snarkvm_console_types_scalar/
from_field_lossy.rs

1// Copyright (c) 2019-2025 Provable Inc.
2// This file is part of the snarkVM library.
3
4// Licensed under the Apache License, Version 2.0 (the "License");
5// you may not use this file except in compliance with the License.
6// You may obtain a copy of the License at:
7
8// http://www.apache.org/licenses/LICENSE-2.0
9
10// Unless required by applicable law or agreed to in writing, software
11// distributed under the License is distributed on an "AS IS" BASIS,
12// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13// See the License for the specific language governing permissions and
14// limitations under the License.
15
16use super::*;
17
18impl<E: Environment> Scalar<E> {
19    /// Casts a scalar from a base field, with lossy truncation.
20    ///
21    /// This method is commonly used by hash-to-scalar algorithms,
22    /// where the hash output does not need to preserve the full base field.
23    pub fn from_field_lossy(field: &Field<E>) -> Self {
24        debug_assert!(Scalar::<E>::size_in_bits() < Field::<E>::size_in_bits());
25
26        // Truncate the field to the size in data bits (1 bit less than the MODULUS) of the scalar.
27        // Slicing here is safe as the base field is larger than the scalar field.
28        let result = Self::from_bits_le(&field.to_bits_le()[..Scalar::<E>::size_in_data_bits()]);
29        debug_assert!(result.is_ok(), "A lossy scalar should always be able to be constructed from field bits");
30        result.unwrap()
31    }
32}
33
34#[cfg(test)]
35mod tests {
36    use super::*;
37    use snarkvm_console_network_environment::Console;
38
39    type CurrentEnvironment = Console;
40
41    const ITERATIONS: u64 = 10_000;
42
43    #[test]
44    fn test_from_field() -> Result<()> {
45        let mut rng = TestRng::default();
46
47        for _ in 0..ITERATIONS {
48            // Sample a random scalar.
49            let size_in_data_bits = Scalar::<CurrentEnvironment>::size_in_data_bits();
50            let prepare = &Scalar::<CurrentEnvironment>::rand(&mut rng).to_bits_le()[0..size_in_data_bits];
51            let expected = Scalar::<CurrentEnvironment>::from_bits_le(prepare)?;
52            // Perform the operation.
53            let candidate = Scalar::from_field_lossy(&expected.to_field()?);
54            assert_eq!(expected, candidate);
55
56            // Sample a random field.
57            let expected = Field::<CurrentEnvironment>::rand(&mut rng);
58            // Perform the operation (should not fail).
59            let _result = Scalar::from_field_lossy(&expected);
60        }
61        Ok(())
62    }
63}
snarkvm_console_types_scalar/from_field_lossy.rs

snarkvm_console_types_scalar/
from_field_lossy.rs
