use std::num::NonZeroUsize;

use instruction::SocketCount;
use serde::{Deserialize, Serialize};

pub mod id;
pub mod instruction;
pub mod semi_human;

pub type Map<K, V> = std::collections::BTreeMap<K, V>;
pub type Set<T> = std::collections::BTreeSet<T>;

/// Gives you a super well typed graph IR for a given human-readable repr.
///
/// Look at [`semi_human::GraphIr`] and the test files in the repo at `testfiles/`
/// to see what the RON should look like.
/// No, we don't want you to write out [`GraphIr`] in full by hand.
/// That's something for the machines to do.
///
/// # Errors
///
/// Returns an error if the parsed source is not a valid human-readable graph IR.
pub fn from_ron(source: &str) -> ron::error::SpannedResult<GraphIr> {
    let human_repr: semi_human::GraphIr = ron::from_str(source)?;
    Ok(human_repr.into())
}

/// The toplevel representation of a whole pipeline.
///
/// # DAGs
///
/// Pipelines may not be fully linear. They may branch out and recombine later on.
/// As such, the representation for them which is currently used is a
/// [**D**irected **A**cyclic **G**raph](https://en.wikipedia.org/wiki/Directed_acyclic_graph).
///
/// For those who are already familiar with graphs, a DAG is one, except that:
///
/// - It is **directed**: Edges have a direction they point to.
///   In this case, edges point from the outputs of streamers to inputs of consumers.
/// - It is **acyclic**: Those directed edges may not form loops.
///   In other words, if one follows edges only in their direction, it must be impossible
///   to come back to an already visited node.
///
/// Here, if an edge points from _A_ to _B_ (`A --> B`),
/// then _A_ is called a **dependency** or an **input source** of _B_,
/// and _B_ is called a **dependent** or an **output target** of _A_.
///
/// The DAG also enables another neat operation:
/// [Topological sorting](https://en.wikipedia.org/wiki/Topological_sorting).
/// This allows to put the entire graph into a linear list,
/// where it's guaranteed that once a vertex is visited,
/// all dependencies of it will have been visited already as well.
///
/// The representation used here in specific is a bit more complicated,
/// since **instructions** directly aren't just connected to one another,
/// but their **sockets** are instead.
///
/// So the vertices of the DAG are the **sockets**
/// (which are either [`id::Input`] or [`id::Output`] depending on the direction),
/// and each **socket** in turn belongs to an **instruction**.
///
/// # Usage
///
/// - If you want to build one from scratch,
///   add a few helper methods like
///     constructing an empty one,
///     adding instructions and
///     adding edges
/// - If you want to construct one from an existing repr,
///   maybe you want to use [`semi_human::GraphIr`].
///
/// # Storing additional data
///
/// Chances are the graph IR seems somewhat fit to put metadata in it.
/// However, most likely you're interacting in context of some other system,
/// and also want to manage and index that data on your own.
///
/// As such, consider using _secondary_ maps instead.
/// That is, store in a data structure _you_ own a mapping
/// from [`id`]s
/// to whatever data you need.
#[derive(Clone, Debug, Default, PartialEq, Eq, Deserialize, Serialize)]
pub struct GraphIr {
    /// "Backbone" storage of all **instruction** IDs to
    /// what **kind of instruction** they are.
    instructions: Map<id::Instruction, instruction::Kind>,

    /// How the data flows forward. **Dependencies** map to **dependents** here.
    edges: Map<id::Output, Set<id::Input>>,
    /// How the data flows backward. **Dependents** map to **dependencies** here.
    rev_edges: Map<id::Input, id::Output>,
}

// TODO: this impl block, but actually the whole module, screams for tests
impl GraphIr {
    /// Look "backwards" in the graph,
    /// and find out what instructions need to be done before this one.
    /// The input slots are visited in order.
    ///
    /// - The iterator returns individually [`Some`]`(`[`None`]`)` if the corresponding slot is
    ///   not connected.
    ///
    /// The same caveats as for [`GraphIr::resolve`] apply.
    #[must_use]
    pub fn input_sources(
        &self,
        subject: &id::Instruction,
    ) -> Option<impl Iterator<Item = Option<&id::Output>> + '_> {
        let (subject, kind) = self.instructions.get_key_value(subject)?;
        let SocketCount { inputs, .. } = kind.socket_count();

        Some((0..inputs).map(|idx| {
            let input = id::Input(socket(subject, idx));
            self.rev_edges.get(&input)
        }))
    }

    /// Look "forwards" in the graph to see what other instructions this instruction feeds into.
    ///
    /// The output slots represent the top-level iterator,
    /// and each one's connections are emitted one level below.
    ///
    /// Just [`Iterator::flatten`] if you are not interested in the slots.
    ///
    /// The same caveats as for [`GraphIr::resolve`] apply.
    #[must_use]
    pub fn output_targets(
        &self,
        subject: &id::Instruction,
    ) -> Option<impl Iterator<Item = Option<&Set<id::Input>>> + '_> {
        let (subject, kind) = self.instructions.get_key_value(subject)?;
        let SocketCount { outputs, .. } = kind.socket_count();

        Some((0..outputs).map(|idx| {
            let output = id::Output(socket(subject, idx));
            self.edges.get(&output)
        }))
    }

    /// Returns the instruction corresponding to the given ID.
    /// Returns [`None`] if there is no such instruction in this graph IR.
    ///
    /// Theoretically this could be fixed easily at the expense of some memory
    /// by just incrementing and storing some global counter,
    /// however, at the moment there's no compelling reason
    /// to actually have multiple [`GraphIr`]s at one point in time.
    /// Open an issue if that poses a problem for you.
    #[must_use]
    pub fn resolve<'ir>(&'ir self, subject: &id::Instruction) -> Option<InstructionRef<'ir>> {
        let (id, kind) = self.instructions.get_key_value(subject)?;

        let input_sources = self.input_sources(subject)?.collect();
        let output_targets = self.output_targets(subject)?.collect();

        Some(InstructionRef {
            id,
            kind,
            input_sources,
            output_targets,
        })
    }

    /// Returns the instruction this input belongs to.
    ///
    /// The same caveats as for [`GraphIr::resolve`] apply.
    #[must_use]
    pub fn owner_of_input<'ir>(&'ir self, input: &id::Input) -> Option<InstructionRef<'ir>> {
        self.resolve(&input.socket().belongs_to)
    }

    /// Returns the instruction this output belongs to.
    ///
    /// The same caveats as for [`GraphIr::resolve`] apply.
    #[must_use]
    pub fn owner_of_output<'ir>(&'ir self, output: &id::Output) -> Option<InstructionRef<'ir>> {
        self.resolve(&output.socket().belongs_to)
    }

    /// Returns the order in which the instructions could be visited
    /// in order to ensure that all dependencies are resolved
    /// before a vertex is visited.
    ///
    /// # Panics
    ///
    /// Panics if there are any cycles in the IR, as it needs to be a DAG.
    #[must_use]
    // yes, this function could probably return an iterator and be lazy
    // no, not today
    pub fn topological_sort(&self) -> Vec<InstructionRef> {
        // count how many incoming edges each vertex has
        let mut nonzero_input_counts: Map<_, NonZeroUsize> =
            self.rev_edges
                .iter()
                .fold(Map::new(), |mut count, (input, _)| {
                    let _ = *count
                        .entry(input.socket().belongs_to.clone())
                        .and_modify(|count| *count = count.saturating_add(1))
                        .or_insert(NonZeroUsize::MIN);
                    count
                });

        // are there any unconnected ones we could start with?
        // TODO: experiment if a VecDeque with some ordering fun is digested better by the executor
        let no_inputs: Vec<_> = {
            let nonzero: Set<_> = nonzero_input_counts.keys().collect();
            let all: Set<_> = self.instructions.keys().collect();
            all.difference(&nonzero).copied().cloned().collect()
        };

        // then let's find the order!
        let mut order = Vec::new();
        let mut active_queue = no_inputs;

        while let Some(current) = active_queue.pop() {
            // now that this vertex is visited and resolved,
            // make sure all dependents notice that

            let dependents = self
                .output_targets(&current)
                .expect("graph to be consistent")
                .flatten()
                .flatten();

            for dependent_input in dependents {
                let dependent = &dependent_input.socket().belongs_to;

                // how many inputs are connected to this dependent without us?
                let count = nonzero_input_counts
                    .get_mut(dependent)
                    .expect("connected output must refer to non-zero input");

                let new = NonZeroUsize::new(count.get() - 1);
                if let Some(new) = new {
                    // aww, still some
                    *count = new;
                    continue;
                }

                // none, that means this one is free now! let's throw it onto the active queue then
                let (now_active, _) = nonzero_input_counts
                    .remove_entry(dependent)
                    .expect("connected output must refer to non-zero input");
                active_queue.push(now_active);
            }

            // TODO: check if this instruction is "well-fed", that is, has all the inputs it needs,
            //       and if not, panic
            order.push(self.resolve(&current).expect("graph to be consistent"));
        }

        assert!(
            nonzero_input_counts.is_empty(),
            concat!(
                "topological sort didn't cover all instructions\n",
                "either there are unconnected inputs, or there is a cycle\n",
                "unresolved instructions:\n",
                "{:#?}"
            ),
            nonzero_input_counts,
        );

        order
    }
}

/// Constructs an [`id::Socket`] a bit more tersely.
fn socket(id: &id::Instruction, idx: u16) -> id::Socket {
    id::Socket {
        belongs_to: id.clone(),
        idx: id::SocketIdx(idx),
    }
}

/// A full instruction bundeled in context, with its inputs and outputs.
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct Instruction {
    pub id: id::Instruction,
    pub kind: instruction::Kind,

    // can't have these two public since then a user might corrupt their length
    input_sources: Vec<Option<id::Output>>,
    output_targets: Vec<Set<id::Input>>,
}

impl Instruction {
    /// Where this instruction gets its inputs from.
    ///
    /// [`None`] means that this input is unfilled,
    /// and must be filled before the instruction can be ran.
    #[must_use]
    pub fn input_sources(&self) -> &[Option<id::Output>] {
        &self.input_sources
    }

    /// To whom outputs are sent.
    #[must_use]
    pub fn output_targets(&self) -> &[Set<id::Input>] {
        &self.output_targets
    }
}

/// [`Instruction`], but every single field is borrowed instead.
/// See its docs.
///
/// Use the [`From`] impl to handily convert into an [`Instruction`].
/// The other way around is unlikely to be wanted — since you already have an [`Instruction`],
/// chances are you just want to take a reference (`&`) of it.
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct InstructionRef<'ir> {
    pub id: &'ir id::Instruction,
    pub kind: &'ir instruction::Kind,

    input_sources: Vec<Option<&'ir id::Output>>,
    output_targets: Vec<Option<&'ir Set<id::Input>>>,
}

impl<'ir> InstructionRef<'ir> {
    /// Where this instruction gets its inputs from.
    ///
    /// [`None`] means that this input is unfilled,
    /// and must be filled before the instruction can be ran.
    #[must_use]
    pub fn input_sources(&self) -> &[Option<&'ir id::Output>] {
        &self.input_sources
    }

    /// To whom outputs are sent.
    #[must_use]
    pub fn output_targets(&self) -> &[Option<&'ir Set<id::Input>>] {
        &self.output_targets
    }
}

// would love to use ToOwned but Rust has no specialization yet
// and it'd hurt a blanket impl of ToOwned otherwise
impl From<InstructionRef<'_>> for Instruction {
    fn from(source: InstructionRef<'_>) -> Self {
        Self {
            id: source.id.clone(),
            kind: source.kind.clone(),
            input_sources: source
                .input_sources
                .into_iter()
                .map(Option::<&_>::cloned)
                .collect(),
            output_targets: source
                .output_targets
                .into_iter()
                .map(|outputs| outputs.map(Clone::clone).unwrap_or_default())
                .collect(),
        }
    }
}