Add fixed stack-backed allocator implementation

src/core/io.zig

@@ -4,11 +4,12 @@ const stack = @import("./stack.zig");
 const testing = @import("./testing.zig");
 
 ///
-///
+/// Allocation options for an [Allocator].
 ///
 pub const Allocation = struct {
     existing: ?[*]u8,
-    size: usize
+    alignment: u29,
+    size: usize,
 };
 
 ///
@@ -17,29 +18,7 @@ pub const Allocation = struct {
 pub const Allocator = meta.Function(Allocation, ?[*]u8);
 
 ///
-///
-///
-pub const ArenaAllocator = struct {
-    region: []u8,
-    cursor: usize = 0,
-
-    ///
-    ///
-    ///
-    pub fn allocator(arena_allocator: *ArenaAllocator) Allocator {
-        return Allocator.fromClosure(arena_allocator, struct {
-            fn call(context: *ArenaAllocator, allocation: Allocation) ?[*]u8 {
-                _ = allocation;
-                _ = context;
-
-                return null;
-            }
-        }.call);
-    }
-};
-
-///
-///
+/// [MakeError.OutOfMemory] if the requested amount of memory could not be allocated.
 ///
 pub const MakeError = error {
     OutOfMemory,
@@ -259,6 +238,21 @@ test "Check memory is equal" {
     try testing.expect(!equals(u8, bytes_sequence, &.{69, 42}));
 }
 
+///
+/// Fills the contents of `target` with `source`.
+///
+pub fn fill(comptime Element: type, target: []Element, source: Element) void {
+    for (target) |_, index| target[index] = source;
+}
+
+test "Fill data" {
+    var buffer = [_]u32{0} ** 8;
+
+    fill(u32, &buffer, 1);
+
+    for (buffer) |element| try testing.expect(element == 1);
+}
+
 ///
 /// Searches for the first instance of an `Element` equal to `needle` in `haystack`, returning its
 /// index or `null` if nothing was found.
@@ -318,7 +312,10 @@ test "Find first of sequence" {
 }
 
 ///
+/// Frees `allocated_memory` using `allocator`.
 ///
+/// *Note* that only memory known to be freeable by `allocator` should be passed via
+/// `allocated_memory`. Anything else will result is considered unreachable logic.
 ///
 pub fn free(allocator: Allocator, allocated_memory: anytype) void {
     if (allocator.call(.{
@@ -332,6 +329,7 @@ pub fn free(allocator: Allocator, allocated_memory: anytype) void {
         }),
 
         .size = 0,
+        .alignment = 0,
     }) != null) unreachable;
 }
 
@@ -356,13 +354,21 @@ test "Hashing bytes" {
 }
 
 ///
-///
+/// 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 {
-    return @ptrCast([*]Element, @alignCast(@alignOf(Element), allocator.call(.{
+    const alignment = @alignOf(Element);
+
+    if (allocator.call(.{
         .existing = null,
-        .size = size,
-    }) orelse return error.OutOfMemory));
+        .size = @sizeOf(Element) * size,
+        .alignment = alignment,
+    })) |buffer| {
+        return @ptrCast([*]Element, @alignCast(alignment, buffer));
+    }
+
+    return error.OutOfMemory;
 }
 
 ///
@@ -385,7 +391,8 @@ test "Data swapping" {
 }
 
 ///
-/// [Writer] that silently consumes all given data without failure and throws it away.
+/// Thread-safe and lock-free [Writer] that silently consumes all given data without failure and
+/// throws it away.
 ///
 /// This is commonly used for testing or redirected otherwise unwanted output data that has to be
 /// sent somewhere for whatever reason.

src/core/meta.zig

@@ -69,18 +69,16 @@ pub fn Function(comptime In: type, comptime Out: type) type {
                 else => @compileError("`context` must be a pointer"),
             }
 
-            var function = Self{
+            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);
+                            Context, @alignCast(@alignOf(Context), erased)), input);
                     }
                 }.callErased,
             };
-
-            return function;
         }
     };
 }

src/core/stack.zig

@@ -39,7 +39,7 @@ pub fn Fixed(comptime Element: type) type {
         }
 
         ///
-        /// Attempts to push `element` into `self`, returning a [FixedPushError] if it failed.
+        /// Attempts to push `element` into `self`, returning a [PushError] if it failed.
         ///
         pub fn push(self: *Self, element: Element) PushError!void {
             if (self.filled == self.buffer.len) return error.OutOfMemory;
@@ -49,8 +49,7 @@ pub fn Fixed(comptime Element: type) type {
         }
 
         ///
-        /// Attempts to push all of `elements` into `self`, returning a [FixedPushError] if it
-        /// failed.
+        /// Attempts to push all of `elements` into `self`, returning a [PushError] if it failed.
         ///
         pub fn pushAll(self: *Self, elements: []const Element) PushError!void {
             const filled = (self.filled + elements.len);
@@ -61,6 +60,20 @@ pub fn Fixed(comptime Element: type) type {
 
             self.filled = filled;
         }
+
+        ///
+        /// Attempts to push `count` instances of `element` into `self`, returning a [PushError] if
+        /// it failed.
+        ///
+        pub fn pushMany(self: *Self, element: Element, count: usize) PushError!void {
+            const filled = (self.filled + count);
+
+            if (filled > self.buffer.len) return error.OutOfMemory;
+
+            io.fill(Element, self.buffer[self.filled ..], element);
+
+            self.filled = filled;
+        }
     };
 }
 
@@ -115,6 +128,45 @@ test "Fixed stack of string literals" {
 ///
 pub const PushError = io.MakeError;
 
+///
+/// Creates and returns a [io.Allocator] value wrapping `fixed_stack`.
+///
+/// The returned [io.Allocator] uses `fixed_stack` and its backing memory buffer as a fixed-length
+/// 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_buffer_address = @ptrToInt(stack.buffer.ptr);
+
+                // 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_buffer_address) or
+                    (buffer_address >= (stack_buffer_address + stack.filled))) return buffer;
+
+                // TODO: Investigate ways of freeing if it is the last allocation.
+                return null;
+            };
+
+            // Reallocate / allocate the memory.
+            // TODO: Remove stdlib dependency.
+            const adjusted_offset = @import("std").mem.alignPointerOffset(stack.buffer.ptr +
+                stack.filled, allocation.alignment) orelse return null;
+
+            const head = stack.filled + adjusted_offset;
+            const tail = head + allocation.size;
+
+            stack.pushMany(0, tail) catch return null;
+
+            return stack.buffer[head .. tail].ptr;
+
+        }
+    }.alloc);
+}
+
 ///
 /// Returns an [io.Writer] wrapping `fixed_stack`.
 ///

src/core/table.zig

@@ -1,4 +1,5 @@
 const io = @import("./io.zig");
+const stack = @import("./stack.zig");
 const testing = @import("./testing.zig");
 
 ///
@@ -48,10 +49,10 @@ pub fn Hashed(comptime Key: type, comptime Value: type,
         /// Returns a new [Self] value or an [io.MakeError] if initializing failed.
         ///
         pub fn init(allocator: Allocator) io.MakeError!Self {
-            const initial_capacity = 4;
+            const capacity = 4;
 
             return Self{
-                .buckets = (try io.makeMany(Bucket, allocator, initial_capacity))[0 .. initial_capacity],
+                .buckets = (try io.makeMany(Bucket, allocator, capacity))[0 .. capacity],
                 .filled = 0,
                 .allocator = allocator,
                 .load_limit = 0.75,
@@ -197,15 +198,15 @@ pub const string_literal_context = KeyContext([]const u8){
 };
 
 test "Hash table manipulation with string literal context" {
-    var buffer = [_]u8{0} ** 1024;
-    var arena_allocator = io.ArenaAllocator{.region = &buffer};
+    var buffer = [_]u8{0} ** 4096;
+    var fixed_stack = stack.Fixed(u8){.buffer = &buffer};
 
-    var table =
-        try Hashed([]const u8, u32, string_literal_context).init(arena_allocator.allocator());
+    var table = try Hashed([]const u8, u32, string_literal_context).
+        init(stack.fixedAllocator(&fixed_stack));
 
     defer table.deinit();
 
-    const foo = @as(u32, 69);
+    const foo = 69;
 
     try testing.expect(table.remove("foo") == null);
     try table.insert("foo", foo);