use std::io;
use std::os::fd::AsFd;
use std::os::fd::AsRawFd;
use std::os::fd::FromRawFd;
use std::os::fd::OwnedFd;

use crate::Error;
use crate::Link;
use crate::Result;

/// Represents a bpf iterator for reading kernel data structures. This requires
/// Linux 5.8.
///
/// This implements [`std::io::Read`] for reading bytes from the iterator.
/// Methods require working with raw bytes. You may find libraries such as
/// [`plain`](https://crates.io/crates/plain) helpful.
#[derive(Debug)]
pub struct Iter {
    fd: OwnedFd,
}

impl Iter {
    /// Create a new `Iter` wrapping the provided `Link`.
    pub fn new(link: &Link) -> Result<Self> {
        let link_fd = link.as_fd().as_raw_fd();
        let fd = unsafe { libbpf_sys::bpf_iter_create(link_fd) };
        if fd < 0 {
            return Err(Error::from(io::Error::last_os_error()));
        }
        Ok(Self {
            fd: unsafe { OwnedFd::from_raw_fd(fd) },
        })
    }
}

impl io::Read for Iter {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let bytes_read =
            unsafe { libc::read(self.fd.as_raw_fd(), buf.as_mut_ptr() as *mut _, buf.len()) };
        if bytes_read < 0 {
            return Err(io::Error::last_os_error());
        }
        Ok(bytes_read as usize)
    }
}