tasm_lib/arithmetic/u64/

shift_right.rs

use std::collections::HashMap;

use triton_vm::prelude::*;

use crate::prelude::*;
use crate::traits::basic_snippet::Reviewer;
use crate::traits::basic_snippet::SignOffFingerprint;

/// [Shift right][shr] for unsigned 64-bit integers.
///
/// # Behavior
///
/// ```text
/// BEFORE: _ [arg: u64] shift_amount
/// AFTER:  _ [result: u64]
/// ```
///
/// # Preconditions
///
/// - input argument `arg` is properly [`BFieldCodec`] encoded
/// - input argument `shift_amount` is in `0..64`
///
/// # Postconditions
///
/// - the output is the input argument `arg` bit-shifted to the right by
///   input argument `shift_amount`
/// - the output is properly [`BFieldCodec`] encoded
///
/// [shr]: core::ops::Shr
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct ShiftRight;

impl ShiftRight {
    pub const SHIFT_AMOUNT_TOO_BIG_ERROR_ID: i128 = 330;
}

impl BasicSnippet for ShiftRight {
    fn parameters(&self) -> Vec<(DataType, String)> {
        let arg = (DataType::U64, "arg".to_string());
        let shift_amount = (DataType::U32, "shift_amount".to_string());

        vec![arg, shift_amount]
    }

    fn return_values(&self) -> Vec<(DataType, String)> {
        vec![(DataType::U64, "shifted_arg".to_string())]
    }

    fn entrypoint(&self) -> String {
        "tasmlib_arithmetic_u64_shift_right".to_string()
    }

    fn code(&self, _: &mut Library) -> Vec<LabelledInstruction> {
        let entrypoint = self.entrypoint();
        let shift_amount_gt_32 = format!("{entrypoint}_shift_amount_gt_32");

        triton_asm!(
            // BEFORE: _ arg_hi arg_lo shift
            // AFTER:  _ (arg >> shift)_hi (arg >> shift)_lo
            {entrypoint}:
                /* bounds check */
                push 64
                dup 1
                lt
                assert error_id {Self::SHIFT_AMOUNT_TOO_BIG_ERROR_ID}
                // _ arg_hi arg_lo shift

                /* special case if shift amount is greater than 32 */
                dup 0
                push 32
                lt
                // _ arg_hi arg_lo shift (32 < shift)

                skiz
                    call {shift_amount_gt_32}
                // _ arg_hi arg_lo shift

                /* Over a finite field, both right shift and integer division are difficult.
                 * However, integer multiplication and therefore left shift are easy, provided
                 * the field elements are within the right ranges.
                 * General strategy: multiply by the correct power of 2 to shift left by
                 * (32 - shift_amount), then `split` and throw away the low limb. For example,
                 * 0b10_0001 >> 5 gives 0b1, as does a left shift by 27 after throwing away the
                 * low limb:
                 *
                 *   0b10_0001 << 27 = 0b1_0000_1000_0000_0000_0000_0000_0000_0000
                 *                         ╰─────────────╴ low limb ╶────────────╯
                 */
                push -1
                mul
                addi 32
                // _ arg_hi arg_lo (32 - shift)

                push 2
                pow
                // _ arg_hi arg_lo (2^(32 - shift))

                pick 2
                dup 1
                mul
                // _ arg_lo (2^(32 - shift)) (arg_hi << (32 - shift))

                split
                // _ arg_lo (2^(32 - shift)) (arg_hi >> shift) carry
                // _ arg_lo (2^(32 - shift)) (arg >> shift)_hi carry

                place 3
                place 3
                // _ (arg >> shift)_hi carry arg_lo (2^(32 - shift))

                mul
                split
                pop 1
                // _ (arg >> shift)_hi carry (arg_lo >> shift)

                add
                // _ (arg >> shift)_hi (arg >> shift)_lo

                return

            // BEFORE: _ arg_hi arg_lo shift
            // AFTER:  _ 0      arg_hi (shift - 32)
            {shift_amount_gt_32}:
                addi -32
                // _ arg_hi arg_lo (shift - 32)

                pick 1
                pop 1
                // _ arg_hi (shift - 32)

                push 0
                place 2
                // _ 0 arg_hi (shift - 32)

                return
        )
    }

    fn sign_offs(&self) -> HashMap<Reviewer, SignOffFingerprint> {
        let mut sign_offs = HashMap::new();
        sign_offs.insert(Reviewer("ferdinand"), 0xa7cfa746b15b2d76.into());
        sign_offs
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::test_prelude::*;

    impl ShiftRight {
        pub fn assert_expected_shift_right_behavior(&self, shift_amount: u32, arg: u64) {
            let initial_stack = self.set_up_test_stack((arg, shift_amount));

            let mut expected_stack = initial_stack.clone();
            self.rust_shadow(&mut expected_stack).unwrap();

            test_rust_equivalence_given_complete_state(
                &ShadowedClosure::new(Self),
                &initial_stack,
                &[],
                &NonDeterminism::default(),
                &None,
                Some(&expected_stack),
            );
        }
    }

    impl Closure for ShiftRight {
        type Args = (u64, u32);

        fn rust_shadow(&self, stack: &mut Vec<BFieldElement>) -> Result<(), RustShadowError> {
            let (arg, shift_amount) = pop_encodable::<Self::Args>(stack)?;
            if shift_amount >= 64 {
                return Err(RustShadowError::ArithmeticOverflow);
            }
            push_encodable(stack, &(arg >> shift_amount));
            Ok(())
        }

        fn pseudorandom_args(
            &self,
            seed: [u8; 32],
            bench_case: Option<BenchmarkCase>,
        ) -> Self::Args {
            let mut rng = StdRng::from_seed(seed);

            match bench_case {
                Some(BenchmarkCase::CommonCase) => (0x642, 15),
                Some(BenchmarkCase::WorstCase) => (0x123, 33),
                None => (rng.random(), rng.random_range(0..64)),
            }
        }

        fn corner_case_args(&self) -> Vec<Self::Args> {
            (0..64).map(|i| (1 << i, i)).collect()
        }
    }

    #[macro_rules_attr::apply(test)]
    fn rust_shadow() {
        ShadowedClosure::new(ShiftRight).test()
    }

    #[macro_rules_attr::apply(test)]
    fn unit_test() {
        ShiftRight.assert_expected_shift_right_behavior(2, 8);
    }

    #[macro_rules_attr::apply(proptest)]
    fn property_test(arg: u64, #[strategy(0_u32..64)] shift_amount: u32) {
        ShiftRight.assert_expected_shift_right_behavior(shift_amount, arg);
    }

    #[macro_rules_attr::apply(proptest)]
    fn negative_property_test(arg: u64, #[strategy(64_u32..)] shift_amount: u32) {
        test_assertion_failure(
            &ShadowedClosure::new(ShiftRight),
            InitVmState::with_stack(ShiftRight.set_up_test_stack((arg, shift_amount))),
            &[ShiftRight::SHIFT_AMOUNT_TOO_BIG_ERROR_ID],
        );
    }
}

#[cfg(test)]
mod benches {
    use super::*;
    use crate::test_prelude::*;

    #[macro_rules_attr::apply(test)]
    fn benchmark() {
        ShadowedClosure::new(ShiftRight).bench();
    }
}