First-pass implementation of basic event loop

.vscode/launch.json

@@ -0,0 +1,16 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Ona",
+            "type": "gdb",
+            "request": "launch",
+            "target": "${workspaceFolder}/zig-out/bin/ona",
+            "cwd": "${workspaceRoot}",
+            "valuesFormatting": "parseText",
+        }
+    ]
+}

.vscode/settings.json

@@ -1,6 +1,5 @@
 {
     "editor.rulers": [100],
-    "git.detectSubmodules": false,
 
     "files.exclude":{
         "**/.git": true,
@@ -12,4 +11,6 @@
         "**/zig-cache": true,
         "**/zig-out": true,
     },
+
+    "git.detectSubmodulesLimit": 0,
 }

.vscode/tasks.json

@@ -3,13 +3,22 @@
 
     "tasks": [
         {
-            "label": "Build Ona",
+            "label": "Build",
             "type": "shell",
             "command": "zig build",
             "group": {
                 "kind": "build",
                 "isDefault": true
             }
+        },
+        {
+            "label": "Test",
+            "type": "shell",
+            "command": "zig build test",
+            "group": {
+                "kind": "test",
+                "isDefault": true
+            }
         }
     ]
 }

build.zig

@@ -71,7 +71,7 @@ pub fn build(builder: *std.build.Builder) void {
 
     // Ona tests.
     {
-        const ona_tests = builder.addTest("src/main.zig");
+        const ona_tests = builder.addTest("src/stack.zig");
 
         ona_tests.setTarget(target);
         ona_tests.setBuildMode(mode);

src/errors.zig

@@ -0,0 +1,7 @@
+
+///
+/// Returns `true` if `value` did not return `Error`, otherwise `false`.
+///
+pub fn isOk(comptime Error: type, value: Error!void) bool {
+    return if (value) |_| true else |_| false;
+}

src/io.zig

@@ -0,0 +1,578 @@
+const stack = @import("./stack.zig");
+const std = @import("std");
+
+///
+///
+///
+pub const Path = struct {
+    length: u16,
+    buffer: [max]u8,
+
+    ///
+    ///
+    ///
+    pub const empty = std.mem.zeroes(Path);
+
+    ///
+    ///
+    ///
+    pub fn equalsText(path: Path, text: []const u8) bool {
+        return std.mem.eql(u8, path.buffer[0 .. path.length], text);
+    }
+
+    ///
+    /// The maximum possible byte-length of a [Path].
+    ///
+    /// Note that paths are encoded using UTF-8, meaning that a character may be bigger than one
+    /// byte. Because of this, it is not safe to asume that a path may hold `2048` individual
+    /// characters.
+    ///
+    pub const max = 2048;
+};
+
+///
+/// Universal resource identifier (URI) that operates atop the operating system to provide a
+/// platform-agnostic interface for local and networked I/O access.
+///
+/// For more information, see [https://en.wikipedia.org/wiki/URI].
+///
+pub const Uri = struct {
+    buffer: [max]u8,
+    scheme_len: u16,
+    user_info_range: Range,
+    host_range: Range,
+    port_number: u16,
+    path_range: Range,
+
+    ///
+    /// Errors that may occur during parsing of a URI from URI-compatible source encoding.
+    ///
+    /// [ParseError.TooLong] occurs when the provided source data is bigger than the max allowed
+    /// data representation in [max].
+    ///
+    /// [ParseError.UnexpectedToken] occurs when the internal tokenization process encounters a
+    /// URI component token in the wrong order.
+    ///
+    /// [ParseError.InvalidEncoding] occurs when the source encoding being parsed is not properly
+    /// encoded in its own format (malformed UTF-8, for example).
+    ///
+    pub const ParseError = error {
+        TooLong,
+        UnexpectedToken,
+        InvalidEncoding,
+    };
+
+    const Range = struct {
+        off: u16,
+        len: u16,
+
+        const none = std.mem.zeroes(Range);
+    };
+
+    ///
+    /// Represents an individual component of a URI sequence.
+    ///
+    pub const Token = union(enum) {
+        scheme: []const u8,
+        user_info: []const u8,
+        host: []const u8,
+        port: []const u8,
+        path: []const u8,
+        query: []const u8,
+        fragment: []const u8,
+    };
+
+    ///
+    /// Tokenizes the data in [Tokenizer.utf8_sequence] into URI tokens.
+    ///
+    /// See [Component] for more information on the supported URI tokens.
+    ///
+    pub const Tokenizer = struct {
+        cursor: usize = 0,
+        utf8_sequence: []const u8,
+
+        ///
+        /// Extracts the next [Token] in sequence from `tokenizer` and returns it or `null` if
+        /// there are no more tokens to be extracted.
+        ///
+        pub fn next(tokenizer: *Tokenizer) ?Token {
+            while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                'A' ... 'Z', 'a' ... 'z' => {
+                    const begin = tokenizer.cursor;
+
+                    tokenizer.cursor += 1;
+
+                    var is_scheme = (begin == 0);
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                        switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+                        '+', '.', '-', '0' ... '9', 'A' ... 'Z', 'a' ... 'z' =>
+                           tokenizer.cursor += 1,
+
+                        ':' => {
+                            if (is_scheme) {
+                                defer tokenizer.cursor += 1;
+
+                                return Token{.scheme =
+                                    tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                            }
+
+                            tokenizer.cursor += 1;
+                        },
+
+                        '#', '?' => break,
+
+                        else => {
+                            tokenizer.cursor += 1;
+                            is_scheme = false;
+                        },
+                    };
+
+                    return Token{.path =
+                        tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+
+                '@' => {
+                    tokenizer.cursor += 1;
+
+                    const begin = tokenizer.cursor;
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                        switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                        '/', ':' => break,
+                        else => tokenizer.cursor += 1,
+                    };
+
+                    return Token{.host =
+                        tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+
+                ':' => {
+                    tokenizer.cursor += 1;
+
+                    const begin = tokenizer.cursor;
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                        switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                        '/' => break,
+                        else => tokenizer.cursor += 1,
+                    };
+
+                    return Token{
+                        .port = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+
+                '/' => {
+                    tokenizer.cursor += 1;
+
+                    if (tokenizer.utf8_sequence[tokenizer.cursor] == '/') {
+                        tokenizer.cursor += 1;
+
+                        const begin = tokenizer.cursor;
+
+                        while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                            switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                            '@' => return Token{.user_info =
+                                tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]},
+
+                            ':', '/' => break,
+                            else => tokenizer.cursor += 1,
+                        };
+
+                        return Token{
+                            .host = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                    } else {
+                        const begin = (tokenizer.cursor - 1);
+
+                        tokenizer.cursor += 1;
+
+                        while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                            switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                            '?', '#' => break,
+                            else => tokenizer.cursor += 1,
+                        };
+
+                        return Token{
+                            .path = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                    }
+                },
+
+                '?' => {
+                    tokenizer.cursor += 1;
+
+                    const begin = tokenizer.cursor;
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                        switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                        '#' => {
+                            tokenizer.cursor -= 1;
+
+                            break;
+                        },
+
+                        else => tokenizer.cursor += 1,
+                    };
+
+                    return Token{
+                        .query = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+
+                '#' => {
+                    tokenizer.cursor += 1;
+
+                    const begin = tokenizer.cursor;
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len) tokenizer.cursor += 1;
+
+                    return Token{
+                        .fragment = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+
+                else => {
+                    const begin = tokenizer.cursor;
+
+                    tokenizer.cursor += 1;
+
+                    while (tokenizer.cursor < tokenizer.utf8_sequence.len)
+                        switch (tokenizer.utf8_sequence[tokenizer.cursor]) {
+
+                        '#', '?' => break,
+                        else => tokenizer.cursor += 1,
+                    };
+
+                    return Token{
+                        .path = tokenizer.utf8_sequence[begin .. (tokenizer.cursor - begin)]};
+                },
+            };
+
+            return null;
+        }
+
+        ///
+        /// A more constrained variant of [next] that accepts a `expected_token` argument to
+        /// validate the component type of a [Token] before it is returned.
+        ///
+        /// If the [Component] of the extracted [Token] is not identical to `expected_token`,
+        /// it will be discarded and `null` is returned instead.
+        ///
+        pub fn nextExpect(tokenizer: *Tokenizer, expected_token: std.meta.Tag(Token)) ?Token {
+            if (tokenizer.next()) |token| {
+                if (token == expected_token) return token;
+            }
+
+            return null;
+        }
+    };
+
+    ///
+    ///
+    ///
+    pub const empty = std.mem.zeroes(Uri);
+
+    ///
+    /// The maximum possible byte-length of a [URI].
+    ///
+    /// Note that a URI character may be encoded using multiple bytes, meaning that `2048` is not
+    /// identical in meaning to `2048` URI characters.
+    ///
+    pub const max = 2048;
+
+    ///
+    /// Returns `true` if `uri_scheme` matches the scheme contained in `uri`, otherwise `false`.
+    ///
+    pub fn isScheme(uri: Uri, uri_scheme: []const u8) bool {
+        return std.mem.eql(u8, uri.buffer[0 .. uri.scheme_len], uri_scheme);
+    }
+
+    ///
+    /// Attempts to parse and return a [URI] value out of `utf8_sequence`, otherwise returning
+    /// [ParseError] if `utf8_sequence` is invalid in any way.
+    ///
+    /// [ParseError.InvalidEncoding] occurs if the data encoded in `utf8_sequence` cannot be
+    /// validated as UTF-8 or it contains an invalid ASCII decimal number encoding for its URL port.
+    ///
+    /// See [ParseError] for more details on the other errors that may be returned.
+    ///
+    pub fn parse(utf8_sequence: []const u8) ParseError!Uri {
+        if (!std.unicode.utf8ValidateSlice(utf8_sequence)) return error.InvalidEncoding;
+
+        var uri = Uri.empty;
+
+        if (utf8_sequence.len != 0) {
+            if (utf8_sequence.len > max) return error.TooLong;
+
+            var tokenizer = Tokenizer{.utf8_sequence = utf8_sequence};
+            const scheme_token = tokenizer.nextExpect(.scheme) orelse return error.UnexpectedToken;
+            var uri_buffer = stack.Unmanaged(u8){.buffer = &uri.buffer};
+            const uri_writer = uri_buffer.asWriter();
+            const assert = std.debug.assert;
+
+            // These write operations will never fail because the uri_buffer will be known to be big
+            // enough by this point.
+            assert(uri_writer.write(scheme_token.scheme) == scheme_token.scheme.len);
+            assert(uri_writer.writeByte(':'));
+
+            // Downcast is safe because utf8_sequence can't be greater than u16 max.
+            uri.scheme_len = @intCast(u16, scheme_token.scheme.len);
+
+            var last_token = scheme_token;
+
+            while (tokenizer.next()) |scheme_specific_token| {
+                switch (scheme_specific_token) {
+                    .scheme => return error.UnexpectedToken,
+
+                    .user_info => |user_info| {
+                        if (last_token != .scheme) return error.UnexpectedToken;
+
+                        const delimiter = "//";
+
+                        assert(uri_writer.write(delimiter) == delimiter.len);
+
+                        uri.user_info_range = .{
+                            .off = @intCast(u16, uri_buffer.filled),
+                            .len = @intCast(u16, user_info.len),
+                        };
+
+                        assert(uri_writer.write(user_info) == user_info.len);
+                        assert(uri_writer.writeByte('@'));
+                    },
+
+                    .host => |host| {
+                        switch (last_token) {
+                            .scheme => {
+                                const delimiter = "//";
+
+                                assert(uri_writer.write(delimiter) == delimiter.len);
+                            },
+
+                            .user_info => {},
+                            else => return error.UnexpectedToken,
+                        }
+
+                        assert(uri_writer.write(host) == host.len);
+                    },
+
+                    .port => |port| {
+                        if (last_token != .host) return error.UnexpectedToken;
+
+                        const port_radix = 10;
+
+                        uri.port_number = std.fmt.parseInt(u16, port, port_radix) catch
+                            return error.InvalidEncoding;
+
+                        assert(uri_writer.writeByte(':'));
+                        assert(uri_writer.write(port) == port.len);
+                    },
+
+                    .path => |path| {
+                        if ((last_token != .scheme) and (last_token != .host) and
+                            (last_token != .port)) return error.UnexpectedToken;
+
+                        uri.path_range = .{
+                            .off = @intCast(u16, uri_buffer.filled),
+                            .len = @intCast(u16, path.len),
+                        };
+
+                        assert(uri_writer.write(path) == path.len);
+                    },
+
+                    .query => |query| {
+                        if ((last_token != .scheme) and (last_token != .host) and
+                            (last_token != .port) and (last_token != .path))
+                            return error.UnexpectedToken;
+
+                        assert(uri_writer.writeByte('?'));
+
+                        uri.path_range = .{
+                            .off = @intCast(u16, uri_buffer.filled),
+                            .len = @intCast(u16, query.len),
+                        };
+
+                        assert(uri_writer.write(query) == query.len);
+                    },
+
+                    .fragment => |fragment| {
+                        if ((last_token != .scheme) or (last_token != .host) or
+                            (last_token != .port) or (last_token != .path) or
+                            (last_token != .query)) return error.UnexpectedToken;
+
+                        assert(uri_writer.writeByte('#'));
+
+                        uri.path_range = .{
+                            .off = @intCast(u16, uri_buffer.filled),
+                            .len = @intCast(u16, fragment.len),
+                        };
+
+                        assert(uri_writer.write(fragment) == fragment.len);
+                    },
+                }
+
+                last_token = scheme_specific_token;
+            }
+        }
+
+        return uri;
+    }
+
+    ///
+    /// Creates and returns a [Path] value from the path component of `uri`.
+    ///
+    pub fn toPath(uri: Uri) Path {
+        var path = Path{
+            .length = uri.path_range.len,
+            .buffer = std.mem.zeroes([Path.max]u8),
+        };
+
+        std.mem.copy(u8, path.buffer[0 ..], uri.buffer[uri.path_range.off .. uri.path_range.len]);
+
+        return path;
+    }
+
+    ///
+    /// Writes the path component of `uri` to `path_writer`, returning `true` if all bytes used to
+    /// encode the path were successfully written, otherwise `false` if it was partially completed
+    /// or not at all.
+    ///
+    pub fn writePath(uri: Uri, path_writer: Writer) bool {
+        return (path_writer.write(uri.buffer[uri.path_range.off ..
+            uri.path_range.len]) == uri.path_range.len);
+    }
+};
+
+test "uri" {
+    const testing = @import("std").testing;
+    const empty_uri = Uri.empty;
+
+    try testing.expect(empty_uri.isScheme(""));
+    try testing.expect(empty_uri.toPath().equalsText(""));
+
+    const scheme_only_uri = try Uri.parse("uri:");
+
+    try testing.expect(scheme_only_uri.isScheme("uri"));
+    try testing.expect(scheme_only_uri.toPath().equalsText(""));
+
+    const absolute_file_path = "/path/to/file";
+    const absolute_file_uri = try Uri.parse("file:" ++ absolute_file_path);
+
+    try testing.expect(absolute_file_uri.isScheme("file"));
+    try testing.expect(absolute_file_uri.toPath().equalsText(absolute_file_path));
+
+    const relative_file_path = "path/to/file";
+    const relative_file_uri = try Uri.parse("file:" ++ relative_file_path);
+
+    try testing.expect(relative_file_uri.isScheme("file"));
+    try testing.expect(relative_file_uri.toPath().equalsText(relative_file_path));
+}
+
+///
+/// Opaque interface to a "writable" resource, such as a block device, memory buffer, or network
+/// socket.
+///
+pub const Writer = struct {
+    context: *anyopaque,
+    operation: fn (*anyopaque, []const u8) usize,
+
+    ///
+    /// Wraps and returns a reference to `write_context` of type `WriteContext` and its associated
+    /// `writeContext` writing operation in a [Writer].
+    ///
+    pub fn wrap(
+        comptime WriteContext: type,
+        write_context: *WriteContext,
+        comptime writeContext: fn (*WriteContext, []const u8) usize
+    ) Writer {
+        return .{
+            .context = write_context,
+
+            .operation = struct {
+                fn write(context: *anyopaque, buffer: []const u8) usize {
+                    return writeContext(@ptrCast(*WriteContext,
+                        @alignCast(@alignOf(WriteContext), context)), buffer);
+                }
+            }.write,
+        };
+    }
+
+    ///
+    /// Attempts to write `buffer` to `writer`, returning the number of bytes from `buffer` that
+    /// were successfully written.
+    ///
+    pub fn write(writer: Writer, buffer: []const u8) usize {
+        return writer.operation(writer.context, buffer);
+    }
+
+    ///
+    /// Writes the singular `byte` to `writer`, returning `true` if it was successfully written,
+    /// otherwise `false`.
+    ///
+    pub fn writeByte(writer: Writer, byte: u8) bool {
+        return (writer.operation(writer.context,
+            @ptrCast([*]const u8, &byte)[0 .. 1]) != 0);
+    }
+
+    ///
+    /// Writes `value` as a ASCII / UTF-8 encoded integer to `writer`, returning `true` if the full
+    /// sequence was successfully written, otherwise `false`.
+    ///
+    /// The `base` argument identifies which base system to encode `value` as, with `10` being
+    /// decimal, `16` being hexadecimal, `8` being octal`, so on and so forth.
+    ///
+    pub fn writeInt(writer: Writer, value: anytype, base: u4) bool {
+        const Int = @TypeOf(value);
+        const type_info = @typeInfo(Int);
+
+        if (type_info != .Int) @compileError("value must be of type int");
+
+        if (value == 0) return writer.writeByte('0');
+
+        var buffer = std.mem.zeroes([28]u8);
+        var buffer_count = @as(usize, 0);
+        var n1 = value;
+
+        if ((type_info.Int.signedness == .signed) and (value < 0)) {
+            // Negative value.
+            n1 = -value;
+            buffer[0] = '-';
+            buffer_count += 1;
+        }
+
+        while (n1 != 0) {
+            buffer[buffer_count] = @intCast(u8, (n1 % base) + '0');
+            n1 = (n1 / base);
+            buffer_count += 1;
+        }
+
+        for (buffer[0 .. (buffer_count / 2)]) |_, i|
+            std.mem.swap(u8, &buffer[i], &buffer[buffer_count - i - 1]);
+
+        return (writer.write(buffer[0 .. buffer_count]) == buffer_count);
+    }
+};
+
+var null_context = @as(usize, 0);
+
+///
+/// Writer that silently throws consumed data away and never fails.
+///
+/// This is commonly used for testing or redirected otherwise unwanted output data that can't not be
+/// sent somewhere for whatever reason.
+///
+pub const null_writer = Writer{
+    .context = (&null_context),
+
+    .operation = struct {
+        fn write(context: *anyopaque, buffer: []const u8) usize {
+            // Validate context canary value.
+            std.debug.assert(@ptrCast(*usize, @alignCast(@alignOf(usize), context)).* == 0);
+
+            return buffer.len;
+        }
+    }.write,
+};

src/main.zig

@@ -5,105 +5,173 @@ const c = @cImport({
     @cInclude("lua/lauxlib.h");
 });
 
+const errors = @import("./errors.zig");
+const io = @import("./io.zig");
+const stack = @import("./stack.zig");
 const std = @import("std");
 
+const Request = struct {
+    next: ?*Request = null,
+    frame: anyframe,
+
+    message: union(enum) {
+        close: struct {
+            file: *c.SDL_RWops,
+            is_closed: *bool,
+        },
+
+        open_readable: struct {
+            uri: *const io.Uri,
+            file: ?*c.SDL_RWops,
+        },
+    },
+};
+
 fn luaAlloc(userdata: ?*anyopaque, ptr: ?*anyopaque, original_size: usize,
     updated_size: usize) callconv(.C) ?*anyopaque {
-
     // Implementation derived from
     // https://github.com/natecraddock/ziglua/blob/master/src/ziglua.zig.
     const alignment = @alignOf(std.c.max_align_t);
     const Allocator = std.mem.Allocator;
     const allocator = @ptrCast(*Allocator, @alignCast(@alignOf(Allocator), userdata.?));
-
     if (@ptrCast(?[*]align(alignment) u8, @alignCast(alignment, ptr))) |prev_ptr| {
         // Allocator is working with an existing pointer.
         const prev_slice = prev_ptr[0 .. original_size];
-
         if (updated_size == 0) {
             // Updated size of `0` to free the existing memory block.
             allocator.free(prev_slice);
-
             return null;
         }
-
         // Resize the existing memory block.
         return (allocator.reallocAdvanced(prev_slice, alignment,
             updated_size, .exact) catch return null).ptr;
     }
-
     // No existing pointer, allocate a new block of memory.
     return (allocator.alignedAlloc(u8, alignment, updated_size) catch return null).ptr;
 }
 
+///
+/// Entry point.
+///
 pub fn main() anyerror!void {
-    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
-
     if (c.SDL_Init(c.SDL_INIT_EVERYTHING) != 0) {
         c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION, "Failed to initialize SDL2 runtime");
 
-        return error.SystemFailure;
+        return error.InitFailure;
     }
 
     defer c.SDL_Quit();
 
-    var lua_allocator = gpa.allocator();
-    const lua_state = c.lua_newstate(luaAlloc, @ptrCast(*anyopaque, &lua_allocator));
+    const pref_path = create_pref_path: {
+        const path = c.SDL_GetPrefPath("ona", "ona") orelse {
+            c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION, "Failed to load user path");
 
-    if (lua_state == null) {
-        c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION,
-            "Failed to initialize Lua virtual machine");
+            return error.InitFailure;
+        };
 
-        return error.SystemFailure;
-    }
+        break: create_pref_path path[0 .. std.mem.len(path)];
+    };
 
-    defer c.lua_close(lua_state);
+    defer c.SDL_free(pref_path.ptr);
 
-    const sdl_window = create_sdl_window: {
+    const window = create_window: {
         const pos = c.SDL_WINDOWPOS_UNDEFINED;
         var flags = @as(u32, 0);
 
-        break: create_sdl_window c.SDL_CreateWindow("Ona", pos, pos, 640, 480, flags);
+        break: create_window c.SDL_CreateWindow("Ona", pos, pos, 640, 480, flags) orelse {
+            c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION, "Failed to load SDL2 window");
+
+            return error.InitFailure;
+        };
     };
 
-    if (sdl_window == null) {
-        c.SDL_LogCritical(c.SDL_LOG_CATEGORY_VIDEO, "Failed to create SDL2 window");
+    defer c.SDL_DestroyWindow(window);
 
-        return error.SystemFailure;
-    }
-
-    defer c.SDL_DestroyWindow(sdl_window);
-
-    const sdl_renderer = create_sdl_renderer: {
+    const renderer = create_renderer: {
         var flags = @as(u32, 0);
 
-        break: create_sdl_renderer c.SDL_CreateRenderer(sdl_window, -1, flags);
+        break: create_renderer c.SDL_CreateRenderer(window, -1, flags) orelse {
+            c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION, "Failed to load SDL2 renderer");
+
+            return error.InitFailure;
+        };
     };
 
-    if (sdl_renderer == null) {
-        c.SDL_LogCritical(c.SDL_LOG_CATEGORY_VIDEO, "Failed to create SDL2 renderer");
+    defer c.SDL_DestroyRenderer(renderer);
 
-        return error.SystemFailure;
-    }
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    var lua_allocator = gpa.allocator();
 
-    defer c.SDL_DestroyRenderer(sdl_renderer);
+    const lua_state = c.lua_newstate(luaAlloc, @ptrCast(*anyopaque, &lua_allocator)) orelse {
+        c.SDL_LogCritical(c.SDL_LOG_CATEGORY_APPLICATION,
+            "Failed to initialize Lua virtual machine");
 
+        return error.InitFailure;
+    };
+
+    defer c.lua_close(lua_state);
+
+    var request_chain = @as(?*Request, null);
     var is_running = true;
 
     while (is_running) {
-        var sdl_event = std.mem.zeroes(c.SDL_Event);
+        var event = std.mem.zeroes(c.SDL_Event);
 
-        while (c.SDL_PollEvent(&sdl_event) != 0) {
-            switch (sdl_event.type) {
+        while (c.SDL_PollEvent(&event) != 0) {
+            switch (event.type) {
                 c.SDL_QUIT => is_running = false,
-
                 else => {},
             }
         }
 
-        _ = c.SDL_SetRenderDrawColor(sdl_renderer, 0, 0, 0, 255);
-        _ = c.SDL_RenderClear(sdl_renderer);
+        if (c.SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255) != 0) {
+            c.SDL_LogError(c.SDL_LOG_CATEGORY_VIDEO, c.SDL_GetError());
+            c.SDL_ClearError();
+        }
 
-        c.SDL_RenderPresent(sdl_renderer);
+        if (c.SDL_RenderClear(renderer) != 0) {
+            c.SDL_LogError(c.SDL_LOG_CATEGORY_VIDEO, c.SDL_GetError());
+            c.SDL_ClearError();
+        }
+
+        c.SDL_RenderPresent(renderer);
+
+        while (request_chain) |request_head| {
+            const request = request_head;
+
+            request_chain = request_head.next;
+
+            switch (request.message) {
+                .close => |*close| close.is_closed.* = (c.SDL_RWclose(close.file) == 0),
+
+                .open_readable => |*open_readable| {
+                    if (open_readable.uri.isScheme("data")) {
+                        var path = stack.Fixed(u8, 4096){};
+
+                        // These can never fail as the sum of the potential bytes written will
+                        // always be less than 4096.
+                        path.pushAll("./") catch unreachable;
+                        std.debug.assert(open_readable.uri.writePath(path.asWriter()));
+
+                        open_readable.file = c.SDL_RWFromFile(&path.buffer, "r");
+                    } else if (open_readable.uri.isScheme("user")) {
+                        var path = stack.Fixed(u8, 4096){};
+                        const isOk = errors.isOk;
+
+                        // Cannot guarantee that the sum of potential bytes written will always be
+                        // less than path max.
+                        if (isOk(stack.FinitePushError, path.pushAll(pref_path)) and
+                            open_readable.uri.writePath(path.asWriter())) {
+
+                            open_readable.file = c.SDL_RWFromFile(&path.buffer, "r");
+                        }
+                    }
+                },
+            }
+
+            resume request.frame;
+        }
+
+        c.SDL_Delay(1);
     }
 }

src/stack.zig

@@ -0,0 +1,203 @@
+const io = @import("./io.zig");
+const std = @import("std");
+
+///
+/// Potential errors that may occur while trying to push one or more elements into a stack of a
+/// known maximum size.
+///
+/// [FinitePushError.Overflow] is returned if the stack does not have sufficient capacity to hold a
+/// given set of elements.
+///
+pub const FinitePushError = error {
+    Overflow,
+};
+
+///
+/// Returns a fixed-size stack collection capable of holding a maximum of `capacity` elements of
+/// type `Element`.
+///
+pub fn Fixed(comptime Element: type, comptime capacity: usize) type {
+    return struct {
+        filled: usize = 0,
+        buffer: [capacity]Element = undefined,
+
+        const Self = @This();
+
+        ///
+        /// Wraps `self` and returns it in a [io.Writer] value.
+        ///
+        /// Note that this will raise a compilation error if [Element] is not `u8`.
+        ///
+        pub fn asWriter(self: *Self) io.Writer {
+            if (Element != u8) @compileError("Cannot coerce fixed stack of type " ++
+                @typeName(Element) ++ " into a Writer");
+
+            return io.Writer.wrap(Self, self, struct {
+                fn write(stack: *Self, buffer: []const u8) usize {
+                    stack.pushAll(buffer) catch |err| switch (err) {
+                        error.Overflow => return 0,
+                    };
+
+                    return buffer.len;
+                }
+            }.write);
+        }
+
+        ///
+        /// Clears all elements from `self`.
+        ///
+        pub fn clear(self: *Self) void {
+            self.filled = 0;
+        }
+
+        ///
+        /// Counts and returns the number of pushed elements in `self`.
+        ///
+        pub fn count(self: Self) usize {
+            return self.filled;
+        }
+
+        ///
+        /// Attempts to pop the tail-end of `self`, returning the element value or `null` if the
+        /// stack is empty.
+        ///
+        pub fn pop(self: *Self) ?Element {
+            if (self.filled == 0) return null;
+
+            self.filled -= 1;
+
+            return self.buffer[self.filled];
+        }
+
+        ///
+        /// Attempts to push `element` into `self`, returning [FinitePushError.Overflow] if the
+        /// stack is full.
+        ///
+        pub fn push(self: *Self, element: Element) FinitePushError!void {
+            if (self.filled == capacity) return error.Overflow;
+
+            self.buffer[self.filled] = element;
+            self.filled += 1;
+        }
+
+        ///
+        /// Attempts to push all of `elements` into `self`, returning [FinitePushError.Overflow] if
+        /// the stack does not have sufficient capacity to hold the new elements.
+        ///
+        pub fn pushAll(self: *Self, elements: []const u8) FinitePushError!void {
+            const filled = (self.filled + elements.len);
+
+            if (filled > capacity) return error.Overflow;
+
+            std.mem.copy(u8, self.buffer[self.filled ..], elements);
+
+            self.filled = filled;
+        }
+    };
+}
+
+pub fn Unmanaged(comptime Element: type) type {
+    return struct {
+        filled: usize = 0,
+        buffer: []Element,
+
+        const Self = @This();
+
+        ///
+        /// Wraps `self` and returns it in a [io.Writer] value.
+        ///
+        /// Note that this will raise a compilation error if [Element] is not `u8`.
+        ///
+        pub fn asWriter(self: *Self) io.Writer {
+            if (Element != u8) @compileError("Cannot coerce fixed stack of type " ++
+                @typeName(Element) ++ " into a Writer");
+
+            return io.Writer.wrap(Self, self, struct {
+                fn write(stack: *Self, buffer: []const u8) usize {
+                    stack.pushAll(buffer) catch |err| switch (err) {
+                        error.Overflow => return 0,
+                    };
+
+                    return buffer.len;
+                }
+            }.write);
+        }
+
+        ///
+        /// Clears all elements from `self`.
+        ///
+        pub fn clear(self: *Self) void {
+            self.filled = 0;
+        }
+
+        ///
+        /// Counts and returns the number of pushed elements in `self`.
+        ///
+        pub fn count(self: Self) usize {
+            return self.filled;
+        }
+
+        ///
+        /// Attempts to pop the tail-end of `self`, returning the element value or `null` if the
+        /// stack is empty.
+        ///
+        pub fn pop(self: *Self) ?Element {
+            if (self.filled == 0) return null;
+
+            self.filled -= 1;
+
+            return self.buffer[self.filled];
+        }
+
+        ///
+        /// Attempts to push `element` into `self`, returning [FinitePushError.Overflow] if the
+        /// stack is full.
+        ///
+        pub fn push(self: *Self, element: Element) FinitePushError!void {
+            if (self.filled == self.buffer.len) return error.Overflow;
+
+            self.buffer[self.filled] = element;
+            self.filled += 1;
+        }
+
+        ///
+        /// Attempts to push all of `elements` into `self`, returning [FinitePushError.Overflow] if
+        /// the stack does not have sufficient capacity to hold the new elements.
+        ///
+        pub fn pushAll(self: *Self, elements: []const u8) FinitePushError!void {
+            const filled = (self.filled + elements.len);
+
+            if (filled > self.buffer.len) return error.Overflow;
+
+            std.mem.copy(u8, self.buffer[self.filled ..], elements);
+
+            self.filled = filled;
+        }
+    };
+}
+
+test "fixed stack" {
+    const testing = @import("std").testing;
+    const expectError = testing.expectError;
+    const expectEqual = testing.expectEqual;
+    var stack = Fixed(u8, 4){};
+
+    try expectEqual(stack.count(), 0);
+    try expectEqual(stack.pop(), null);
+    try stack.push(69);
+    try expectEqual(stack.count(), 1);
+    try expectEqual(stack.pop(), 69);
+    try stack.pushAll(&.{42, 10, 95, 0});
+    try expectEqual(stack.count(), 4);
+    try expectError(FinitePushError.Overflow, stack.push(1));
+    try expectError(FinitePushError.Overflow, stack.pushAll(&.{1, 11, 11}));
+
+    stack.clear();
+
+    try expectEqual(stack.count(), 0);
+
+    const writer = stack.asWriter();
+
+    try expectEqual(writer.write(&.{0, 0, 0, 0}), 4);
+    try expectEqual(writer.writeByte(0), false);
+}