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const std = @import("std");

const Writer = struct {
  // These two fields are the same as before
  ptr: *anyopaque,
  writeAllFn: *const fn (ptr: *anyopaque, data: []const u8) anyerror!void,

  // This is new
  fn init(ptr: anytype) Writer {
      const T = @TypeOf(ptr);
      const ptr_info = @typeInfo(T);

      const gen = struct {
          pub fn writeAll(pointer: *anyopaque, data: []const u8) anyerror!void {
              const self: T = @ptrCast(@alignCast(pointer));
              return ptr_info.Pointer.child.writeAll(self, data);
          }
      };

      return .{
          .ptr = ptr,
          .writeAllFn = gen.writeAll,
      };
  }

  // This is the same as before
  pub fn writeAll(self: Writer, data: []const u8) !void {
      return self.writeAllFn(self.ptr, data);
  }
};

const File = struct {
  fd: std.os.fd_t,

  fn writeAll(ptr: *anyopaque, data: []const u8) !void {
      const self: *File = @ptrCast(@alignCast(ptr));
      // os.write might not write all of `data`, we should really look at the
      // returned value, the number of bytes written, and handle cases where data
      // wasn't all written.
      _ = try std.os.write(self.fd, data);
  }

  fn writer(self: *File) Writer {
      return Writer.init(self);
  }
};

pub fn main() !void {
  var file = try std.fs.createFileAbsolute("/tmp/demo.txt", .{});
  var my_file = File{ .fd = file.handle };
  const writer = my_file.writer();
  try writer.writeAll("hello world");
}