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//! Arcs are integer values which exist within an OID's hierarchy.
use crate::{Error, ObjectIdentifier, Result};
use core::mem;
/// Type alias used to represent an "arc" (i.e. integer identifier value).
///
/// X.660 does not define a maximum size of an arc.
///
/// The current representation is `u32`, which has been selected as being
/// sufficient to cover the current PKCS/PKIX use cases this library has been
/// used in conjunction with.
///
/// Future versions may potentially make it larger if a sufficiently important
/// use case is discovered.
pub type Arc = u32;
/// Maximum value of the first arc in an OID.
pub(crate) const ARC_MAX_FIRST: Arc = 2;
/// Maximum value of the second arc in an OID.
pub(crate) const ARC_MAX_SECOND: Arc = 39;
/// Maximum number of bytes supported in an arc.
const ARC_MAX_BYTES: usize = mem::size_of::<Arc>();
/// Maximum value of the last byte in an arc.
const ARC_MAX_LAST_OCTET: u8 = 0b11110000; // Max bytes of leading 1-bits
/// [`Iterator`] over [`Arc`] values (a.k.a. nodes) in an [`ObjectIdentifier`].
///
/// This iterates over all arcs in an OID, including the root.
pub struct Arcs<'a> {
/// OID we're iterating over
oid: &'a ObjectIdentifier,
/// Current position within the serialized DER bytes of this OID
cursor: Option<usize>,
}
impl<'a> Arcs<'a> {
/// Create a new iterator over the arcs of this OID
pub(crate) fn new(oid: &'a ObjectIdentifier) -> Self {
Self { oid, cursor: None }
}
/// Try to parse the next arc in this OID.
///
/// This method is fallible so it can be used as a first pass to determine
/// that the arcs in the OID are well-formed.
pub(crate) fn try_next(&mut self) -> Result<Option<Arc>> {
match self.cursor {
// Indicates we're on the root OID
None => {
let root = RootArcs::try_from(self.oid.as_bytes()[0])?;
self.cursor = Some(0);
Ok(Some(root.first_arc()))
}
Some(0) => {
let root = RootArcs::try_from(self.oid.as_bytes()[0])?;
self.cursor = Some(1);
Ok(Some(root.second_arc()))
}
Some(offset) => {
let mut result = 0;
let mut arc_bytes = 0;
loop {
let len = checked_add!(offset, arc_bytes);
match self.oid.as_bytes().get(len).cloned() {
// The arithmetic below includes advance checks
// against `ARC_MAX_BYTES` and `ARC_MAX_LAST_OCTET`
// which ensure the operations will not overflow.
#[allow(clippy::integer_arithmetic)]
Some(byte) => {
arc_bytes = checked_add!(arc_bytes, 1);
if (arc_bytes > ARC_MAX_BYTES) && (byte & ARC_MAX_LAST_OCTET != 0) {
return Err(Error::ArcTooBig);
}
result = result << 7 | (byte & 0b1111111) as Arc;
if byte & 0b10000000 == 0 {
self.cursor = Some(checked_add!(offset, arc_bytes));
return Ok(Some(result));
}
}
None => {
if arc_bytes == 0 {
return Ok(None);
} else {
return Err(Error::Base128);
}
}
}
}
}
}
}
}
impl<'a> Iterator for Arcs<'a> {
type Item = Arc;
fn next(&mut self) -> Option<Arc> {
// ObjectIdentifier constructors should ensure the OID is well-formed
self.try_next().expect("OID malformed")
}
}
/// Byte containing the first and second arcs of an OID.
///
/// This is represented this way in order to reduce the overall size of the
/// [`ObjectIdentifier`] struct.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
struct RootArcs(u8);
impl RootArcs {
/// Create [`RootArcs`] from the first and second arc values represented
/// as `Arc` integers.
pub(crate) const fn new(first_arc: Arc, second_arc: Arc) -> Result<Self> {
if first_arc > ARC_MAX_FIRST {
return Err(Error::ArcInvalid { arc: first_arc });
}
if second_arc > ARC_MAX_SECOND {
return Err(Error::ArcInvalid { arc: second_arc });
}
// The checks above ensure this operation will not overflow
#[allow(clippy::integer_arithmetic)]
let byte = (first_arc * (ARC_MAX_SECOND + 1)) as u8 + second_arc as u8;
Ok(Self(byte))
}
/// Get the value of the first arc
#[allow(clippy::integer_arithmetic)]
pub(crate) const fn first_arc(self) -> Arc {
self.0 as Arc / (ARC_MAX_SECOND + 1)
}
/// Get the value of the second arc
#[allow(clippy::integer_arithmetic)]
pub(crate) const fn second_arc(self) -> Arc {
self.0 as Arc % (ARC_MAX_SECOND + 1)
}
}
impl TryFrom<u8> for RootArcs {
type Error = Error;
// Ensured not to overflow by constructor invariants
#[allow(clippy::integer_arithmetic)]
fn try_from(octet: u8) -> Result<Self> {
let first = octet as Arc / (ARC_MAX_SECOND + 1);
let second = octet as Arc % (ARC_MAX_SECOND + 1);
let result = Self::new(first, second)?;
debug_assert_eq!(octet, result.0);
Ok(result)
}
}
impl From<RootArcs> for u8 {
fn from(root_arcs: RootArcs) -> u8 {
root_arcs.0
}
}
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