use std::fmt::Debug;
use std::marker::PhantomData;
use derivative::Derivative;
use crate::dag::{RxDAG, RxSubDAG, RxContext, MutRxContext};
use crate::dag_uid::RxDAGUid;
use crate::clone_set_fn::CloneSetFn;
use crate::rx_impl::{CurrentOrNext, Rx};

/// Index into the DAG which will give you a node, which may be a variable or computed value.
///
/// To actually get or set the value you need a shared reference to the [RxDAG].
///
/// The DAG and refs have an ID so that you can't use one ref on another DAG, however this is checked at runtime.
/// The lifetimes are checked at compile-time though.
#[derive(Debug, Derivative)]
#[derivative(Clone(bound = ""), Copy(bound = ""))]
pub(crate) struct RxRef<'c, T> {
    index: usize,
    graph_id: RxDAGUid<'c>,
    phantom: PhantomData<T>
}

/// Index into the [RxDAG] which will give you a variable.
///
/// To actually get or set the value you need a shared reference to the [RxDAG].
#[derive(Debug, Derivative)]
#[derivative(Clone(bound = ""), Copy(bound = ""))]
pub struct Var<'c, T>(RxRef<'c, T>);

/// Index into the [RxDAG] which will give you a computed value.
///
/// To actually get the value you need a shared reference to the [RxDAG].
/// You cannot set the value, instead it's computed from other values.
#[derive(Debug, Derivative)]
#[derivative(Clone(bound = ""), Copy(bound = ""))]
pub struct CRx<'c, T>(RxRef<'c, T>);

/// View and mutate a part of a [Var].
#[derive(Debug)]
pub struct DVar<'c, S, T, GetFn: Fn(&S) -> &T, SetFn: Fn(&S, T) -> S> {
    source: RxRef<'c, S>,
    get: GetFn,
    set: SetFn
}

/// View a part of a [CRx].
#[derive(Debug)]
pub struct DCRx<'c, S, T, GetFn: Fn(&S) -> &T> {
    source: RxRef<'c, S>,
    get: GetFn
}

impl<'c, T> RxRef<'c, T> {
    pub(crate) fn new(graph: &RxDAG<'c>, index: usize) -> Self {
        RxRef {
            index,
            graph_id: graph.id(),
            phantom: PhantomData
        }
    }

    /// Read the node
    fn get<'a>(self, graph: RxSubDAG<'a, 'c>) -> &'a T where 'c: 'a {
        unsafe { self.get_rx(graph).get_dyn() }
    }

    /// Take `next` if set, otherwise returns a reference to `current`.
    /// The value should then be re-passed to `next` via `set`
    fn take_latest<'a>(self, graph: RxSubDAG<'a, 'c>) -> CurrentOrNext<'a, T> where 'c: 'a {
        unsafe { self.get_rx(graph).take_latest_dyn() }
    }

    /// Write a new value to the node. The changes will be applied on recompute.
    fn set(self, graph: RxSubDAG<'_, 'c>, value: T) {
        unsafe { self.get_rx(graph).set_dyn(value); }
    }

    /// Apply a transformation to the latest value. If `set` this will apply to the recently-set value.
    /// This must be used instead of chaining [RxRef::set] and [RxRef::get], since setting a value doesn't make it
    /// returned by [RxRef::get] until the graph is recomputed.
    ///
    /// Like `set` the changes only actually reflect in [RxRef::get] on recompute.
    fn modify<F: FnOnce(&T) -> T>(self, graph: RxSubDAG<'_, 'c>, modify: F) {
        let latest = self.take_latest(graph);
        let next = modify(latest.as_ref());
        self.set(graph, next);
    }

    /// Get the underlying [Rx] where the data is stored.
    fn get_rx<'a>(self, graph: RxSubDAG<'a, 'c>) -> &'a Rx<'c> where 'c: 'a {
        debug_assert!(self.graph_id == graph.id, "RxRef::get_rx: different graph");
        debug_assert!(self.index < graph.before.len(), "RxRef refers to a future node (not a DAG?)");
        // Since we already checked the index, we can use get_unchecked
        let elem = unsafe { graph.before.get_unchecked(self.index) };
        elem.as_node().expect("RxRef is corrupt: it points to an edge")
    }
}

impl<'c, T> Var<'c, T> {
    pub(crate) fn new(internal: RxRef<'c, T>) -> Self {
        Var(internal)
    }

    /// Read the variable
    pub fn get<'a>(self, c: impl RxContext<'a, 'c>) -> &'a T where 'c: 'a {
        let graph = c.sub_dag();
        self.0.get(graph)
    }

    /// Write a new value to the variable. The changes will be applied on recompute.
    pub fn set<'a>(self, c: impl MutRxContext<'a, 'c>, value: T) where 'c: 'a {
        let graph = c.sub_dag();
        self.0.set(graph, value);
    }

    /// Apply a transformation to the latest value. If [Var::set] this will apply to the recently-set value.
    /// This must be used instead of chaining [Var::set] and [Var::get], since setting a value doesn't make it
    /// returned by [Var::get] until the graph is recomputed.
    ///
    /// Like `set` the changes only actually reflect in [Var::get] on recompute.
    pub fn modify<'a, F: FnOnce(&T) -> T>(self, c: impl MutRxContext<'a, 'c>, modify: F) where 'c: 'a {
        let graph = c.sub_dag();
        self.0.modify(graph, modify)
    }

    /// Create a view of part of the variable.
    ///
    /// Do know that `SetFn` will take the most recently-set value even if the graph hasn't been recomputed.
    /// This means you can create multiple `derive`s and set them all before recompute, and you don't have to worry
    /// about the later derived values setting their part on the stale whole.
    pub fn derive<U, GetFn: Fn(&T) -> &U, SetFn: Fn(&T, U) -> T>(self, get: GetFn, set: SetFn) -> DVar<'c, T, U, GetFn, SetFn> {
        DVar {
            source: self.0,
            get,
            set
        }
    }

    /// Create a view of part of the variable, which clones the value on set.
    ///
    /// Do know that `SetFn` will take the most recently-set value even if the graph hasn't been recomputed.
    /// This means you can create multiple `derive`s and set them all before recompute, and you don't have to worry
    /// about the later derived values setting their part on the stale whole.
    pub fn derive_using_clone<U, GetFn: Fn(&T) -> &U, SetFn: Fn(&mut T, U)>(self, get: GetFn, set: SetFn) -> DVar<'c, T, U, GetFn, CloneSetFn<T, U, SetFn>> where T: Clone {
        self.derive(get, CloneSetFn::new(set))
    }
}

impl<'c, T> CRx<'c, T> {
    pub(crate) fn new(internal: RxRef<'c, T>) -> Self {
        CRx(internal)
    }

    /// Read the computed value
    pub fn get<'a>(self, c: impl RxContext<'a, 'c>) -> &'a T where 'c: 'a {
        let graph = c.sub_dag();
        self.0.get(graph)
    }

    /// Create a view of part of the computed value.
    pub fn derive<U, GetFn: Fn(&T) -> &U>(self, get: GetFn) -> DCRx<'c, T, U, GetFn> {
        DCRx {
            source: self.0,
            get
        }
    }
}

impl<'c, S, T, GetFn: Fn(&S) -> &T, SetFn: Fn(&S, T) -> S> DVar<'c, S, T, GetFn, SetFn> {
    /// Read the part of the variable this view gets.
    pub fn get<'a>(&self, c: impl RxContext<'a, 'c>) -> &'a T where 'c: 'a, S: 'a {
        let graph = c.sub_dag();
        (self.get)(self.source.get(graph))
    }

    /// Write a new value to the part of the variable this view gets.
    ///
    /// Do know that this uses the most recently-set value even if the graph hasn't been recomputed.
    /// This means you can create multiple `derive`s and set them all before recompute, and you don't have to worry
    /// about the later derived values setting their part on the stale whole.
    pub fn set<'a>(&self, c: impl MutRxContext<'a, 'c>, value: T) where 'c: 'a, S: 'a {
        let graph = c.sub_dag();
        let old_value = self.source.take_latest(graph);
        let new_value = (self.set)(old_value.as_ref(), value);
        self.source.set(graph, new_value)
    }
}

impl<'c, S, T, GetFn: Fn(&S) -> &T> DCRx<'c, S, T, GetFn> {
    /// Read the part of the computed value this view gets.
    pub fn get<'a>(&self, c: impl RxContext<'a, 'c>) -> &'a T where 'c: 'a, S: 'a {
        let graph = c.sub_dag();
        (self.get)(self.source.get(graph))
    }
}