keyfork/crates/derive/keyfork-derive-util/src/index.rs

use serde::{Deserialize, Serialize};
use thiserror::Error;

/// Errors associated with creating a [`DerivationIndex`].
#[derive(Error, Debug)]
pub enum Error {
    /// The index was too large and should be less than 2^31.
    #[error("Index is too large, must be less than 0x80000000: {0}")]
    IndexTooLarge(u32),

    /// An integer could not be parsed from the string.
    #[error("Unable to parse integer for index")]
    IntParseError(#[from] std::num::ParseIntError),
}

type Result<T, E = Error> = std::result::Result<T, E>;

/// Index for a given extended private key.
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct DerivationIndex(pub(crate) u32);

impl DerivationIndex {
    /// Creates a new [`DerivationIndex`].
    ///
    /// # Errors
    /// Returns an error if the index is larger than the hardened flag.
    ///
    /// # Examples
    /// ```rust
    /// # use keyfork_derive_util::*;
    /// let bip44 = DerivationIndex::new(44, true);
    /// ```
    ///
    /// Using a derivation index that is higher than 2^31 returns an error:
    ///
    /// ```rust,should_panic
    /// # use keyfork_derive_util::*;
    /// let too_high = DerivationIndex::new(u32::MAX, true).unwrap();
    /// ```
    pub const fn new(index: u32, hardened: bool) -> Result<Self> {
        if index & (0b1 << 31) > 0 {
            return Err(Error::IndexTooLarge(index));
        }
        Ok(Self(index | ((hardened as u32) << 31)))
    }

    #[doc(hidden)]
    pub const fn new_unchecked(index: u32, hardened: bool) -> Self {
        Self(index | ((hardened as u32) << 31))
    }

    /*
     * Probably never used.
    pub(crate) fn from_bytes(bytes: [u8; 4]) -> Self {
        Self(u32::from_be_bytes(bytes))
    }
    */

    /// Return the internal derivation index. Note that if the derivation index is hardened, the
    /// highest bit will be set, and the value can't be used to create a new derivation index.
    ///
    /// # Examples
    /// ```rust
    /// # use keyfork_derive_util::*;
    /// assert_eq!(DerivationIndex::new(44, true).unwrap().inner(), 2147483692);
    /// assert_eq!(DerivationIndex::new(200, false).unwrap().inner(), 200);
    /// ```
    pub fn inner(&self) -> u32 {
        self.0
    }

    pub(crate) fn to_bytes(&self) -> [u8; 4] {
        self.0.to_be_bytes()
    }

    /// Whether or not the index is hardened, allowing deriving the key from a known parent public
    /// key.
    ///
    /// # Examples
    /// ```rust
    /// # use keyfork_derive_util::*;
    /// assert_eq!(DerivationIndex::new(0, true).unwrap().is_hardened(), true);
    /// assert_eq!(DerivationIndex::new(0, false).unwrap().is_hardened(), false);
    /// ```
    pub fn is_hardened(&self) -> bool {
        self.0 & (0b1 << 31) != 0
    }
}

impl std::str::FromStr for DerivationIndex {
    type Err = Error;

    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
        // Returns &str without suffix if suffix is found
        let (s, is_hardened) = match s.strip_suffix('\'') {
            Some(subslice) => (subslice, true),
            None => (s, false),
        };
        let index: u32 = s.parse()?;
        Self::new(index, is_hardened)
    }
}

impl std::fmt::Display for DerivationIndex {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0 & (u32::MAX >> 1))?;
        if self.0 & (0b1 << 31) != 0 {
            write!(f, "'")?;
        }
        Ok(())
    }
}

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

    #[test]
    #[should_panic]
    fn fails_on_high_index() {
        DerivationIndex::new(0x8000_0001, false).unwrap();
    }

    #[test]
    fn has_hardened_bit() {
        assert_eq!(DerivationIndex::new(0x0, true).unwrap().0, 0b1 << 31);
    }

    #[test]
    fn misc_values() -> Result<()> {
        assert_eq!(DerivationIndex::new(0x8000_0000 - 1, true)?.0, u32::MAX);
        assert_eq!(DerivationIndex::new(0x2, false)?.0, 2);
        assert_eq!(DerivationIndex::new(0x00AB_CDEF, true)?.0, 0x80AB_CDEF);
        assert_eq!(DerivationIndex::new(0x00AB_CDEF, false)?.0, 0x00AB_CDEF);
        Ok(())
    }

    #[test]
    fn from_str() -> Result<()> {
        assert_eq!(DerivationIndex::from_str("100000")?.0, 100_000);
        assert_eq!(
            DerivationIndex::from_str("100000'")?.0,
            (0b1 << 31) + 100_000
        );
        Ok(())
    }

    #[test]
    fn display() -> Result<()> {
        assert_eq!(&DerivationIndex::new(3232, false)?.to_string(), "3232");
        assert_eq!(&DerivationIndex::new(3232, true)?.to_string(), "3232'");
        Ok(())
    }

    #[test]
    fn equivalency() -> Result<()> {
        let values = ["123456'", "123456", "1726562", "0'", "0"];
        for value in values {
            assert_eq!(value, DerivationIndex::from_str(value)?.to_string());
        }
        Ok(())
    }

    #[test]
    #[should_panic]
    fn from_str_fails_on_high_index() {
        DerivationIndex::from_str(&0x8000_0001u32.to_string()).unwrap();
    }
}