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

//! Deserialization-related parts of the VM.

use std::borrow::Cow;
use std::marker::PhantomData;

use indexmap::IndexMap;

use serde::Serialize;
use serde::de::Error as _;
use serde::de::IntoDeserializer as _;

use smallvec::SmallVec;


use super::Frame;
use super::Interpreter;
use super::Pack;
use super::PatternConstants;
use super::PatternElement;
use super::SerdeObject;
use super::Type;
use super::Value;
use crate::errors::MatchError;

/// A `DeserializeSeed` for Datafu input.
///
/// This converts from Serde to Datafu's internal representation (a "pack").
pub(crate) struct Packer<'pat, 'state, O: Serialize> {
    /// The global interpreter state.
    interp: Interpreter<'pat, 'state, O>,
    /// Current call limit.
    call_limit: usize,
    /// Whether we're collecting values.
    collecting: bool,
}

struct FramesMut<'packer, 'pat> {
    frames: &'packer mut Vec<Frame<'pat>>,
}

struct Frames<'packer, 'pat> {
    frames: &'packer Vec<Frame<'pat>>,
}

impl<'packer, 'pat> FramesMut<'packer, 'pat> {
    fn iter_mut<'a>(
        &'a mut self,
    ) -> impl Iterator<Item=&'a mut Frame<'pat>> + DoubleEndedIterator
    where
        'packer: 'a,
    {
        self.frames.iter_mut()
    }

    fn iter_active_mut<'a>(
        &'a mut self,
    ) -> impl Iterator<Item=&'a mut Frame<'pat>> + DoubleEndedIterator
    where
        'packer: 'a,
    {
        self.iter_mut().filter(|frame| {
            frame.active()
        })
    }
}

impl<'packer, 'pat> Frames<'packer, 'pat> {
    fn iter<'a>(
        &'a self,
    ) -> impl Iterator<Item=&'a Frame<'pat>> + DoubleEndedIterator
    where
        'packer: 'a,
    {
        self.frames.iter()
    }

    fn iter_active<'a>(
        &'a self,
    ) -> impl Iterator<Item=&'a Frame<'pat>> + DoubleEndedIterator
    where
        'packer: 'a,
    {
        self.iter().filter(|frame| {
            frame.active()
        })
    }
}

impl<'pat, 'state, 'de, O: Serialize> Packer<'pat, 'state, O> {
    /// Creates a new Packer.
    pub(crate) fn new(
        interp: Interpreter<'pat, 'state, O>,
        call_limit: usize,
    ) -> Self {
        Self {
            interp: interp,
            call_limit: call_limit,
            collecting: false,
        }
    }

    fn frames_mut(&mut self) -> FramesMut<'_, 'pat> {
        FramesMut {
            frames: &mut *self.interp.frames,
        }
    }

    fn frames(&mut self) -> Frames<'_, 'pat> {
        Frames {
            frames: &*self.interp.frames,
        }
    }

    /// Steps the VM into the next operation.
    fn step_in<E: serde::de::Error>(&mut self) -> Result<(), E> {
        if self.call_limit > 0 {
            self.call_limit -= 1;
        } else {
            self.interp.error.insert(MatchError::StackOverflow);
            return Err(E::custom("stack overflow"));
        }
        // iterate up to the *live* length (i.e. the loop is allowed to modify
        // the length).
        // NOTE: we need to use while-let so as to not borrow anything in an
        // iterator. filtering happens on the *result* of the iterator.
        let mut index_iter = 0..;
        while let Some(index) = index_iter.next().filter(|&i| {
            i < self.interp.frames.len()
        }) {
            let frame = &mut self.interp.frames[index];
            if frame.overstep > 0 || !frame.matches {
                // overstepped and non-matching frames
                frame.overstep += 1;
                // FIXME check if this is correct (it probably isn't)
                //frame.matches = false;
            } else {
                if !frame.next() {
                    // empty/end-of frames
                    // 1 layer of step-in.
                    // step-out will undo this.
                    // this is correct because this branch implies overstep = 0
                    frame.overstep = 1;
                    // FIXME we still don't think this is correct
                    frame.matches = false;
                } else if matches!(
                    frame.op(),
                    PatternElement::SubtreeMarker,
                ) {
                    // subtrees!
                    // these are tricky, because the current frame can be moved
                    // in memory. so we have to use indexing every time.
                    // tho first we set it as overstep because it has special
                    // handling.
                    frame.overstep = 1;
                    frame.matches = false;
                    let mut at = index + 1;
                    while self.interp.frames[index].next() {
                        let op = self.interp.frames[index].raw_op();
                        if let PatternElement::ValueSubtree {
                            index: subtree, ..
                        } = op {
                            let new_frame = Frame {
                                ops: &self.interp.pat.protos[subtree][..],
                                iar: None,
                                overstep: 0,
                                matches: true,
                            };
                            // we want the "newest" frame last, so it is
                            // easier to unwind back.
                            self.interp.frames.insert(at, new_frame);
                            at += 1;
                        } else {
                            unreachable!()
                        }
                    }
                }
            }
        }
        Ok(())
    }

    /// Steps the VM back into the previous operation.
    fn step_out<E: serde::de::Error>(
        &mut self,
        mut packs: Vec<Pack<'pat, 'de>>,
    ) -> Result<Vec<Pack<'pat, 'de>>, E> {
        // this code attempts to maintain the logical invariant of:
        // self.frames().iter_active().count() == packs.len()
        self.call_limit += 1;
        let mut index_iter = 0..;
        let mut pack_index = packs.len();
        let orig_len = self.interp.frames.len();
        while let Some(index) = index_iter.next().filter(|&i| {
            i < orig_len
        }) {
            // iterate backwards
            let index = orig_len - index - 1;
            let frame = &mut self.interp.frames[index];
            let has_pack = frame.matches;
            if frame.overstep > 0 {
                // handle overstep
                frame.overstep -= 1;
            } else {
                if has_pack {
                    pack_index -= 1;
                }
                // unwind frame
                if frame.prev() {
                    // successfully unwound. do nothing.
                } else {
                    // find parent frame.
                    let mut count = 1;
                    let mut target = index;
                    let mut target_pack = pack_index;
                    while count > 0 && target > 0 {
                        target -= 1;
                        if self.interp.frames[target].matches {
                            debug_assert!(target_pack > 0);
                            target_pack -= 1;
                        }
                        match self.interp.frames[target].num_subtrees() {
                            Some((num, _)) if num < count => {
                                count -= num;
                            },
                            Some((num, _)) => {
                                count = 0;
                            },
                            None => {
                                count += 1;
                            },
                        }
                    }
                    if count == 0 {
                        // found target frame
                        let frame = self.interp.frames.remove(index);
                        let target_frame = &mut self.interp.frames[target];
                        let (_, optional) = target_frame.value_subtree();
                        target_frame.prev().then(|| ()).unwrap();
                        if has_pack {
                            let pack = packs.remove(pack_index);
                            if !target_frame.matches {
                                packs.insert(target_pack, pack);
                                target_frame.matches = true;
                                pack_index += 1;
                            } else {
                                packs[target_pack].merge_from(pack);
                            }
                        } else {
                            if !optional {
                                self.interp.error.insert({
                                    MatchError::ValidationError
                                });
                                return Err(E::custom("subtree failed"));
                            }
                        }
                        if let Some((0, _)) = target_frame.num_subtrees() {
                            target_frame.overstep = 0;
                        }
                    }
                }
            }
        }
        Ok(packs)
    }
}

impl<'pat, 'state, 'de, O> serde::de::DeserializeSeed<'de>
for &mut Packer<'pat, 'state, O>
where
    O: Serialize,
{
    type Value = (Vec<Pack<'pat, 'de>>, Option<SerdeObject<'de>>);
    fn deserialize<D>(
        mut self,
        deserializer: D,
    ) -> Result<Self::Value, D::Error>
    where
        D: serde::Deserializer<'de>
    {
        if let Err(e) = self.step_in() { return Err(e); }
        let pat = self.interp.pat;
        let target_type = self.frames().iter_active().try_fold(
            Type::IgnoredAny,
            |target_type, frame| {
                Ok(match (target_type, frame.get_type()) {
                    // required type binds stronger than any/ignored_any
                    (Type::IgnoredAny, Some((ty, true))) => ty,
                    (Type::Any, Some((ty, true))) => ty,
                    // and also stronger than optional any/ignored_any
                    (ty, Some((Type::IgnoredAny, _))) => ty,
                    (ty, Some((Type::Any, _))) => ty,
                    // None effectively falls back into any
                    (Type::IgnoredAny, None) => Type::Any,
                    (ty, None) => ty,
                    // prefer owned if any branch prefers owned
                    (Type::String, Some((Type::Str, true))) => {
                        Type::String
                    },
                    (Type::Str, Some((Type::String, true))) => {
                        Type::String
                    },
                    (Type::Bytes, Some((Type::ByteBuf, true))) => {
                        Type::ByteBuf
                    },
                    (Type::ByteBuf, Some((Type::Bytes, true))) => {
                        Type::ByteBuf
                    },
                    // types which are the same are okay
                    (left, Some((right, _))) if left == right => {
                        left
                    },
                    // optional type vs Any/IgnoredAny prefers Any
                    (Type::IgnoredAny, Some((_, false))) => Type::Any,
                    (Type::Any, Some((_, false))) => Type::Any,
                    // types which are not the same are an error because we
                    // only request a specific type if it's actually required
                    (left, Some((right, _))) => {
                        return Err(MatchError::Unsatisfiable);
                    },
                })
            },
        );
        let target_type = match target_type {
            Ok(target_type) => target_type,
            Err(e) => {
                self.interp.error.insert(e);
                return Err(D::Error::custom("type conflict"));
            },
        };
        match target_type {
            Type::Any => deserializer.deserialize_any(&mut *self),
            Type::IgnoredAny => {
                deserializer.deserialize_ignored_any(&mut *self)
            },
            Type::Bool => deserializer.deserialize_bool(&mut *self),
            Type::I8 => deserializer.deserialize_i8(&mut *self),
            Type::I16 => deserializer.deserialize_i16(&mut *self),
            Type::I32 => deserializer.deserialize_i32(&mut *self),
            Type::I64 => deserializer.deserialize_i64(&mut *self),
            Type::I128 => deserializer.deserialize_i128(&mut *self),
            Type::U8 => deserializer.deserialize_u8(&mut *self),
            Type::U16 => deserializer.deserialize_u16(&mut *self),
            Type::U32 => deserializer.deserialize_u32(&mut *self),
            Type::U64 => deserializer.deserialize_u64(&mut *self),
            Type::U128 => deserializer.deserialize_u128(&mut *self),
            Type::F32 => deserializer.deserialize_f32(&mut *self),
            Type::F64 => deserializer.deserialize_f64(&mut *self),
            Type::Char => deserializer.deserialize_char(&mut *self),
            Type::Str if !self.collecting => {
                deserializer.deserialize_str(&mut *self)
            },
            Type::Str | Type::String => {
                deserializer.deserialize_string(&mut *self)
            },
            Type::Bytes if !self.collecting => {
                deserializer.deserialize_bytes(&mut *self)
            },
            Type::Bytes | Type::ByteBuf => {
                deserializer.deserialize_byte_buf(&mut *self)
            },
            Type::Option => deserializer.deserialize_option(&mut *self),
            Type::Unit => deserializer.deserialize_unit(&mut *self),
            Type::Seq => deserializer.deserialize_seq(&mut *self),
            Type::Map => deserializer.deserialize_map(&mut *self),
            Type::Identifier => {
                deserializer.deserialize_identifier(&mut *self)
            },
            Type::Tuple(len) => {
                deserializer.deserialize_tuple(len, &mut *self)
            },
            Type::UnitStruct(name) => {
                deserializer.deserialize_unit_struct(name, &mut *self)
            },
            Type::NewtypeStruct(name) => {
                deserializer.deserialize_newtype_struct(name, &mut *self)
            },
            Type::TupleStruct { name, len } => {
                deserializer.deserialize_tuple_struct(name, len, &mut *self)
            },
            Type::Struct { name, fields } => {
                deserializer.deserialize_struct(name, fields, &mut *self)
            },
            Type::Enum { name, variants } => {
                deserializer.deserialize_enum(name, variants, &mut *self)
            },
        }.and_then(|(packs, obj)| Ok((self.step_out(packs)?, obj)))
    }
}

/// visit method generator for simple values (primitives).
///
/// can generate whole function or just the glue.
macro_rules! vs {
    (fn $visit:ident $obj:ident ($data_type:pat) $rust_type:ty) => {
        fn $visit<E>(mut self, v: $rust_type) -> Result<Self::Value, E>
        where
            E: serde::de::Error,
        {
            vs!(self (v) $obj ($data_type))
        }
    };
    ($this:ident $v:tt $obj:ident ($data_type:pat)) => {
        {
            let pat = $this.interp.pat;
            let mut obj = None;
            if $this.collecting {
                obj = Some(SerdeObject::$obj$v);
            }
            let mut packs = Vec::new();
            let result = {
                $this.frames_mut().iter_active_mut().try_for_each(|frame| {
                    let ty = frame.get_type();
                    match ty {
                        | Some(($data_type, _))
                        | Some((Type::Any, _))
                        | Some((Type::IgnoredAny, _))
                        | None
                        => {},
                        Some((_, false)) => {
                            frame.matches = false;
                            return Ok(());
                        },
                        Some((_, true)) => {
                            return Err(MatchError::ValidationError)
                        },
                    }
                    let mut pack = Pack::default();
                    if let Some(name) = frame.get_name(pat) {
                        let mut map = IndexMap::new();
                        map.insert(name, (Pack::default(), SerdeObject::$obj$v));
                        pack.subpacks.push(map);
                    }
                    packs.push(pack);
                    Ok(())
                })
            };
            match result {
                Err(e) => {
                    $this.interp.error.insert(e);
                    return Err(serde::de::Error::custom("type mismatch"));
                },
                _ => (),
            }
            Ok((packs, obj))
        }
    };
}

impl<'pat, 'state, 'de, O> serde::de::Visitor<'de>
for &mut Packer<'pat, 'state, O>
where
    O: Serialize,
{
    type Value = (Vec<Pack<'pat, 'de>>, Option<SerdeObject<'de>>);
    fn expecting(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "unsure")
    }

    vs!(fn visit_bool Bool (Type::Bool) bool);
    vs!(fn visit_i8 I8 (Type::I8) i8);
    vs!(fn visit_i16 I16 (Type::I16) i16);
    vs!(fn visit_i32 I32 (Type::I32) i32);
    vs!(fn visit_i64 I64 (Type::I64) i64);
    vs!(fn visit_i128 I128 (Type::I128) i128);
    vs!(fn visit_u8 U8 (Type::U8) u8);
    vs!(fn visit_u16 U16 (Type::U16) u16);
    vs!(fn visit_u32 U32 (Type::U32) u32);
    vs!(fn visit_u64 U64 (Type::U64) u64);
    vs!(fn visit_u128 U128 (Type::U128) u128);
    vs!(fn visit_f32 F32 (Type::F32) f32);
    vs!(fn visit_f64 F64 (Type::F64) f64);
    vs!(fn visit_char Char (Type::Char) char);

    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        let pat = self.interp.pat;
        let mut obj = None;
        if self.collecting {
            obj = Some(SerdeObject::Str(Cow::Owned(v.into())));
        }
        let mut packs = Vec::new();
        let result = {
            self.frames_mut().iter_active_mut().try_for_each(|frame| {
                let ty = frame.get_type();
                match ty {
                    | Some((Type::String, _))
                    | Some((Type::Str, _))
                    | Some((Type::Any, _))
                    | Some((Type::IgnoredAny, _))
                    | None
                    => {},
                    Some((_, false)) => {
                        frame.matches = false;
                        return Ok(());
                    },
                    Some((_, true)) => {
                        return Err(MatchError::ValidationError)
                    },
                }
                match frame.op() {
                    PatternElement::Value { value: Some(value), .. } => {
                        match value {
                            | Value::String { index, skippable }
                            if pat.strings[index] != v => {
                                if skippable {
                                    frame.matches = false;
                                    return Ok(());
                                } else {
                                    return Err(MatchError::ValidationError);
                                }
                            },
                            | Value::Regex { index, skippable }
                            if !pat.regices[index].is_match(v) => {
                                if skippable {
                                    frame.matches = false;
                                    return Ok(());
                                } else {
                                    return Err(MatchError::ValidationError);
                                }
                            },
                            | Value::Type { .. }
                            | Value::Regex { .. }
                            | Value::String { .. }
                            => {}, // ok
                        }
                    },
                    PatternElement::Value { value: None, .. } => {},
                    _ => unreachable!(),
                }
                let mut pack = Pack::default();
                if let Some(name) = frame.get_name(pat) {
                    let mut map = IndexMap::new();
                    map.insert(
                        name,
                        (
                            Pack::default(),
                            SerdeObject::Str(Cow::Owned(v.into())),
                        ),
                    );
                    pack.subpacks.push(map);
                }
                packs.push(pack);
                Ok(())
            })
        };
        match result {
            Err(e) => {
                self.interp.error.insert(e);
                return Err(E::custom("type/value mismatch"));
            },
            _ => (),
        }
        Ok((packs, obj))
    }
    fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        let pat = self.interp.pat;
        let mut obj = None;
        if self.collecting {
            obj = Some(SerdeObject::Str(Cow::Borrowed(v)));
        }
        let mut packs = Vec::new();
        let result = {
            self.frames_mut().iter_active_mut().try_for_each(|frame| {
                let ty = frame.get_type();
                match ty {
                    | Some((Type::String, _))
                    | Some((Type::Str, _))
                    | Some((Type::Any, _))
                    | Some((Type::IgnoredAny, _))
                    | None
                    => {},
                    Some((_, false)) => {
                        frame.matches = false;
                        return Ok(());
                    },
                    Some((_, true)) => {
                        return Err(MatchError::ValidationError)
                    },
                }
                match frame.op() {
                    PatternElement::Value { value: Some(value), .. } => {
                        match value {
                            | Value::String { index, skippable }
                            if pat.strings[index] != v => {
                                if skippable {
                                    frame.matches = false;
                                    return Ok(());
                                } else {
                                    return Err(MatchError::ValidationError);
                                }
                            },
                            | Value::Regex { index, skippable }
                            if !pat.regices[index].is_match(v) => {
                                if skippable {
                                    frame.matches = false;
                                    return Ok(());
                                } else {
                                    return Err(MatchError::ValidationError);
                                }
                            },
                            | Value::Type { .. }
                            | Value::Regex { .. }
                            | Value::String { .. }
                            => {}, // ok
                        }
                    },
                    PatternElement::Value { value: None, .. } => {},
                    _ => unreachable!(),
                }
                let mut pack = Pack::default();
                if let Some(name) = frame.get_name(pat) {
                    let mut map = IndexMap::new();
                    map.insert(
                        name,
                        (
                            Pack::default(),
                            SerdeObject::Str(Cow::Borrowed(v)),
                        ),
                    );
                    pack.subpacks.push(map);
                }
                packs.push(pack);
                Ok(())
            })
        };
        match result {
            Err(e) => {
                self.interp.error.insert(e);
                return Err(E::custom("type/value mismatch"));
            },
            _ => (),
        }
        Ok((packs, obj))
    }
    fn visit_string<E>(self, v: String) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        // TODO try to avoid cloning
        self.visit_str(&*v)
    }
    fn visit_bytes<E>(self, v: &[u8]) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        vs!(self (Cow::Owned(v.to_owned())) Bytes (Type::Bytes | Type::ByteBuf))
    }
    fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        vs!(self (Cow::Borrowed(v)) Bytes (Type::Bytes | Type::ByteBuf))
    }
    fn visit_byte_buf<E>(self, v: Vec<u8>) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        // TODO try to avoid cloning
        self.visit_bytes(&*v)
    }
    fn visit_none<E>(self) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        vs!(self {} None (Type::Option))
    }
    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
    where
        D: serde::de::Deserializer<'de>,
    {
        todo!()
    }
    fn visit_unit<E>(self) -> Result<Self::Value, E>
    where
        E: serde::de::Error,
    {
        vs!(self {} Unit (Type::Unit))
    }
    fn visit_newtype_struct<D>(
        self,
        deserializer: D
    ) -> Result<Self::Value, D::Error>
    where
        D: serde::de::Deserializer<'de>,
    {
        todo!()
    }
    fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
    where
        A: serde::de::SeqAccess<'de>,
    {
        let mut obj = None;
        if self.collecting {
            obj = Some(SerdeObject::Seq(Vec::new()));
        }
        todo!()
    }
    fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
    where
        A: serde::de::MapAccess<'de>,
    {
        let old_collecting = self.collecting;
        let pat = self.interp.pat;
        let mut collecting = old_collecting;
        let typeck = self.frames_mut().iter_active_mut().try_for_each(|frame| {
            let ty = frame.get_type();
            match ty {
                | Some((Type::Map, _))
                | Some((Type::Any, _))
                | Some((Type::IgnoredAny, _))
                | None
                => {},
                Some((_, false)) => {
                    frame.matches = false;
                    return Ok(());
                },
                Some((_, true)) => {
                    return Err(MatchError::ValidationError)
                },
            }
            if frame.get_name(pat).is_some() {
                collecting = true;
            }
            Ok(())
        });
        match typeck {
            Err(e) => {
                self.interp.error.insert(e);
                return Err(A::Error::custom("type mismatch"));
            },
            _ => (),
        }
        if let Err(e) = self.step_in() { return Err(e); }
        self.collecting = collecting;
        let mut subframes = Vec::new();
        self.frames().iter_active().for_each(|frame| {
            if let Some(key_subtree) = frame.key_subtree() {
                subframes.push(Frame {
                    ops: &pat.protos[key_subtree],
                    iar: None,
                    overstep: 0,
                    matches: true,
                });
            }
        });
        let mut obj_inner = Vec::new();
        let mut output_packs = Vec::new();
        while let Some(packed_key) = {
            let subinterp = Interpreter {
                pat: pat,
                frames: &mut subframes,
                error: self.interp.error,
            };
            let mut subpacker = Packer {
                interp: subinterp,
                collecting: self.collecting,
                call_limit: self.call_limit,
            };
            map.next_key_seed(&mut subpacker)?
        } {
            self.frames_mut().iter_active_mut().filter(|frame| {
                frame.key_subtree().is_some()
            }).zip(&mut subframes).for_each(|(frame, subframe)| {
                frame.matches = subframe.matches;
                // reset subframe for next iteration
                // NOTE wait to reset subframe.matches when merging packs!!!
                subframe.iar = None;
            });
            self.frames_mut().iter_active_mut().for_each(|frame| {
                // mark every non-subtree key as matching.
                if frame.key_subtree().is_none() {
                    frame.matches = true;
                }
            });
            let packed_value = map.next_value_seed(&mut *self)?;
            if self.collecting {
                obj_inner.push(
                    (packed_key.1.unwrap(), packed_value.1.unwrap()),
                );
            }
            let mut key_packs_per_frame = packed_key.0.into_iter();
            let mut value_packs_per_frame = packed_value.0;
            // whatever is active in self.frames(), if matches, has a pack
            // whatever is in subframes, if matches, has a pack
            // count(active self.frames() with subtree which match) is always
            // smaller than count(subframes which match) because the former
            // gets updated by next_value_seed
            // count(active self.frames() with subtree) == count(subframes)
            // tl;dr: need to figure out which packs produced by subframes line
            // up with which packs produced by self, discarding extra subframes
            // (where the corresponding self frame doesn't match) and accepting
            // extra packs produced by self.
            // NOTE: key_packs_per_frame ~ subframes
            // value_packs_per_frame ~ self
            // keys come first tho (key.merge_from(value))
            let mut iter_subframes = subframes.iter_mut();
            // related to value_packs_per_frame
            let mut pack_index = 0;
            for frame in self.frames().iter_active() {
                // check if this frame has an associated subframe
                let subframe = if frame.key_subtree().is_some() {
                    // if there are more frames with associated subframes
                    // than there are subframes, panic
                    Some(iter_subframes.next().unwrap())
                } else {
                    None
                };
                if frame.matches && subframe.is_some() {
                    // this already implies subframe.matches
                    let key_pack = key_packs_per_frame.next().unwrap();
                    let value_pack = &mut value_packs_per_frame[pack_index];
                    key_pack.merge_into(value_pack);
                    pack_index += 1;
                } else if frame.matches {
                    // value matches but there's no subframe, carry on
                    pack_index += 1;
                } else if !frame.matches && subframe.is_some() {
                    // frame didn't match but there was a subframe
                    let subframe = subframe.unwrap();
                    if subframe.matches {
                        // subframe matched, remove key pack
                        let _ = key_packs_per_frame.next().unwrap();
                    } else {
                        // neither matched, no relevant packs
                        // do reset subframe for next_key_seed tho!
                        subframe.matches = true;
                    }
                } else {
                    // no relevant packs
                }
            }
            if output_packs.is_empty() {
                output_packs = value_packs_per_frame;
            } else {
                for (left, right) in output_packs.iter_mut().zip(
                    value_packs_per_frame,
                ) {
                    left.subpacks.extend(right.subpacks)
                }
            }
        }
        let obj = SerdeObject::Map(obj_inner);
        let mut final_packs = self.step_out(output_packs)?;
        let mut iter_final_packs = 0..;
        self.frames_mut().iter_active_mut().for_each(|frame| {
            let ty = frame.get_type();
            match ty {
                | Some((Type::Map, _))
                | Some((Type::Any, _))
                | Some((Type::IgnoredAny, _))
                | None
                => {
                    let matched = std::mem::replace(&mut frame.matches, true);
                    if !matched {
                        final_packs.insert(
                            iter_final_packs.start,
                            Pack::default(),
                        );
                    }
                },
                _ => return,
            }
            let pack = &mut final_packs[iter_final_packs.next().unwrap()];
            if let Some(name) = frame.get_name(pat) {
                // we can assume collecting == true
                let old_pack = std::mem::take(pack);
                let mut map = IndexMap::new();
                map.insert(name, (old_pack, obj.clone()));
                pack.subpacks.push(map);
            }
        });
        self.collecting = old_collecting;
        Ok((final_packs, collecting.then(|| obj)))
    }
    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
    where
        A: serde::de::EnumAccess<'de>,
    {
        let mut obj = None;
        if self.collecting {
            obj = Some(SerdeObject::Enum {
                variant: todo!(),
                data: todo!(),
            });
        }
        todo!()
    }
}

/// A `Deserializer` for Datafu output.
///
/// This converts from Datafu's internal representation (a "pack") into the
/// desired output type.
pub struct Unpacker<'pat, 'de> {
    pack: Pack<'pat, 'de>,
    call_limit: usize,
}

impl<'pat, 'de> Unpacker<'pat, 'de> {
    /// Unpacks a Datafu "pack".
    pub fn new(pack: Pack<'pat, 'de>, call_limit: usize) -> Self {
        Self {
            pack, call_limit,
        }
    }
}

impl<'pat, 'de> serde::Deserializer<'de> for Unpacker<'pat, 'de> {
    // TODO datafu errors
    type Error = serde::de::value::Error;
    fn deserialize_any<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_bool<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_i8<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_i16<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_i32<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_i64<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_u8<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_u16<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_u32<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_u64<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_f32<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_f64<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_char<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_str<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_string<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_bytes<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_byte_buf<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_option<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_unit<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_unit_struct<V>(self, _: &'static str, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_newtype_struct<V>(self, _: &'static str, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_seq<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_tuple<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_tuple_struct<V>(self, _: &'static str, _: usize, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_map<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_struct<V>(
        self,
        _: &'static str,
        fields: &'static [&'static str],
        visitor: V,
    ) -> Result<V::Value, Self::Error>
    where
        V: serde::de::Visitor<'de>,
    {
        todo!()
    }
    fn deserialize_enum<V>(self, _: &'static str, _: &'static [&'static str], _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_identifier<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
    fn deserialize_ignored_any<V>(self, _: V) -> Result<V::Value, Self::Error> where V: serde::de::Visitor<'de> { todo!() }
}

/// Deserializes a SerdeObject
pub(crate) struct SerdeObjectDeserializer<'de, E> {
    pub(crate) obj: SerdeObject<'de>,
    pub(crate) value: Option<SerdeObject<'de>>,
    pub(crate) _e: PhantomData<fn() -> E>,
}

impl<'de, E> serde::de::Deserializer<'de> for SerdeObjectDeserializer<'de, E>
where
    E: serde::de::Error,
{
    type Error = E;
    fn deserialize_any<V>(self, v: V) -> Result<V::Value, Self::Error>
    where
        V: serde::de::Visitor<'de>,
    {
        match self.obj {
            SerdeObject::Bool(x) => v.visit_bool(x),
            SerdeObject::I8(x) => v.visit_i8(x),
            SerdeObject::I16(x) => v.visit_i16(x),
            SerdeObject::I32(x) => v.visit_i32(x),
            SerdeObject::I64(x) => v.visit_i64(x),
            SerdeObject::I128(x) => v.visit_i128(x),
            SerdeObject::U8(x) => v.visit_u8(x),
            SerdeObject::U16(x) => v.visit_u16(x),
            SerdeObject::U32(x) => v.visit_u32(x),
            SerdeObject::U64(x) => v.visit_u64(x),
            SerdeObject::U128(x) => v.visit_u128(x),
            SerdeObject::F32(x) => v.visit_f32(x),
            SerdeObject::F64(x) => v.visit_f64(x),
            SerdeObject::Char(x) => v.visit_char(x),
            SerdeObject::Str(Cow::Owned(x)) => v.visit_string(x),
            SerdeObject::Str(Cow::Borrowed(x)) => v.visit_borrowed_str(x),
            SerdeObject::Bytes(Cow::Owned(x)) => v.visit_byte_buf(x),
            SerdeObject::Bytes(Cow::Borrowed(x)) => v.visit_borrowed_bytes(x),
            SerdeObject::Some(x) => v.visit_some(x.into_deserializer()),
            SerdeObject::None => v.visit_none(),
            SerdeObject::Unit => v.visit_unit(),
            SerdeObject::Seq(x) => todo!(),
            SerdeObject::Map(x) => todo!(),
            SerdeObject::NewtypeStruct(x) => {
                v.visit_newtype_struct(x.into_deserializer())
            },
            SerdeObject::Enum { variant, data } => todo!(),
        }
    }
    fn deserialize_ignored_any<V>(self, v: V) -> Result<V::Value, Self::Error>
    where
        V: serde::de::Visitor<'de>,
    {
        drop(self);
        v.visit_unit()
    }
    serde::forward_to_deserialize_any! {
        bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
        bytes byte_buf option unit unit_struct newtype_struct seq tuple
        tuple_struct map struct enum identifier
    }
}

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

    use crate::vm::MAX_CALLS;
    use crate::vm::Interpreter;
    use crate::vm::Type;
    use crate::vm::Value;
    use crate::vm::PatternElement;
    use crate::vm::SerdeObject;
    use crate::vm::Frame;

    use postcard::Deserializer as PostcardDeserializer;
    use serde::de::DeserializeSeed as _;
    use serde_json::Deserializer as JsonDeserializer;

    use crate::errors::MatchError;

    #[test]
    #[should_panic]
    fn test_broken() {
        // broken pattern, should never be emitted by parser. make sure it's
        // not accepted.
        let consts = PatternConstants::<()>::default();
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let _ = Packer::new(interp, MAX_CALLS);
    }

    #[test]
    fn test_empty_create() {
        // test creating the parser with an empty pattern.
        let mut consts = PatternConstants::<()>::default();
        consts.protos.push(Vec::new());
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let _ = Packer::new(interp, MAX_CALLS);
    }

    #[test]
    fn test_empty_match() {
        // test matching something with an empty pattern.
        let mut consts = PatternConstants::<()>::default();
        consts.protos.push(Vec::new());
        let mut der = JsonDeserializer::from_str("{}");
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let pack = Packer::new(interp, MAX_CALLS).deserialize(&mut der).unwrap();
    }

    #[test]
    fn test_simple_match() {
        // test matching a simple value
        let mut consts = PatternConstants::<()>::default();
        consts.strings.push("hello".into());
        consts.protos.push(vec![
            PatternElement::Value {
                name: Some(0),
                value: Some(Value::Type {
                    ty: Type::U64,
                    skippable: false,
                }),
            },
        ]);
        let mut der = JsonDeserializer::from_str("3");
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let packed = Packer::new(interp, MAX_CALLS).deserialize(&mut der);
        let (packs, obj) = packed.unwrap();
        assert!(obj.is_none());
        assert_eq!(packs[0].subpacks[0]["hello"].1, SerdeObject::U64(3));
    }

    #[test]
    fn test_simple_error() {
        // test a value that doesn't match (serde_json error)
        let mut consts = PatternConstants::<()>::default();
        consts.strings.push("hello".into());
        consts.protos.push(vec![
            PatternElement::Value {
                name: Some(0),
                value: Some(Value::Type {
                    ty: Type::U64,
                    skippable: false,
                }),
            },
        ]);
        let mut der = JsonDeserializer::from_str("\"hello\"");
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let packed = Packer::new(interp, MAX_CALLS).deserialize(&mut der);
        // error produced by serde_json
        assert!(packed.is_err());
    }

    #[test]
    fn test_basic_multiframe() {
        // test multiple frames (matching and non-matching)
        let mut consts = PatternConstants::<()>::default();
        consts.strings.push("a".into());
        consts.strings.push("b".into());
        consts.protos.push(vec![
            PatternElement::Value {
                name: Some(0),
                value: Some(Value::Type {
                    ty: Type::U64,
                    skippable: true,
                }),
            },
        ]);
        consts.protos.push(vec![
            PatternElement::Value {
                name: Some(1),
                value: Some(Value::Type {
                    ty: Type::Bool,
                    skippable: true,
                }),
            },
        ]);
        let mut der = JsonDeserializer::from_str(r#"10"#);
        let mut err = Default::default();
        let mut frames: Vec<_> = Default::default();
        frames.push(Frame {
            ops: &consts.protos[0],
            iar: None,
            matches: true,
            overstep: 0,
        });
        frames.push(Frame {
            ops: &consts.protos[1],
            iar: None,
            matches: true,
            overstep: 0,
        });
        let interp = Interpreter {
            pat: &consts,
            error: &mut err,
            frames: &mut frames,
        };
        let packed = Packer::new(interp, MAX_CALLS).deserialize(&mut der);
        let (packs, obj) = packed.unwrap();
        assert!(obj.is_none());
        assert_eq!(
            packs[0].subpacks[0]["a"].1,
            SerdeObject::U64(10),
        );
        assert_eq!(packs.len(), 1);
        assert!(frames[0].matches);
        assert!(!frames[1].matches);
    }

    #[test]
    fn test_map() {
        // test visit_map
        let mut consts = PatternConstants::<()>::default();
        consts.strings.push("key".into());
        consts.strings.push("value".into());
        consts.protos.push(vec![
            PatternElement::Value {
                name: Some(0),
                value: None,
            },
        ]);
        consts.protos.push(vec![
            PatternElement::Value {
                name: None,
                value: Some(Value::Type {
                    ty: Type::Map,
                    skippable: false,
                }),
            },
            PatternElement::Tag {
                key_subtree: Some(0),
            },
            PatternElement::Value {
                name: Some(1),
                value: Some(Value::Type {
                    ty: Type::U64,
                    skippable: false,
                }),
            },
        ]);
        let mut der = JsonDeserializer::from_str(r#"{"hello": 0, "world": 1}"#);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(&consts, &mut err, &mut frames);
        let packed = Packer::new(interp, MAX_CALLS).deserialize(&mut der);
        let (packs, obj) = packed.unwrap();
        assert!(obj.is_none());
        assert_eq!(
            packs[0].subpacks[0]["key"].1,
            SerdeObject::Str("hello".into()),
        );
        assert_eq!(
            packs[0].subpacks[0]["value"].1,
            SerdeObject::U64(0),
        );
        assert_eq!(
            packs[0].subpacks[1]["key"].1,
            SerdeObject::Str("world".into()),
        );
        assert_eq!(
            packs[0].subpacks[1]["value"].1,
            SerdeObject::U64(1),
        );
    }

    #[test]
    fn test_parser_empty() {
        // use a parsed empty pattern to test Packer
        let consts = crate::parser::parse::<&'static str, &'static str, ()>(
            "",
            None,
            None,
        ).unwrap();
        let mut der = JsonDeserializer::from_str(r#"{"hello": 0, "world": 1}"#);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(
            &consts,
            &mut err,
            &mut frames,
            //&mut output,
        );
        let (mut packs, obj) = Packer::new(
            interp,
            MAX_CALLS,
        ).deserialize(&mut der).unwrap();
        assert!(obj.is_none());
        assert_eq!(packs.len(), 1);
        let pack = packs.pop().unwrap();
        assert!(pack.subpacks.is_empty());
    }

    #[test]
    fn test_parser_basic() {
        // use a basic parsed pattern to test Packer
        let consts = crate::parser::parse::<&'static str, &'static str, ()>(
            ":map->[name:str]value:str",
            None,
            None,
        ).unwrap();
        let data = &[
            0x02, // map length (2)
            0x04, // string length (4)
            0x6E, 0x61, 0x6D, 0x65, // b'name'
            0x01, // string length (1)
            0x61, // b'a'
            0x05, // string length (5)
            0x76, 0x61, 0x6C, 0x75, 0x65, // b'value'
            0x01, // string length (1)
            0x62, // b'b'
        ];
        let mut der = PostcardDeserializer::from_bytes(data);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(
            &consts,
            &mut err,
            &mut frames,
            //&mut output,
        );
        let result = Packer::new(
            interp,
            MAX_CALLS,
        ).deserialize(&mut der);
        let (mut packs, obj) = result.unwrap();
        assert!(obj.is_none());
        assert_eq!(packs.len(), 1);
        let pack = packs.pop().unwrap();
        assert_eq!(pack.subpacks.len(), 2);
    }

    #[test]
    fn test_parser_basic_subtree() {
        // use a basic parsed pattern with a subtree to test Packer
        let consts = crate::parser::parse::<&'static str, &'static str, ()>(
            ":map(->[name:str]value:str)",
            None,
            None,
        ).unwrap();
        let data = &[
            0x02, // map length (2)
            0x04, // string length (4)
            0x6E, 0x61, 0x6D, 0x65, // b'name'
            0x01, // string length (1)
            0x61, // b'a'
            0x05, // string length (5)
            0x76, 0x61, 0x6C, 0x75, 0x65, // b'value'
            0x01, // string length (1)
            0x62, // b'b'
        ];
        let mut der = PostcardDeserializer::from_bytes(data);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(
            &consts,
            &mut err,
            &mut frames,
            //&mut output,
        );
        let result = Packer::new(
            interp,
            MAX_CALLS,
        ).deserialize(&mut der);
        let (mut packs, obj) = result.unwrap();
        assert!(obj.is_none());
        assert_eq!(packs.len(), 1);
        let pack = packs.pop().unwrap();
        assert_eq!(pack.subpacks.len(), 2);
    }

    #[test]
    fn test_parser_subtrees() {
        // use a parsed pattern with subtrees to test Packer
        // also test a non-self-describing format (postcard)
        let consts = crate::parser::parse::<&'static str, &'static str, ()>(
            ":map(->['name'?]name:str)?(->['value'?]value:u32)?(->[:str]:?ignored_any)",
            None,
            None,
        ).unwrap();
        let data = &[
            0x02, // map length (2)
            0x04, // string length (4)
            0x6E, 0x61, 0x6D, 0x65, // b'name'
            0x01, // string length (1)
            0x61, // b'a'
            0x05, // string length (5)
            0x76, 0x61, 0x6C, 0x75, 0x65, // b'value'
            0x01, // 1
        ];
        let mut der = PostcardDeserializer::from_bytes(data);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(
            &consts,
            &mut err,
            &mut frames,
            //&mut output,
        );
        let result = Packer::new(
            interp,
            MAX_CALLS,
        ).deserialize(&mut der);
        let (mut packs, obj) = result.unwrap();
        assert!(obj.is_none());
        assert_eq!(packs.len(), 1);
        let pack = packs.pop().unwrap();
        assert_eq!(pack.subpacks.len(), 2);
        assert_eq!(pack.subpacks[0]["name"].1, SerdeObject::Str(From::from("a")));
        assert_eq!(pack.subpacks[1]["value"].1, SerdeObject::U32(1));
    }

    #[test]
    fn test_parser_subtrees_strict() {
        // use a parsed pattern with subtrees to test Packer
        // also test a non-self-describing format (postcard)
        // also require at least one subtree to match on every iteration.
        // also this checks for validation error
        let consts = crate::parser::parse::<&'static str, &'static str, ()>(
            ":map((->['name'?]name)?(->['value'?]value)?)(->[:str]:u32)",
            None,
            None,
        ).unwrap();
        let data = &[
            0x03, // map length (3)
            0x04, // string length (4)
            0x6E, 0x61, 0x6D, 0x65, // b'name'
            0x01, // 1
            0x05, // string length (5)
            0x76, 0x61, 0x6C, 0x75, 0x65, // b'value'
            0x01, // 1
            0x05, // string length (5)
            0x76, 0x65, 0x6C, 0x75, 0x65, // b'velue'
            0x01, // 1
        ];
        let mut der = PostcardDeserializer::from_bytes(data);
        let mut err = Default::default();
        let mut frames = Default::default();
        let interp = Interpreter::new(
            &consts,
            &mut err,
            &mut frames,
            //&mut output,
        );
        let result = Packer::new(
            interp,
            MAX_CALLS,
        ).deserialize(&mut der);
        assert!(matches!(err, Some(MatchError::ValidationError)));
        assert!(result.is_err());
    }
}