//! The tagged object stream inside `doc.dat`: a self-describing tree of objects whose fields carry
//! a type code and a four-character tag. Parsing needs no schema; unknown fields are kept, so the
//! document mapping can name what it could not import.

use crate::Error;
use crate::container::Cursor;

/// A four-character tag in its readable order (`Desc`, `Chld`).
#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct Tag(pub u32);

impl Tag {
    pub const fn of(s: &[u8; 4]) -> Self {
        Self(u32::from_be_bytes(*s))
    }
}

impl std::fmt::Debug for Tag {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", String::from_utf8_lossy(&self.0.to_be_bytes()))
    }
}

impl std::fmt::Display for Tag {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        std::fmt::Debug::fmt(self, f)
    }
}

/// Index of an object in [`Stream::objects`].
pub type ObjId = usize;

#[derive(Debug, Clone, PartialEq)]
pub enum Value {
    Int(i64),
    UInt(u64),
    Float(f64),
    Bool(bool),
    Enum {
        id: u16,
        version: u16,
    },
    Str(String),
    /// Fixed-length vectors of i32/f32/f64 (points, rectangles, transforms).
    Ints(Vec<i64>),
    Floats(Vec<f64>),
    /// Fixed-size record (curve nodes, ids) or fixed-size struct (colour components).
    Bytes(Vec<u8>),
    Blob(Vec<u8>),
    /// Name of another archive entry (raster tiles, embedded files).
    Entry(String),
    Flags(u64),
    /// `None` is a null object.
    Obj(Option<ObjId>),
    Array(Vec<Value>),
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Kind {
    Root,
    /// Defined once and referenced by id elsewhere.
    Shared,
    Inline,
    /// Untagged fields identified by position (curve data).
    Positional,
}

#[derive(Debug, Clone, PartialEq)]
pub struct Object {
    pub class: Tag,
    pub kind: Kind,
    /// Class chain, most derived first (`ShpN`, `VNod`, `Node`).
    pub chain: Vec<Tag>,
    /// Fields in stream order; positional fields have tag 0.
    pub fields: Vec<(Tag, Value)>,
    /// For objects with many fields: each tag with the positions of its first and last field,
    /// sorted by tag, so a lookup isn't a scan of every field (a shared object is looked up
    /// again on every visit). Built by [`Object::index`]; empty for small objects.
    index: Vec<(Tag, u32, u32)>,
}

/// Objects with more fields than this get an [`Object::index`]; fewer are scanned.
const INDEXED_FIELDS: usize = 16;

impl Object {
    pub fn new(class: Tag, kind: Kind, chain: Vec<Tag>, fields: Vec<(Tag, Value)>) -> Self {
        let mut o = Self { class, kind, chain, fields, index: Vec::new() };
        o.index();
        o
    }

    /// (Re)build the tag index after `fields` changed.
    fn index(&mut self) {
        self.index.clear();
        if self.fields.len() <= INDEXED_FIELDS {
            return;
        }
        let mut index: Vec<(Tag, u32, u32)> = Vec::new();
        if index.try_reserve_exact(self.fields.len()).is_err() {
            return;
        }
        // Fields beyond u32 positions are impossible: a stream holds at most MAX_VALUES values.
        index.extend(self.fields.iter().enumerate().filter_map(|(i, (t, _))| u32::try_from(i).ok().map(|i| (*t, i, i))));
        // Stable: equal tags stay in stream order, so the run of a tag starts at its first field.
        index.sort_by_key(|(t, _, _)| *t);
        index.dedup_by(|later, first| {
            if later.0 == first.0 {
                first.2 = later.2;
                true
            } else {
                false
            }
        });
        index.shrink_to_fit();
        self.index = index;
    }

    fn position(&self, tag: Tag, last: bool) -> Option<usize> {
        if self.index.is_empty() {
            let mut fields = self.fields.iter().enumerate();
            let hit = if last { fields.rev().find(|(_, (t, _))| *t == tag) } else { fields.find(|(_, (t, _))| *t == tag) };
            return hit.map(|(i, _)| i);
        }
        let at = self.index.binary_search_by_key(&tag, |(t, _, _)| *t).ok()?;
        let (_, first, end) = self.index.get(at)?;
        usize::try_from(if last { *end } else { *first }).ok()
    }

    /// The first field tagged `tag`.
    pub fn get(&self, tag: Tag) -> Option<&Value> {
        self.fields.get(self.position(tag, false)?).map(|(_, v)| v)
    }

    /// The last field tagged `tag` (a shape changed from one kind to another keeps a stale first one).
    pub fn get_last(&self, tag: Tag) -> Option<&Value> {
        self.fields.get(self.position(tag, true)?).map(|(_, v)| v)
    }
}

#[derive(Debug)]
pub struct Stream {
    pub root: ObjId,
    pub objects: Vec<Object>,
}

/// Deepest object nesting accepted; real documents stay well below (each layer level adds three).
pub const MAX_DEPTH: usize = 384;

const MAX_OBJECTS: usize = 8_000_000;

/// Most values (fields and array elements) one stream may decode. A value takes about 40 bytes
/// in memory however few it took in the file (a bool array packs eight in a byte), so the size
/// of the input alone doesn't bound what parsing allocates: a small compressed entry could
/// otherwise ask for tens of gigabytes. Real documents stay far below.
pub const MAX_VALUES: usize = 1 << 24;

struct Parser<'a> {
    c: Cursor<'a>,
    objects: Vec<Object>,
    /// Shared ids, defined at first use and referenced backwards.
    shared: std::collections::HashMap<u32, ObjId>,
    depth: usize,
    /// Values decoded so far, against [`MAX_VALUES`].
    values: usize,
}

/// Parse a stream (`00 FF 4B 53` header). Bytes after the root's terminator are ignored.
pub fn parse(bytes: &[u8]) -> Result<Stream, Error> {
    let mut c = Cursor::new(bytes, 0);
    if c.take(4)? != b"\x00\xffKS" {
        return Err(Error::Malformed("document stream signature"));
    }
    let version = c.u16()?;
    if version > 2 {
        return Err(Error::Unsupported("document stream version"));
    }
    let class = Tag(c.u32()?);
    c.u16()?;
    if version == 2 {
        c.u32()?;
    }
    let mut p = Parser { c, objects: Vec::new(), shared: std::collections::HashMap::new(), depth: 0, values: 0 };
    let root = p.push(Object::new(class, Kind::Root, vec![class], Vec::new()))?;
    let fields = p.fields(true, class)?;
    if let Some(o) = p.objects.get_mut(root) {
        o.fields = fields;
        o.index();
    }
    Ok(Stream { root, objects: p.objects })
}

impl Parser<'_> {
    fn push(&mut self, o: Object) -> Result<ObjId, Error> {
        if self.objects.len() >= MAX_OBJECTS {
            return Err(Error::Limit("too many objects"));
        }
        self.objects.push(o);
        Ok(self.objects.len() - 1)
    }

    /// Counts `n` more values against [`MAX_VALUES`], before anything is allocated for them.
    fn spend(&mut self, n: usize) -> Result<(), Error> {
        self.values = self.values.checked_add(n).filter(|&v| v <= MAX_VALUES).ok_or(Error::Limit("too many values in the document stream"))?;
        Ok(())
    }

    /// An empty array with room for `n` values, paid for first; an allocation that still fails
    /// is an error, not an abort.
    fn vec<T>(&mut self, n: usize) -> Result<Vec<T>, Error> {
        self.spend(n)?;
        let mut v = Vec::new();
        v.try_reserve_exact(n).map_err(|_| Error::Limit("not enough memory for the document stream"))?;
        Ok(v)
    }

    /// Element count of an array: each element takes at least `min` bytes, so a count the
    /// remaining input cannot hold is rejected before anything is allocated.
    fn count(&mut self, min: usize) -> Result<usize, Error> {
        let n = usize::try_from(self.c.u32()?).map_err(|_| Error::Malformed("array length"))?;
        if n.checked_mul(min.max(1)).is_none_or(|bytes| bytes > self.c.remaining()) {
            return Err(Error::Malformed("array longer than its data"));
        }
        Ok(n)
    }

    fn fields(&mut self, tagged: bool, class: Tag) -> Result<Vec<(Tag, Value)>, Error> {
        self.depth += 1;
        if self.depth > MAX_DEPTH {
            return Err(Error::Limit("objects nested too deeply"));
        }
        let mut out = Vec::new();
        loop {
            let t = self.c.u8()?;
            let code = t & 0x7F;
            if code == 0 {
                break;
            }
            self.spend(1)?;
            let tag = if tagged { Tag(self.c.u32()?) } else { Tag(0) };
            let v = if t & 0x80 != 0 { self.array(code, class)? } else { self.scalar(code, tagged, class)? };
            out.push((tag, v));
        }
        self.depth -= 1;
        Ok(out)
    }

    fn string(&mut self) -> Result<String, Error> {
        let n = usize::try_from(self.c.u32()?).map_err(|_| Error::Malformed("string length"))?;
        Ok(String::from_utf8_lossy(self.c.take(n)?).into_owned())
    }

    fn floats(&mut self, n: usize, wide: bool) -> Result<Value, Error> {
        let mut v = Vec::with_capacity(n);
        for _ in 0..n {
            v.push(if wide { f64::from_le_bytes(self.c.array()?) } else { f64::from(f32::from_le_bytes(self.c.array()?)) });
        }
        Ok(Value::Floats(v))
    }

    fn scalar(&mut self, code: u8, tagged: bool, class: Tag) -> Result<Value, Error> {
        let c = &mut self.c;
        Ok(match code {
            0x01 => Value::UInt(c.u8()?.into()),
            0x02 => Value::UInt(c.u16()?.into()),
            0x03 | 0x2f | 0x34 => Value::UInt(c.u32()?.into()),
            0x04 => Value::UInt(c.u64()?),
            0x05 => Value::Int(i8::from_le_bytes(c.array()?).into()),
            0x06 => Value::Int(i16::from_le_bytes(c.array()?).into()),
            0x07 => Value::Int(i32::from_le_bytes(c.array()?).into()),
            0x08 => Value::Int(i64::from_le_bytes(c.array()?)),
            0x09 => Value::Float(f32::from_le_bytes(c.array()?).into()),
            0x0a => Value::Float(f64::from_le_bytes(c.array()?)),
            0x15..=0x19 => {
                let n = usize::from(code - 0x13);
                let mut v = Vec::with_capacity(n);
                for _ in 0..n {
                    v.push(i32::from_le_bytes(c.array()?).into());
                }
                Value::Ints(v)
            }
            0x1f..=0x23 => return self.floats(usize::from(code - 0x1d), false),
            0x24..=0x28 => return self.floats(usize::from(code - 0x22), true),
            0x29 => Value::Bool(c.u8()? != 0),
            0x2a => Value::Enum { id: c.u16()?, version: c.u16()? },
            0x2b | 0x2e => Value::Str(self.string()?),
            0x2c => {
                let n = usize::from(c.u16()?);
                Value::Bytes(c.take(n)?.to_vec())
            }
            0x35..=0x74 => Value::Bytes(c.take(usize::from(code - 0x34))?.to_vec()),
            0x2d => {
                let n = usize::try_from(c.u32()?).map_err(|_| Error::Malformed("blob length"))?;
                Value::Blob(c.take(n)?.to_vec())
            }
            0x33 => {
                c.u32()?;
                let n = usize::try_from(c.u32()?).map_err(|_| Error::Malformed("entry name length"))?;
                Value::Entry(String::from_utf8_lossy(c.take(n)?).into_owned())
            }
            0x75 => {
                c.u16()?;
                let n = usize::from(c.u8()?);
                if n > 8 {
                    return Err(Error::Malformed("flags wider than 64 bits"));
                }
                let mut b = [0u8; 8];
                b.get_mut(..n).ok_or(Error::Malformed("flags"))?.copy_from_slice(c.take(n)?);
                Value::Flags(u64::from_le_bytes(b))
            }
            0x30 => {
                let fields = self.fields(false, class)?;
                let id = self.push(Object::new(class, Kind::Positional, Vec::new(), fields))?;
                Value::Obj(Some(id))
            }
            0x31 | 0x32 if !tagged => return Err(Error::Malformed("object without a field tag")),
            0x31 => self.shared()?,
            0x32 => self.inline(None)?,
            _ => return Err(Error::Unsupported("unknown field type in document stream")),
        })
    }

    fn array(&mut self, code: u8, class: Tag) -> Result<Value, Error> {
        let min = match code {
            0x01 | 0x05 | 0x29 => 1,
            0x02 | 0x06 => 2,
            0x03 | 0x07 | 0x09 | 0x2f | 0x34 => 4,
            0x04 | 0x08 | 0x0a => 8,
            0x15..=0x19 => 4 * usize::from(code - 0x13),
            0x1f..=0x23 => 4 * usize::from(code - 0x1d),
            0x24..=0x28 => 8 * usize::from(code - 0x22),
            0x35..=0x74 => usize::from(code - 0x34),
            _ => 1,
        };
        match code {
            0x29 => {
                let n = usize::try_from(self.c.u32()?).map_err(|_| Error::Malformed("array length"))?;
                let bits = self.c.take(n.div_ceil(8))?;
                let mut v = self.vec(n)?;
                v.extend((0..n).map(|i| Value::Bool(bits.get(i / 8).is_some_and(|b| b >> (i % 8) & 1 != 0))));
                Ok(Value::Array(v))
            }
            0x2a => {
                let n = self.count(2)?;
                let version = self.c.u16()?;
                let mut v = self.vec(n)?;
                for _ in 0..n {
                    v.push(Value::Enum { id: self.c.u16()?, version });
                }
                Ok(Value::Array(v))
            }
            0x2b | 0x2e => {
                self.c.u32()?;
                let n = self.count(4)?;
                let mut v = self.vec(n)?;
                for _ in 0..n {
                    v.push(Value::Str(self.string()?));
                }
                Ok(Value::Array(v))
            }
            0x2c => {
                let n = usize::try_from(self.c.u32()?).map_err(|_| Error::Malformed("array length"))?;
                let size = usize::from(self.c.u16()?);
                // Records of no bytes take nothing from the data, whatever their number.
                if n > 0 && size == 0 {
                    return Err(Error::Malformed("array of records without bytes"));
                }
                if n.checked_mul(size).is_none_or(|b| b > self.c.remaining()) {
                    return Err(Error::Malformed("array longer than its data"));
                }
                let mut v = self.vec(n)?;
                for _ in 0..n {
                    v.push(Value::Bytes(self.c.take(size)?.to_vec()));
                }
                Ok(Value::Array(v))
            }
            0x32 => {
                let n = self.count(1)?;
                let class = Tag(self.c.u32()?);
                self.c.u16()?;
                let mut v = self.vec(n)?;
                for _ in 0..n {
                    v.push(self.inline(Some(class))?);
                }
                Ok(Value::Array(v))
            }
            0x2d | 0x33 | 0x75 => Err(Error::Malformed("array of an unarrayable type")),
            _ => {
                let n = self.count(min)?;
                let mut v = self.vec(n)?;
                for _ in 0..n {
                    v.push(self.scalar(code, true, class)?);
                }
                Ok(Value::Array(v))
            }
        }
    }

    /// Inline object; `class` is given once for a whole array of them.
    fn inline(&mut self, class: Option<Tag>) -> Result<Value, Error> {
        match self.c.u8()? {
            0 => return Ok(Value::Obj(None)),
            1 => {}
            _ => return Err(Error::Malformed("object marker")),
        }
        let class = match class {
            Some(c) => c,
            None => {
                let c = Tag(self.c.u32()?);
                self.c.u16()?;
                c
            }
        };
        let fields = self.fields(true, class)?;
        let id = self.push(Object::new(class, Kind::Inline, vec![class], fields))?;
        Ok(Value::Obj(Some(id)))
    }

    fn shared(&mut self) -> Result<Value, Error> {
        let flag = self.c.u8()?;
        if flag == 0 {
            return Ok(Value::Obj(None));
        }
        let key = self.c.u32()?;
        match flag {
            2 => return self.shared.get(&key).map(|id| Value::Obj(Some(*id))).ok_or(Error::Malformed("reference to an undefined object")),
            1 => {}
            _ => return Err(Error::Malformed("object marker")),
        }
        if self.shared.contains_key(&key) {
            return Err(Error::Malformed("object defined twice"));
        }
        // Register before the fields: an object's own fields may refer back to it.
        let id = self.push(Object::new(Tag(0), Kind::Shared, Vec::new(), Vec::new()))?;
        self.shared.insert(key, id);
        let mut chain = Vec::new();
        let mut fields = Vec::new();
        loop {
            match self.c.u8()? {
                0 => {
                    let tag = Tag(self.c.u32()?);
                    self.c.u16()?;
                    chain.push(tag);
                    fields.extend(self.fields(true, tag)?);
                }
                1 => {
                    chain.push(Tag(self.c.u32()?));
                    break;
                }
                2 => break,
                _ => return Err(Error::Malformed("object class chain")),
            }
            if chain.len() > 64 {
                return Err(Error::Limit("object class chain too long"));
            }
        }
        let class = chain.first().copied().unwrap_or(Tag(0));
        fields.extend(self.fields(true, class)?);
        if let Some(o) = self.objects.get_mut(id) {
            *o = Object::new(class, Kind::Shared, chain, fields);
        }
        Ok(Value::Obj(Some(id)))
    }
}

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

    #[test]
    fn lookups_in_a_large_object_find_the_first_and_last_fields_of_a_tag() {
        let tag = |i: u32| Tag(0x4100_0000 + i);
        let mut fields: Vec<(Tag, Value)> = (0..1000).map(|i| (tag(i % 300), Value::UInt(u64::from(i)))).collect();
        fields.push((tag(7), Value::Str("last".into())));
        let o = Object::new(Tag::of(b"Grup"), Kind::Shared, Vec::new(), fields.clone());
        assert!(!o.index.is_empty());
        // The index has one entry per distinct tag, not per field.
        assert_eq!(o.index.len(), 300);
        for t in 0..300 {
            let first = fields.iter().find(|(k, _)| *k == tag(t)).map(|(_, v)| v);
            let last = fields.iter().rev().find(|(k, _)| *k == tag(t)).map(|(_, v)| v);
            assert_eq!(o.get(tag(t)), first);
            assert_eq!(o.get_last(tag(t)), last);
        }
        assert_eq!(o.get(tag(300)), None);
        assert_eq!(o.get_last(tag(7)), Some(&Value::Str("last".into())));
        let small = Object::new(Tag::of(b"Grup"), Kind::Shared, Vec::new(), vec![(tag(1), Value::Bool(true)), (tag(1), Value::Bool(false))]);
        assert!(small.index.is_empty());
        assert_eq!(small.get(tag(1)), Some(&Value::Bool(true)));
        assert_eq!(small.get_last(tag(1)), Some(&Value::Bool(false)));
    }
}