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//! The segregated witness program as defined by [BIP141].
//!
//! > A scriptPubKey (or redeemScript as defined in BIP16/P2SH) that consists of a 1-byte push
//! > opcode (for 0 to 16) followed by a data push between 2 and 40 bytes gets a new special
//! > meaning. The value of the first push is called the "version byte". The following byte
//! > vector pushed is called the "witness program".
//!
//! [BIP141]: <https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki>

use core::convert::TryInto;
use core::fmt;

use crate::blockdata::script::witness_version::WitnessVersion;
use crate::blockdata::script::{PushBytes, PushBytesBuf, PushBytesErrorReport};

/// The segregated witness program.
///
/// The segregated witness program is technically only the program bytes _excluding_ the witness
/// version, however we maintain length invariants on the `program` that are governed by the version
/// number, therefore we carry the version number around along with the program bytes.
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct WitnessProgram {
    /// The segwit version associated with this witness program.
    version: WitnessVersion,
    /// The witness program (between 2 and 40 bytes).
    program: PushBytesBuf,
}

impl WitnessProgram {
    /// Creates a new witness program.
    pub fn new<P>(version: WitnessVersion, program: P) -> Result<Self, Error>
    where
        P: TryInto<PushBytesBuf>,
        <P as TryInto<PushBytesBuf>>::Error: PushBytesErrorReport,
    {
        use Error::*;

        let program = program.try_into().map_err(|error| InvalidLength(error.input_len()))?;
        if program.len() < 2 || program.len() > 40 {
            return Err(InvalidLength(program.len()));
        }

        // Specific segwit v0 check. These addresses can never spend funds sent to them.
        if version == WitnessVersion::V0 && (program.len() != 20 && program.len() != 32) {
            return Err(InvalidSegwitV0Length(program.len()));
        }
        Ok(WitnessProgram { version, program })
    }

    /// Returns the witness program version.
    pub fn version(&self) -> WitnessVersion { self.version }

    /// Returns the witness program.
    pub fn program(&self) -> &PushBytes { &self.program }
}

/// Witness program error.
#[derive(Clone, Debug, PartialEq, Eq)]
#[non_exhaustive]
pub enum Error {
    /// The witness program must be between 2 and 40 bytes in length.
    InvalidLength(usize),
    /// A v0 witness program must be either of length 20 or 32.
    InvalidSegwitV0Length(usize),
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use Error::*;

        match *self {
            InvalidLength(len) =>
                write!(f, "witness program must be between 2 and 40 bytes: length={}", len),
            InvalidSegwitV0Length(len) =>
                write!(f, "a v0 witness program must be either 20 or 32 bytes: length={}", len),
        }
    }
}

#[cfg(feature = "std")]
impl std::error::Error for Error {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        use Error::*;

        match *self {
            InvalidLength(_) | InvalidSegwitV0Length(_) => None,
        }
    }
}