// Copyright (C) 2021, 2022, 2023 Soni L.
// SPDX-License-Identifier: MIT OR Apache-2.0

//! The Datafu Virtual Machine.
//!
//! This is the stuff that actually matches the pattern.

use std::borrow::Cow;
use std::cell::Cell;
use std::cell::RefCell;
use std::collections::BTreeMap;
use std::collections::VecDeque;
use std::marker::PhantomData;

use indexmap::IndexMap;
use indexmap::IndexSet;
use regex::Regex;
use serde::Serialize;

use crate::Predicate;
//use crate::errors::MatchError;

mod de;

pub(crate) use de::Unpacker;
pub(crate) use de::Packer;

/// Max depth for VM/serde recursion.
pub(crate) const MAX_CALLS: usize = 250;

//type Matches<'a, 'b, T> = BTreeMap<&'a str, KVPair<'b, T>>;

// maybe we should use a builder for this?
/// The constant pool for a pattern.
pub(crate) struct PatternConstants<O: Serialize> {
    /// The protos ("functions") in a pattern.
    ///
    /// The last proto is implicitly the main function/entry point.
    pub(crate) protos: Vec<Vec<PatternElement>>,
    // Note that we can borrow these when creating the output map.
    // https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=da26f9175e96273fa0b94971a4e6172f
    pub(crate) strings: IndexSet<String>,
    pub(crate) regices: Vec<Regex>,
    pub(crate) predicates: Vec<Box<Predicate>>,
    pub(crate) defs: Vec<O>,
}

impl<O: Serialize> Default for PatternConstants<O> {
    fn default() -> Self {
        Self {
            protos: Default::default(),
            strings: Default::default(),
            regices: Default::default(),
            predicates: Default::default(),
            defs: Default::default(),
        }
    }
}

impl<O: Serialize + std::fmt::Debug> std::fmt::Debug for PatternConstants<O> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct(
            "PatternConstants"
        ).field(
            "protos", &self.protos,
        ).field(
            "strings", &self.strings,
        ).field(
            "regices", &self.regices,
        ).field(
            "predicates",
            &format_args!("({} predicates)", self.predicates.len()),
        ).field(
            "defs",
            &self.defs,
        ).finish()
    }
}

/// A datafu pattern element.
#[derive(Copy, Clone, Debug)]
pub(crate) enum PatternElement {
    /// A value is the core capturing element.
    Value {
        /// The index of the (string) name to apply to this value.
        name: Option<usize>,
        /// The expected value of this entry.
        value: Option<Value>,
    },
    /// A tag is the core iterative element. It is always followed by a value.
    ///
    /// This one is empty.
    EmptyTag,
    /// A tag is the core iterative element. It is always followed by a value.
    Tag {
        /// The index of the (proto) key to match against.
        key_subtree: usize,
        /// Whether to allow this tree subtree to match nothing.
        ///
        /// By default, a datafu pattern only matches a tree if every branch of
        /// the tree matches something. This enables opting out of that.
        // TODO this isn't currently implemented.
        optional: bool,
    },
    /// Marks the end of pattern iteration and the start of subtrees (if any).
    SubtreeMarker,
    /// A value subtree is a subtree for values.
    ///
    /// It is applied *after* tags, and thus any value subtrees come last in
    /// a pattern's elements.
    ValueSubtree {
        /// The proto index of the subtree.
        index: usize,
        /// Whether to allow this value subtree to match nothing.
        ///
        /// By default, a datafu pattern only matches a tree if every branch of
        /// the tree matches something. This enables opting out of that.
        optional: bool,
    },
}

/// A value matcher.
#[derive(Copy, Clone, Debug)]
pub(crate) enum Value {
    /// The value must match the specified string.
    String {
        /// The index of the string.
        index: usize,
        /// Whether to skip non-matching values, instead of erroring.
        skippable: bool,
    },
    /// The value must match the specified regex.
    Regex {
        /// The index of the regex.
        index: usize,
        /// Whether to skip non-matching values, instead of erroring.
        skippable: bool,
    },
    // /// The value must match the specified integer.
    // Integer {
    //     /// The integer.
    //     value: usize,
    //     /// Whether to skip non-matching values, instead of erroring.
    //     skippable: bool,
    // },
    // /// The value must match the specified integer range.
    // Range {
    //     /// The range.
    //     value: Range<usize>,
    //     /// Whether to skip non-matching values, instead of erroring.
    //     skippable: bool,
    // },
    // /// The value must match the specified predicate.
    // Predicate {
    //     /// The index of the predicate.
    //     index: usize,
    //     /// Whether to skip non-matching values, instead of erroring.
    //     skippable: bool,
    // },
    // /// The value must match the specified parameter.
    // Paameter {
    //     /// The index of the parameter.
    //     index: usize,
    //     /// Whether to skip non-matching values, instead of erroring.
    //     skippable: bool,
    // },
    /// The value must have the specified type.
    Type {
        /// The expected type.
        ty: Type,
        /// Whether to skip non-matching values, instead of erroring.
        skippable: bool,
    },
}

/// A pattern token.
// TODO docs
#[derive(Copy, Clone, Debug)]
pub(crate) enum PatternToken {
    /// Start of a tag.
    Arrow,

    Identifier(usize),

    String(usize, bool),
    Regex(usize, bool),
    Parameter(usize, bool),
    KeySubtree(usize, bool),
    ValueSubtree(usize, bool),

    /// Represents a predicate which must be applied.
    ///
    /// These are custom, arbitrary predicates, powered by serde. They're
    /// represented by `:$foo` in a pattern.
    ApplyPredicate(
        /// The predicate index (in `PatternConstants.predicates`).
        usize,
        /// Whether to skip non-matching values, instead of erroring.
        bool,
    ),

    /// Represents a type expectation.
    ///
    /// These are similar to predicates. They're represented by `:foo`, but are
    /// built-in and provide functionality not supported by predicates.
    ///
    /// Specifically, predicates cannot ask serde for a map or a list directly.
    /// Instead, they'd be required to parse a whole map/list/etc, which could
    /// cause issues which datafu is designed to avoid. (Datafu is designed to
    /// resist malicious input more so than arbitrary serde deserializers.)
    Type(
        /// The expected type.
        Type,
        /// Whether to skip non-matching values, instead of erroring.
        bool,
    ),

    /// End of a tag.
    End,
}

/// The types datafu and serde currently support.
///
/// These are used as expectations for serde (e.g.
/// `Deserializer::deserialize_string`).
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub(crate) enum Type {
    Any,
    IgnoredAny,
    Bool,
    I8,
    I16,
    I32,
    I64,
    I128,
    U8,
    U16,
    U32,
    U64,
    U128,
    F32,
    F64,
    Char,
    Str,
    String,
    Bytes,
    ByteBuf,
    Option,
    Unit,
    Seq,
    Tuple(usize),
    Map,
    UnitStruct(&'static str),
    NewtypeStruct(&'static str),
    TupleStruct {
        name: &'static str,
        len: usize,
    },
    Identifier,
    Struct {
        name: &'static str,
        fields: &'static [&'static str],
    },
    Enum {
        name: &'static str,
        variants: &'static [&'static str],
    },
}

/// The types which can be deserialized by serde.
///
/// We guess this is basically the same thing as a serde_value?
#[derive(Clone, Debug, PartialEq)]
pub(crate) enum SerdeObject<'de> {
    Bool(bool),
    I8(i8),
    I16(i16),
    I32(i32),
    I64(i64),
    I128(i128),
    U8(u8),
    U16(u16),
    U32(u32),
    U64(u64),
    U128(u128),
    F32(f32),
    F64(f64),
    Char(char),
    Str(Cow<'de, str>),
    Bytes(Cow<'de, [u8]>),
    Some(Box<SerdeObject<'de>>),
    None,
    Unit,
    Seq(Vec<SerdeObject<'de>>),
    // NOTE: support for multimaps!
    Map(Vec<(SerdeObject<'de>, SerdeObject<'de>)>),
    NewtypeStruct(Box<SerdeObject<'de>>),
    Enum {
        variant: Box<SerdeObject<'de>>,
        data: Box<SerdeObject<'de>>,
    },
}

impl<'de> SerdeObject<'de> {
    ///// Checks the type of this object.
    //fn check<E: serde::de::Error>(&self, ty: Option<Type>) -> Result<(), E> {
    //    let ty = match ty {
    //        None => return Ok(()),
    //        Some(ty) => ty,
    //    };
    //    match (ty, self) {
    //        | (Type::Any, v)
    //        | (Type::IgnoredAny, v)
    //        => Ok(()),
    //        | (Type::Bool, v @ SerdeObject::Bool(_))
    //        | (Type::I8, v @ SerdeObject::I8(_))
    //        | (Type::I16, v @ SerdeObject::I16(_))
    //        | (Type::I32, v @ SerdeObject::I32(_))
    //        | (Type::I64, v @ SerdeObject::I64(_))
    //        | (Type::I128, v @ SerdeObject::I128(_))
    //        | (Type::U8, v @ SerdeObject::U8(_))
    //        | (Type::U16, v @ SerdeObject::U16(_))
    //        | (Type::U32, v @ SerdeObject::U32(_))
    //        | (Type::U64, v @ SerdeObject::U64(_))
    //        | (Type::U128, v @ SerdeObject::U128(_))
    //        | (Type::F32, v @ SerdeObject::F32(_))
    //        | (Type::F64, v @ SerdeObject::F64(_))
    //        | (Type::Char, v @ SerdeObject::Char(_))
    //        | (Type::Str, v @ SerdeObject::Str(_))
    //        | (Type::String, v @ SerdeObject::Str(_))
    //        | (Type::Bytes, v @ SerdeObject::Bytes(_))
    //        | (Type::ByteBuf, v @ SerdeObject::Bytes(_))
    //        | (Type::Option, v @ SerdeObject::None)
    //        | (Type::Option, v @ SerdeObject::Some(_))
    //        | (Type::Unit, v @ SerdeObject::Unit)
    //        | (Type::Seq, v @ SerdeObject::Seq(_))
    //        | (Type::Map, v @ SerdeObject::Map(_))
    //        => Ok(()),
    //        _ => todo!(),
    //    }
    //}
}

impl<'de, E> serde::de::IntoDeserializer<'de, E> for SerdeObject<'de>
where
    E: serde::de::Error,
{
    type Deserializer = self::de::SerdeObjectDeserializer<'de, E>;

    fn into_deserializer(self) -> Self::Deserializer {
        Self::Deserializer {
            obj: self,
            _e: PhantomData,
        }
    }
}

/// Packed serde objects and datafu internal representation.
///
/// This is an iterative store of key-value pairs.
///
/// It's effectively a tree node.
#[derive(Clone, Debug, Default)]
pub struct Pack<'pat, 'de> {
    subpacks: VecDeque<(IndexMap<&'pat str, SerdeObject<'de>>, Pack<'pat, 'de>)>,
}

impl<'pat, 'de> Pack<'pat, 'de> {
    /// Adds the elements of the `other` pack to the matching iterarions of the
    /// current pack. If either pack is empty, pick the non-empty pack.
    ///
    /// The current pack will have the same number of iterations, but will
    /// contain captures from both packs. In case of captures of the same name,
    /// `other` will override `self`.
    ///
    /// # Panics
    ///
    /// Panics if the packs have different iteration lengths.
    fn zip(&mut self, mut other: Self) {
        match (self.subpacks.len(), other.subpacks.len()) {
            (0, _) => {
                *self = other;
            },
            (_, 0) => {},
            (a, b) if a == b => {
                for (l, r) in self.subpacks.iter_mut().zip(other.subpacks) {
                    // note that we can't actually recurse deeper than the VM
                    // actually does itself, so we don't need to worry about
                    // blowing the stack.
                    l.0.extend(r.0);
                    l.1.zip(r.1);
                }
            },
            _ => unreachable!("zip unbalanced iterations"),
        }
    }

    /// Adds the elements of the `other` pack to all iterations captured by
    /// this pack, such as to form a cartesian product. If either pack is
    /// empty, pick the non-empty pack. If both packs have a length of 1, merge
    /// their captures.
    ///
    /// The current pack will contain captures from both packs. In case of
    /// captures of the same name, `other` will override `self`.
    fn cartesian_product(&mut self, mut other: Self) {
        match (self.subpacks.len(), other.subpacks.len()) {
            (_, 0) => {
                return;
            },
            (0, _) => {
                *self = other;
                return;
            },
            (1, 1) => {
                let (robjects, rpack) = other.subpacks.pop_back().unwrap();
                let (ref mut lobjects, ref mut lpack) = self.subpacks[0];
                lobjects.extend(robjects);
                lpack.cartesian_product(rpack);
                return;
            },
            (1, _) => {
                self.subpacks[0].1.cartesian_product(other);
                return;
            },
            (_, 1) => {
                // FIXME: need to be careful with this one.
                // we want `other` to override entries from `self`, so we need
                // to scan `self` for captures of the same name as those in
                // `other`, and remove those captures. only then can we swap
                // `self` and `other` and merge them.
                // for now we can just do the inefficient thing tho.
            },
            _ => {},
        }
        // FIXME instead of doing this, perhaps we should find the smaller one,
        // and put clones of the larger one into it?
        self.subpacks.iter_mut().for_each(|&mut (_, ref mut lpack)| {
            lpack.cartesian_product(other.clone())
        });
    }

    /// Returns the serde object with the given name at the given iteration of
    /// this pack.
    #[cfg(test)]
    fn get_object_at(
        &self,
        iter: usize,
        name: &str,
    ) -> Option<&SerdeObject<'de>> {
        self.subpacks.get(iter).map(|x| &x.0).and_then(|x| x.get(name))
    }

    /// Returns the subpack related to the given name at the given iteration of
    /// this pack.
    #[cfg(test)]
    fn get_subpack_at(
        &self,
        iter: usize,
        name: &str,
    ) -> Option<&Pack<'pat, 'de>> {
        let _ = name;
        self.subpacks.get(iter).map(|x| &x.1)
    }
}

/// The Datafu interpreter, sorta.
#[derive(Debug)]
pub(crate) struct Interpreter<'pat, 'state, O: Serialize> {
    /// The pattern currently being processed.
    pat: &'pat PatternConstants<O>,
    /// The error override (if any).
    error: &'state mut Option<crate::errors::MatchError>,
    /// The current interpreter frames.
    frames: &'state mut Vec<Frame<'pat>>,
}

#[derive(Debug)]
pub(crate) struct Frame<'pat> {
    /// The instructions/function currently being processed.
    ops: &'pat [PatternElement],
    /// The instruction index being processed.
    iar: Option<usize>,
    /// How many steps this frame has not actually advanced for.
    ///
    /// This is used at end of frame and on match failure.
    overstep: usize,
    /// Whether this frame matches the data so far.
    matches: bool,
    /// Whether this frame must not be allowed to match in the key step.
    poison: bool,
}

impl<'pat, 'state, O: Serialize> Interpreter<'pat, 'state, O> {
    pub(crate) fn new(
        pat: &'pat PatternConstants<O>,
        error: &'state mut Option<crate::errors::MatchError>,
        frames: &'state mut Vec<Frame<'pat>>,
    ) -> Self {
        debug_assert!(frames.is_empty());
        frames.push(Frame {
            ops: &pat.protos.last().unwrap(),
            iar: None,
            overstep: 0,
            matches: true,
            poison: false,
        });
        Self {
            pat: pat,
            error: error,
            frames: frames,
        }
    }
}

impl<'pat> Frame<'pat> {
    /// Gets the type currently associated with this frame.
    ///
    /// Returns the type and whether it is required to match.
    fn get_type(
        &self,
    ) -> Option<(Type, bool)> {
        match self.op() {
            PatternElement::Value { value: Some(value), .. } => {
                match value {
                    | Value::String { skippable, .. }
                    | Value::Regex { skippable, .. }
                    => {
                        Some((Type::Str, !skippable))
                    },
                    Value::Type { ty, skippable } => {
                        Some((ty, !skippable))
                    },
                }
            },
            | PatternElement::EmptyTag
            | PatternElement::Tag { .. }
            => panic!("attempt to get type of tag"),
            _ => None,
        }
    }

    /// Gets the name currently associated with this frame.
    fn get_name<O: Serialize>(
        &self,
        pat: &'pat PatternConstants<O>,
    ) -> Option<&'pat str> {
        let strings = &pat.strings;
        match self.op() {
            PatternElement::Value { name: Some(name), .. } => {
                Some(&*strings[name])
            },
            | PatternElement::EmptyTag
            | PatternElement::Tag { .. }
            => panic!("attempt to get name of tag"),
            _ => None,
        }
    }

    /// Advances the instruction address register.
    ///
    /// # Returns
    ///
    /// `true` if successful, `false` otherwise.
    fn next(&mut self) -> bool {
        let new = self.iar.map_or(0, |v| v + 1);
        new < self.ops.len() && {
            self.iar = Some(new);
            true
        }
    }

    /// Returns the current instruction.
    ///
    /// # Panics
    ///
    /// Panics if called on a non-matching frame or if iteration hasn't begun.
    fn op(&self) -> PatternElement {
        assert!(self.active(), "op() called on inactive frame");
        self.raw_op()
    }

    /// Counts the number of *active* subtrees, if any, and whether any
    /// subtrees have been unwound.
    ///
    /// # Panics
    ///
    /// Panics if iteration hasn't begun.
    fn num_subtrees(&self) -> Option<(usize, bool)> {
        let iar = self.iar.unwrap();
        // check if there are any subtrees
        matches!(
            self.ops[iar],
            | PatternElement::ValueSubtree { .. }
            | PatternElement::SubtreeMarker
        ).then(|| {
            // count the number of subtrees
            (
                self.ops[0..=iar].iter().rev().take_while(|x| {
                    matches!(x, PatternElement::ValueSubtree { .. })
                }).count(),
                self.ops.len() - 1 != iar,
            )
        })
    }

    /// Returns whether this key has a subtree, and if so, its index and
    /// whether it is optional, as an `(index, optional)` pair.
    ///
    /// # Panics
    ///
    /// Panics if iteration hasn't begun, or this isn't a key.
    fn key_subtree(&self) -> Option<(usize, bool)> {
        match self.op() {
            PatternElement::Tag { key_subtree, optional } => {
                Some((key_subtree, optional))
            },
            PatternElement::EmptyTag => None,
            _ => unreachable!(),
        }
    }

    /// Returns whether this frame is in a value operation.
    ///
    /// # Panics
    ///
    /// Panics if the frame isn't active or iteraction hasn't begun.
    #[inline]
    fn is_value(&self) -> bool {
        self.active() && matches!(
            self.raw_op(),
            PatternElement::Value { .. },
        )
    }

    /// Returns this value subtree, as an `(index, optional)` pair.
    ///
    /// # Panics
    ///
    /// Panics if iteration hasn't begun, or this isn't a value subtree.
    fn value_subtree(&self) -> (usize, bool) {
        if let PatternElement::ValueSubtree {
            index,
            optional,
        } = self.raw_op() {
            (index, optional)
        } else {
            unreachable!()
        }
    }

    /// Returns the raw instruction.
    ///
    /// # Panics
    ///
    /// Panics if iteration hasn't begun.
    fn raw_op(&self) -> PatternElement {
        self.ops[self.iar.expect("ops[iar]")]
    }

    /// Returns whether this frame is active (not overstepped).
    #[inline]
    fn active(&self) -> bool {
        self.overstep == 0
    }

    /// Rewinds the instruction address register.
    ///
    /// # Returns
    ///
    /// `true` if successful, `false` otherwise.
    fn prev(&mut self) -> bool {
        let new = self.iar.expect("iar").checked_sub(1);
        new.is_some() && {
            self.iar = new;
            true
        }
    }
}

///// Stores a single match.
/////
///// See also Holder.
//enum HolderState<'a, 'b, T: PatternTypes> {
//    /// Empty holder, for a key-value pair.
//    EmptyKey,
//    /// Empty holder, for a Matcher and a key-value pair.
//    EmptyKeySubtree,
//    // /// Empty holder, for a Matcher and a value.
//    // EmptyValueSubtree,
//    /// Occupied holder, for a key-value pair..
//    Key(KVPair<'b, T>),
//    /// Occupied holder, for a Matcher and a key-value pair.
//    KeySubtree(Peekable<Matcher<'a, 'b, T>>, KVPair<'b, T>),
//    /// Occupied holder, for a Matcher and a value. The empty variant is
//    /// omitted as it would never be used otherwise.
//    ValueSubtree(Peekable<Matcher<'a, 'b, T>>, RefOwn<'b, T::Ref, T::Own>),
//    /// Occupied holder, for a value. The empty variant is omitted as it would
//    /// never be used otherwise.
//    Value(RefOwn<'b, T::Ref, T::Own>),
//}
//
///// Helper enum for HolderState.
//#[derive(Copy, Clone, Debug, Eq, PartialEq)]
//enum HolderKind {
//    Key,
//    KeySubtree,
//    ValueSubtree,
//    Value
//}
//
////impl<'a, 'b, T: PatternTypes> Clone for HolderState<'a, 'b, T> {
////    fn clone(&self) -> Self {
////        match self {
////            HolderState::EmptyKey => HolderState::EmptyKey,
////            HolderState::EmptySubtree => HolderState::EmptySubtree,
////            HolderState::Key(v) => HolderState::Key(*v),
////            HolderState::KeySubtree(m, v) => HolderState::KeySubtree(m.clone(), *v),
////            HolderState::ValueSubtree(m, v) => HolderState::ValueSubtree(m.clone(), *v),
////            HolderState::Value(v) => HolderState::Value(*v),
////        }
////    }
////}
//
//impl<'a, 'b, T: PatternTypes> HolderState<'a, 'b, T> {
//    #[rustfmt::skip]
//    fn is_empty(&self) -> bool {
//        match self {
//            | HolderState::EmptyKey
//            | HolderState::EmptyKeySubtree
//            //| HolderState::EmptyValueSubtree
//            => true, _ => false
//        }
//    }
//
//    fn has_value(&self) -> bool {
//        !self.is_empty()
//    }
//
//    fn kind(&self) -> HolderKind {
//        match self {
//            | HolderState::EmptyKey
//            | HolderState::Key(_)
//            => HolderKind::Key,
//            | HolderState::EmptyKeySubtree
//            | HolderState::KeySubtree(_, _)
//            => HolderKind::KeySubtree,
//            //| HolderState::EmptyValueSubtree
//            | HolderState::ValueSubtree(_, _)
//            => HolderKind::ValueSubtree,
//            | HolderState::Value(_)
//            => HolderKind::Value,
//        }
//    }
//
//    fn value(&self) -> Option<RefOwn<'b, T::Ref, T::Own>> {
//        match *self {
//            HolderState::Key((_, value)) => Some(value),
//            HolderState::KeySubtree(_, (_, value)) => Some(value),
//            HolderState::ValueSubtree(_, value) => Some(value),
//            HolderState::Value(value) => Some(value),
//            _ => None
//        }
//    }
//
//    fn key(&self) -> Option<RefOwn<'b, T::Ref, T::Own>> {
//        match *self {
//            HolderState::Key((key, _)) => Some(key),
//            HolderState::KeySubtree(_, (key, _)) => Some(key),
//            _ => None
//        }
//    }
//
//    fn pair(&self) -> Option<KVPair<'b, T>> {
//        match *self {
//            HolderState::Key(pair) => Some(pair),
//            HolderState::KeySubtree(_, pair) => Some(pair),
//            _ => None
//        }
//    }
//
//    fn subtree(&mut self) -> Option<&mut Peekable<Matcher<'a, 'b, T>>> {
//        match *self {
//            HolderState::KeySubtree(ref mut subtree, _) => Some(subtree),
//            HolderState::ValueSubtree(ref mut subtree, _) => Some(subtree),
//            _ => None
//        }
//    }
//
//    fn clear(&mut self) {
//        *self = match self.kind() {
//            HolderKind::Key => HolderState::EmptyKey,
//            HolderKind::KeySubtree => HolderState::EmptyKeySubtree,
//            HolderKind::ValueSubtree => unreachable!(), //HolderState::EmptyValueSubtree,
//            HolderKind::Value => unreachable!(),
//        };
//        assert!(self.is_empty());
//    }
//}
//
///// Stores a single match and associated metadata.
/////
///// A single match is generally a key-value pair, but may be a collection of
///// named pairs in the case of subtree matches, or just a value for the initial
///// holder.
//struct Holder<'a, 'b, T: PatternTypes> {
//     name: Option<&'a str>,
//     value: HolderState<'a, 'b, T>,
//     parent: Option<RefOwn<'b, T::Ref, T::Own>>,
//     iterator: Option<Box<dyn Iterator<Item=KVPair<'b, T>> + 'b>>,
//     filters: Vec<Box<dyn (for<'c> Fn(&'c mut HolderState<'a, 'b, T>) -> Result<(), MatchError>) + 'a>>,
//}
//
//impl<'a, 'b, T: PatternTypes> Holder<'a, 'b, T> {
//    fn next(&mut self) -> Result<bool, MatchError> {
//        self.ensure_iterator()?;
//        if let Self {
//            value: ref mut v,
//            iterator: Some(ref mut it),
//            ref filters,
//            ..
//        } = self {
//            // check if we're in a subtree and (not) done.
//            if let Some(matcher) = v.subtree() {
//                if let Some(res) = matcher.peek() {
//                    // report any errors
//                    return res.as_ref().map(|_| true).map_err(|e| e.clone());
//                }
//            }
//            let kind = v.kind();
//            let mut next_v;
//            loop {
//                next_v = match it.next() {
//                    Some(pair) => HolderState::Key(pair),
//                    None => return Ok(false)
//                };
//                for filter in filters {
//                    filter(&mut next_v)?;
//                    if next_v.is_empty() {
//                        break;
//                    }
//                }
//                if next_v.has_value() {
//                    break;
//                }
//            }
//            assert!(next_v.has_value());
//            assert_eq!(next_v.kind(), kind);
//            *v = next_v;
//            Ok(true)
//        } else {
//            unreachable!()
//        }
//    }
//
//    /// Ensure `self.iterator.is_some()`, creating an iterator if necessary.
//    fn ensure_iterator(&mut self) -> Result<(), MatchError> {
//        if self.iterator.is_none() {
//            let iter = T::pairs(self.parent.unwrap());
//            if iter.is_none() {
//                return Err(MatchError::UnsupportedOperation);
//            }
//            self.iterator = iter;
//        }
//        assert!(self.iterator.is_some());
//        Ok(())
//    }
//}
//
//impl<'a, 'b, T: PatternTypes> Default for Holder<'a, 'b, T> {
//    fn default() -> Self {
//        Self {
//            name: Default::default(),
//            value: HolderState::EmptyKey,
//            parent: Default::default(),
//            iterator: Default::default(),
//            filters: Default::default(),
//        }
//    }
//}
//
//pub struct Matcher<'a, 'b, T: PatternTypes> {
//    defs: &'a PatternConstants<T>,
//    frame: Frame<'a, 'b, T>,
//}
//
//// TODO:
////
//// [x] Arrow
//// [x] StringKey
//// [x] RegexKey
//// [x] KeySubtree
//// [x] ValueSubtree
//// [x] Ident
//// [x] Param (untested)
//// [x] ApplyPredicate
//// [x] End
//
///// Helper for `PatternElement::StringKey`.
//fn on_string_key<'a, 'b, T: PatternTypes>(
//    matcher: &mut Matcher<'a, 'b, T>,
//    id: usize,
//    skippable: bool,
//) -> Result<bool, MatchError> {
//    let path = matcher.frame.path.last_mut().unwrap();
//    assert!(path.iterator.is_none());
//    let key = &matcher.defs.strings[id];
//    let iter = T::get(path.parent.unwrap(), RefOwn::Str(key));
//    match iter {
//        Some(None) if !skippable => Err(MatchError::ValidationError),
//        Some(opt) => {
//            path.iterator = Some(Box::new(opt.into_iter()));
//            Ok(true)
//        }
//        None => Err(MatchError::UnsupportedOperation),
//    }
//}
//
///// Helper for `PatternElement::ParameterKey`.
//fn on_parameter_key<'a, 'b, T: PatternTypes>(
//    matcher: &mut Matcher<'a, 'b, T>,
//    id: usize,
//    skippable: bool,
//) -> Result<bool, MatchError> {
//    let path = matcher.frame.path.last_mut().unwrap();
//    assert!(path.iterator.is_none());
//    let key = matcher.defs.defs[id];
//    let iter = T::get(path.parent.unwrap(), RefOwn::Own(key));
//    match iter {
//        Some(None) if !skippable => Err(MatchError::ValidationError),
//        Some(opt) => {
//            path.iterator = Some(Box::new(opt.into_iter()));
//            Ok(true)
//        }
//        None => Err(MatchError::UnsupportedOperation),
//    }
//}
//
///// Helper for `PatternElement::RegexKey`.
//fn on_regex_key<'a, 'b, T: PatternTypes>(
//    matcher: &mut Matcher<'a, 'b, T>,
//    id: usize,
//    skippable: bool,
//) -> Result<bool, MatchError> {
//    matcher.frame.path.last_mut().unwrap().ensure_iterator()?;
//    let re = &matcher.defs.regices[id];
//    let path = matcher.frame.path.last_mut().unwrap();
//    path.filters.push(Box::new(move |value| {
//        let s = T::as_str(value.key().unwrap());
//        match (s.map_or(false, |s| re.is_match(s)), skippable) {
//            (true, _) => Ok(()),
//            (false, true) => {
//                value.clear();
//                Ok(())
//            },
//            (false, false) => Err(MatchError::ValidationError),
//        }
//    }));
//    Ok(true)
//}
//
///// Helper for `PatternElement::KeySubtree`.
//fn on_key_subtree<'a, 'b, T: PatternTypes>(
//    matcher: &mut Matcher<'a, 'b, T>,
//    id: usize,
//    skippable: bool,
//) -> Result<bool, MatchError> {
//    let _ = skippable; // FIXME what should a skippable KeySubtree even do?!
//    matcher.frame.path.last_mut().unwrap().ensure_iterator()?;
//    let defs = matcher.defs;
//    let rlimit: usize = matcher.frame.depth;
//    let path = matcher.frame.path.last_mut().unwrap();
//    assert!(path.value.is_empty());
//    assert_eq!(path.value.kind(), HolderKind::Key);
//    path.value = HolderState::EmptyKeySubtree;
//    path.filters.push(Box::new(move |value| {
//        let key = value.key().unwrap();
//        let mut subtree = Matcher::new(key, defs, id, rlimit)?.peekable();
//        match subtree.peek() {
//            Some(&Ok(_)) => {
//                *value = HolderState::KeySubtree(subtree, value.pair().unwrap());
//                Ok(())
//            },
//            Some(&Err(ref e)) => {
//                Err(e.clone())
//            },
//            None => {
//                value.clear();
//                Ok(())
//            }
//        }
//    }));
//    Ok(true)
//}
//
//const DUMMY_OPS: &'static [PatternElement] = &[];
//
//impl<'a, 'b, T: PatternTypes> Matcher<'a, 'b, T> {
//    pub(crate) fn new(obj: RefOwn<'b, T::Ref, T::Own>, defs: &'a PatternConstants<T>, proto: usize, rlimit: usize) -> Result<Self, MatchError> {
//        let ops: &[_] = &defs.protos[proto];
//        Self::with_ops(obj, defs, ops, rlimit)
//    }
//
//    /// Constructs a Matcher that yields a single dummy result.
//    fn with_ops(obj: RefOwn<'b, T::Ref, T::Own>, defs: &'a PatternConstants<T>, ops: &'a [PatternElement], rlimit: usize) -> Result<Self, MatchError> {
//        let depth = rlimit.checked_sub(1).ok_or(MatchError::StackOverflow)?;
//        Ok(Self {
//            defs: defs,
//            frame: Frame {
//                //obj: obj,
//                ops: ops,
//                iar: None,
//                depth: depth,
//                path: {
//                    let mut holder = Holder::default();
//                    holder.value = HolderState::Value(obj);
//                    holder.iterator = Some(Box::new(std::iter::empty()));
//                    vec![holder]
//                },
//                in_key: false,
//            },
//        })
//    }
//
//    fn on_in_key(&mut self) -> Result<bool, MatchError> {
//        match self.frame.op() {
//            PatternElement::End => {
//                let path = self.frame.path.last_mut().unwrap();
//                if path.next()? {
//                    Ok(false)
//                } else {
//                    drop(path);
//                    self.frame.path.pop().unwrap();
//                    // stop at previous End, or start of frame
//                    while self.frame.prev() {
//                        if matches!(self.frame.op(), PatternElement::End) {
//                            break;
//                        }
//                    }
//                    // is start of frame?
//                    if !self.frame.prev() {
//                        self.frame.path.clear();
//                    }
//                    Ok(true)
//                }
//            },
//            PatternElement::ApplyPredicate(id, skippable) => {
//                // failing on T::get() is already handled, but we may need a
//                // T::pairs(). construct it here.
//                self.frame.path.last_mut().unwrap().ensure_iterator()?;
//                let pred = &self.defs.predicates[id];
//                let path = self.frame.path.last_mut().unwrap();
//                path.filters.push(Box::new(move |value| {
//                    match (pred(value.value().unwrap()), skippable) {
//                        (true, _) => Ok(()),
//                        (false, true) => {
//                            value.clear();
//                            Ok(())
//                        },
//                        (false, false) => Err(MatchError::ValidationError),
//                    }
//                }));
//                Ok(true)
//            },
//            PatternElement::StringKey(id, skippable) => {
//                on_string_key(self, id, skippable)
//            },
//            PatternElement::ParameterKey(id, skippable) => {
//                on_parameter_key(self, id, skippable)
//            },
//            PatternElement::RegexKey(id, skippable) => {
//                on_regex_key(self, id, skippable)
//            },
//            PatternElement::KeySubtree(id, skippable) => {
//                on_key_subtree(self, id, skippable)
//            },
//            _ => unreachable!("on_in_key")
//        }
//    }
//
//    fn on_not_in_key(&mut self) -> Result<bool, MatchError> {
//        match self.frame.op() {
//            PatternElement::Arrow => {
//                // this *should* always pass.
//                assert!(self.frame.path.last().unwrap().iterator.is_some());
//                let mut holder = Holder::default();
//                holder.parent = self.frame.path.last().unwrap().value.value();
//                assert!(holder.parent.is_some());
//                self.frame.path.push(holder);
//                Ok(false)
//            },
//            PatternElement::Identifier(id) => {
//                let name = self.defs.strings.get(id).map(|s| &**s);
//                let path = self.frame.path.last_mut().unwrap();
//                path.name = name;
//                assert!(path.iterator.is_none());
//                // we don't actually create the iterator here,
//                // as we may still wanna use T::get() instead.
//                Ok(true)
//            },
//            PatternElement::ApplyPredicate(id, skippable) => {
//                assert!(self.frame.path.len() == 1);
//                let pred = &self.defs.predicates[id];
//                let value = self.frame.path.last().unwrap().value.value();
//                match (pred(value.unwrap()), skippable) {
//                    (true, _) => Ok(false),
//                    (false, true) => {
//                        self.frame.path.clear();
//                        // any Ok(_) will do
//                        Ok(false)
//                    },
//                    (false, false) => Err(MatchError::ValidationError),
//                }
//            },
//            PatternElement::StringKey(id, skippable) => {
//                on_string_key(self, id, skippable)
//            },
//            PatternElement::ParameterKey(id, skippable) => {
//                on_parameter_key(self, id, skippable)
//            },
//            PatternElement::RegexKey(id, skippable) => {
//                on_regex_key(self, id, skippable)
//            },
//            PatternElement::KeySubtree(id, skippable) => {
//                on_key_subtree(self, id, skippable)
//            },
//            PatternElement::ValueSubtree(id, skippable) => {
//                let value = self.frame.path.last().unwrap().value.value().unwrap();
//                let mut subtree = Matcher::new(
//                    value,
//                    self.defs,
//                    id,
//                    self.frame.depth
//                )?.peekable();
//                let mut dummy = Matcher::with_ops(
//                    value,
//                    self.defs,
//                    DUMMY_OPS,
//                    self.frame.depth
//                )?.peekable();
//                // may panic.
//                let peeked = subtree.peek();
//                // shouldn't panic.
//                let _ = dummy.peek();
//                // push Holder after peek.
//                self.frame.path.push(Holder::default());
//                let mut holder = self.frame.path.last_mut().unwrap();
//                holder.parent = Some(value);
//                holder.iterator = Some(Box::new(std::iter::empty()));
//                match peeked {
//                    None if skippable => {
//                        holder.value = HolderState::ValueSubtree(dummy, value);
//                        Ok(true)
//                    },
//                    Some(&Ok(_)) | None => {
//                        drop(peeked);
//                        holder.value = HolderState::ValueSubtree(subtree, value);
//                        Ok(true)
//                    },
//                    Some(&Err(ref e)) => {
//                        Err(e.clone())
//                    },
//                }
//            },
//            _ => unreachable!("on_not_in_key")
//        }
//    }
//
//    fn collect_results(&mut self) -> Matches<'a, 'b, T> {
//        let mut res: Matches<'a, 'b, T> = Default::default();
//        for holder in &mut self.frame.path {
//            // make sure it's not empty.
//            assert!(holder.value.has_value());
//            // handle subtrees.
//            if let Some(matcher) = holder.value.subtree() {
//                if let Some(matches) = matcher.next() {
//                    // NOTE: we have checked these already.
//                    // (and if we haven't, that's a bug.)
//                    res.extend(matches.unwrap());
//                }
//            }
//            // handle pairs.
//            if let Some(pair) = holder.value.pair() {
//                if let Some(name) = holder.name {
//                    res.insert(name, pair);
//                }
//            }
//        }
//        res
//    }
//
//    fn on_end(&mut self) -> (bool, Matches<'a, 'b, T>) {
//        match self.frame.op() {
//            PatternElement::End => {
//                assert!(!self.frame.path.last().expect("path").value.is_empty());
//                let res = self.collect_results();
//                if !self.frame.prev() {
//                    // NOTE: frame.prev() must always be called, even if this
//                    // gets replaced with debug_assert!() in the future.
//                    assert!(false, "frame.prev()");
//                }
//                (true, res)
//            }
//            PatternElement::ApplyPredicate {..} => {
//                assert!(!self.frame.in_key);
//                let res = self.collect_results();
//                self.frame.path.clear();
//                (false, res)
//            }
//            _ => unreachable!("on_end")
//        }
//    }
//}
//
//impl<'a, 'b, T: PatternTypes> Iterator for Matcher<'a, 'b, T> {
//    type Item = Result<BTreeMap<&'a str, KVPair<'b, T>>, MatchError>;
//
//    fn next(&mut self) -> Option<Self::Item> {
//        if self.frame.ops.is_empty() {
//            if !self.frame.path.is_empty() {
//                self.frame.path.clear();
//                return Some(Ok(Default::default()));
//            }
//        }
//        while !self.frame.path.is_empty() {
//            if !self.frame.next() {
//                let (in_key, res) = self.on_end();
//                self.frame.in_key = in_key;
//                return Some(Ok(res));
//            } else {
//                let in_key = if self.frame.in_key {
//                    self.on_in_key()
//                } else {
//                    self.on_not_in_key()
//                };
//                match in_key {
//                    Ok(in_key) => self.frame.in_key = in_key,
//                    Err(e) => {
//                        self.frame.path.clear();
//                        return Some(Err(e))
//                    },
//                }
//            }
//        }
//        None
//    }
//}