//! Creating immutable images of state.
use super::*;
use std::{error,fmt};


/// An image of the entire post container
#[derive(Debug, Default, Serialize, Deserialize)]
pub struct Freeze
{
    posts: Vec<post::Post>,
}

impl From<Freeze> for Imouto
{
    #[inline] fn from(from: Freeze) -> Self
    {
	Self::from_freeze(from)
    }
}

impl TryFrom<Imouto> for Freeze
{
    type Error = FreezeError;

    #[inline] fn try_from(from: Imouto) -> Result<Self, Self::Error>
    {
	from.try_into_freeze()
    }
}


/// Error returned when a freeze operation fails
#[derive(Debug)]
pub struct FreezeError{
    held: Option<Arc<RwLock<post::Post>>>,
}

impl FreezeError
{
    /// The post associated with this error, if there is one.
    pub fn post(&self) -> Option<&Arc<RwLock<post::Post>>>
    {
	self.held.as_ref()
    }
}

impl error::Error for FreezeError{}
impl fmt::Display for FreezeError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f,"Failed to create freeze image")?;
	
	if let Some(aref) = &self.held
	{
	    let cnt = Arc::strong_count(&aref) - 1;
	    if cnt > 0 {
		write!(f, "Post reference still held in {} other places.", cnt)
	    } else {
		write!(f, "Post reference was still held in another place at the time, but no longer is.")
	    }
	} else {
	    Ok(())
	}
    }
}

impl Imouto
{
    /// Create a serialisable image of this store by cloning each post into it.
    pub async fn freeze(&self) -> Freeze
    {
	let read = &self.all;
	let mut sis = Freeze{
	    posts: Vec::with_capacity(read.len()),
	};
	for (_, post) in read.iter()
	{
	    sis.posts.push(post.read().await.clone());
	}

	sis
    }

    /// Consume into a serialisable image of this store.
    ///
    /// # Fails
    /// If references to any posts are still held elsewhere.
    pub fn try_into_freeze(self) -> Result<Freeze, FreezeError>
    {
	let read = self.all;
	let mut sis = Freeze{
	    posts: Vec::with_capacity(read.len()),
	};
	for post in read.into_iter()
	{
	    sis.posts.push(match Arc::try_unwrap(post) {
		Ok(val) => val.into_inner(),
		Err(arc) => return Err(FreezeError{held: Some(arc)}),
	//	Err(_arc) => panic!("Reference to post is still being used"),//arc.read().await.clone(), // something else holds the reference, we can't consume it.
	    });
	}

	Ok(sis)
    }
    
    /// Consume into a serialisable image of this store.
    ///
    /// # Panics
    /// If references to any posts are still held elsewhere.
    pub fn into_freeze(self) -> Freeze
    {
	self.try_into_freeze().expect("Failed to consume into freeze")
    }

    /// Create a new store from a serialisable image of one by cloning each post in it
    pub fn unfreeze(freeze: &Freeze) -> Self
    {
	let mut posts = Arena::new();
	let mut user_map = HashMap::new();
	
	for post in freeze.posts.iter()
	{   
	    let idx = posts.insert(Arc::new(RwLock::new(post.clone())));
	    user_map.entry(post.owner().clone())
		.or_insert_with(|| MaybeVec::new())
		.push(idx);   
	}

	Self {
	    all: posts,
	    user_map,
	}
    }
    
    /// Create a new store by consuming serialisable image of one by cloning each post in it
    pub fn from_freeze(freeze: Freeze) -> Self
    {
	let mut posts = Arena::new();
	let mut user_map = HashMap::new();
	
	for post in freeze.posts.into_iter()
	{
	    let mapping = user_map.entry(post.owner().clone())
		.or_insert_with(|| MaybeVec::new());
	    let idx = posts.insert(Arc::new(RwLock::new(post)));
	    mapping.push(idx);
	}

	Self {
	    all: posts,
	    user_map,
	}
    }
}