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// Copyright (C) 2019-2023 Aleo Systems Inc.
// This file is part of the snarkVM library.

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at:
// http://www.apache.org/licenses/LICENSE-2.0

// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use super::*;

impl<E: Environment, const RATE: usize> HashToScalar for Poseidon<E, RATE> {
    type Input = Field<E>;
    type Output = Scalar<E>;

    /// Returns a scalar from hashing the input.
    /// This method uses truncation (up to data bits) to project onto the scalar field.
    #[inline]
    fn hash_to_scalar(&self, input: &[Self::Input]) -> Result<Self::Output> {
        // Note: We are reconstituting the base field into a scalar field.
        // This is safe as the scalar field modulus is less than the base field modulus,
        // and thus will always fit within a single base field element.
        debug_assert!(Scalar::<E>::size_in_bits() < Field::<E>::size_in_bits());

        // Hash the input to the base field.
        let output = self.hash(input)?;

        // Truncate the output to the size in data bits (1 bit less than the MODULUS) of the scalar.
        // Slicing here is safe as the base field is larger than the scalar field.
        let bits = &output.to_bits_le()[..Scalar::<E>::size_in_data_bits()];

        // Output the scalar.
        Scalar::from_bits_le(bits)
    }
}