Solve day 9 part 1

src/day09.zig

@@ -0,0 +1,76 @@
+const std = @import("std");
+
+pub const input = @embedFile("input/day09.txt");
+const test_input1 = @embedFile("input/day09_test1.txt");
+
+var gpa = std.heap.GeneralPurposeAllocator(.{ .safety = true }){};
+const allocator = gpa.allocator();
+
+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 tail = [2]isize{ 0, 0 };
+    try visited.put(tail, {});
+    while (lines.next()) |line| {
+        const direction = line[0];
+        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;
+    return 0;
+}
+
+test "solves part1" {
+    try std.testing.expectEqual(solve_part1(test_input1), 13);
+}
+
+test "solves part2" {
+    try std.testing.expectEqual(solve_part2(test_input1), 0);
+}

src/input/day09_test1.txt

@@ -0,0 +1,8 @@
+R 4
+U 4
+L 3
+D 1
+R 4
+D 1
+L 5
+R 2

src/main.zig

@@ -8,6 +8,7 @@ const day05 = @import("day05.zig");
 const day06 = @import("day06.zig");
 const day07 = @import("day07.zig");
 const day08 = @import("day08.zig");
+const day09 = @import("day09.zig");
 const utils = @import("utils.zig");
 
 fn solve_day(comptime day_num: u8, day: anytype, stdout: anytype, timer: *std.time.Timer) !void {
@@ -48,6 +49,7 @@ pub fn main() !void {
     try solve_day(6, day06, &stdout, &timer);
     try solve_day(7, day07, &stdout, &timer);
     try solve_day(8, day08, &stdout, &timer);
+    try solve_day(9, day09, &stdout, &timer);
 
     try bw.flush();
 }
@@ -63,5 +65,6 @@ test {
     _ = day06;
     _ = day07;
     _ = day08;
+    _ = day09;
     _ = utils;
 }