    const std = @import("../std.zig");
    const io = std.io;
    const testing = std.testing;

    /// A Reader that counts how many bytes has been read from it.
    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.Reader(*@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 };
            }
        };
    }

    pub fn countingReader(reader: anytype) CountingReader(@TypeOf(reader)) {
        return .{ .child_reader = reader };
    }

    test "io.CountingReader" {
        const bytes = "yay" ** 100;
        var fbs = io.fixedBufferStream(bytes);

        var counting_stream = countingReader(fbs.reader());
        const stream = counting_stream.reader();

        //read and discard all bytes

        while (stream.readByte()) |_| {} else |err| {
            try testing.expect(err == error.EndOfStream);
        }

        try testing.expect(counting_stream.bytes_read == bytes.len);
    }