#![allow(unused)]

use core::fmt;

use managed::{ManagedMap, ManagedSlice};

use crate::config::{REASSEMBLY_BUFFER_COUNT, REASSEMBLY_BUFFER_SIZE};
use crate::storage::Assembler;
use crate::time::{Duration, Instant};

#[cfg(feature = "alloc")]
type Buffer = alloc::vec::Vec<u8>;
#[cfg(not(feature = "alloc"))]
type Buffer = [u8; REASSEMBLY_BUFFER_SIZE];

/// Problem when assembling: something was out of bounds.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub struct AssemblerError;

impl fmt::Display for AssemblerError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "AssemblerError")
    }
}

#[cfg(feature = "std")]
impl std::error::Error for AssemblerError {}

/// Packet assembler is full
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
#[cfg_attr(feature = "defmt", derive(defmt::Format))]
pub struct AssemblerFullError;

impl fmt::Display for AssemblerFullError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "AssemblerFullError")
    }
}

#[cfg(feature = "std")]
impl std::error::Error for AssemblerFullError {}

/// Holds different fragments of one packet, used for assembling fragmented packets.
///
/// The buffer used for the `PacketAssembler` should either be dynamically sized (ex: Vec<u8>)
/// or should be statically allocated based upon the MTU of the type of packet being
/// assembled (ex: 1280 for a IPv6 frame).
#[derive(Debug)]
pub struct PacketAssembler<K> {
    key: Option<K>,
    buffer: Buffer,

    assembler: Assembler,
    total_size: Option<usize>,
    expires_at: Instant,
}

impl<K> PacketAssembler<K> {
    /// Create a new empty buffer for fragments.
    pub const fn new() -> Self {
        Self {
            key: None,

            #[cfg(feature = "alloc")]
            buffer: Buffer::new(),
            #[cfg(not(feature = "alloc"))]
            buffer: [0u8; REASSEMBLY_BUFFER_SIZE],

            assembler: Assembler::new(),
            total_size: None,
            expires_at: Instant::ZERO,
        }
    }

    pub(crate) fn reset(&mut self) {
        self.key = None;
        self.assembler.clear();
        self.total_size = None;
        self.expires_at = Instant::ZERO;
    }

    /// Set the total size of the packet assembler.
    pub(crate) fn set_total_size(&mut self, size: usize) -> Result<(), AssemblerError> {
        if let Some(old_size) = self.total_size {
            if old_size != size {
                return Err(AssemblerError);
            }
        }

        #[cfg(not(feature = "alloc"))]
        if self.buffer.len() < size {
            return Err(AssemblerError);
        }

        #[cfg(feature = "alloc")]
        if self.buffer.len() < size {
            self.buffer.resize(size, 0);
        }

        self.total_size = Some(size);
        Ok(())
    }

    /// Return the instant when the assembler expires.
    pub(crate) fn expires_at(&self) -> Instant {
        self.expires_at
    }

    pub(crate) fn add_with(
        &mut self,
        offset: usize,
        f: impl Fn(&mut [u8]) -> Result<usize, AssemblerError>,
    ) -> Result<(), AssemblerError> {
        if self.buffer.len() < offset {
            return Err(AssemblerError);
        }

        let len = f(&mut self.buffer[offset..])?;
        assert!(offset + len <= self.buffer.len());

        net_debug!(
            "frag assembler: receiving {} octets at offset {}",
            len,
            offset
        );

        self.assembler.add(offset, len);
        Ok(())
    }

    /// Add a fragment into the packet that is being reassembled.
    ///
    /// # Errors
    ///
    /// - Returns [`Error::PacketAssemblerBufferTooSmall`] when trying to add data into the buffer at a non-existing
    /// place.
    pub(crate) fn add(&mut self, data: &[u8], offset: usize) -> Result<(), AssemblerError> {
        #[cfg(not(feature = "alloc"))]
        if self.buffer.len() < offset + data.len() {
            return Err(AssemblerError);
        }

        #[cfg(feature = "alloc")]
        if self.buffer.len() < offset + data.len() {
            self.buffer.resize(offset + data.len(), 0);
        }

        let len = data.len();
        self.buffer[offset..][..len].copy_from_slice(data);

        net_debug!(
            "frag assembler: receiving {} octets at offset {}",
            len,
            offset
        );

        self.assembler.add(offset, data.len());
        Ok(())
    }

    /// Get an immutable slice of the underlying packet data, if reassembly complete.
    /// This will mark the assembler as empty, so that it can be reused.
    pub(crate) fn assemble(&mut self) -> Option<&'_ [u8]> {
        if !self.is_complete() {
            return None;
        }

        // NOTE: we can unwrap because `is_complete` already checks this.
        let total_size = self.total_size.unwrap();
        self.reset();
        Some(&self.buffer[..total_size])
    }

    /// Returns `true` when all fragments have been received, otherwise `false`.
    pub(crate) fn is_complete(&self) -> bool {
        self.total_size == Some(self.assembler.peek_front())
    }

    /// Returns `true` when the packet assembler is free to use.
    fn is_free(&self) -> bool {
        self.key.is_none()
    }
}

/// Set holding multiple [`PacketAssembler`].
#[derive(Debug)]
pub struct PacketAssemblerSet<K: Eq + Copy> {
    assemblers: [PacketAssembler<K>; REASSEMBLY_BUFFER_COUNT],
}

impl<K: Eq + Copy> PacketAssemblerSet<K> {
    const NEW_PA: PacketAssembler<K> = PacketAssembler::new();

    /// Create a new set of packet assemblers.
    pub fn new() -> Self {
        Self {
            assemblers: [Self::NEW_PA; REASSEMBLY_BUFFER_COUNT],
        }
    }

    /// Get a [`PacketAssembler`] for a specific key.
    ///
    /// If it doesn't exist, it is created, with the `expires_at` timestamp.
    ///
    /// If the assembler set is full, in which case an error is returned.
    pub(crate) fn get(
        &mut self,
        key: &K,
        expires_at: Instant,
    ) -> Result<&mut PacketAssembler<K>, AssemblerFullError> {
        let mut empty_slot = None;
        for slot in &mut self.assemblers {
            if slot.key.as_ref() == Some(key) {
                return Ok(slot);
            }
            if slot.is_free() {
                empty_slot = Some(slot)
            }
        }

        let slot = empty_slot.ok_or(AssemblerFullError)?;
        slot.key = Some(*key);
        slot.expires_at = expires_at;
        Ok(slot)
    }

    /// Remove all [`PacketAssembler`]s that are expired.
    pub fn remove_expired(&mut self, timestamp: Instant) {
        for frag in &mut self.assemblers {
            if !frag.is_free() && frag.expires_at < timestamp {
                frag.reset();
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Copy)]
    struct Key {
        id: usize,
    }

    #[test]
    fn packet_assembler_overlap() {
        let mut p_assembler = PacketAssembler::<Key>::new();

        p_assembler.set_total_size(5).unwrap();

        let data = b"Rust";
        p_assembler.add(&data[..], 0);
        p_assembler.add(&data[..], 1);

        assert_eq!(p_assembler.assemble(), Some(&b"RRust"[..]))
    }

    #[test]
    fn packet_assembler_assemble() {
        let mut p_assembler = PacketAssembler::<Key>::new();

        let data = b"Hello World!";

        p_assembler.set_total_size(data.len()).unwrap();

        p_assembler.add(b"Hello ", 0).unwrap();
        assert_eq!(p_assembler.assemble(), None);

        p_assembler.add(b"World!", b"Hello ".len()).unwrap();

        assert_eq!(p_assembler.assemble(), Some(&b"Hello World!"[..]));
    }

    #[test]
    fn packet_assembler_out_of_order_assemble() {
        let mut p_assembler = PacketAssembler::<Key>::new();

        let data = b"Hello World!";

        p_assembler.set_total_size(data.len()).unwrap();

        p_assembler.add(b"World!", b"Hello ".len()).unwrap();
        assert_eq!(p_assembler.assemble(), None);

        p_assembler.add(b"Hello ", 0).unwrap();

        assert_eq!(p_assembler.assemble(), Some(&b"Hello World!"[..]));
    }

    #[test]
    fn packet_assembler_set() {
        let key = Key { id: 1 };

        let mut set = PacketAssemblerSet::new();

        assert!(set.get(&key, Instant::ZERO).is_ok());
    }

    #[test]
    fn packet_assembler_set_full() {
        let mut set = PacketAssemblerSet::new();
        for i in 0..REASSEMBLY_BUFFER_COUNT {
            set.get(&Key { id: i }, Instant::ZERO).unwrap();
        }
        assert!(set.get(&Key { id: 4 }, Instant::ZERO).is_err());
    }

    #[test]
    fn packet_assembler_set_assembling_many() {
        let mut set = PacketAssemblerSet::new();

        let key = Key { id: 0 };
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assert_eq!(assr.assemble(), None);
        assr.set_total_size(0).unwrap();
        assr.assemble().unwrap();

        // Test that `.assemble()` effectively deletes it.
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assert_eq!(assr.assemble(), None);
        assr.set_total_size(0).unwrap();
        assr.assemble().unwrap();

        let key = Key { id: 1 };
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assr.set_total_size(0).unwrap();
        assr.assemble().unwrap();

        let key = Key { id: 2 };
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assr.set_total_size(0).unwrap();
        assr.assemble().unwrap();

        let key = Key { id: 2 };
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assr.set_total_size(2).unwrap();
        assr.add(&[0x00], 0).unwrap();
        assert_eq!(assr.assemble(), None);
        let assr = set.get(&key, Instant::ZERO).unwrap();
        assr.add(&[0x01], 1).unwrap();
        assert_eq!(assr.assemble(), Some(&[0x00, 0x01][..]));
    }
}