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use triton_vm::prelude::*;
use crate::prelude::*;
/// A u32 `pow` that behaves like Rustc's `pow` method on `u32`, crashing in case of overflow.
#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
pub struct SafePow;
impl BasicSnippet for SafePow {
fn parameters(&self) -> Vec<(DataType, String)> {
vec![
(DataType::U32, "base".to_owned()),
(DataType::U32, "exponent".to_owned()),
]
}
fn return_values(&self) -> Vec<(DataType, String)> {
vec![(DataType::U32, "result".to_owned())]
}
fn entrypoint(&self) -> String {
"tasmlib_arithmetic_u32_safe_pow".to_string()
}
fn code(&self, _: &mut Library) -> Vec<LabelledInstruction> {
// This algorithm is implemented below. `bpow2` has type
// `u64` because it would otherwise erroneously overflow
// in the last iteration of the loop when e.g. calculating
// 2.pow(31).
// fn safe_pow(base: u32, exponent: u64) -> Self {
// let mut bpow2: u64 = base as u64;
// let mut acc = 1u32;
// let mut i = exponent;
// while i != 0 {
// if i & 1 == 1 {
// acc *= bpow2;
// }
// bpow2 *= bpow2;
// i >>= 1;
// }
// acc
// }
let entrypoint = self.entrypoint();
let while_acc_label = format!("{entrypoint}_while_acc");
let mul_acc_with_bpow2_label = format!("{entrypoint}_mul_acc_with_bpow2");
triton_asm!(
{entrypoint}:
// _ base exponent
push 0
swap 2
swap 1
// _ 0 base exponent
push 1
// _ [base_u64] exponent acc
// rename: `exponent` -> `i`, `base_u64` -> `bpow2_u64`
// _ [bpow2_u64] i acc
call {while_acc_label}
// _ [bpow2_u64] 0 acc
swap 3
pop 3
return
// INVARIANT: _ [bpow2_u64] i acc
{while_acc_label}:
// check condition
dup 1 push 0 eq
skiz
return
// _ [bpow2_u64] i acc
// Verify that `bpow2_u64` does not exceed `u32::MAX`
dup 3 push 0 eq assert error_id 120
// _ 0 bpow2 i acc
dup 1
push 1
and
// _ 0 bpow2 i acc (i & 1)
skiz
call {mul_acc_with_bpow2_label}
// _ 0 bpow2 i acc
swap 2
// _ 0 acc i bpow2
dup 0 mul split
// _ 0 acc i bpow2_next_hi bpow2_next_lo
// GOAL: // _ [bpow_u64] i acc
swap 3
// _ 0 bpow2_next_lo i bpow2_next_hi acc
swap 1
// _ 0 bpow2_next_lo i acc bpow2_next_hi
swap 4 pop 1
// _ bpow2_next_hi bpow2_next_lo i acc
// _ [bpow2_next_u64] i acc
push 2
// _ [bpow2_next_u64] i acc 2
dup 2
// _ [bpow2_next_u64] i acc 2 i
div_mod
// _ [bpow2_u64] i acc (i >> 2) (i % 2)
pop 1 swap 2 pop 1
// _ [bpow2_u64] (i >> 2) acc
recurse
{mul_acc_with_bpow2_label}:
// _ 0 bpow2 i acc
dup 2
mul
// _ 0 bpow2 i (acc * bpow2)
split swap 1 push 0 eq assert error_id 121
// _ 0 bpow2 i new_acc
return
)
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::test_prelude::*;
impl Closure for SafePow {
type Args = (u32, u32);
fn rust_shadow(&self, stack: &mut Vec<BFieldElement>) {
let (base, exponent) = pop_encodable::<Self::Args>(stack);
push_encodable(stack, &(base.pow(exponent)));
}
fn pseudorandom_args(
&self,
seed: [u8; 32],
bench_case: Option<BenchmarkCase>,
) -> Self::Args {
let Some(bench_case) = bench_case else {
let mut seeded_rng = StdRng::from_seed(seed);
let base = seeded_rng.random_range(0..0x10);
let exponent = seeded_rng.random_range(0..0x8);
return (base, exponent);
};
match bench_case {
BenchmarkCase::CommonCase => (10, 5),
BenchmarkCase::WorstCase => (2, 31),
}
}
fn corner_case_args(&self) -> Vec<Self::Args> {
vec![(0, 0)]
}
}
#[test]
fn ruts_shadow() {
ShadowedClosure::new(SafePow).test()
}
#[test]
fn u32_pow_unit_test() {
for (base, exp) in [
(0, 0),
(0, 1),
(1, 0),
(1, 1),
(2, 30),
(2, 31),
(3, 20),
(4, 15),
(5, 13),
(6, 12),
(7, 11),
(8, 10),
(9, 10),
(10, 9),
(11, 9),
(12, 8),
(u32::MAX, 0),
(u32::MAX, 1),
(1, u32::MAX),
(0, u32::MAX),
(1, u32::MAX - 1),
(0, u32::MAX - 1),
(1, u32::MAX - 2),
(0, u32::MAX - 2),
(1, u32::MAX - 3),
(0, u32::MAX - 3),
] {
let initial_stack = SafePow.set_up_test_stack((base, exp));
let mut expected_final_stack = initial_stack.clone();
SafePow.rust_shadow(&mut expected_final_stack);
let _vm_output_state = test_rust_equivalence_given_complete_state(
&ShadowedClosure::new(SafePow),
&initial_stack,
&[],
&NonDeterminism::default(),
&None,
Some(&expected_final_stack),
);
}
}
#[test]
fn u32_pow_negative_test() {
for (base, exp) in [
(2, 32),
(3, 21),
(4, 16),
(5, 14),
(6, 13),
(7, 12),
(8, 11),
(9, 11),
(10, 10),
(11, 10),
(12, 10),
(u32::MAX, 2),
(u32::MAX, 3),
(u32::MAX, 4),
(u32::MAX, 5),
(u32::MAX, 6),
(u32::MAX, 7),
(u32::MAX, 8),
(u32::MAX, 9),
(1 << 16, 2),
(1 << 16, 3),
(1 << 16, 4),
(1 << 16, 5),
(1 << 16, 6),
(1 << 16, 7),
(1 << 16, 8),
(1 << 8, 4),
(1 << 8, 8),
(1 << 8, 16),
(1 << 8, 32),
] {
test_assertion_failure(
&ShadowedClosure::new(SafePow),
InitVmState::with_stack(SafePow.set_up_test_stack((base, exp))),
&[120, 121],
);
}
}
}
#[cfg(test)]
mod benches {
use super::*;
use crate::test_prelude::*;
#[test]
fn benchmark() {
ShadowedClosure::new(SafePow).bench()
}
}