CountingReader

pub fn CountingReader(comptime ReaderType: anytype) type {
    return struct {
        child_reader: ReaderType,
        bytes_read: u64 = 0,

        pub const Error = ReaderType.Error;
        pub const Reader = io.GenericReader(*@This(), Error, read);

        pub fn read(self: *@This(), buf: []u8) Error!usize {
            const amt = try self.child_reader.read(buf);
            self.bytes_read += amt;
            return amt;
        }

        pub fn reader(self: *@This()) Reader {
            return .{ .context = self };
        }
    };
}