1use super::{Digest, Felt, StarkField, DIGEST_SIZE, ZERO};
  2use crate::utils::{
  3    string::String, ByteReader, ByteWriter, Deserializable, DeserializationError, Serializable,
  4};
  5use core::{cmp::Ordering, ops::Deref};
  6
  7// DIGEST TRAIT IMPLEMENTATIONS
  8// ================================================================================================
  9
 10#[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
 11pub struct RpoDigest([Felt; DIGEST_SIZE]);
 12
 13impl RpoDigest {
 14    pub fn new(value: [Felt; DIGEST_SIZE]) -> Self {
 15        Self(value)
 16    }
 17
 18    pub fn as_elements(&self) -> &[Felt] {
 19        self.as_ref()
 20    }
 21
 22    pub fn as_bytes(&self) -> [u8; 32] {
 23        <Self as Digest>::as_bytes(self)
 24    }
 25
 26    pub fn digests_as_elements<'a, I>(digests: I) -> impl Iterator<Item = &'a Felt>
 27    where
 28        I: Iterator<Item = &'a Self>,
 29    {
 30        digests.flat_map(|d| d.0.iter())
 31    }
 32}
 33
 34impl Digest for RpoDigest {
 35    fn as_bytes(&self) -> [u8; 32] {
 36        let mut result = [0; 32];
 37
 38        result[..8].copy_from_slice(&self.0[0].as_int().to_le_bytes());
 39        result[8..16].copy_from_slice(&self.0[1].as_int().to_le_bytes());
 40        result[16..24].copy_from_slice(&self.0[2].as_int().to_le_bytes());
 41        result[24..].copy_from_slice(&self.0[3].as_int().to_le_bytes());
 42
 43        result
 44    }
 45}
 46
 47impl Serializable for RpoDigest {
 48    fn write_into<W: ByteWriter>(&self, target: &mut W) {
 49        target.write_u8_slice(&self.as_bytes());
 50    }
 51}
 52
 53impl Deserializable for RpoDigest {
 54    fn read_from<R: ByteReader>(source: &mut R) -> Result<Self, DeserializationError> {
 55        let mut inner: [Felt; DIGEST_SIZE] = [ZERO; DIGEST_SIZE];
 56        for inner in inner.iter_mut() {
 57            let e = source.read_u64()?;
 58            if e >= Felt::MODULUS {
 59                return Err(DeserializationError::InvalidValue(String::from(
 60                    "Value not in the appropriate range",
 61                )));
 62            }
 63            *inner = Felt::new(e);
 64        }
 65
 66        Ok(Self(inner))
 67    }
 68}
 69
 70impl From<[Felt; DIGEST_SIZE]> for RpoDigest {
 71    fn from(value: [Felt; DIGEST_SIZE]) -> Self {
 72        Self(value)
 73    }
 74}
 75
 76impl From<RpoDigest> for [Felt; DIGEST_SIZE] {
 77    fn from(value: RpoDigest) -> Self {
 78        value.0
 79    }
 80}
 81
 82impl From<RpoDigest> for [u8; 32] {
 83    fn from(value: RpoDigest) -> Self {
 84        value.as_bytes()
 85    }
 86}
 87
 88impl Deref for RpoDigest {
 89    type Target = [Felt; DIGEST_SIZE];
 90
 91    fn deref(&self) -> &Self::Target {
 92        &self.0
 93    }
 94}
 95
 96impl Ord for RpoDigest {
 97    fn cmp(&self, other: &Self) -> Ordering {
 98        // compare the inner u64 of both elements.
 99        //
100        // it will iterate the elements and will return the first computation different than
101        // `Equal`. Otherwise, the ordering is equal.
102        //
103        // the endianness is irrelevant here because since, this being a cryptographically secure
104        // hash computation, the digest shouldn't have any ordered property of its input.
105        //
106        // finally, we use `Felt::inner` instead of `Felt::as_int` so we avoid performing a
107        // montgomery reduction for every limb. that is safe because every inner element of the
108        // digest is guaranteed to be in its canonical form (that is, `x in [0,p)`).
109        self.0
110            .iter()
111            .map(Felt::inner)
112            .zip(other.0.iter().map(Felt::inner))
113            .fold(Ordering::Equal, |ord, (a, b)| match ord {
114                Ordering::Equal => a.cmp(&b),
115                _ => ord,
116            })
117    }
118}
119
120impl PartialOrd for RpoDigest {
121    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
122        Some(self.cmp(other))
123    }
124}
125
126// TESTS
127// ================================================================================================
128
129#[cfg(test)]
130mod tests {
131
132    use super::{Deserializable, Felt, RpoDigest, Serializable};
133    use crate::utils::SliceReader;
134    use rand_utils::rand_value;
135
136    #[test]
137    fn digest_serialization() {
138        let e1 = Felt::new(rand_value());
139        let e2 = Felt::new(rand_value());
140        let e3 = Felt::new(rand_value());
141        let e4 = Felt::new(rand_value());
142
143        let d1 = RpoDigest([e1, e2, e3, e4]);
144
145        let mut bytes = vec![];
146        d1.write_into(&mut bytes);
147        assert_eq!(32, bytes.len());
148
149        let mut reader = SliceReader::new(&bytes);
150        let d2 = RpoDigest::read_from(&mut reader).unwrap();
151
152        assert_eq!(d1, d2);
153    }
154}