miden-standards 0.17.0

// NOTE EXECUTION HINT
// ================================================================================================

use miden_protocol::Felt;
use miden_protocol::block::BlockNumber;
use miden_protocol::errors::NoteError;

/// Specifies the conditions under which a note is ready to be consumed.
/// These conditions are meant to be encoded in the note script as well.
///
/// This struct can be represented as the combination of a tag, and a payload.
/// The tag specifies the variant of the hint, and the payload encodes the hint data.
///
/// # Felt layout
///
/// [`NoteExecutionHint`] can be encoded into a [`Felt`] with the following layout:
///
/// ```text
/// [24 zero bits | payload (32 bits) | tag (8 bits)]
/// ```
///
/// This way, hints such as [NoteExecutionHint::Always], are represented by `Felt::ONE`.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum NoteExecutionHint {
    /// Unspecified note execution hint. Implies it is not known under which conditions the note
    /// is consumable.
    None,
    /// The note's script can be executed at any time.
    Always,
    /// The note's script can be executed after the specified block number.
    AfterBlock { block_num: BlockNumber },
    /// The note's script can be executed in the specified slot within the specified round.
    ///
    /// The slot is defined as follows:
    /// - First we define the length of the round in powers of 2. For example, round_len = 10 is a
    ///   round of 1024 blocks.
    /// - Then we define the length of a slot within the round also using powers of 2. For example,
    ///   slot_len = 7 is a slot of 128 blocks.
    /// - Lastly, the offset specifies the index of the slot within the round - i.e., 0 is the first
    ///   slot, 1 is the second slot etc.
    ///
    /// For example: { round_len: 10, slot_len: 7, slot_offset: 1 } means that the note can
    /// be executed in any second 128 block slot of a 1024 block round. These would be blocks
    /// 128..255, 1152..1279, 2176..2303 etc.
    OnBlockSlot {
        round_len: u8,
        slot_len: u8,
        slot_offset: u8,
    },
    /// An encoding that this version does not recognize, preserved verbatim.
    Unknown(Felt),
}

impl NoteExecutionHint {
    // CONSTANTS
    // ------------------------------------------------------------------------------------------------

    pub(crate) const NONE_TAG: u8 = 0;
    pub(crate) const ALWAYS_TAG: u8 = 1;
    pub(crate) const AFTER_BLOCK_TAG: u8 = 2;
    pub(crate) const ON_BLOCK_SLOT_TAG: u8 = 3;

    // CONSTRUCTORS
    // ------------------------------------------------------------------------------------------------

    /// Creates a [NoteExecutionHint::None] variant
    pub fn none() -> Self {
        NoteExecutionHint::None
    }

    /// Creates a [NoteExecutionHint::Always] variant
    pub fn always() -> Self {
        NoteExecutionHint::Always
    }

    /// Creates a [NoteExecutionHint::AfterBlock] variant based on the given `block_num`
    pub fn after_block(block_num: BlockNumber) -> Self {
        NoteExecutionHint::AfterBlock { block_num }
    }

    /// Creates a [NoteExecutionHint::OnBlockSlot] for the given parameters. See the variants
    /// documentation for details on the parameters.
    pub fn on_block_slot(round_len: u8, slot_len: u8, slot_offset: u8) -> Self {
        NoteExecutionHint::OnBlockSlot { round_len, slot_len, slot_offset }
    }

    pub fn from_parts(tag: u8, payload: u32) -> Result<NoteExecutionHint, NoteError> {
        match tag {
            Self::NONE_TAG => {
                if payload != 0 {
                    return Err(NoteError::InvalidNoteExecutionHintPayload(tag, payload));
                }
                Ok(NoteExecutionHint::None)
            },
            Self::ALWAYS_TAG => {
                if payload != 0 {
                    return Err(NoteError::InvalidNoteExecutionHintPayload(tag, payload));
                }
                Ok(NoteExecutionHint::Always)
            },
            Self::AFTER_BLOCK_TAG => Ok(NoteExecutionHint::after_block(BlockNumber::from(payload))),
            Self::ON_BLOCK_SLOT_TAG => {
                let remainder = ((payload >> 24) & 0xff) as u8;
                if remainder != 0 {
                    return Err(NoteError::InvalidNoteExecutionHintPayload(tag, payload));
                }

                let round_len = ((payload >> 16) & 0xff) as u8;
                let slot_len = ((payload >> 8) & 0xff) as u8;
                let slot_offset = (payload & 0xff) as u8;
                let hint = NoteExecutionHint::OnBlockSlot { round_len, slot_len, slot_offset };

                Ok(hint)
            },
            _ => Err(NoteError::other(format!(
                "note execution hint tag {tag} must be in range 0..={}",
                Self::ON_BLOCK_SLOT_TAG
            ))),
        }
    }

    /// Returns whether the note execution conditions validate for the given `block_num`
    ///
    /// # Returns
    /// - `None` if we don't know whether the note can be consumed.
    /// - `Some(true)` if the note is consumable for the given `block_num`
    /// - `Some(false)` if the note is not consumable for the given `block_num`
    pub fn can_be_consumed(&self, block_num: BlockNumber) -> Option<bool> {
        let block_num = block_num.as_u32();
        match self {
            NoteExecutionHint::None | NoteExecutionHint::Unknown(_) => None,
            NoteExecutionHint::Always => Some(true),
            NoteExecutionHint::AfterBlock { block_num: hint_block_num } => {
                Some(block_num >= hint_block_num.as_u32())
            },
            NoteExecutionHint::OnBlockSlot { round_len, slot_len, slot_offset } => {
                let round_len_blocks: u32 = 1 << round_len;
                let slot_len_blocks: u32 = 1 << slot_len;

                let block_round_index = block_num / round_len_blocks;

                let slot_start_block =
                    block_round_index * round_len_blocks + (*slot_offset as u32) * slot_len_blocks;
                let slot_end_block = slot_start_block + slot_len_blocks;

                let can_be_consumed = block_num >= slot_start_block && block_num < slot_end_block;
                Some(can_be_consumed)
            },
        }
    }

    /// Encodes the [`NoteExecutionHint`] into an 8-bit tag and a 32-bit payload, or `None` for
    /// [`NoteExecutionHint::Unknown`], which by definition has no valid decomposition.
    pub fn into_parts(&self) -> Option<(u8, u32)> {
        match self {
            NoteExecutionHint::None => Some((Self::NONE_TAG, 0)),
            NoteExecutionHint::Always => Some((Self::ALWAYS_TAG, 0)),
            NoteExecutionHint::AfterBlock { block_num } => {
                Some((Self::AFTER_BLOCK_TAG, block_num.as_u32()))
            },
            NoteExecutionHint::OnBlockSlot { round_len, slot_len, slot_offset } => {
                let payload: u32 =
                    ((*round_len as u32) << 16) | ((*slot_len as u32) << 8) | (*slot_offset as u32);
                Some((Self::ON_BLOCK_SLOT_TAG, payload))
            },
            NoteExecutionHint::Unknown(_) => None,
        }
    }
}

/// Converts a [`NoteExecutionHint`] into a [`Felt`] with the layout documented on the type.
impl From<NoteExecutionHint> for Felt {
    fn from(value: NoteExecutionHint) -> Self {
        match value {
            NoteExecutionHint::Unknown(felt) => felt,
            hint => {
                let (tag, payload) =
                    hint.into_parts().expect("every hint but `Unknown` decomposes into parts");
                // The composed value occupies the low 40 bits, so it is always canonical.
                Felt::new_unchecked(((payload as u64) << 8) | (tag as u64))
            },
        }
    }
}

/// Converts a [`Felt`] into a [`NoteExecutionHint`] with the layout documented on the type.
impl From<Felt> for NoteExecutionHint {
    fn from(value: Felt) -> Self {
        let encoded = value.as_canonical_u64();
        let tag = (encoded & 0b1111_1111) as u8;

        // A felt with bits set above the documented layout does not encode a hint, so it must not
        // truncate into one - that would lose those bits on re-encoding.
        u32::try_from(encoded >> 8)
            .ok()
            .and_then(|payload| Self::from_parts(tag, payload).ok())
            .unwrap_or(NoteExecutionHint::Unknown(value))
    }
}

// TESTS
// ================================================================================================

#[cfg(test)]
mod tests {

    use super::*;

    fn assert_hint_serde(note_execution_hint: NoteExecutionHint) {
        let (tag, payload) = note_execution_hint.into_parts().unwrap();
        let deserialized = NoteExecutionHint::from_parts(tag, payload).unwrap();
        assert_eq!(deserialized, note_execution_hint);
    }

    #[test]
    fn test_serialization_round_trip() {
        assert_hint_serde(NoteExecutionHint::None);
        assert_hint_serde(NoteExecutionHint::Always);
        assert_hint_serde(NoteExecutionHint::after_block(15.into()));
        assert_hint_serde(NoteExecutionHint::OnBlockSlot {
            round_len: 9,
            slot_len: 12,
            slot_offset: 18,
        });
    }

    #[test]
    fn test_encode_round_trip() {
        for hint in [
            NoteExecutionHint::None,
            NoteExecutionHint::Always,
            NoteExecutionHint::after_block(15.into()),
            NoteExecutionHint::OnBlockSlot {
                round_len: 22,
                slot_len: 33,
                slot_offset: 44,
            },
        ] {
            let encoded = Felt::from(hint);
            assert_eq!(NoteExecutionHint::from(encoded), hint);
        }

        assert_eq!(Felt::from(NoteExecutionHint::always()).as_canonical_u64(), 1);
    }

    /// A felt that does not encode a recognized hint decodes as `Unknown`.
    #[test]
    fn unknown_hint_round_trip() {
        // A tag above the highest known one, a non-zero payload on a tag that requires an empty
        // one, a non-zero remainder on the `OnBlockSlot` payload, and a felt with bits set above
        // the documented 40-bit layout.
        for encoded in [7u64, (1 << 8) | 1, (1 << 32) | 3, 1 << 40] {
            let encoded = Felt::new(encoded).unwrap();
            let hint = NoteExecutionHint::from(encoded);

            assert_eq!(hint, NoteExecutionHint::Unknown(encoded));
            assert_eq!(hint.into_parts(), None);
            assert_eq!(hint.can_be_consumed(100.into()), None);
            assert_eq!(Felt::from(hint), encoded);
        }
    }

    #[test]
    fn test_can_be_consumed() {
        let none = NoteExecutionHint::none();
        assert!(none.can_be_consumed(100.into()).is_none());

        let always = NoteExecutionHint::always();
        assert!(always.can_be_consumed(100.into()).unwrap());

        let after_block = NoteExecutionHint::after_block(12345.into());
        assert!(!after_block.can_be_consumed(12344.into()).unwrap());
        assert!(after_block.can_be_consumed(12345.into()).unwrap());

        let on_block_slot = NoteExecutionHint::on_block_slot(10, 7, 1);
        assert!(!on_block_slot.can_be_consumed(127.into()).unwrap()); // Block 127 is not in the slot 128..255
        assert!(on_block_slot.can_be_consumed(128.into()).unwrap()); // Block 128 is in the slot 128..255
        assert!(on_block_slot.can_be_consumed(255.into()).unwrap()); // Block 255 is in the slot 128..255
        assert!(!on_block_slot.can_be_consumed(256.into()).unwrap()); // Block 256 is not in the slot 128..255
        assert!(on_block_slot.can_be_consumed(1152.into()).unwrap()); // Block 1152 is in the slot 1152..1279
        assert!(on_block_slot.can_be_consumed(1279.into()).unwrap()); // Block 1279 is in the slot 1152..1279
        assert!(on_block_slot.can_be_consumed(2176.into()).unwrap()); // Block 2176 is in the slot 2176..2303
        assert!(!on_block_slot.can_be_consumed(2175.into()).unwrap()); // Block 1279 is in the slot
        // 2176..2303
    }

    #[test]
    fn test_parts_validity() {
        NoteExecutionHint::from_parts(NoteExecutionHint::NONE_TAG, 1).unwrap_err();
        NoteExecutionHint::from_parts(NoteExecutionHint::ALWAYS_TAG, 12).unwrap_err();
        // 4th byte should be blank for tag 3 (OnBlockSlot)
        NoteExecutionHint::from_parts(NoteExecutionHint::ON_BLOCK_SLOT_TAG, 1 << 24).unwrap_err();
        NoteExecutionHint::from_parts(NoteExecutionHint::ON_BLOCK_SLOT_TAG, 0).unwrap();

        NoteExecutionHint::from_parts(10, 1).unwrap_err();
    }
}