Solve day 9

src/day09.zig

@@ -2,69 +2,73 @@ const std = @import("std");
 
 pub const input = @embedFile("input/day09.txt");
 const test_input1 = @embedFile("input/day09_test1.txt");
+const test_input2 = @embedFile("input/day09_test2.txt");
 
 var gpa = std.heap.GeneralPurposeAllocator(.{ .safety = true }){};
 const allocator = gpa.allocator();
 
+fn move_rope(comptime S: usize, knots: *[S][2]isize, line: []const u8, visited: *std.AutoHashMap([2]isize, void)) !void {
+    const direction = line[0];
+    const steps = try std.fmt.parseInt(u8, line[2..], 10);
+    var step: usize = 0;
+    while (step < steps) : (step += 1) {
+        switch (direction) {
+            'R' => {
+                knots[0][0] += 1;
+            },
+            'L' => {
+                knots[0][0] -= 1;
+            },
+            'U' => {
+                knots[0][1] -= 1;
+            },
+            'D' => {
+                knots[0][1] += 1;
+            },
+            else => unreachable,
+        }
+
+        var i: usize = 1;
+        while (i < knots.len) : (i += 1) {
+            var x_diff = knots[i][0] - knots[i - 1][0];
+            var y_diff = knots[i][1] - knots[i - 1][1];
+            if (x_diff < -1 or x_diff > 1 or y_diff < -1 or y_diff > 1) {
+                if (x_diff < 0) {
+                    knots[i][0] += 1;
+                } else if (x_diff > 0) {
+                    knots[i][0] -= 1;
+                }
+                if (y_diff < 0) {
+                    knots[i][1] += 1;
+                } else if (y_diff > 0) {
+                    knots[i][1] -= 1;
+                }
+                if (i == knots.len - 1) try visited.put(knots[i], {});
+            }
+        }
+    }
+}
+
 pub fn solve_part1(data: []const u8) !usize {
     var lines = std.mem.tokenize(u8, data, "\n");
     var visited = std.AutoHashMap([2]isize, void).init(allocator);
-    var head = [2]isize{ 0, 0 };
+    var knots = [2][2]isize{ [2]isize{ 0, 0 }, [2]isize{ 0, 0 } };
-    var tail = [2]isize{ 0, 0 };
+    try visited.put(knots[0], {});
-    try visited.put(tail, {});
     while (lines.next()) |line| {
-        const direction = line[0];
+        try move_rope(2, &knots, line, &visited);
-        const steps = try std.fmt.parseInt(u8, line[2..], 10);
-        // std.debug.print("{s} {d}\n", .{ [_]u8{direction}, steps });
-        var step: usize = 0;
-        while (step < steps) : (step += 1) {
-            // std.debug.print("move head: {s}\n", .{[_]u8{direction}});
-            switch (direction) {
-                'R' => {
-                    head[0] += 1;
-                },
-                'L' => {
-                    head[0] -= 1;
-                },
-                'U' => {
-                    head[1] -= 1;
-                },
-                'D' => {
-                    head[1] += 1;
-                },
-                else => unreachable,
-            }
-            var x_diff = tail[0] - head[0];
-            var y_diff = tail[1] - head[1];
-            // std.debug.print("x_diff: {d}\n", .{x_diff});
-            // std.debug.print("y_diff: {d}\n", .{y_diff});
-            if (x_diff < -1 or x_diff > 1 or y_diff < -1 or y_diff > 1) {
-                if (x_diff < 0) {
-                    // std.debug.print("move tail x: {d}\n", .{-1});
-                    tail[0] += 1;
-                } else if (x_diff > 0) {
-                    // std.debug.print("move tail x: {d}\n", .{1});
-                    tail[0] -= 1;
-                }
-                if (y_diff < 0) {
-                    // std.debug.print("move tail y: {d}\n", .{-1});
-                    tail[1] += 1;
-                } else if (y_diff > 0) {
-                    // std.debug.print("move tail y: {d}\n", .{1});
-                    tail[1] -= 1;
-                }
-                try visited.put(tail, {});
-            }
-            // std.debug.print("head: {d}\n", .{head});
-            // std.debug.print("tail: {d}\n", .{tail});
-        }
     }
     return visited.count();
 }
 
 pub fn solve_part2(data: []const u8) !usize {
-    _ = data;
+    var lines = std.mem.tokenize(u8, data, "\n");
-    return 0;
+    var visited = std.AutoHashMap([2]isize, void).init(allocator);
+    var knots = [_][2]isize{[_]isize{ 0, 0 }} ** 10;
+    try visited.put(knots[0], {});
+    while (lines.next()) |line| {
+        try move_rope(10, &knots, line, &visited);
+    }
+    return visited.count();
 }
 
 test "solves part1" {
@@ -72,5 +76,6 @@ test "solves part1" {
 }
 
 test "solves part2" {
-    try std.testing.expectEqual(solve_part2(test_input1), 0);
+    try std.testing.expectEqual(solve_part2(test_input1), 1);
+    try std.testing.expectEqual(solve_part2(test_input2), 36);
 }

src/input/day09_test2.txt

@@ -0,0 +1,8 @@
+R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20