diff --git a/day04-hard/build.zig b/day04-hard/build.zig
new file mode 100644
index 0000000..2649974
--- /dev/null
+++ b/day04-hard/build.zig
@@ -0,0 +1,70 @@
+const std = @import("std");
+
+// Although this function looks imperative, note that its job is to
+// declaratively construct a build graph that will be executed by an external
+// runner.
+pub fn build(b: *std.Build) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard optimization options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
+    // set a preferred release mode, allowing the user to decide how to optimize.
+    const optimize = b.standardOptimizeOption(.{});
+
+    const exe = b.addExecutable(.{
+        .name = "day04-hard",
+        // In this case the main source file is merely a path, however, in more
+        // complicated build scripts, this could be a generated file.
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // This declares intent for the executable to be installed into the
+    // standard location when the user invokes the "install" step (the default
+    // step when running `zig build`).
+    b.installArtifact(exe);
+
+    // This *creates* a Run step in the build graph, to be executed when another
+    // step is evaluated that depends on it. The next line below will establish
+    // such a dependency.
+    const run_cmd = b.addRunArtifact(exe);
+
+    // By making the run step depend on the install step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    // This is not necessary, however, if the application depends on other installed
+    // files, this ensures they will be present and in the expected location.
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    // This allows the user to pass arguments to the application in the build
+    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+
+    // Creates a step for unit testing. This only builds the test executable
+    // but does not run it.
+    const unit_tests = b.addTest(.{
+        .root_source_file = .{ .path = "src/main.zig" },
+        .target = target,
+        .optimize = optimize,
+    });
+
+    const run_unit_tests = b.addRunArtifact(unit_tests);
+
+    // Similar to creating the run step earlier, this exposes a `test` step to
+    // the `zig build --help` menu, providing a way for the user to request
+    // running the unit tests.
+    const test_step = b.step("test", "Run unit tests");
+    test_step.dependOn(&run_unit_tests.step);
+}
diff --git a/day04-hard/sample.in b/day04-hard/sample.in
new file mode 100644
index 0000000..71f208a
--- /dev/null
+++ b/day04-hard/sample.in
@@ -0,0 +1,6 @@
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
\ No newline at end of file
diff --git a/day04-hard/src/main.zig b/day04-hard/src/main.zig
new file mode 100644
index 0000000..0daa1c9
--- /dev/null
+++ b/day04-hard/src/main.zig
@@ -0,0 +1,83 @@
+const std = @import("std");
+
+fn StackList(comptime T: type, comptime capacity_type: type, comptime capacity: capacity_type) type {
+    return struct {
+        const Self = @This();
+        mem: [capacity]T,
+        length: capacity_type,
+
+        fn add(self: *Self, value: T) void {
+            self.mem[self.length] = value;
+            self.length += 1;
+        }
+
+        fn has(self: *Self, needle: T) bool {
+            for (0..self.length) |i| {
+                if (self.mem[i] == needle) {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        fn init() Self {
+            return Self{
+                .mem = undefined,
+                .length = 0,
+            };
+        }
+    };
+}
+
+fn getWinningNumbersCount(line: []const u8) u32 {
+    var result: u32 = 0;
+    var winning_numbers = StackList(u16, u8, 20).init();
+
+    var index: usize = 0;
+    while (index < line.len and line[index] != ':') : (index += 1) {}
+
+    index += 2;
+    while (index < line.len and line[index] != '|') : (index += 3) {
+        winning_numbers.add((@as(u16, line[index]) << 8) | @as(u16, line[index + 1]));
+    }
+
+    index += 2;
+    while (index < line.len) : (index += 3) {
+        const current_card = (@as(u16, line[index]) << 8) | @as(u16, line[index + 1]);
+        if (winning_numbers.has(current_card)) {
+            result += 1;
+        }
+    }
+
+    return result;
+}
+
+fn solve(line: []const u8, state: *[]u32) u32 {
+    const winning_numbers_count = getWinningNumbersCount(line);
+    state.*[0] += 1;
+    //std.debug.print("state: {d} long, starts with {d}; winning numbers: {d}\n", .{ state.len, state.*[0], winning_numbers_count });
+    for (0..winning_numbers_count) |i| {
+        state.*[i + 1] += state.*[0];
+    }
+
+    return state.*[0];
+}
+
+pub fn main() !void {
+    const stdout = std.io.getStdOut().writer();
+
+    const raw_in = std.io.getStdIn();
+    var buffered_reader = std.io.bufferedReader(raw_in.reader());
+    var reader = buffered_reader.reader();
+
+    var result: u32 = 0;
+    var line_buffer: [1000]u8 = undefined;
+    var state = std.mem.zeroes([1000]u32);
+    var slice: []u32 = state[0..];
+    while (try reader.readUntilDelimiterOrEof(&line_buffer, '\n')) |line| {
+        result += solve(line, &slice);
+        slice = slice[1..];
+    }
+    try stdout.print("{d}\n", .{result});
+}