tasm_lib/arithmetic/u32/
shift_right.rs

1use std::collections::HashMap;
2
3use triton_vm::prelude::*;
4
5use crate::prelude::*;
6use crate::traits::basic_snippet::Reviewer;
7use crate::traits::basic_snippet::SignOffFingerprint;
8
9/// [Shift right][shr] for unsigned 32-bit integers.
10///
11/// # Behavior
12///
13/// ```text
14/// BEFORE: _ [arg: u32] shift_amount
15/// AFTER:  _ [result: u32]
16/// ```
17///
18/// # Preconditions
19///
20/// - input argument `arg` is properly [`BFieldCodec`] encoded
21/// - input argument `shift_amount` is in `0..32`
22///
23/// # Postconditions
24///
25/// - the output is the input argument `arg` bit-shifted to the right by
26///   input argument `shift_amount`
27/// - the output is properly [`BFieldCodec`] encoded
28///
29/// [shr]: core::ops::Shr
30#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Hash)]
31pub struct ShiftRight;
32
33impl ShiftRight {
34    pub const SHIFT_AMOUNT_TOO_BIG_ERROR_ID: i128 = 490;
35}
36
37impl BasicSnippet for ShiftRight {
38    fn parameters(&self) -> Vec<(DataType, String)> {
39        let arg = (DataType::U32, "arg".to_string());
40        let shift_amount = (DataType::U32, "shift_amount".to_string());
41
42        vec![arg, shift_amount]
43    }
44
45    fn return_values(&self) -> Vec<(DataType, String)> {
46        vec![(DataType::U32, "shifted_arg".to_string())]
47    }
48
49    fn entrypoint(&self) -> String {
50        "tasmlib_arithmetic_u32_shift_right".to_string()
51    }
52
53    fn code(&self, _: &mut Library) -> Vec<LabelledInstruction> {
54        triton_asm!(
55            // BEFORE: _ value shift
56            // AFTER:  _ (value >> shift)
57            {self.entrypoint()}:
58                /* bounds check mimics Rust's behavior */
59                push 32
60                dup 1
61                lt
62                assert error_id {Self::SHIFT_AMOUNT_TOO_BIG_ERROR_ID}
63
64                /* for an explanation of how & why this works, see `u64::shift_right` */
65                push -1
66                mul
67                addi 32
68                push 2
69                pow
70                // _ value (2 ^ (32 - shift))
71
72                mul
73                // _ (value << (32 - shift))
74
75                split
76                pop 1
77                // _ (value >> shift))
78
79                return
80        )
81    }
82
83    fn sign_offs(&self) -> HashMap<Reviewer, SignOffFingerprint> {
84        let mut sign_offs = HashMap::new();
85        sign_offs.insert(Reviewer("ferdinand"), 0x1b65003ff2618480.into());
86        sign_offs
87    }
88}
89
90#[cfg(test)]
91mod tests {
92    use super::*;
93    use crate::test_prelude::*;
94
95    impl Closure for ShiftRight {
96        type Args = (u32, u32);
97
98        fn rust_shadow(&self, stack: &mut Vec<BFieldElement>) -> Result<(), RustShadowError> {
99            let (arg, shift_amount) = pop_encodable::<Self::Args>(stack)?;
100            if shift_amount >= 32 {
101                return Err(RustShadowError::ArithmeticOverflow);
102            }
103            push_encodable(stack, &(arg >> shift_amount));
104            Ok(())
105        }
106
107        fn pseudorandom_args(
108            &self,
109            seed: [u8; 32],
110            bench_case: Option<BenchmarkCase>,
111        ) -> Self::Args {
112            let mut rng = StdRng::from_seed(seed);
113            match bench_case {
114                Some(BenchmarkCase::CommonCase) => ((1 << 16) - 1, 16),
115                Some(BenchmarkCase::WorstCase) => (u32::MAX, 1),
116                None => (rng.random(), rng.random_range(0..32)),
117            }
118        }
119
120        fn corner_case_args(&self) -> Vec<Self::Args> {
121            (0..32).map(|i| (u32::MAX, i)).collect()
122        }
123    }
124
125    #[macro_rules_attr::apply(test)]
126    fn rust_shadow() {
127        ShadowedClosure::new(ShiftRight).test();
128    }
129
130    #[macro_rules_attr::apply(proptest)]
131    fn too_big_shift_amount_crashes_vm(arg: u32, #[strategy(32_u32..)] shift_amount: u32) {
132        test_assertion_failure(
133            &ShadowedClosure::new(ShiftRight),
134            InitVmState::with_stack(ShiftRight.set_up_test_stack((arg, shift_amount))),
135            &[ShiftRight::SHIFT_AMOUNT_TOO_BIG_ERROR_ID],
136        )
137    }
138}
139
140#[cfg(test)]
141mod benches {
142    use super::*;
143    use crate::test_prelude::*;
144
145    #[macro_rules_attr::apply(test)]
146    fn benchmark() {
147        ShadowedClosure::new(ShiftRight).bench();
148    }
149}
tasm_lib/arithmetic/u32/shift_right.rs

tasm_lib/arithmetic/u32/
shift_right.rs
