dist, diff);
+
+                        // If one axis is at a +2 distance, snap the other one
+                        const otherAxis = (axis + 1) % 2;
+                        tail[otherAxis] = head[otherAxis];
+                        break;
+                    }
+                }
+            }
+
+            // Register the poisition again if it hasn't been saved before
+            const tail = knots[knots.len - 1];
+            for (visited.items) |coord| {
+                if (coord[0] == tail[0] and coord[1] == tail[1]) break;
+            } else {
+                visited.append(tail) catch unreachable;
+            }
+        }
+    }
+
+    return visited.items.len;
+}