Implement loading of Oar archive entry-backed files

src/core/io.zig

@@ -4,31 +4,129 @@ const stack = @import("./stack.zig");
 const testing = @import("./testing.zig");
 
 ///
-/// Allocation options for an [Allocator].
+/// [AccessError.Inacessible] is a generic catch-all for IO resources that are inaccessible for
+/// implementation-specific reasons.
 ///
-pub const Allocation = struct {
-    existing: ?[*]u8,
-    alignment: u29,
-    size: usize,
+pub const AccessError = error {
+    Inaccessible,
 };
 
 ///
-/// Closure for dynamic memory allocation through the referenced allocator state machine capture.
+/// [AllocationError.OutOfMemory] if the requested amount of memory could not be allocated.
 ///
-pub const Allocator = meta.Function(Allocation, ?[*]u8);
-
-///
-/// [MakeError.OutOfMemory] if the requested amount of memory could not be allocated.
-///
-pub const MakeError = error {
+pub const AllocationError = error {
     OutOfMemory,
 };
 
 ///
-/// Closure that captures a reference to readable resources like block devices, memory buffers,
-/// network sockets, and more.
+/// Memory layout description for a memory allocation.
 ///
-pub const Reader = meta.Function([]u8, usize);
+pub const AllocationLayout = struct {
+    length: usize,
+    alignment: u29 = 8,
+};
+
+///
+/// Interface for dynamic memory allocation through the state machine of the wrapped allocator
+/// implementation.
+///
+pub const Allocator = struct {
+    context: *anyopaque,
+
+    vtable: *const struct {
+        alloc: fn (*anyopaque, AllocationLayout) AllocationError![*]u8,
+        dealloc: fn (*anyopaque, [*]u8) void,
+        realloc: fn (*anyopaque, [*]u8, AllocationLayout) AllocationError![*]u8,
+    },
+
+    ///
+    /// Attempts to allocate a block of memory from `allocator` according to `layout`, returning it
+    /// or [AllocationError] if it failed.
+    ///
+    pub fn alloc(allocator: Allocator, layout: AllocationLayout) AllocationError![*]u8 {
+        return allocator.vtable.alloc(allocator.context, layout);
+    }
+
+    ///
+    /// Deallocates the block of memory from `allocator` referenced by `allocation`.
+    ///
+    pub fn dealloc(allocator: Allocator, allocation: [*]u8) void {
+        allocator.vtable.dealloc(allocator.context, allocation);
+    }
+
+    ///
+    /// Attempts to reallocate the existing block of memory from `allocator` referenced by
+    /// `allocation` according to `layout`, returning it or [AllocationError] if it failed.
+    ///
+    pub fn realloc(allocator: Allocator, allocation: [*]u8,
+        layout: AllocationLayout) AllocationError![*]u8 {
+
+        return allocator.vtable.realloc(allocator.context, allocation, layout);
+    }
+
+    ///
+    /// Wraps `implementation`, returning the [Allocator] value.
+    ///
+    pub fn wrap(implementation: anytype) Allocator {
+        const Implementation = @TypeOf(implementation.*);
+
+        return .{
+            .context = @ptrCast(*anyopaque, implementation),
+
+            .vtable = switch (@typeInfo(Implementation)) {
+                .Struct => &.{
+                    .alloc = struct {
+                        fn call(context: *anyopaque, layout: AllocationLayout) AllocationError![*]u8 {
+                            return @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).alloc(layout);
+                        }
+                    }.call,
+
+                    .dealloc = struct {
+                        fn call(context: *anyopaque, allocation: [*]u8) void {
+                            return @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).dealloc(allocation);
+                        }
+                    }.call,
+
+                    .realloc = struct {
+                        fn call(context: *anyopaque, allocation: [*]u8,
+                            layout: AllocationLayout) AllocationError![*]u8 {
+
+                            return @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).realloc(allocation, layout);
+                        }
+                    }.call,
+                },
+
+                .Opaque => &.{
+                    .alloc = struct {
+                        fn call(context: *anyopaque, layout: AllocationLayout) AllocationError![*]u8 {
+                            return @ptrCast(*Implementation, context).alloc(layout);
+                        }
+                    }.call,
+
+                    .dealloc = struct {
+                        fn call(context: *anyopaque, allocation: [*]u8) void {
+                            return @ptrCast(*Implementation, context).dealloc(allocation);
+                        }
+                    }.call,
+
+                    .realloc = struct {
+                        fn call(context: *anyopaque, allocation: [*]u8,
+                            layout: AllocationLayout) AllocationError![*]u8 {
+
+                            return @ptrCast(*Implementation, context).realloc(allocation, layout);
+                        }
+                    }.call,
+                },
+
+                else => @compileError(
+                    "`context` must a single-element pointer referencing a struct or opaque type"),
+            },
+        };
+    }
+};
 
 ///
 /// Returns a state machine for lazily computing all `Element` components of a given source input
@@ -143,10 +241,57 @@ test "Spliterator(u8)" {
 }
 
 ///
-/// Closure that captures a reference to writable resources like block devices, memory buffers,
+/// Interface for capturing a reference to a writable resource like block devices, memory buffers,
 /// network sockets, and more.
 ///
-pub const Writer = meta.Function([]const u8, usize);
+pub const Writer = struct {
+    context: *anyopaque,
+
+    vtable: *const struct {
+        write: fn (*anyopaque, []const u8) AccessError!usize,
+    },
+
+    ///
+    /// Wraps `implementation`, returning the [Writer] value.
+    ///
+    pub fn wrap(implementation: anytype) Writer {
+        const Implementation = @TypeOf(implementation.*);
+
+        return .{
+            .context = @ptrCast(*anyopaque, implementation),
+
+            .vtable = switch (@typeInfo(Implementation)) {
+                .Struct => &.{
+                    .write = struct {
+                        fn call(context: *anyopaque, buffer: []const u8) AccessError!usize {
+                            return @ptrCast(*Implementation,
+                                @alignCast(@alignOf(Implementation), context)).write(buffer);
+                        }
+                    }.call,
+                },
+
+                .Opaque => &.{
+                    .write = struct {
+                        fn call(context: *anyopaque, buffer: []const u8) AccessError!usize {
+                            return @ptrCast(*Implementation, context).write(buffer);
+                        }
+                    }.call,
+                },
+
+                else => @compileError(
+                    "`context` must a single-element pointer referencing a struct or opaque type"),
+            },
+        };
+    }
+
+    ///
+    /// Attempts to write to `buffer` to `writer`, returning the number of successfully written or
+    /// [AccessError] if it failed.
+    ///
+    pub fn write(writer: Writer, buffer: []const u8) AccessError!usize {
+        return writer.vtable.write(writer.context, buffer);
+    }
+};
 
 ///
 /// Returns a sliced reference of the raw bytes in `pointer`.
@@ -320,19 +465,14 @@ test "findFirstOf" {
 /// `allocated_memory`. Anything else will result is considered unreachable logic.
 ///
 pub fn free(allocator: Allocator, allocated_memory: anytype) void {
-    if (allocator.call(.{
-        .existing = @ptrCast([*]u8, switch (@typeInfo(@TypeOf(allocated_memory))) {
-            .Pointer => |info| switch (info.size) {
-                .One, .Many, .C => allocated_memory,
-                .Slice => allocated_memory.ptr,
-            },
+    allocator.dealloc(@ptrCast([*]u8, switch (@typeInfo(@TypeOf(allocated_memory))) {
+        .Pointer => |info| switch (info.size) {
+            .One, .Many, .C => allocated_memory,
+            .Slice => allocated_memory.ptr,
+        },
 
-            else => @compileError("`allocated_memory` must be a pointer"),
-        }),
-
-        .size = 0,
-        .alignment = 0,
-    }) != null) unreachable;
+        else => @compileError("`allocated_memory` must be a pointer"),
+    }));
 }
 
 test "free" {
@@ -369,14 +509,13 @@ test "hashBytes" {
 /// Attempts to allocate a buffer of `size` `Element`s using `allocator`, returning it or a
 /// [MakeError] if it failed.
 ///
-pub fn makeMany(comptime Element: type, allocator: Allocator, size: usize) MakeError![*]Element {
+pub fn makeMany(comptime Element: type, allocator: Allocator, size: usize) AllocationError![*]Element {
     const alignment = @alignOf(Element);
 
-    return @ptrCast([*]Element, @alignCast(alignment, allocator.call(.{
-        .existing = null,
-        .size = @sizeOf(Element) * size,
+    return @ptrCast([*]Element, @alignCast(alignment, try allocator.alloc(.{
+        .length = @sizeOf(Element) * size,
         .alignment = alignment,
-    }) orelse return error.OutOfMemory));
+    })));
 }
 
 test "makeMany" {
@@ -392,14 +531,13 @@ test "makeMany" {
 /// Attempts to allocate a buffer of `1` `Element` using `allocator`, returning it or a [MakeError]
 /// if it failed.
 ///
-pub fn makeOne(comptime Element: type, allocator: Allocator) MakeError!*Element {
+pub fn makeOne(comptime Element: type, allocator: Allocator) AllocationError!*Element {
     const alignment = @alignOf(Element);
 
-    return @ptrCast(*Element, @alignCast(alignment, allocator.call(.{
-        .existing = null,
-        .size = @sizeOf(Element),
+    return @ptrCast(*Element, @alignCast(alignment, try allocator.alloc(.{
+        .length = @sizeOf(Element),
         .alignment = alignment,
-    }) orelse return error.OutOfMemory));
+    })));
 }
 
 test "makeOne" {
@@ -429,6 +567,11 @@ test "swap" {
     try testing.expect(b == 0);
 }
 
+///
+/// Mandatory context variable used by [null_writer].
+///
+const null_context: u64 = 0;
+
 ///
 /// Thread-safe and lock-free [Writer] that silently consumes all given data without failure and
 /// throws it away.
@@ -436,11 +579,13 @@ test "swap" {
 /// This is commonly used for testing or redirected otherwise unwanted output data that has to be
 /// sent somewhere for whatever reason.
 ///
-pub const null_writer = Writer.from(struct {
-    fn write(buffer: []const u8) usize {
+pub const null_writer = Writer.wrap(@ptrCast(*const opaque {
+    const Self = @This();
+
+    fn write(_: Self, buffer: []const u8) usize {
         return buffer.len;
     }
-}.write);
+}, &null_context));
 
 test "null_writer" {
     const sequence = "foo";

src/core/meta.zig

@@ -8,77 +8,3 @@ pub fn FnReturn(comptime Fn: type) type {
 
     return type_info.Fn.return_type orelse void;
 }
-
-///
-/// Returns a single-input single-output closure type where `In` represents the input type, `Out`
-/// represents the output type, and `captures_size` represents the size of the closure context.
-///
-pub fn Function(comptime In: type, comptime Out: type) type {
-    return struct {
-        callErased: fn (*anyopaque, In) Out,
-        context: *anyopaque,
-
-        fn Invoker(comptime Context: type) type {
-            return if (Context == void) fn (In) Out else fn (Context, In) Out;
-        }
-
-        ///
-        /// Function type.
-        ///
-        const Self = @This();
-
-        ///
-        /// Invokes `self` with `input`, producing a result according to the current context data.
-        ///
-        pub fn call(self: Self, input: In) Out {
-            return self.callErased(self.context, input);
-        }
-
-        ///
-        /// Creates and returns a [Self] using the `invoke` as the behavior executed when [call] or
-        /// [callErased] is called.
-        ///
-        /// For creating a closure-style function, see [fromClosure].
-        ///
-        pub fn from(comptime invoke: fn (In) Out) Self {
-            return .{
-                .context = undefined,
-
-                .callErased = struct {
-                    fn callErased(_: *anyopaque, input: In) Out {
-                        return invoke(input);
-                    }
-                }.callErased,
-            };
-        }
-
-        ///
-        /// Creates and returns a [Self] by capturing the `context` value as the capture context and
-        /// `invoke` as the behavior executed when [call] or [callErased] is called.
-        ///
-        /// The newly created [Self] is returned.
-        ///
-        pub fn fromClosure(context: anytype, comptime invoke: fn (@TypeOf(context), In) Out) Self {
-            const Context = @TypeOf(context);
-
-            switch (@typeInfo(Context)) {
-                .Pointer => |info| if (info.size == .Slice)
-                    @compileError("`context` cannot be a slice"),
-
-                .Void => {},
-                else => @compileError("`context` must be a pointer"),
-            }
-
-            return Self{
-                .context = @ptrCast(*anyopaque, context),
-
-                .callErased = struct {
-                    fn callErased(erased: *anyopaque, input: In) Out {
-                        return if (Context == void) invoke(input) else invoke(@ptrCast(
-                            Context, @alignCast(@alignOf(Context), erased)), input);
-                    }
-                }.callErased,
-            };
-        }
-    };
-}

src/core/stack.zig

@@ -145,38 +145,69 @@ pub const FixedPushError = error {
 /// memory pool to linearly allocate memory from.
 ///
 pub fn fixedAllocator(fixed_stack: *Fixed(u8)) io.Allocator {
-    return io.Allocator.fromClosure(fixed_stack, struct {
-        fn alloc(stack: *Fixed(u8), allocation: io.Allocation) ?[*]u8 {
-            if (allocation.existing) |buffer| if (allocation.size == 0) {
-                // Deallocate the memory.
-                const buffer_address = @ptrToInt(buffer);
-                const stack_address = @ptrToInt(stack.buffer.ptr);
+    const FixedStack = @TypeOf(fixed_stack.*);
 
-                // Check the buffer is within the address space of the stack buffer. If not, it
-                // should just be returned to let the caller know it cannot be freed.
-                if (buffer_address < stack_address or buffer_address >=
-                    (stack_address + stack.filled)) return buffer;
+    return io.Allocator.wrap(@ptrCast(*opaque {
+        const Self = @This();
 
-                // TODO: Investigate ways of actually freeing if it is the last allocation.
-                return null;
-            } else {
-                // TODO: Investigate ways of in-place relocating if it is the last allocation.
-            };
-
-            // Reallocate / allocate the memory.
+        pub fn alloc(self: *Self, layout: io.AllocationLayout) io.AllocationError![*]u8 {
             // TODO: Remove stdlib dependency.
+            const stack = self.stackCast();
+
             const adjusted_offset = @import("std").mem.alignPointerOffset(stack.buffer.ptr +
-                stack.filled, allocation.alignment) orelse return null;
+                stack.filled, layout.alignment) orelse return error.OutOfMemory;
 
             const head = stack.filled + adjusted_offset;
-            const tail = head + allocation.size;
+            const tail = head + layout.length;
 
-            stack.pushMany(0, tail) catch return null;
+            stack.pushMany(0, tail) catch return error.OutOfMemory;
 
             return stack.buffer[head .. tail].ptr;
-
         }
-    }.alloc);
+
+        pub fn dealloc(self: *Self, allocation: [*]u8) void {
+            // Deallocate the memory.
+            const stack = self.stackCast();
+            const allocation_address = @ptrToInt(allocation);
+            const stack_address = @ptrToInt(stack.buffer.ptr);
+
+            // Check the buffer is within the address space of the stack buffer. If not, it cannot
+            // be freed.
+            if (allocation_address < stack_address or allocation_address >=
+                (stack_address + stack.filled)) unreachable;
+
+            // TODO: Investigate ways of actually freeing if it is the last allocation.
+        }
+
+        pub fn realloc(self: *Self, allocation: [*]u8,
+            layout: io.AllocationLayout) io.AllocationError![*]u8 {
+
+            // TODO: Investigate ways of in-place relocating if it is the last allocation.
+            // TODO: Remove stdlib dependency.
+            const stack = self.stackCast();
+            const allocation_address = @ptrToInt(allocation);
+            const stack_address = @ptrToInt(stack.buffer.ptr);
+
+            // Check the buffer is within the address space of the stack buffer. If not, it cannot
+            // be reallocated.
+            if (allocation_address < stack_address or allocation_address >=
+                (stack_address + stack.filled)) unreachable;
+
+            const adjusted_offset = @import("std").mem.alignPointerOffset(stack.buffer.ptr +
+                stack.filled, layout.alignment) orelse return error.OutOfMemory;
+
+            const head = stack.filled + adjusted_offset;
+            const tail = head + layout.length;
+
+            stack.pushMany(0, tail) catch return error.OutOfMemory;
+
+            return stack.buffer[head .. tail].ptr;
+        }
+
+        fn stackCast(self: *Self) *Fixed(u8) {
+            return @ptrCast(*FixedStack, @alignCast(@alignOf(FixedStack), self));
+        }
+    }, fixed_stack));
 }
 
 test "fixedAllocator" {
@@ -185,14 +216,11 @@ test "fixedAllocator" {
     const allocator = fixedAllocator(&stack);
 
     // Allocation
-    var block_memory = allocator.call(.{
-        .existing = null,
+    var block_memory = try allocator.alloc(.{
         .alignment = @alignOf(u64),
-        .size = @sizeOf(u64),
+        .length = @sizeOf(u64),
     });
 
-    try testing.expect(block_memory != null);
-
     const buffer_address_head = @ptrToInt(&buffer);
     const buffer_address_tail = @ptrToInt(&buffer) + buffer.len;
 
@@ -204,14 +232,11 @@ test "fixedAllocator" {
     }
 
     // Reallocation.
-    block_memory = allocator.call(.{
-        .existing = block_memory,
+    block_memory = try allocator.realloc(block_memory, .{
         .alignment = @alignOf(u64),
-        .size = @sizeOf(u64),
+        .length = @sizeOf(u64),
     });
 
-    try testing.expect(block_memory != null);
-
     {
         const block_memory_address = @ptrToInt(block_memory);
 
@@ -220,11 +245,7 @@ test "fixedAllocator" {
     }
 
     // Deallocation.
-    try testing.expect(allocator.call(.{
-        .existing = block_memory,
-        .alignment = 0,
-        .size = 0,
-    }) == null);
+    allocator.dealloc(block_memory);
 }
 
 ///
@@ -234,15 +255,23 @@ test "fixedAllocator" {
 /// referenced by `fixed_stack` until it is full.
 ///
 pub fn fixedWriter(fixed_stack: *Fixed(u8)) io.Writer {
-    return io.Writer.fromClosure(fixed_stack, struct {
-        fn write(stack: *Fixed(u8), buffer: []const u8) usize {
-            stack.pushAll(buffer) catch |err| switch (err) {
+    const FixedStack = @TypeOf(fixed_stack.*);
+
+    return io.Writer.wrap(@ptrCast(*opaque {
+        const Self = @This();
+
+        fn stackCast(self: *Self) *Fixed(u8) {
+            return @ptrCast(*FixedStack, @alignCast(@alignOf(FixedStack), self));
+        }
+
+        pub fn write(self: *Self, buffer: []const u8) io.AccessError!usize {
+            self.stackCast().pushAll(buffer) catch |err| switch (err) {
                 error.BufferOverflow => return 0,
             };
 
             return buffer.len;
         }
-    }.write);
+    }, fixed_stack));
 }
 
 test "fixedWriter" {
@@ -250,6 +279,8 @@ test "fixedWriter" {
     var sequence_stack = Fixed(u8){.buffer = &buffer};
     const sequence_data = [_]u8{8, 16, 32, 64};
 
-    try testing.expect(fixedWriter(&sequence_stack).call(&sequence_data) == sequence_data.len);
+    try testing.expect((try fixedWriter(&sequence_stack).
+        write(&sequence_data)) == sequence_data.len);
+
     try testing.expect(io.equals(u8, sequence_stack.buffer, &sequence_data));
 }

src/core/table.zig

@@ -29,6 +29,11 @@ pub fn Hashed(comptime Key: type, comptime Value: type,
             maybe_next_index: ?usize = null,
         };
 
+        ///
+        /// Errors that may occur during initialization of a hash table.
+        ///
+        pub const InitError = io.AllocationError;
+
         ///
         /// Hash table type.
         ///
@@ -46,9 +51,9 @@ pub fn Hashed(comptime Key: type, comptime Value: type,
         ///
         /// Initializes a [Self] using `allocator` as the memory allocation strategy.
         ///
-        /// Returns a new [Self] value or an [io.MakeError] if initializing failed.
+        /// Returns a new [Self] value or an [InitError] if initializing failed.
         ///
-        pub fn init(allocator: Allocator) io.MakeError!Self {
+        pub fn init(allocator: Allocator) InitError!Self {
             const capacity = 4;
 
             return Self{
@@ -166,7 +171,7 @@ pub fn Hashed(comptime Key: type, comptime Value: type,
 /// [InsertError.KeyExists] occurs when an insertion was attempted on a table with a matching key
 /// already present.
 ///
-pub const InsertError = io.MakeError || error {
+pub const InsertError = io.AllocationError || error {
     KeyExists,
 };
 

src/core/unicode.zig

@@ -7,7 +7,7 @@ const testing = @import("./testing.zig");
 /// [PrintError.WriteFailure] occurs when the underlying [io.Writer] implementation failed to write
 /// the entirety of a the requested print operation.
 ///
-pub const PrintError = error {
+pub const PrintError = io.AccessError || error {
     WriteFailure,
 };
 
@@ -69,7 +69,7 @@ pub fn printInt(writer: io.Writer, radix: Radix, value: anytype) PrintError!void
             if (value == 0) {
                 const zero = "0";
 
-                if (writer.call(zero) != zero.len) return error.WriteFailure;
+                if ((try writer.write(zero)) != zero.len) return error.WriteFailure;
             } else {
                 // Big enough to hold the hexadecimal representation of the integer type, which is
                 // the largest number format accomodated for in [Radix].
@@ -95,7 +95,7 @@ pub fn printInt(writer: io.Writer, radix: Radix, value: anytype) PrintError!void
                 for (buffer[0 .. (buffer_count / 2)]) |_, i|
                     io.swap(u8, &buffer[i], &buffer[buffer_count - i - 1]);
 
-                if (writer.call(buffer[0 .. buffer_count]) != buffer_count)
+                if ((try writer.write(buffer[0 .. buffer_count])) != buffer_count)
                     return error.WriteFailure;
             }
         },

src/ona/main.zig

@@ -13,22 +13,19 @@ pub fn main() anyerror!void {
 /// Runs the game engine.
 ///
 fn runEngine(app: *sys.App, graphics: *sys.Graphics) anyerror!void {
-    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
-
-    defer _ = gpa.deinit();
-
     {
-        var file_reader = try app.data.openRead(try sys.Path.from(&.{"ona.lua"}));
+        const path = try sys.Path.from(&.{"ona.lua"});
+        var file_reader = try app.data.openFileReader(path);
 
         defer file_reader.close();
 
-        const file_size = try file_reader.size();
-        const allocator = gpa.allocator();
-        const buffer = try allocator.alloc(u8, file_size);
+        const file_size = (try app.data.query(path)).length;
+        const allocator = sys.threadSafeAllocator();
+        const buffer = (try core.io.makeMany(u8, allocator, file_size))[0 .. file_size];
 
-        defer allocator.free(buffer);
+        defer core.io.free(allocator, buffer);
 
-        if ((try file_reader.read(0, buffer)) != file_size) return error.ScriptLoadFailure;
+        if ((try file_reader.read(buffer)) != file_size) return error.ScriptLoadFailure;
 
         app.log(.debug, buffer);
     }

src/ona/oar.zig

@@ -2,13 +2,13 @@ const core = @import("core");
 const sys = @import("./sys.zig");
 
 ///
-/// Metadata of an Oar archive entry.
+/// Metadata of an Oar archive file entry.
 ///
-const Block = extern struct {
+const Entry = extern struct {
     signature: [signature_magic.len]u8 = signature_magic,
     path: sys.Path = sys.Path.empty,
-    data_size: u64 = 0,
-    data_head: u64 = 0,
+    data_offset: u64 = 0,
+    data_length: u64 = 0,
     padding: [232]u8 = [_]u8{0} ** 232,
 
     comptime {
@@ -19,84 +19,14 @@ const Block = extern struct {
 };
 
 ///
-/// Reference to a file entry in an Oar archive, denoting the starting offset from the top of head
-/// of the file and its size.
+/// [FindError.ArchiveUnsupported] occurs when trying to read a file that does not follow an Oar
+/// archive format considered valid by this implemenatation.
 ///
-pub const Entry = struct {
-    head: u64,
-    size: u64,
-
-    ///
-    /// [FindError.EntryNotFound] occurs when no entry matched the parameters of the find operation.
-    ///
-    /// [FindError.ArchiveUnsupported] occurs if the file provided to the find operation is not a
-    /// valid archive file.
-    ///
-    pub const FindError = error {
-        EntryNotFound,
-        ArchiveUnsupported,
-    };
-
-    ///
-    /// Attempts to perform a binary search on the entry blocks defined in `archive_file` for one
-    /// matching `entry_path`, returning an [Entry] referencing its data or a [FindError] if it
-    /// failed.
-    ///
-    /// **Note** that this operation has `O(log n)` time complexity.
-    ///
-    pub fn find(archive_file: *sys.ReadableFile, entry_path: sys.Path) FindError!Entry {
-        var header = Header{};
-        const header_size = @sizeOf(Header);
-        const io = core.io;
-
-        if (((archive_file.read(0, io.bytesOf(&header)) catch
-            return error.ArchiveUnsupported) != header_size) or
-                (!io.equals(u8, &header.signature, &signature_magic)) or
-                (header.revision != revision_magic) or
-                (header.entry_head <= header_size)) return error.ArchiveUnsupported;
-
-        // Read file table.
-        var head: usize = 0;
-        var tail: usize = (header.entry_count - 1);
-        const block_size = @sizeOf(Block);
-
-        while (head <= tail) {
-            var block = Block{};
-            const midpoint = (head + (tail - head) / 2);
-
-            if ((archive_file.read(header.entry_head + (block_size * midpoint), io.bytesOf(&block))
-                catch return error.ArchiveUnsupported) != block_size) return error.EntryNotFound;
-
-            const comparison = entry_path.compare(block.path);
-
-            if (comparison == 0) return Entry{
-                .head = block.data_head,
-                .size = block.data_size,
-            };
-
-            if (comparison > 0) {
-                head = (midpoint + 1);
-            } else {
-                tail = (midpoint - 1);
-            }
-        }
-
-        return error.EntryNotFound;
-    }
-
-    ///
-    /// Reads the data from `entry` in `archive_file` from the byte at the entry-relative `offset`
-    /// into `buffer` until either the end of the entry data, end of archive file, or end of buffer
-    /// is reached.
-    ///
-    /// The number of bytes read is returned or [sys.FileError] if it failed.
-    ///
-    pub fn read(entry: Entry, archive_file: *sys.ReadableFile,
-        offset: u64, buffer: []u8) sys.FileError!usize {
-
-        return archive_file.read(entry.head + offset,
-            buffer[0 .. core.math.min(usize, buffer.len, entry.size)]);
-    }
+/// [FindError.EntryNotFound] occurs when the queried entry was not found in the archive file.
+///
+pub const FindError = core.io.AccessError || error {
+    ArchiveUnsupported,
+    EntryNotFound,
 };
 
 ///
@@ -106,8 +36,7 @@ const Header = extern struct {
     signature: [signature_magic.len]u8 = signature_magic,
     revision: u8 = revision_magic,
     entry_count: u32 = 0,
-    entry_head: u64 = 0,
-    padding: [496]u8 = [_]u8{0} ** 496,
+    padding: [502]u8 = [_]u8{0} ** 502,
 
     comptime {
         const size = @sizeOf(@This());
@@ -117,7 +46,54 @@ const Header = extern struct {
 };
 
 ///
-/// The magic revision number that this Oar software implementation understands.
+/// Attempts to find an [Entry] with a path name matching `path` in `archive_reader`.
+///
+/// An [Entry] value is returned if a match was found, otherwise [FindError] if it failed.
+///
+pub fn findEntry(archive_reader: sys.FileReader, path: sys.Path) FindError!Entry {
+    var header = Header{};
+    const header_size = @sizeOf(Header);
+    const io = core.io;
+
+    if ((try archive_reader.read(io.bytesOf(&header))) != header_size)
+        return error.ArchiveUnsupported;
+
+    if (!io.equals(u8, &header.signature, &signature_magic))
+        return error.ArchiveUnsupported;
+
+    if (header.revision != revision_magic) return error.ArchiveUnsupported;
+
+    // Read file table.
+    var head: u64 = 0;
+    var tail: u64 = (header.entry_count - 1);
+    const entry_size = @sizeOf(Entry);
+
+    while (head <= tail) {
+        var entry = Entry{};
+        const midpoint = head + ((tail - head) / 2);
+        const offset = header_size + (entry_size * midpoint);
+
+        try archive_reader.seek(offset);
+
+        if ((try archive_reader.read(io.bytesOf(&entry))) != entry_size)
+            return error.ArchiveUnsupported;
+
+        const comparison = path.compare(entry.path);
+
+        if (comparison == 0) return entry;
+
+        if (comparison > 0) {
+            head = (midpoint + 1);
+        } else {
+            tail = (midpoint - 1);
+        }
+    }
+
+    return error.EntryNotFound;
+}
+
+///
+/// Magic revision number that this Oar software implementation understands.
 ///
 const revision_magic = 0;
 

src/ona/sys.zig

@@ -93,86 +93,106 @@ pub const App = struct {
 };
 
 ///
+/// Snapshotted information about the status of a file.
 ///
-///
-pub const ReadableFile = opaque {
-    ///
-    ///
-    ///
-    pub fn close(readable_file: *ReadableFile) void {
-        if (ext.SDL_RWclose(readable_file.rwOpsCast()) != 0)
-            return ext.SDL_LogWarn(ext.SDL_LOG_CATEGORY_APPLICATION,
-                "Attempt to close an invalid file reference");
-    }
-
-    ///
-    ///
-    ///
-    pub fn read(readable_file: *ReadableFile, offset: u64, buffer: []u8) FileError!u64 {
-        const rw_ops = readable_file.rwOpsCast();
-
-        {
-            ext.SDL_ClearError();
-
-            const math = core.math;
-            const min = math.min;
-            const maxIntValue = math.maxIntValue;
-            var sought = min(u64, offset, maxIntValue(i64));
-
-            if (ext.SDL_RWseek(rw_ops, @intCast(i64, sought), ext.RW_SEEK_SET) < 0)
-                return error.FileInaccessible;
-
-            var to_seek = offset - sought;
-
-            while (to_seek != 0) {
-                sought = min(u64, to_seek, maxIntValue(i64));
-
-                ext.SDL_ClearError();
-
-                if (ext.SDL_RWseek(rw_ops, @intCast(i64, sought), ext.RW_SEEK_CUR) < 0)
-                    return error.FileInaccessible;
-
-                to_seek -= sought;
-            }
-        }
-
-        ext.SDL_ClearError();
-
-        const buffer_read = ext.SDL_RWread(rw_ops, buffer.ptr, @sizeOf(u8), buffer.len);
-
-        if ((buffer_read == 0) and (ext.SDL_GetError() != null))
-            return error.FileInaccessible;
-
-        return buffer_read;
-    }
-
-    ///
-    ///
-    ///
-    pub fn rwOpsCast(readable_file: *ReadableFile) *ext.SDL_RWops {
-        return @ptrCast(*ext.SDL_RWops, @alignCast(@alignOf(ext.SDL_RWops), readable_file));
-    }
-
-    ///
-    ///
-    ///
-    pub fn size(readable_file: *ReadableFile) FileError!u64 {
-        ext.SDL_ClearError();
-
-        const byte_size = ext.SDL_RWsize(readable_file.rwOpsCast());
-
-        if (byte_size < 0) return error.FileInaccessible;
-
-        return @intCast(u64, byte_size);
-    }
+pub const FileStatus = struct {
+    length: u64,
 };
 
 ///
-/// [Error.FileInaccessible] is a generic catch-all for a [FileAccess] reference no longer pointing
-/// to a file or the file becomming invalid for whatever reason.
+/// Interface for working with bi-directional, streamable resources accessed through a file-system.
 ///
-pub const FileError = error {
-    FileInaccessible,
+pub const FileReader = struct {
+    context: *anyopaque,
+
+    vtable: *const struct {
+        close: fn (*anyopaque) void,
+        read: fn (*anyopaque, []u8) core.io.AccessError!u64,
+        seek: fn (*anyopaque, u64) core.io.AccessError!void,
+    },
+
+    ///
+    /// Closes the `file_reader`, logging a wraning if the `file_reader` is already considered
+    /// closed.
+    ///
+    pub fn close(file_reader: FileReader) void {
+        file_reader.vtable.close(file_reader.context);
+    }
+
+    ///
+    /// Attempts to read from `file_reader` into `buffer`, returning the number of bytes
+    /// successfully read or [core.io.AccessError] if it failed.
+    ///
+    pub fn read(file_reader: FileReader, buffer: []u8) core.io.AccessError!u64 {
+        return file_reader.vtable.read(file_reader.context, buffer);
+    }
+
+    ///
+    /// Attempts to seek from the beginning of `file_reader` to `cursor` bytes in, returning
+    /// [core.io.AccessError] if it failed.
+    ///
+    pub fn seek(file_reader: FileReader, cursor: u64) core.io.AccessError!void {
+        return file_reader.vtable.seek(file_reader.context, cursor);
+    }
+
+    ///
+    /// Wraps `implementation`, returning a [FileReader] value.
+    ///
+    pub fn wrap(implementation: anytype) FileReader {
+        const Implementation = @TypeOf(implementation.*);
+
+        return .{
+            .context = @ptrCast(*anyopaque, implementation),
+
+            .vtable = switch (@typeInfo(Implementation)) {
+                .Struct => &.{
+                    .close = struct {
+                        fn call(context: *anyopaque) void {
+                            @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).close();
+                        }
+                    }.call,
+
+                    .read = struct {
+                        fn call(context: *anyopaque, buffer: []u8) core.io.AccessError!u64 {
+                            return @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).read(buffer);
+                        }
+                    }.call,
+
+                    .seek = struct {
+                        fn call(context: *anyopaque, cursor: u64) core.io.AccessError!void {
+                            return @ptrCast(*Implementation, @alignCast(
+                                @alignOf(Implementation), context)).seek(cursor);
+                        }
+                    }.call,
+                },
+
+                .Opaque => &.{
+                    .close = struct {
+                        fn call(context: *anyopaque) void {
+                            @ptrCast(*Implementation, context).close();
+                        }
+                    }.call,
+
+                    .read = struct {
+                        fn call(context: *anyopaque, buffer: []u8) core.io.AccessError!u64 {
+                            return @ptrCast(*Implementation, context).read(buffer);
+                        }
+                    }.call,
+
+                    .seek = struct {
+                        fn call(context: *anyopaque, cursor: u64) core.io.AccessError!void {
+                            return @ptrCast(*Implementation, context).seek(cursor);
+                        }
+                    }.call,
+                },
+
+                else => @compileError(
+                    "`context` must a single-element pointer referencing a struct or opaque type"),
+            },
+        };
+    }
 };
 
 ///
@@ -181,42 +201,84 @@ pub const FileError = error {
 ///
 pub const FileSystem = union(enum) {
     native: []const u8,
-    archive_file: *ReadableFile,
+
+    archive: struct {
+        file_system: *const FileSystem,
+        path: Path,
+    },
 
     ///
-    /// With files typically being backed by a block device, they can produce a variety of errors -
-    /// from physical to virtual errors - these are all encapsulated by the API as general
-    /// [OpenError.FileNotFound] errors.
+    /// [AccessError.FileNotFound] occurs when a queried file could not be found on the file-system
+    /// by the process. This may mean the file does not exist, however it may also mean that the
+    /// process does not have sufficient rights to read it.
     ///
-    /// When a given [FileSystem] does not support a specified [OpenMode],
-    /// [OpenError.ModeUnsupported] is used to inform the consuming code that another [OpenMode]
-    /// should be tried or, if no mode other is suitable, that the resource is effectively
-    /// unavailable.
+    /// [AccessError.FileSystemfailure] denotes a file-system implementation-specific failure to
+    /// access resources has occured and therefore cannot proceed to access the file.
     ///
-    /// If the number of known [FileAccess] handles has been exhausted, [OpenError.OutOfFiles] is
-    /// used to communicate this.
-    ///
-    pub const OpenError = error {
+    pub const AccessError = error {
         FileNotFound,
-        ModeUnsupported,
-        OutOfFiles,
+        FileSystemFailure,
     };
 
     ///
-    /// Attempts to open the file identified by `path` with `mode` as the mode for opening the file.
+    /// Attempts to open the file identified by `path` on `file_system` for reading, returning a
+    /// [FileReader] value that provides access to the opened file or [AccessError] if it failed.
     ///
-    /// Returns a [ReadableFile] reference that provides access to the file referenced by `path`or a
-    /// [OpenError] if it failed.
-    ///
-    pub fn openRead(file_system: *const FileSystem, path: Path) OpenError!*ReadableFile {
-        switch (file_system.*) {
-            .archive_file => |archive_file| {
-                const entry = oar.Entry.find(archive_file, path) catch return error.FileNotFound;
+    pub fn openFileReader(file_system: FileSystem, path: Path) AccessError!FileReader {
+        switch (file_system) {
+            .archive => |archive| {
+                const archive_reader = try archive.file_system.openFileReader(archive.path);
 
-                _ = entry;
-                // TODO: Alloc file context.
+                errdefer archive_reader.close();
 
-                return error.FileNotFound;
+                const entry = oar.findEntry(archive_reader, path) catch |err| return switch (err) {
+                    error.ArchiveUnsupported, error.Inaccessible => error.FileSystemFailure,
+                    error.EntryNotFound => error.FileNotFound,
+                };
+
+                archive_reader.seek(entry.data_offset) catch return error.FileSystemFailure;
+
+                const io = core.io;
+
+                const allocator = threadSafeAllocator();
+
+                const entry_reader = io.makeOne(struct {
+                    allocator: io.Allocator,
+                    base_reader: FileReader,
+                    cursor: u64,
+                    offset: u64,
+                    length: u64,
+
+                    const Self = @This();
+
+                    pub fn close(self: *Self) void {
+                        self.base_reader.close();
+                        io.free(self.allocator, self);
+                    }
+
+                    pub fn read(self: *Self, buffer: []u8) io.AccessError!u64 {
+                        try self.base_reader.seek(self.offset + self.cursor);
+
+                        return self.base_reader.read(buffer[0 ..
+                            core.math.min(usize, buffer.len, self.length)]);
+                    }
+
+                    pub fn seek(self: *Self, cursor: u64) io.AccessError!void {
+                        self.cursor = cursor;
+                    }
+                }, allocator) catch return error.FileSystemFailure;
+
+                errdefer io.free(allocator, entry_reader);
+
+                entry_reader.* = .{
+                    .allocator = allocator,
+                    .base_reader = archive_reader,
+                    .cursor = 0,
+                    .offset = entry.data_offset,
+                    .length = entry.data_length,
+                };
+
+                return FileReader.wrap(entry_reader);
             },
 
             .native => |native| {
@@ -239,8 +301,124 @@ pub const FileSystem = union(enum) {
 
                 ext.SDL_ClearError();
 
-                return @ptrCast(*ReadableFile, ext.SDL_RWFromFile(&path_buffer, "rb")
-                    orelse return error.FileNotFound);
+                const rw_ops =
+                    ext.SDL_RWFromFile(&path_buffer, "rb") orelse return error.FileNotFound;
+
+                errdefer _ = ext.SDL_RWclose(rw_ops);
+
+                return FileReader.wrap(@ptrCast(*opaque {
+                    const Self = @This();
+
+                    fn rwOpsCast(self: *Self) *ext.SDL_RWops {
+                        return @ptrCast(*ext.SDL_RWops, @alignCast(@alignOf(ext.SDL_RWops), self));
+                    }
+
+                    pub fn read(self: *Self, buffer: []u8) core.io.AccessError!u64 {
+                        ext.SDL_ClearError();
+
+                        const bytes_read =
+                            ext.SDL_RWread(self.rwOpsCast(), buffer.ptr, @sizeOf(u8), buffer.len);
+
+                        if ((bytes_read == 0) and (ext.SDL_GetError() != null))
+                            return error.Inaccessible;
+
+                        return bytes_read;
+                    }
+
+                    pub fn seek(self: *Self, cursor: u64) core.io.AccessError!void {
+                        ext.SDL_ClearError();
+
+                        const math = core.math;
+                        const min = math.min;
+                        const maxIntValue = math.maxIntValue;
+                        var sought = min(u64, cursor, maxIntValue(i64));
+                        const ops = self.rwOpsCast();
+
+                        if (ext.SDL_RWseek(ops, @intCast(i64, sought), ext.RW_SEEK_SET) < 0)
+                            return error.Inaccessible;
+
+                        var to_seek = cursor - sought;
+
+                        while (to_seek != 0) {
+                            sought = min(u64, to_seek, maxIntValue(i64));
+
+                            ext.SDL_ClearError();
+
+                            if (ext.SDL_RWseek(ops, @intCast(i64, sought), ext.RW_SEEK_CUR) < 0)
+                                return error.Inaccessible;
+
+                            to_seek -= sought;
+                        }
+                    }
+
+                    pub fn close(self: *Self) void {
+                        ext.SDL_ClearError();
+
+                        if (ext.SDL_RWclose(self.rwOpsCast()) != 0)
+                            return ext.SDL_LogWarn(ext.SDL_LOG_CATEGORY_APPLICATION,
+                                "Attempt to close an invalid file reference");
+                    }
+                }, rw_ops));
+            },
+        }
+    }
+
+    ///
+    /// Attempts to query the status of the file identified by `path` on `file_system` for reading,
+    /// returning a [FileStatus] value containing a the state of the file at the moment or
+    /// [AccessError] if it failed.
+    ///
+    pub fn query(file_system: FileSystem, path: Path) AccessError!FileStatus {
+        switch (file_system) {
+            .archive => |archive| {
+                const archive_reader = try archive.file_system.openFileReader(archive.path);
+
+                defer archive_reader.close();
+
+                const entry = oar.findEntry(archive_reader, path) catch |err| return switch (err) {
+                    error.ArchiveUnsupported, error.Inaccessible => error.FileSystemFailure,
+                    error.EntryNotFound => error.FileNotFound,
+                };
+
+                return FileStatus{
+                    .length = entry.data_length,
+                };
+            },
+
+            .native => |native| {
+                if (native.len == 0) return error.FileNotFound;
+
+                var path_buffer = [_]u8{0} ** 4096;
+                const seperator_length = @boolToInt(native[native.len - 1] != Path.seperator);
+
+                if ((native.len + seperator_length + path.length) >= path_buffer.len)
+                    return error.FileNotFound;
+
+                const io = core.io;
+
+                io.copy(u8, &path_buffer, native);
+
+                if (seperator_length != 0) path_buffer[native.len] = Path.seperator;
+
+                io.copy(u8, path_buffer[native.len .. path_buffer.len],
+                    path.buffer[0 .. path.length]);
+
+                ext.SDL_ClearError();
+
+                const rw_ops =
+                    ext.SDL_RWFromFile(&path_buffer, "rb") orelse return error.FileSystemFailure;
+
+                defer if (ext.SDL_RWclose(rw_ops) != 0) unreachable;
+
+                ext.SDL_ClearError();
+
+                const size = ext.SDL_RWsize(rw_ops);
+
+                if (size < 0) return error.FileSystemFailure;
+
+                return FileStatus{
+                    .length = @intCast(u64, size),
+                };
             },
         }
     }
@@ -346,7 +524,8 @@ pub const Path = extern struct {
     };
 
     ///
-    ///
+    /// Returns a value above `0` if the path of `this` is greater than `that`, below `0` if it is
+    /// less, or `0` if they are identical.
     ///
     pub fn compare(this: Path, that: Path) isize {
         return core.io.compareBytes(this.buffer[0 ..this.length], that.buffer[0 .. that.length]);
@@ -432,16 +611,29 @@ pub const RunError = error {
 };
 
 ///
-/// Returns a [core.io.Allocator] bound to the underlying system allocator.
+/// Returns a thread-safe [core.io.Allocator] value based on the default system allocation strategy.
 ///
-pub fn allocator() core.io.Allocator {
-    // TODO: Add leak detection.
-    return .{
-        .bound = .{
-            .alloc = ext.SDL_alloc,
-            .dealloc = ext.SDL_free,
-        },
-    };
+pub fn threadSafeAllocator() core.io.Allocator {
+    const io = core.io;
+
+    return io.Allocator.wrap(@as(*opaque {
+        const Self = @This();
+
+        pub fn alloc(_: *Self, layout: io.AllocationLayout) io.AllocationError![*]u8 {
+            return @ptrCast([*]u8, ext.SDL_malloc(layout.length) orelse return error.OutOfMemory);
+        }
+
+        pub fn realloc(_: *Self, allocation: [*]u8,
+            layout: io.AllocationLayout) io.AllocationError![*]u8 {
+
+            return @ptrCast([*]u8, ext.SDL_realloc(allocation, layout.length)
+                orelse return error.OutOfMemory);
+        }
+
+        pub fn dealloc(_: *Self, allocation: [*]u8) void {
+            ext.SDL_free(allocation);
+        }
+    }, undefined));
 }
 
 ///
@@ -453,16 +645,11 @@ pub fn allocator() core.io.Allocator {
 pub fn display(comptime Error: anytype,
     comptime run: fn (*App, *Graphics) callconv(.Async) Error!void) (RunError || Error)!void {
 
-    var cwd = FileSystem{.native = "./"};
+    const cwd = FileSystem{.native = "./"};
     const user_prefix = ext.SDL_GetPrefPath("ona", "ona") orelse return error.InitFailure;
 
     defer ext.SDL_free(user_prefix);
 
-    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
-
-    defer if (gpa.deinit())
-        ext.SDL_LogWarn(ext.SDL_LOG_CATEGORY_APPLICATION, "Runtime allocator leaked memory");
-
     var app = App{
         .user = .{.native = std.mem.sliceTo(user_prefix, 0)},
 
@@ -480,17 +667,13 @@ pub fn display(comptime Error: anytype,
             return error.InitFailure;
         },
 
-        .data = .{
-            .archive_file = cwd.openRead(try Path.from(&.{"./data.oar"})) catch {
-                ext.SDL_LogCritical(ext.SDL_LOG_CATEGORY_APPLICATION, "Failed to load ./data.oar");
-
-                return error.InitFailure;
-            },
-        },
+        .data = .{.archive = .{
+            .file_system = &cwd,
+            .path = try Path.from(&.{"./data.oar"}),
+        }},
     };
 
     defer {
-        app.data.archive_file.close();
         ext.SDL_DestroySemaphore(app.message_semaphore);
         ext.SDL_DestroyMutex(app.message_mutex);
     }