first commit

.gitignore

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+.zig-cache/

build.zig

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+const std = @import("std");
+
+// Although this function looks imperative, it does not perform the build
+// directly and instead it mutates the build graph (`b`) that will be then
+// executed by an external runner. The functions in `std.Build` implement a DSL
+// for defining build steps and express dependencies between them, allowing the
+// build runner to parallelize the build automatically (and the cache system to
+// know when a step doesn't need to be re-run).
+pub fn build(b: *std.Build) void {
+    // Standard target options allow 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(.{});
+    // It's also possible to define more custom flags to toggle optional features
+    // of this build script using `b.option()`. All defined flags (including
+    // target and optimize options) will be listed when running `zig build --help`
+    // in this directory.
+
+    // This creates a module, which represents a collection of source files alongside
+    // some compilation options, such as optimization mode and linked system libraries.
+    // Zig modules are the preferred way of making Zig code available to consumers.
+    // addModule defines a module that we intend to make available for importing
+    // to our consumers. We must give it a name because a Zig package can expose
+    // multiple modules and consumers will need to be able to specify which
+    // module they want to access.
+    const mod = b.addModule("_2048zig", .{
+        // The root source file is the "entry point" of this module. Users of
+        // this module will only be able to access public declarations contained
+        // in this file, which means that if you have declarations that you
+        // intend to expose to consumers that were defined in other files part
+        // of this module, you will have to make sure to re-export them from
+        // the root file.
+        .root_source_file = b.path("src/root.zig"),
+        // Later on we'll use this module as the root module of a test executable
+        // which requires us to specify a target.
+        .target = target,
+    });
+
+    // Here we define an executable. An executable needs to have a root module
+    // which needs to expose a `main` function. While we could add a main function
+    // to the module defined above, it's sometimes preferable to split business
+    // logic and the CLI into two separate modules.
+    //
+    // If your goal is to create a Zig library for others to use, consider if
+    // it might benefit from also exposing a CLI tool. A parser library for a
+    // data serialization format could also bundle a CLI syntax checker, for example.
+    //
+    // If instead your goal is to create an executable, consider if users might
+    // be interested in also being able to embed the core functionality of your
+    // program in their own executable in order to avoid the overhead involved in
+    // subprocessing your CLI tool.
+    //
+    // If neither case applies to you, feel free to delete the declaration you
+    // don't need and to put everything under a single module.
+    const exe = b.addExecutable(.{
+        .name = "_2048zig",
+        .root_module = b.createModule(.{
+            // b.createModule defines a new module just like b.addModule but,
+            // unlike b.addModule, it does not expose the module to consumers of
+            // this package, which is why in this case we don't have to give it a name.
+            .root_source_file = b.path("src/main.zig"),
+            // Target and optimization levels must be explicitly wired in when
+            // defining an executable or library (in the root module), and you
+            // can also hardcode a specific target for an executable or library
+            // definition if desireable (e.g. firmware for embedded devices).
+            .target = target,
+            .optimize = optimize,
+            // List of modules available for import in source files part of the
+            // root module.
+            .imports = &.{
+                // Here "_2048zig" is the name you will use in your source code to
+                // import this module (e.g. `@import("_2048zig")`). The name is
+                // repeated because you are allowed to rename your imports, which
+                // can be extremely useful in case of collisions (which can happen
+                // importing modules from different packages).
+                .{ .name = "_2048zig", .module = mod },
+            },
+        }),
+    });
+
+    // This declares intent for the executable to be installed into the
+    // install prefix when running `zig build` (i.e. when executing the default
+    // step). By default the install prefix is `zig-out/` but can be overridden
+    // by passing `--prefix` or `-p`.
+    b.installArtifact(exe);
+
+    // This creates a top level step. Top level steps have a name and can be
+    // invoked by name when running `zig build` (e.g. `zig build run`).
+    // This will evaluate the `run` step rather than the default step.
+    // For a top level step to actually do something, it must depend on other
+    // steps (e.g. a Run step, as we will see in a moment).
+    const run_step = b.step("run", "Run the app");
+
+    // This creates a RunArtifact step in the build graph. A RunArtifact step
+    // invokes an executable compiled by Zig. Steps will only be executed by the
+    // runner if invoked directly by the user (in the case of top level steps)
+    // or if another step depends on it, so it's up to you to define when and
+    // how this Run step will be executed. In our case we want to run it when
+    // the user runs `zig build run`, so we create a dependency link.
+    const run_cmd = b.addRunArtifact(exe);
+    run_step.dependOn(&run_cmd.step);
+
+    // By making the run step depend on the default step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    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);
+    }
+
+    // Creates an executable that will run `test` blocks from the provided module.
+    // Here `mod` needs to define a target, which is why earlier we made sure to
+    // set the releative field.
+    const mod_tests = b.addTest(.{
+        .root_module = mod,
+    });
+
+    // A run step that will run the test executable.
+    const run_mod_tests = b.addRunArtifact(mod_tests);
+
+    // Creates an executable that will run `test` blocks from the executable's
+    // root module. Note that test executables only test one module at a time,
+    // hence why we have to create two separate ones.
+    const exe_tests = b.addTest(.{
+        .root_module = exe.root_module,
+    });
+
+    // A run step that will run the second test executable.
+    const run_exe_tests = b.addRunArtifact(exe_tests);
+
+    // A top level step for running all tests. dependOn can be called multiple
+    // times and since the two run steps do not depend on one another, this will
+    // make the two of them run in parallel.
+    const test_step = b.step("test", "Run tests");
+    test_step.dependOn(&run_mod_tests.step);
+    test_step.dependOn(&run_exe_tests.step);
+
+    // Just like flags, top level steps are also listed in the `--help` menu.
+    //
+    // The Zig build system is entirely implemented in userland, which means
+    // that it cannot hook into private compiler APIs. All compilation work
+    // orchestrated by the build system will result in other Zig compiler
+    // subcommands being invoked with the right flags defined. You can observe
+    // these invocations when one fails (or you pass a flag to increase
+    // verbosity) to validate assumptions and diagnose problems.
+    //
+    // Lastly, the Zig build system is relatively simple and self-contained,
+    // and reading its source code will allow you to master it.
+}

build.zig.zon

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+.{
+    // This is the default name used by packages depending on this one. For
+    // example, when a user runs `zig fetch --save <url>`, this field is used
+    // as the key in the `dependencies` table. Although the user can choose a
+    // different name, most users will stick with this provided value.
+    //
+    // It is redundant to include "zig" in this name because it is already
+    // within the Zig package namespace.
+    .name = ._2048zig,
+    // This is a [Semantic Version](https://semver.org/).
+    // In a future version of Zig it will be used for package deduplication.
+    .version = "0.0.0",
+    // Together with name, this represents a globally unique package
+    // identifier. This field is generated by the Zig toolchain when the
+    // package is first created, and then *never changes*. This allows
+    // unambiguous detection of one package being an updated version of
+    // another.
+    //
+    // When forking a Zig project, this id should be regenerated (delete the
+    // field and run `zig build`) if the upstream project is still maintained.
+    // Otherwise, the fork is *hostile*, attempting to take control over the
+    // original project's identity. Thus it is recommended to leave the comment
+    // on the following line intact, so that it shows up in code reviews that
+    // modify the field.
+    .fingerprint = 0x8d138a4e0b2ef51e, // Changing this has security and trust implications.
+    // Tracks the earliest Zig version that the package considers to be a
+    // supported use case.
+    .minimum_zig_version = "0.16.0",
+    // This field is optional.
+    // Each dependency must either provide a `url` and `hash`, or a `path`.
+    // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.
+    // Once all dependencies are fetched, `zig build` no longer requires
+    // internet connectivity.
+    .dependencies = .{
+        // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
+        //.example = .{
+        //    // When updating this field to a new URL, be sure to delete the corresponding
+        //    // `hash`, otherwise you are communicating that you expect to find the old hash at
+        //    // the new URL. If the contents of a URL change this will result in a hash mismatch
+        //    // which will prevent zig from using it.
+        //    .url = "https://example.com/foo.tar.gz",
+        //
+        //    // This is computed from the file contents of the directory of files that is
+        //    // obtained after fetching `url` and applying the inclusion rules given by
+        //    // `paths`.
+        //    //
+        //    // This field is the source of truth; packages do not come from a `url`; they
+        //    // come from a `hash`. `url` is just one of many possible mirrors for how to
+        //    // obtain a package matching this `hash`.
+        //    //
+        //    // Uses the [multihash](https://multiformats.io/multihash/) format.
+        //    .hash = "...",
+        //
+        //    // When this is provided, the package is found in a directory relative to the
+        //    // build root. In this case the package's hash is irrelevant and therefore not
+        //    // computed. This field and `url` are mutually exclusive.
+        //    .path = "foo",
+        //
+        //    // When this is set to `true`, a package is declared to be lazily
+        //    // fetched. This makes the dependency only get fetched if it is
+        //    // actually used.
+        //    .lazy = false,
+        //},
+    },
+    // Specifies the set of files and directories that are included in this package.
+    // Only files and directories listed here are included in the `hash` that
+    // is computed for this package. Only files listed here will remain on disk
+    // when using the zig package manager. As a rule of thumb, one should list
+    // files required for compilation plus any license(s).
+    // Paths are relative to the build root. Use the empty string (`""`) to refer to
+    // the build root itself.
+    // A directory listed here means that all files within, recursively, are included.
+    .paths = .{
+        "build.zig",
+        "build.zig.zon",
+        "src",
+        // For example...
+        //"LICENSE",
+        //"README.md",
+    },
+}

src/game.zig

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+//! game.zig — the 2048 board and everything it can do.
+const std = @import("std");
+
+/// The 4x4 playing grid. A cell holds the tile VALUE directly:
+/// 0 means "empty", anything else is the number shown on screen.
+pub const Board = struct {
+    cells: [4][4]u16 = .{.{0} ** 4} ** 4,
+
+    /// THE core algorithm: slides ONE row left, closing gaps.
+    /// (Merging equal tiles will be added here later.)
+    /// Used by every direction — right/up/down are just disguises.
+    fn slideRowLeft(row: *[4]u16) void {
+        var write: usize = 0; // where the next tile should land
+
+        // Pass 1: copy every non-zero tile to the next free slot.
+        // `cell` = WHAT we're carrying, `write` = WHERE it lands.
+        for (row) |cell| {
+            if (cell != 0) {
+                row[write] = cell;
+                write += 1;
+            }
+        }
+
+        // Pass 2: everything after the last packed tile becomes 0.
+        for (write..4) |i| {
+            row[i] = 0;
+        }
+    }
+
+    /// Reverses one row in place: [2,0,4,0] becomes [0,4,0,2].
+    /// Only visits the first half (i < 2) — otherwise every swap
+    /// would be undone by a later swap.
+    fn reverse(row: *[4]u16) void {
+        for (0..2) |i| {
+            const tmp = row[i]; // remember one cup's contents
+            row[i] = row[3 - i]; // pour the opposite cup into this one
+            row[3 - i] = tmp; // pour the remembered contents back
+        }
+    }
+
+    /// Slides every row to the LEFT, closing up gaps.
+    pub fn slideLeft(self: *Board) void {
+        // &self.cells = iterate the REAL grid (not a copy),
+        // |*row| = each row by pointer, so edits stick.
+        for (&self.cells) |*row| {
+            slideRowLeft(row); // "hey, slide THIS row"
+        }
+    }
+
+    /// Slides RIGHT = mirror trick: flip each row, slide left,
+    /// flip back. What was "toward index 0" becomes "toward index 3".
+    pub fn slideRight(self: *Board) void {
+        for (&self.cells) |*row| {
+            reverse(row);
+            slideRowLeft(row);
+            reverse(row);
+        }
+    }
+
+    /// Slides UP: pull each COLUMN out into a temp row, slide it,
+    /// push it back. In the temp array "left" means "toward row 0",
+    /// which is exactly UP on the board.
+    pub fn slideUp(self: *Board) void {
+        for (0..4) |c| { // c = column number
+            var tmp: [4]u16 = undefined;
+
+            for (0..4) |r| tmp[r] = self.cells[r][c]; // pull column out (top→bottom)
+            slideRowLeft(&tmp); // slide toward top
+            for (0..4) |r| self.cells[r][c] = tmp[r]; // push back in
+        }
+    }
+
+    /// Slides DOWN: same as slideUp, but read/write the column
+    /// BOTTOM→top using `3 - r`. The flip makes "toward index 0"
+    /// in the temp array mean DOWN on the board.
+    pub fn slideDown(self: *Board) void {
+        for (0..4) |c| { // c = column number
+            var tmp: [4]u16 = undefined;
+
+            for (0..4) |r| tmp[r] = self.cells[3 - r][c]; // pull column out (bottom→top)
+            slideRowLeft(&tmp); // slide toward index 0...
+            for (0..4) |r| self.cells[3 - r][c] = tmp[r]; // ...which is the bottom, thanks to the flip
+        }
+    }
+    /// Spawns a new tile (2 or 4) in a random empty cell.
+    pub fn spawn(self: *Board, rand: std.Random) void {
+        // --- Phase 1: find every empty cell -------------------
+        // Roll 0..9 once now; we'll use it at the end to decide
+        // 2 vs 4 (a 1-in-10 chance of a 4, like the real game).
+        const n = rand.intRangeAtMost(usize, 0, 9);
+
+        // Fixed-size shelf for cell indexes. The board has at most
+        // 16 empties, so no dynamic memory is ever needed.
+        var empties: [16]usize = undefined;
+        var count: usize = 0; // how many slots of `empties` we filled
+
+        // Walk all 16 cells. Flat index i maps to the grid as:
+        //   row = i / 4,  col = i % 4
+        for (0..16) |i| {
+            if (self.cells[i / 4][i % 4] == 0) {
+                empties[count] = i; // remember WHERE an empty is
+                count += 1;
+            }
+        }
+
+        // Board full → nothing to spawn into, just leave.
+        if (count == 0) {
+            return;
+        }
+
+        // --- Phase 2: pick one empty and fill it --------------
+        const pick = rand.uintLessThan(usize, count); // random 0..count-1
+        const spot = empties[pick]; // flat index of the chosen cell
+
+        const row = spot / 4; // undo the flat-index trick
+        const col = spot % 4;
+
+        var value: u16 = 2; // usually spawn a 2...
+        if (n == 9) value = 4; // ...but our early roll said 9 → lucky 4
+
+        self.cells[row][col] = value;
+    }
+};

src/main.zig

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+//! main.zig — program entry point: sets up a board and shows it.
+const game = @import("game.zig");
+const std = @import("std");
+
+pub fn main() !void {
+    // `var` (not const) because the board changes during play.
+    var board: game.Board = .{};
+
+    // Random number generator. The seed (42) makes every run
+    // identical for now — handy for debugging, we'll fix it later.
+    var prng = std.Random.DefaultPrng.init(42);
+    const rand = prng.random();
+
+    board.spawn(rand); // drop one starting tile onto the board
+
+    // --- TEMPORARY test setup ---------------------------------
+    // Hand-made rows to check the slide functions while we build.
+    // Once the real game loop exists, this block gets deleted and
+    // the board starts empty (two spawned tiles, like real 2048).
+    board.cells[0] = .{ 2, 0, 2, 0 };
+    board.cells[1] = .{ 0, 4, 4, 0 };
+    board.cells[2] = .{ 2, 2, 2, 2 };
+    board.cells[3] = .{ 0, 2, 0, 4 };
+    board.slideUp(); // ← swap in any direction to test it
+
+    // Print the grid row by row. `{d:4}` = decimal, 4 chars wide,
+    // so the columns line up.
+    for (board.cells) |row| {
+        for (row) |cell| {
+            std.debug.print("{d:4}", .{cell});
+        }
+        std.debug.print("\n", .{});
+    }
+}

src/root.zig

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+//! By convention, root.zig is the root source file when making a package.
+const std = @import("std");
+const Io = std.Io;
+
+/// This is a documentation comment to explain the `printAnotherMessage` function below.
+///
+/// Accepting an `Io.Writer` instance is a handy way to write reusable code.
+pub fn printAnotherMessage(writer: *Io.Writer) Io.Writer.Error!void {
+    try writer.print("Run `zig build test` to run the tests.\n", .{});
+}
+
+pub fn add(a: i32, b: i32) i32 {
+    return a + b;
+}
+
+test "basic add functionality" {
+    try std.testing.expect(add(3, 7) == 10);
+}