tasm_lib/arithmetic/u32/
safe_sub.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/// Subtract two `u32`s and crash on overflow.
10///
11/// ### Behavior
12///
13/// ```text
14/// BEFORE: _ [right: 32] [left: u32]
15/// AFTER:  _ [left - right: u32]
16/// ```
17///
18/// ### Preconditions
19///
20/// - all input arguments are properly [`BFieldCodec`] encoded
21/// - `left` is greater than or equal to `right`
22///
23/// ### Postconditions
24///
25/// - the output is properly [`BFieldCodec`] encoded
26#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, Hash)]
27pub struct SafeSub;
28
29impl SafeSub {
30    pub const OVERFLOW_ERROR_ID: i128 = 470;
31}
32
33impl BasicSnippet for SafeSub {
34    fn parameters(&self) -> Vec<(DataType, String)> {
35        ["right", "left"]
36            .map(|s| (DataType::U32, s.to_string()))
37            .to_vec()
38    }
39
40    fn return_values(&self) -> Vec<(DataType, String)> {
41        vec![(DataType::U32, "left - right".to_string())]
42    }
43
44    fn entrypoint(&self) -> String {
45        "tasmlib_arithmetic_u32_safe_sub".to_string()
46    }
47
48    fn code(&self, _: &mut Library) -> Vec<LabelledInstruction> {
49        triton_asm!(
50            // BEFORE: _ right left
51            // AFTER:  _ (left - right)
52            {self.entrypoint()}:
53                pick 1
54                push -1
55                mul
56                add
57                dup 0   // _ diff diff
58                split   // _ diff hi lo
59                pop 1   // _ diff hi
60                push 0  // _ diff hi 0
61                eq      // _ diff (hi == 0)
62                assert error_id {Self::OVERFLOW_ERROR_ID}
63                return
64        )
65    }
66
67    fn sign_offs(&self) -> HashMap<Reviewer, SignOffFingerprint> {
68        let mut sign_offs = HashMap::new();
69        sign_offs.insert(Reviewer("ferdinand"), 0x75a4191cfa996c3f.into());
70        sign_offs
71    }
72}
73
74#[cfg(test)]
75mod tests {
76    use super::*;
77    use crate::test_prelude::*;
78
79    impl Closure for SafeSub {
80        type Args = (u32, u32);
81
82        fn rust_shadow(&self, stack: &mut Vec<BFieldElement>) -> Result<(), RustShadowError> {
83            let (right, left) = pop_encodable::<Self::Args>(stack)?;
84            let diff = left
85                .checked_sub(right)
86                .ok_or(RustShadowError::ArithmeticOverflow)?;
87            push_encodable(stack, &diff);
88            Ok(())
89        }
90
91        fn pseudorandom_args(
92            &self,
93            seed: [u8; 32],
94            bench_case: Option<BenchmarkCase>,
95        ) -> Self::Args {
96            let Some(bench_case) = bench_case else {
97                let mut rng = StdRng::from_seed(seed);
98                let left = rng.random();
99                let right = rng.random_range(0..=left);
100
101                return (right, left);
102            };
103
104            match bench_case {
105                BenchmarkCase::CommonCase => (1 << 15, 1 << 16),
106                BenchmarkCase::WorstCase => (0, u32::MAX),
107            }
108        }
109
110        fn corner_case_args(&self) -> Vec<Self::Args> {
111            vec![(0, 0), (0, u32::MAX), (u32::MAX, u32::MAX)]
112        }
113    }
114
115    #[macro_rules_attr::apply(test)]
116    fn rust_shadow() {
117        ShadowedClosure::new(SafeSub).test();
118    }
119
120    #[macro_rules_attr::apply(proptest)]
121    fn overflow_crashes_vm(
122        #[filter(#left != u32::MAX)] left: u32,
123        #[strategy(#left..)] right: u32,
124    ) {
125        test_assertion_failure(
126            &ShadowedClosure::new(SafeSub),
127            InitVmState::with_stack(SafeSub.set_up_test_stack((right, left))),
128            &[SafeSub::OVERFLOW_ERROR_ID],
129        )
130    }
131}
132
133#[cfg(test)]
134mod benches {
135    use super::*;
136    use crate::test_prelude::*;
137
138    #[macro_rules_attr::apply(test)]
139    fn safe_sub_benchmark() {
140        ShadowedClosure::new(SafeSub).bench();
141    }
142}
tasm_lib/arithmetic/u32/safe_sub.rs

tasm_lib/arithmetic/u32/
safe_sub.rs
