//! Serialization for Zcash.
//!
//! This module contains four traits: `ZcashSerialize` and `ZcashDeserialize`,
//! analogs of the Serde `Serialize` and `Deserialize` traits but intended for
//! consensus-critical Zcash serialization formats, and `WriteZcashExt` and
//! `ReadZcashExt`, extension traits for `io::Read` and `io::Write` with utility functions
//! for reading and writing data (e.g., the Bitcoin variable-integer format).

use std::io;
use std::net::{IpAddr, SocketAddr};

use byteorder::{BigEndian, LittleEndian, ReadBytesExt, WriteBytesExt};
use thiserror::Error;

/// A serialization error.
// XXX refine error types -- better to use boxed errors?
#[derive(Error, Debug)]
pub enum SerializationError {
    /// An underlying IO error.
    #[error("io error")]
    Io(#[from] io::Error),
    /// The data to be deserialized was malformed.
    // XXX refine errors
    #[error("parse error: {0}")]
    Parse(&'static str),
}

/// Consensus-critical serialization for Zcash.
///
/// This trait provides a generic serialization for consensus-critical
/// formats, such as network messages, transactions, blocks, etc. It is intended
/// for use only in consensus-critical contexts; in other contexts, such as
/// internal storage, it would be preferable to use Serde.
pub trait ZcashSerialize: Sized {
    /// Write `self` to the given `writer` using the canonical format.
    ///
    /// This function has a `zcash_` prefix to alert the reader that the
    /// serialization in use is consensus-critical serialization, rather than
    /// some other kind of serialization.
    fn zcash_serialize<W: io::Write>(&self, writer: W) -> Result<(), SerializationError>;
}

/// Consensus-critical serialization for Zcash.
///
/// This trait provides a generic deserialization for consensus-critical
/// formats, such as network messages, transactions, blocks, etc. It is intended
/// for use only in consensus-critical contexts; in other contexts, such as
/// internal storage, it would be preferable to use Serde.
pub trait ZcashDeserialize: Sized {
    /// Try to read `self` from the given `reader`.
    ///
    /// This function has a `zcash_` prefix to alert the reader that the
    /// serialization in use is consensus-critical serialization, rather than
    /// some other kind of serialization.
    fn zcash_deserialize<R: io::Read>(reader: R) -> Result<Self, SerializationError>;
}

/// Extends [`Write`] with methods for writing Zcash/Bitcoin types.
///
/// [`Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
pub trait WriteZcashExt: io::Write {
    /// Writes a `u64` using the Bitcoin `CompactSize` encoding.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use zebra_chain::serialization::WriteZcashExt;
    ///
    /// let mut buf = Vec::new();
    /// buf.write_compactsize(0x12).unwrap();
    /// assert_eq!(buf, b"\x12");
    ///
    /// let mut buf = Vec::new();
    /// buf.write_compactsize(0xfd).unwrap();
    /// assert_eq!(buf, b"\xfd\xfd\x00");
    ///
    /// let mut buf = Vec::new();
    /// buf.write_compactsize(0xaafd).unwrap();
    /// assert_eq!(buf, b"\xfd\xfd\xaa");
    ///
    /// let mut buf = Vec::new();
    /// buf.write_compactsize(0xbbaafd).unwrap();
    /// assert_eq!(buf, b"\xfe\xfd\xaa\xbb\x00");
    ///
    /// let mut buf = Vec::new();
    /// buf.write_compactsize(0x22ccbbaafd).unwrap();
    /// assert_eq!(buf, b"\xff\xfd\xaa\xbb\xcc\x22\x00\x00\x00");
    /// ```
    #[inline]
    fn write_compactsize(&mut self, n: u64) -> io::Result<()> {
        match n {
            0x0000_0000..=0x0000_00fc => self.write_u8(n as u8),
            0x0000_00fd..=0x0000_ffff => {
                self.write_u8(0xfd)?;
                self.write_u16::<LittleEndian>(n as u16)
            }
            0x0001_0000..=0xffff_ffff => {
                self.write_u8(0xfe)?;
                self.write_u32::<LittleEndian>(n as u32)
            }
            _ => {
                self.write_u8(0xff)?;
                self.write_u64::<LittleEndian>(n)
            }
        }
    }

    /// Write an `IpAddr` in Bitcoin format.
    #[inline]
    fn write_ip_addr(&mut self, addr: IpAddr) -> io::Result<()> {
        use std::net::IpAddr::*;
        let v6_addr = match addr {
            V4(ref v4) => v4.to_ipv6_mapped(),
            V6(v6) => v6,
        };
        self.write_all(&v6_addr.octets())
    }

    /// Write a `SocketAddr` in Bitcoin format.
    #[inline]
    fn write_socket_addr(&mut self, addr: SocketAddr) -> io::Result<()> {
        self.write_ip_addr(addr.ip())?;
        self.write_u16::<BigEndian>(addr.port())
    }

    /// Write a string in Bitcoin format.
    #[inline]
    fn write_string(&mut self, string: &str) -> io::Result<()> {
        self.write_compactsize(string.len() as u64)?;
        self.write_all(string.as_bytes())
    }
}

/// Mark all types implementing `Write` as implementing the extension.
impl<W: io::Write + ?Sized> WriteZcashExt for W {}

/// Extends [`Read`] with methods for writing Zcash/Bitcoin types.
///
/// [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html
pub trait ReadZcashExt: io::Read {
    /// Reads a `u64` using the Bitcoin `CompactSize` encoding.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use zebra_chain::serialization::ReadZcashExt;
    ///
    /// use std::io::Cursor;
    /// assert_eq!(
    ///     0x12,
    ///     Cursor::new(b"\x12")
    ///         .read_compactsize().unwrap()
    /// );
    /// assert_eq!(
    ///     0xfd,
    ///     Cursor::new(b"\xfd\xfd\x00")
    ///         .read_compactsize().unwrap()
    /// );
    /// assert_eq!(
    ///     0xaafd,
    ///     Cursor::new(b"\xfd\xfd\xaa")
    ///         .read_compactsize().unwrap()
    /// );
    /// assert_eq!(
    ///     0xbbaafd,
    ///     Cursor::new(b"\xfe\xfd\xaa\xbb\x00")
    ///         .read_compactsize().unwrap()
    /// );
    /// assert_eq!(
    ///     0x22ccbbaafd,
    ///     Cursor::new(b"\xff\xfd\xaa\xbb\xcc\x22\x00\x00\x00")
    ///         .read_compactsize().unwrap()
    /// );
    /// ```
    #[inline]
    fn read_compactsize(&mut self) -> io::Result<u64> {
        let flag_byte = self.read_u8()?;
        match flag_byte {
            0xff => self.read_u64::<LittleEndian>(),
            0xfe => Ok(self.read_u32::<LittleEndian>()? as u64),
            0xfd => Ok(self.read_u16::<LittleEndian>()? as u64),
            n => Ok(n as u64),
        }
    }

    /// Read an IP address in Bitcoin format.
    #[inline]
    fn read_ip_addr(&mut self) -> io::Result<IpAddr> {
        use std::net::{IpAddr::*, Ipv6Addr};

        let mut octets = [0u8; 16];
        self.read_exact(&mut octets)?;
        let v6_addr = Ipv6Addr::from(octets);

        match v6_addr.to_ipv4() {
            Some(v4_addr) => Ok(V4(v4_addr)),
            None => Ok(V6(v6_addr)),
        }
    }

    /// Read a Bitcoin-encoded `SocketAddr`.
    #[inline]
    fn read_socket_addr(&mut self) -> io::Result<SocketAddr> {
        let ip_addr = self.read_ip_addr()?;
        let port = self.read_u16::<BigEndian>()?;
        Ok(SocketAddr::new(ip_addr, port))
    }

    /// Read a Bitcoin-encoded UTF-8 string.
    #[inline]
    fn read_string(&mut self) -> io::Result<String> {
        let len = self.read_compactsize()?;
        let mut buf = vec![0; len as usize];
        self.read_exact(&mut buf)?;
        String::from_utf8(buf).map_err(|_| io::ErrorKind::InvalidData.into())
    }

    /// Convenience method to read a `[u8; 4]`.
    #[inline]
    fn read_4_bytes(&mut self) -> io::Result<[u8; 4]> {
        let mut bytes = [0; 4];
        self.read_exact(&mut bytes)?;
        Ok(bytes)
    }

    /// Convenience method to read a `[u8; 12]`.
    #[inline]
    fn read_12_bytes(&mut self) -> io::Result<[u8; 12]> {
        let mut bytes = [0; 12];
        self.read_exact(&mut bytes)?;
        Ok(bytes)
    }

    /// Convenience method to read a `[u8; 32]`.
    #[inline]
    fn read_32_bytes(&mut self) -> io::Result<[u8; 32]> {
        let mut bytes = [0; 32];
        self.read_exact(&mut bytes)?;
        Ok(bytes)
    }

    /// Convenience method to read a `Vec<T>` with a leading count in
    /// a safer manner.
    ///
    /// This method preallocates a buffer, performing a single
    /// allocation in the honest case. It's possible for someone to
    /// send a short message with a large count field, so if we
    /// naively trust the count field we could be tricked into
    /// preallocating a large buffer. Instead, we rely on the passed
    /// maximum count for a valid message and select the min of the
    /// two values.
    #[inline]
    fn read_list<T: ZcashDeserialize>(
        &mut self,
        max_count: usize,
    ) -> Result<Vec<T>, SerializationError> {
        // This prevents the inferred type for zcash_deserialize from
        // taking ownership of &mut self. This wouldn't really be an
        // issue if the target impl's `Copy`, but we need to own it.
        let mut self2 = self;

        let count = self2.read_compactsize()? as usize;

        let mut items = Vec::with_capacity(std::cmp::min(count, max_count));

        for _ in 0..count {
            items.push(T::zcash_deserialize(&mut self2)?);
        }

        return Ok(items);
    }
}

/// Mark all types implementing `Read` as implementing the extension.
impl<R: io::Read + ?Sized> ReadZcashExt for R {}

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

    use std::io::Cursor;

    proptest! {
        // The test below is cheap so we can run it a lot.
        #![proptest_config(ProptestConfig::with_cases(100_000))]

        #[test]
        fn compactsize_round_trip(s in 0u64..0x2_0000u64) {
            // Maximum encoding size of a compactsize is 9 bytes.
            let mut buf = [0u8; 8+1];
            Cursor::new(&mut buf[..]).write_compactsize(s).unwrap();
            let expect_s = Cursor::new(&buf[..]).read_compactsize().unwrap();
            prop_assert_eq!(s, expect_s);
        }
    }
}