feat: add anywriter

Author: RossComputerGuy

README.md

@@ -6,4 +6,4 @@ Any+ is a Zig library for handling anytypes in the runtime.
 
 - [x] Runtime `anytype`
 - [ ] Any-reader
-- [ ] Any-writer
+- [x] Any-writer

any+.zig

@@ -1,6 +1,10 @@
 /// Anytype in runtime
 pub const Anytype = @import("any+/anytype.zig");
 
+/// An opaque writer type
+pub const Anywriter = @import("any+/anywriter.zig");
+
 test {
     _ = Anytype;
+    _ = Anywriter;
 }

any+/anywriter.zig

@@ -0,0 +1,33 @@
+const std = @import("std");
+const Self = @This();
+
+pub const Error = anyerror;
+const PointerWrite = *const fn (*const Self, []const u8) Error!void;
+
+pub const Writer = std.io.Writer(Self, Error, write);
+
+type: []const u8,
+ptr: *anyopaque,
+ptrWrite: PointerWrite,
+
+pub inline fn init(writer: anytype) Self {
+    return initExplicit(@TypeOf(writer), writer);
+}
+
+pub inline fn initExplicit(comptime T: type, writer: T) Self {
+    return .{
+        .type = @typeName(T),
+        .ptr = @constCast(&writer),
+        .ptrWrite = (struct {
+            fn func(self: *const Self, buf: []const u8) Error!void {
+                if (!std.mem.eql(u8, self.type, @typeName(T))) return error.InvalidCast;
+                const w: *T = @ptrCast(@alignCast(self.ptr));
+                return w.write(buf);
+            }
+        }).func,
+    };
+}
+
+pub fn write(self: Self, buf: []const u8) Error!void {
+    return self.ptrWrite(&self, buf);
+}