//! Compact serde-encoding of vectors with small length.

#![allow(clippy::integer_arithmetic)]
use {
    serde::{
        de::{self, Deserializer, SeqAccess, Visitor},
        ser::{self, SerializeTuple, Serializer},
        Deserialize, Serialize,
    },
    std::{convert::TryFrom, fmt, marker::PhantomData},
};

/// Same as u16, but serialized with 1 to 3 bytes. If the value is above
/// 0x7f, the top bit is set and the remaining value is stored in the next
/// bytes. Each byte follows the same pattern until the 3rd byte. The 3rd
/// byte, if needed, uses all 8 bits to store the last byte of the original
/// value.
#[derive(AbiExample)]
pub struct ShortU16(pub u16);

impl Serialize for ShortU16 {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        // Pass a non-zero value to serialize_tuple() so that serde_json will
        // generate an open bracket.
        let mut seq = serializer.serialize_tuple(1)?;

        let mut rem_val = self.0;
        loop {
            let mut elem = (rem_val & 0x7f) as u8;
            rem_val >>= 7;
            if rem_val == 0 {
                seq.serialize_element(&elem)?;
                break;
            } else {
                elem |= 0x80;
                seq.serialize_element(&elem)?;
            }
        }
        seq.end()
    }
}

enum VisitStatus {
    Done(u16),
    More(u16),
}

#[derive(Debug)]
enum VisitError {
    TooLong(usize),
    TooShort(usize),
    Overflow(u32),
    Alias,
    ByteThreeContinues,
}

impl VisitError {
    fn into_de_error<'de, A>(self) -> A::Error
    where
        A: SeqAccess<'de>,
    {
        match self {
            VisitError::TooLong(len) => de::Error::invalid_length(len, &"three or fewer bytes"),
            VisitError::TooShort(len) => de::Error::invalid_length(len, &"more bytes"),
            VisitError::Overflow(val) => de::Error::invalid_value(
                de::Unexpected::Unsigned(val as u64),
                &"a value in the range [0, 65535]",
            ),
            VisitError::Alias => de::Error::invalid_value(
                de::Unexpected::Other("alias encoding"),
                &"strict form encoding",
            ),
            VisitError::ByteThreeContinues => de::Error::invalid_value(
                de::Unexpected::Other("continue signal on byte-three"),
                &"a terminal signal on or before byte-three",
            ),
        }
    }
}

type VisitResult = Result<VisitStatus, VisitError>;

const MAX_ENCODING_LENGTH: usize = 3;
fn visit_byte(elem: u8, val: u16, nth_byte: usize) -> VisitResult {
    if elem == 0 && nth_byte != 0 {
        return Err(VisitError::Alias);
    }

    let val = u32::from(val);
    let elem = u32::from(elem);
    let elem_val = elem & 0x7f;
    let elem_done = (elem & 0x80) == 0;

    if nth_byte >= MAX_ENCODING_LENGTH {
        return Err(VisitError::TooLong(nth_byte.saturating_add(1)));
    } else if nth_byte == MAX_ENCODING_LENGTH.saturating_sub(1) && !elem_done {
        return Err(VisitError::ByteThreeContinues);
    }

    let shift = u32::try_from(nth_byte)
        .unwrap_or(std::u32::MAX)
        .saturating_mul(7);
    let elem_val = elem_val.checked_shl(shift).unwrap_or(std::u32::MAX);

    let new_val = val | elem_val;
    let val = u16::try_from(new_val).map_err(|_| VisitError::Overflow(new_val))?;

    if elem_done {
        Ok(VisitStatus::Done(val))
    } else {
        Ok(VisitStatus::More(val))
    }
}

struct ShortU16Visitor;

impl<'de> Visitor<'de> for ShortU16Visitor {
    type Value = ShortU16;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("a ShortU16")
    }

    fn visit_seq<A>(self, mut seq: A) -> Result<ShortU16, A::Error>
    where
        A: SeqAccess<'de>,
    {
        // Decodes an unsigned 16 bit integer one-to-one encoded as follows:
        // 1 byte  : 0xxxxxxx                   => 00000000 0xxxxxxx :      0 -    127
        // 2 bytes : 1xxxxxxx 0yyyyyyy          => 00yyyyyy yxxxxxxx :    128 - 16,383
        // 3 bytes : 1xxxxxxx 1yyyyyyy 000000zz => zzyyyyyy yxxxxxxx : 16,384 - 65,535
        let mut val: u16 = 0;
        for nth_byte in 0..MAX_ENCODING_LENGTH {
            let elem: u8 = seq.next_element()?.ok_or_else(|| {
                VisitError::TooShort(nth_byte.saturating_add(1)).into_de_error::<A>()
            })?;
            match visit_byte(elem, val, nth_byte).map_err(|e| e.into_de_error::<A>())? {
                VisitStatus::Done(new_val) => return Ok(ShortU16(new_val)),
                VisitStatus::More(new_val) => val = new_val,
            }
        }

        Err(VisitError::ByteThreeContinues.into_de_error::<A>())
    }
}

impl<'de> Deserialize<'de> for ShortU16 {
    fn deserialize<D>(deserializer: D) -> Result<ShortU16, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserializer.deserialize_tuple(3, ShortU16Visitor)
    }
}

/// If you don't want to use the ShortVec newtype, you can do ShortVec
/// serialization on an ordinary vector with the following field annotation:
///
/// #[serde(with = "short_vec")]
///
pub fn serialize<S: Serializer, T: Serialize>(
    elements: &[T],
    serializer: S,
) -> Result<S::Ok, S::Error> {
    // Pass a non-zero value to serialize_tuple() so that serde_json will
    // generate an open bracket.
    let mut seq = serializer.serialize_tuple(1)?;

    let len = elements.len();
    if len > std::u16::MAX as usize {
        return Err(ser::Error::custom("length larger than u16"));
    }
    let short_len = ShortU16(len as u16);
    seq.serialize_element(&short_len)?;

    for element in elements {
        seq.serialize_element(element)?;
    }
    seq.end()
}

struct ShortVecVisitor<T> {
    _t: PhantomData<T>,
}

impl<'de, T> Visitor<'de> for ShortVecVisitor<T>
where
    T: Deserialize<'de>,
{
    type Value = Vec<T>;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("a Vec with a multi-byte length")
    }

    fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
    where
        A: SeqAccess<'de>,
    {
        let short_len: ShortU16 = seq
            .next_element()?
            .ok_or_else(|| de::Error::invalid_length(0, &self))?;
        let len = short_len.0 as usize;

        let mut result = Vec::with_capacity(len);
        for i in 0..len {
            let elem = seq
                .next_element()?
                .ok_or_else(|| de::Error::invalid_length(i, &self))?;
            result.push(elem);
        }
        Ok(result)
    }
}

/// If you don't want to use the ShortVec newtype, you can do ShortVec
/// deserialization on an ordinary vector with the following field annotation:
///
/// #[serde(with = "short_vec")]
///
pub fn deserialize<'de, D, T>(deserializer: D) -> Result<Vec<T>, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de>,
{
    let visitor = ShortVecVisitor { _t: PhantomData };
    deserializer.deserialize_tuple(std::usize::MAX, visitor)
}

pub struct ShortVec<T>(pub Vec<T>);

impl<T: Serialize> Serialize for ShortVec<T> {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        serialize(&self.0, serializer)
    }
}

impl<'de, T: Deserialize<'de>> Deserialize<'de> for ShortVec<T> {
    fn deserialize<D>(deserializer: D) -> Result<ShortVec<T>, D::Error>
    where
        D: Deserializer<'de>,
    {
        deserialize(deserializer).map(ShortVec)
    }
}

/// Return the decoded value and how many bytes it consumed.
#[allow(clippy::result_unit_err)]
pub fn decode_shortu16_len(bytes: &[u8]) -> Result<(usize, usize), ()> {
    let mut val = 0;
    for (nth_byte, byte) in bytes.iter().take(MAX_ENCODING_LENGTH).enumerate() {
        match visit_byte(*byte, val, nth_byte).map_err(|_| ())? {
            VisitStatus::More(new_val) => val = new_val,
            VisitStatus::Done(new_val) => {
                return Ok((usize::from(new_val), nth_byte.saturating_add(1)));
            }
        }
    }
    Err(())
}

#[cfg(test)]
mod tests {
    use {
        super::*,
        assert_matches::assert_matches,
        bincode::{deserialize, serialize},
    };

    /// Return the serialized length.
    fn encode_len(len: u16) -> Vec<u8> {
        bincode::serialize(&ShortU16(len)).unwrap()
    }

    fn assert_len_encoding(len: u16, bytes: &[u8]) {
        assert_eq!(encode_len(len), bytes, "unexpected usize encoding");
        assert_eq!(
            decode_shortu16_len(bytes).unwrap(),
            (usize::from(len), bytes.len()),
            "unexpected usize decoding"
        );
    }

    #[test]
    fn test_short_vec_encode_len() {
        assert_len_encoding(0x0, &[0x0]);
        assert_len_encoding(0x7f, &[0x7f]);
        assert_len_encoding(0x80, &[0x80, 0x01]);
        assert_len_encoding(0xff, &[0xff, 0x01]);
        assert_len_encoding(0x100, &[0x80, 0x02]);
        assert_len_encoding(0x7fff, &[0xff, 0xff, 0x01]);
        assert_len_encoding(0xffff, &[0xff, 0xff, 0x03]);
    }

    fn assert_good_deserialized_value(value: u16, bytes: &[u8]) {
        assert_eq!(value, deserialize::<ShortU16>(bytes).unwrap().0);
    }

    fn assert_bad_deserialized_value(bytes: &[u8]) {
        assert!(deserialize::<ShortU16>(bytes).is_err());
    }

    #[test]
    fn test_deserialize() {
        assert_good_deserialized_value(0x0000, &[0x00]);
        assert_good_deserialized_value(0x007f, &[0x7f]);
        assert_good_deserialized_value(0x0080, &[0x80, 0x01]);
        assert_good_deserialized_value(0x00ff, &[0xff, 0x01]);
        assert_good_deserialized_value(0x0100, &[0x80, 0x02]);
        assert_good_deserialized_value(0x07ff, &[0xff, 0x0f]);
        assert_good_deserialized_value(0x3fff, &[0xff, 0x7f]);
        assert_good_deserialized_value(0x4000, &[0x80, 0x80, 0x01]);
        assert_good_deserialized_value(0xffff, &[0xff, 0xff, 0x03]);

        // aliases
        // 0x0000
        assert_bad_deserialized_value(&[0x80, 0x00]);
        assert_bad_deserialized_value(&[0x80, 0x80, 0x00]);
        // 0x007f
        assert_bad_deserialized_value(&[0xff, 0x00]);
        assert_bad_deserialized_value(&[0xff, 0x80, 0x00]);
        // 0x0080
        assert_bad_deserialized_value(&[0x80, 0x81, 0x00]);
        // 0x00ff
        assert_bad_deserialized_value(&[0xff, 0x81, 0x00]);
        // 0x0100
        assert_bad_deserialized_value(&[0x80, 0x82, 0x00]);
        // 0x07ff
        assert_bad_deserialized_value(&[0xff, 0x8f, 0x00]);
        // 0x3fff
        assert_bad_deserialized_value(&[0xff, 0xff, 0x00]);

        // too short
        assert_bad_deserialized_value(&[]);
        assert_bad_deserialized_value(&[0x80]);

        // too long
        assert_bad_deserialized_value(&[0x80, 0x80, 0x80, 0x00]);

        // too large
        // 0x0001_0000
        assert_bad_deserialized_value(&[0x80, 0x80, 0x04]);
        // 0x0001_8000
        assert_bad_deserialized_value(&[0x80, 0x80, 0x06]);
    }

    #[test]
    fn test_short_vec_u8() {
        let vec = ShortVec(vec![4u8; 32]);
        let bytes = serialize(&vec).unwrap();
        assert_eq!(bytes.len(), vec.0.len() + 1);

        let vec1: ShortVec<u8> = deserialize(&bytes).unwrap();
        assert_eq!(vec.0, vec1.0);
    }

    #[test]
    fn test_short_vec_u8_too_long() {
        let vec = ShortVec(vec![4u8; std::u16::MAX as usize]);
        assert_matches!(serialize(&vec), Ok(_));

        let vec = ShortVec(vec![4u8; std::u16::MAX as usize + 1]);
        assert_matches!(serialize(&vec), Err(_));
    }

    #[test]
    fn test_short_vec_json() {
        let vec = ShortVec(vec![0, 1, 2]);
        let s = serde_json::to_string(&vec).unwrap();
        assert_eq!(s, "[[3],0,1,2]");
    }

    #[test]
    fn test_short_vec_aliased_length() {
        let bytes = [
            0x81, 0x80, 0x00, // 3-byte alias of 1
            0x00,
        ];
        assert!(deserialize::<ShortVec<u8>>(&bytes).is_err());
    }
}