made build.zig.

Has runner and tester for each day,
as well as a runner and tester for all the days.
Also has check step for zls.

build.zig

@@ -0,0 +1,75 @@
+const std = @import("std");
+
+pub fn build(b: *std.Build) !void {
+    const allocator = b.allocator;
+
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    const run_all = b.step("run", "Run all days");
+    const test_all = b.step("test", "Test all days");
+    const check = b.step("check", "run checks");
+
+    const utils_mod = b.createModule(.{ .root_source_file = b.path("src/utils/mod.zig") });
+
+    var dirIt = (try std.fs.cwd().openDir(
+        "./src/days",
+        .{ .iterate = true },
+    )).iterate();
+
+    while (try dirIt.next()) |entry| {
+        if (entry.kind == .directory) {}
+        // each day entry will have a main.zig
+        const source_file = try std.fs.path.join(allocator, &.{ "src", "days", entry.name, "main.zig" });
+        defer allocator.free(source_file);
+        //make sure the main.zig exists
+        _ = std.fs.cwd().openFile(source_file, .{}) catch continue;
+
+        const exe = b.addExecutable(.{
+            .name = entry.name,
+            .root_source_file = b.path(source_file),
+            .target = target,
+            .optimize = optimize,
+        });
+        exe.root_module.addImport("utils", utils_mod);
+
+        b.installArtifact(exe);
+        const install_cmd = b.addInstallArtifact(exe, .{});
+        const install_step = b.step(
+            b.fmt("install_{s}", .{entry.name}),
+            b.fmt("install {s}", .{entry.name}),
+        );
+        install_step.dependOn(&install_cmd.step);
+        b.installArtifact(exe);
+
+        const run_cmd = b.addRunArtifact(exe);
+        const run_step = b.step(
+            b.fmt("run_{s}", .{entry.name}),
+            b.fmt("run {s}", .{entry.name}),
+        );
+        run_step.dependOn(&run_cmd.step);
+        run_all.dependOn(&run_cmd.step);
+
+        const exe_test = b.addTest(.{
+            .root_source_file = b.path(source_file),
+            .target = target,
+            .optimize = optimize,
+        });
+        const test_cmd = b.addRunArtifact(exe_test);
+        const test_step = b.step(
+            b.fmt("test_{s}", .{entry.name}),
+            b.fmt("test {s}", .{entry.name}),
+        );
+        test_step.dependOn(&test_cmd.step);
+        test_all.dependOn(&test_cmd.step);
+
+        const exe_check = b.addExecutable(.{
+            .name = entry.name,
+            .root_source_file = b.path(source_file),
+            .target = target,
+            .optimize = optimize,
+        });
+        exe_check.root_module.addImport("utils", utils_mod);
+        check.dependOn(&exe_check.step);
+    }
+}