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chore: move implementation of bitwise operations into `blackbox_solve…
…r` (#5209) # Description ## Problem\* Related to #5055 ## Summary\* We don't need the field element to know about AND and XOR as we just need to be able to convert into byte arrays and back. I've then moved this into the blackbox solver crate (the only place we were actually using this) to simplify the `AcirField` trait. ## Additional Context ## Documentation\* Check one: - [x] No documentation needed. - [ ] Documentation included in this PR. - [ ] **[For Experimental Features]** Documentation to be submitted in a separate PR. # PR Checklist\* - [x] I have tested the changes locally. - [x] I have formatted the changes with [Prettier](https://prettier.io/) and/or `cargo fmt` on default settings.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,87 @@ | ||
| use acir::AcirField; | ||
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| pub fn bit_and<F: AcirField>(lhs: F, rhs: F, num_bits: u32) -> F { | ||
| bitwise_op(lhs, rhs, num_bits, |lhs_byte, rhs_byte| lhs_byte & rhs_byte) | ||
| } | ||
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| pub fn bit_xor<F: AcirField>(lhs: F, rhs: F, num_bits: u32) -> F { | ||
| bitwise_op(lhs, rhs, num_bits, |lhs_byte, rhs_byte| lhs_byte ^ rhs_byte) | ||
| } | ||
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| fn bitwise_op<F: AcirField>(lhs: F, rhs: F, num_bits: u32, op: fn(u8, u8) -> u8) -> F { | ||
| // XXX: Gadgets like SHA256 need to have their input be a multiple of 8 | ||
| // This is not a restriction caused by SHA256, as it works on bits | ||
| // but most backends assume bytes. | ||
| // We could implicitly pad, however this may not be intuitive for users. | ||
| // assert!( | ||
| // num_bits % 8 == 0, | ||
| // "num_bits is not a multiple of 8, it is {}", | ||
| // num_bits | ||
| // ); | ||
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| let lhs_bytes = mask_to_be_bytes(lhs, num_bits); | ||
| let rhs_bytes = mask_to_be_bytes(rhs, num_bits); | ||
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| let and_byte_arr: Vec<_> = | ||
| lhs_bytes.into_iter().zip(rhs_bytes).map(|(left, right)| op(left, right)).collect(); | ||
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| F::from_be_bytes_reduce(&and_byte_arr) | ||
| } | ||
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| // mask_to methods will not remove any bytes from the field | ||
| // they are simply zeroed out | ||
| // Whereas truncate_to will remove those bits and make the byte array smaller | ||
| fn mask_to_be_bytes<F: AcirField>(field: F, num_bits: u32) -> Vec<u8> { | ||
| let mut bytes = field.to_be_bytes(); | ||
| mask_vector_le(&mut bytes, num_bits as usize); | ||
| bytes | ||
| } | ||
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| fn mask_vector_le(bytes: &mut [u8], num_bits: usize) { | ||
| // reverse to big endian format | ||
| bytes.reverse(); | ||
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| let mask_power = num_bits % 8; | ||
| let array_mask_index = num_bits / 8; | ||
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| for (index, byte) in bytes.iter_mut().enumerate() { | ||
| match index.cmp(&array_mask_index) { | ||
| std::cmp::Ordering::Less => { | ||
| // do nothing if the current index is less than | ||
| // the array index. | ||
| } | ||
| std::cmp::Ordering::Equal => { | ||
| let mask = 2u8.pow(mask_power as u32) - 1; | ||
| // mask the byte | ||
| *byte &= mask; | ||
| } | ||
| std::cmp::Ordering::Greater => { | ||
| // Anything greater than the array index | ||
| // will be set to zero | ||
| *byte = 0; | ||
| } | ||
| } | ||
| } | ||
| // reverse back to little endian | ||
| bytes.reverse(); | ||
| } | ||
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| #[cfg(test)] | ||
| mod tests { | ||
| use acir::{AcirField, FieldElement}; | ||
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| use crate::bit_and; | ||
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| #[test] | ||
| fn and() { | ||
| let max = 10_000u32; | ||
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| let num_bits = (std::mem::size_of::<u32>() * 8) as u32 - max.leading_zeros(); | ||
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| for x in 0..max { | ||
| let x = FieldElement::from(x as i128); | ||
| let res = bit_and(x, x, num_bits); | ||
| assert_eq!(res.to_be_bytes(), x.to_be_bytes()); | ||
| } | ||
| } | ||
| } |