use crate::leb128::{self, read_signed_leb128, write_signed_leb128};
use crate::serialize;
use std::borrow::Cow;
use std::mem::MaybeUninit;
use std::ptr;

// -----------------------------------------------------------------------------
// Encoder
// -----------------------------------------------------------------------------

pub type EncodeResult = Result<(), !>;

pub struct Encoder {
    pub data: Vec<u8>,
}

impl Encoder {
    pub fn new(data: Vec<u8>) -> Encoder {
        Encoder { data }
    }

    pub fn into_inner(self) -> Vec<u8> {
        self.data
    }

    #[inline]
    pub fn emit_raw_bytes(&mut self, s: &[u8]) {
        self.data.extend_from_slice(s);
    }
}

macro_rules! write_uleb128 {
    ($enc:expr, $value:expr, $fun:ident) => {{
        leb128::$fun(&mut $enc.data, $value);
        Ok(())
    }};
}

macro_rules! write_sleb128 {
    ($enc:expr, $value:expr) => {{
        write_signed_leb128(&mut $enc.data, $value as i128);
        Ok(())
    }};
}

impl serialize::Encoder for Encoder {
    type Error = !;

    #[inline]
    fn emit_unit(&mut self) -> EncodeResult {
        Ok(())
    }

    #[inline]
    fn emit_usize(&mut self, v: usize) -> EncodeResult {
        write_uleb128!(self, v, write_usize_leb128)
    }

    #[inline]
    fn emit_u128(&mut self, v: u128) -> EncodeResult {
        write_uleb128!(self, v, write_u128_leb128)
    }

    #[inline]
    fn emit_u64(&mut self, v: u64) -> EncodeResult {
        write_uleb128!(self, v, write_u64_leb128)
    }

    #[inline]
    fn emit_u32(&mut self, v: u32) -> EncodeResult {
        write_uleb128!(self, v, write_u32_leb128)
    }

    #[inline]
    fn emit_u16(&mut self, v: u16) -> EncodeResult {
        write_uleb128!(self, v, write_u16_leb128)
    }

    #[inline]
    fn emit_u8(&mut self, v: u8) -> EncodeResult {
        self.data.push(v);
        Ok(())
    }

    #[inline]
    fn emit_isize(&mut self, v: isize) -> EncodeResult {
        write_sleb128!(self, v)
    }

    #[inline]
    fn emit_i128(&mut self, v: i128) -> EncodeResult {
        write_sleb128!(self, v)
    }

    #[inline]
    fn emit_i64(&mut self, v: i64) -> EncodeResult {
        write_sleb128!(self, v)
    }

    #[inline]
    fn emit_i32(&mut self, v: i32) -> EncodeResult {
        write_sleb128!(self, v)
    }

    #[inline]
    fn emit_i16(&mut self, v: i16) -> EncodeResult {
        write_sleb128!(self, v)
    }

    #[inline]
    fn emit_i8(&mut self, v: i8) -> EncodeResult {
        let as_u8: u8 = unsafe { std::mem::transmute(v) };
        self.emit_u8(as_u8)
    }

    #[inline]
    fn emit_bool(&mut self, v: bool) -> EncodeResult {
        self.emit_u8(if v { 1 } else { 0 })
    }

    #[inline]
    fn emit_f64(&mut self, v: f64) -> EncodeResult {
        let as_u64: u64 = v.to_bits();
        self.emit_u64(as_u64)
    }

    #[inline]
    fn emit_f32(&mut self, v: f32) -> EncodeResult {
        let as_u32: u32 = v.to_bits();
        self.emit_u32(as_u32)
    }

    #[inline]
    fn emit_char(&mut self, v: char) -> EncodeResult {
        self.emit_u32(v as u32)
    }

    #[inline]
    fn emit_str(&mut self, v: &str) -> EncodeResult {
        self.emit_usize(v.len())?;
        self.emit_raw_bytes(v.as_bytes());
        Ok(())
    }
}

impl Encoder {
    #[inline]
    pub fn position(&self) -> usize {
        self.data.len()
    }
}

// -----------------------------------------------------------------------------
// Decoder
// -----------------------------------------------------------------------------

pub struct Decoder<'a> {
    pub data: &'a [u8],
    position: usize,
}

impl<'a> Decoder<'a> {
    #[inline]
    pub fn new(data: &'a [u8], position: usize) -> Decoder<'a> {
        Decoder { data, position }
    }

    #[inline]
    pub fn position(&self) -> usize {
        self.position
    }

    #[inline]
    pub fn set_position(&mut self, pos: usize) {
        self.position = pos
    }

    #[inline]
    pub fn advance(&mut self, bytes: usize) {
        self.position += bytes;
    }

    #[inline]
    pub fn read_raw_bytes(&mut self, s: &mut [MaybeUninit<u8>]) -> Result<(), String> {
        let start = self.position;
        let end = start + s.len();
        assert!(end <= self.data.len());

        // SAFETY: Both `src` and `dst` point to at least `s.len()` elements:
        // `src` points to at least `s.len()` elements by above assert, and
        // `dst` points to `s.len()` elements by derivation from `s`.
        unsafe {
            let src = self.data.as_ptr().add(start);
            let dst = s.as_mut_ptr() as *mut u8;
            ptr::copy_nonoverlapping(src, dst, s.len());
        }

        self.position = end;

        Ok(())
    }
}

macro_rules! read_uleb128 {
    ($dec:expr, $fun:ident) => {{
        let (value, bytes_read) = leb128::$fun(&$dec.data[$dec.position..]);
        $dec.position += bytes_read;
        Ok(value)
    }};
}

macro_rules! read_sleb128 {
    ($dec:expr, $t:ty) => {{
        let (value, bytes_read) = read_signed_leb128($dec.data, $dec.position);
        $dec.position += bytes_read;
        Ok(value as $t)
    }};
}

impl<'a> serialize::Decoder for Decoder<'a> {
    type Error = String;

    #[inline]
    fn read_nil(&mut self) -> Result<(), Self::Error> {
        Ok(())
    }

    #[inline]
    fn read_u128(&mut self) -> Result<u128, Self::Error> {
        read_uleb128!(self, read_u128_leb128)
    }

    #[inline]
    fn read_u64(&mut self) -> Result<u64, Self::Error> {
        read_uleb128!(self, read_u64_leb128)
    }

    #[inline]
    fn read_u32(&mut self) -> Result<u32, Self::Error> {
        read_uleb128!(self, read_u32_leb128)
    }

    #[inline]
    fn read_u16(&mut self) -> Result<u16, Self::Error> {
        read_uleb128!(self, read_u16_leb128)
    }

    #[inline]
    fn read_u8(&mut self) -> Result<u8, Self::Error> {
        let value = self.data[self.position];
        self.position += 1;
        Ok(value)
    }

    #[inline]
    fn read_usize(&mut self) -> Result<usize, Self::Error> {
        read_uleb128!(self, read_usize_leb128)
    }

    #[inline]
    fn read_i128(&mut self) -> Result<i128, Self::Error> {
        read_sleb128!(self, i128)
    }

    #[inline]
    fn read_i64(&mut self) -> Result<i64, Self::Error> {
        read_sleb128!(self, i64)
    }

    #[inline]
    fn read_i32(&mut self) -> Result<i32, Self::Error> {
        read_sleb128!(self, i32)
    }

    #[inline]
    fn read_i16(&mut self) -> Result<i16, Self::Error> {
        read_sleb128!(self, i16)
    }

    #[inline]
    fn read_i8(&mut self) -> Result<i8, Self::Error> {
        let as_u8 = self.data[self.position];
        self.position += 1;
        unsafe { Ok(::std::mem::transmute(as_u8)) }
    }

    #[inline]
    fn read_isize(&mut self) -> Result<isize, Self::Error> {
        read_sleb128!(self, isize)
    }

    #[inline]
    fn read_bool(&mut self) -> Result<bool, Self::Error> {
        let value = self.read_u8()?;
        Ok(value != 0)
    }

    #[inline]
    fn read_f64(&mut self) -> Result<f64, Self::Error> {
        let bits = self.read_u64()?;
        Ok(f64::from_bits(bits))
    }

    #[inline]
    fn read_f32(&mut self) -> Result<f32, Self::Error> {
        let bits = self.read_u32()?;
        Ok(f32::from_bits(bits))
    }

    #[inline]
    fn read_char(&mut self) -> Result<char, Self::Error> {
        let bits = self.read_u32()?;
        Ok(std::char::from_u32(bits).unwrap())
    }

    #[inline]
    fn read_str(&mut self) -> Result<Cow<'_, str>, Self::Error> {
        let len = self.read_usize()?;
        let s = std::str::from_utf8(&self.data[self.position..self.position + len]).unwrap();
        self.position += len;
        Ok(Cow::Borrowed(s))
    }

    #[inline]
    fn error(&mut self, err: &str) -> Self::Error {
        err.to_string()
    }
}

// Specializations for contiguous byte sequences follow. The default implementations for slices
// encode and decode each element individually. This isn't necessary for `u8` slices when using
// opaque encoders and decoders, because each `u8` is unchanged by encoding and decoding.
// Therefore, we can use more efficient implementations that process the entire sequence at once.

// Specialize encoding byte slices. This specialization also applies to encoding `Vec<u8>`s, etc.,
// since the default implementations call `encode` on their slices internally.
impl serialize::Encodable<Encoder> for [u8] {
    fn encode(&self, e: &mut Encoder) -> EncodeResult {
        serialize::Encoder::emit_usize(e, self.len())?;
        e.emit_raw_bytes(self);
        Ok(())
    }
}

// Specialize decoding `Vec<u8>`. This specialization also applies to decoding `Box<[u8]>`s, etc.,
// since the default implementations call `decode` to produce a `Vec<u8>` internally.
impl<'a> serialize::Decodable<Decoder<'a>> for Vec<u8> {
    fn decode(d: &mut Decoder<'a>) -> Result<Self, String> {
        let len = serialize::Decoder::read_usize(d)?;

        let mut v = Vec::with_capacity(len);
        let buf = &mut v.spare_capacity_mut()[..len];
        d.read_raw_bytes(buf)?;

        unsafe {
            v.set_len(len);
        }

        Ok(v)
    }
}