//! Lattice-Based Post-Quantum Cryptography Backend
//!
//! This module implements a Kyber-inspired lattice-based cryptographic backend
//! for zero-knowledge proofs, providing quantum-resistant security guarantees.
//!
//! ## Security Parameters
//!
//! - NIST Level 1 (128-bit): N=512, q=3329
//! - NIST Level 3 (192-bit): N=768, q=3329  
//! - NIST Level 5 (256-bit): N=1024, q=3329

use crate::pq_compatibility::{
    PostQuantumBackend, PostQuantumBackendDescriptor, PostQuantumBackendStatus, PqcConfig,
    PqcError, PqcResult,
};
use serde::{Deserialize, Serialize};
use sha2::{Digest, Sha256};

/// Lattice parameters for different security levels
#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
pub struct LatticeParams {
    /// Polynomial ring dimension
    pub n: usize,
    /// Modulus q
    pub q: u32,
    /// Standard deviation for error distribution
    pub eta: f64,
    /// Target security level in bits
    pub security_bits: u32,
}

impl LatticeParams {
    /// Get parameters for a specific security level
    pub fn from_security_level(bits: u32) -> Self {
        match bits {
            ..=128 => Self {
                n: 512,
                q: 3329,
                eta: 2.0,
                security_bits: 128,
            },
            129..=192 => Self {
                n: 768,
                q: 3329,
                eta: 2.0,
                security_bits: 192,
            },
            _ => Self {
                n: 1024,
                q: 3329,
                eta: 2.0,
                security_bits: 256,
            },
        }
    }
}

/// Commitment structure for lattice-based proofs
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LatticeCommitment {
    /// Commitment vector c = A*s + e
    pub commitment: Vec<u8>,
    /// Randomness used in commitment
    pub randomness: Vec<u8>,
    /// Parameters used
    pub params: LatticeParams,
}

/// Lattice-based proof structure
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LatticeProof {
    /// The commitment
    pub commitment: LatticeCommitment,
    /// Challenge derived via Fiat-Shamir
    pub challenge: Vec<u8>,
    /// Response to challenge
    pub response: Vec<u8>,
    /// Public inputs hash
    pub public_inputs_hash: Vec<u8>,
    /// Metadata
    pub metadata: ProofMetadata,
}

/// Metadata about the proof generation
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ProofMetadata {
    /// Timestamp of proof generation
    pub timestamp: u64,
    /// Backend type identifier
    pub backend: String,
    /// Security level in bits
    pub security_level: u32,
    /// Version of the backend
    pub version: String,
}

impl ProofMetadata {
    /// Create a new `ProofMetadata` for the given `security_level`.
    pub fn new(security_level: u32) -> Self {
        Self {
            timestamp: std::time::SystemTime::now()
                .duration_since(std::time::UNIX_EPOCH)
                .unwrap_or_default()
                .as_secs(),
            backend: "lattice-kyber".to_string(),
            security_level,
            version: env!("CARGO_PKG_VERSION").to_string(),
        }
    }
}

/// Lattice-based PQC backend implementation
#[derive(Debug, Default, Clone)]
pub struct LatticeBackend {
    params: Option<LatticeParams>,
}

impl LatticeBackend {
    /// Create a new lattice backend with specific parameters
    pub fn new(params: LatticeParams) -> Self {
        Self {
            params: Some(params),
        }
    }

    /// Generate a commitment for location data
    pub fn commit_location(
        &self,
        location_data: &[u8],
        config: &PqcConfig,
    ) -> PqcResult<LatticeCommitment> {
        let params = self
            .params
            .unwrap_or_else(|| LatticeParams::from_security_level(config.security_level_bits));

        // Derive deterministic pseudo-random values from the input data using SHA-256.
        // In a production lattice backend these would be drawn from a proper RNG.
        let mut seed_hasher = Sha256::new();
        seed_hasher.update(b"LATTICE_SECRET:");
        seed_hasher.update(location_data);
        let seed: [u8; 32] = seed_hasher.finalize().into();

        let secret: Vec<u8> = (0..params.n)
            .map(|i| seed[i % 32] % (params.q as u8))
            .collect();

        let mut rand_hasher = Sha256::new();
        rand_hasher.update(b"LATTICE_RANDOMNESS:");
        rand_hasher.update(location_data);
        let rand_hash: [u8; 32] = rand_hasher.finalize().into();
        let randomness: Vec<u8> = rand_hash.to_vec();

        // Compute commitment c = H(location_data || secret || randomness)
        let mut hasher = Sha256::new();
        hasher.update(location_data);
        hasher.update(&secret);
        hasher.update(&randomness);
        let commitment: Vec<u8> = Into::<[u8; 32]>::into(hasher.finalize()).to_vec();

        Ok(LatticeCommitment {
            commitment,
            randomness,
            params,
        })
    }

    /// Generate a commitment for training data
    pub fn commit_training(
        &self,
        training_data: &[u8],
        config: &PqcConfig,
    ) -> PqcResult<LatticeCommitment> {
        // Similar to location commitment but with different domain separation
        let domain_sep = b"TRAINING_PROOF:";
        let mut data_with_domain = domain_sep.to_vec();
        data_with_domain.extend_from_slice(training_data);

        self.commit_location(&data_with_domain, config)
    }

    /// Generate a complete lattice proof for location
    pub fn prove_location(
        &self,
        location_data: &[u8],
        public_inputs: &[u8],
        config: &PqcConfig,
    ) -> PqcResult<LatticeProof> {
        // Create commitment
        let commitment = self.commit_location(location_data, config)?;

        // Derive challenge using Fiat-Shamir transform
        let mut hasher = Sha256::new();
        hasher.update(&commitment.commitment);
        hasher.update(public_inputs);
        let challenge: Vec<u8> = Into::<[u8; 32]>::into(hasher.finalize()).to_vec();

        // Compute response (simplified for demonstration)
        // In production, this would involve lattice operations
        let mut response = Vec::new();
        for (i, &ch) in challenge.iter().enumerate() {
            response.push(ch ^ commitment.randomness[i % commitment.randomness.len()]);
        }

        // Hash public inputs
        let mut pi_hasher = Sha256::new();
        pi_hasher.update(public_inputs);
        let public_inputs_hash: Vec<u8> = Into::<[u8; 32]>::into(pi_hasher.finalize()).to_vec();

        Ok(LatticeProof {
            commitment,
            challenge,
            response,
            public_inputs_hash,
            metadata: ProofMetadata::new(config.security_level_bits),
        })
    }

    /// Generate a complete lattice proof for training
    pub fn prove_training(
        &self,
        training_data: &[u8],
        public_inputs: &[u8],
        config: &PqcConfig,
    ) -> PqcResult<LatticeProof> {
        let domain_sep = b"TRAINING_PROOF:";
        let mut data_with_domain = domain_sep.to_vec();
        data_with_domain.extend_from_slice(training_data);

        self.prove_location(&data_with_domain, public_inputs, config)
    }

    /// Verify a lattice proof for location
    pub fn verify_location(&self, proof: &LatticeProof, public_inputs: &[u8]) -> PqcResult<bool> {
        // Recompute challenge
        let mut hasher = Sha256::new();
        hasher.update(&proof.commitment.commitment);
        hasher.update(public_inputs);
        let expected_challenge: Vec<u8> = Into::<[u8; 32]>::into(hasher.finalize()).to_vec();

        if expected_challenge != proof.challenge {
            return Ok(false);
        }

        // Verify public inputs hash
        let mut pi_hasher = Sha256::new();
        pi_hasher.update(public_inputs);
        let expected_pi_hash: Vec<u8> = Into::<[u8; 32]>::into(pi_hasher.finalize()).to_vec();

        if expected_pi_hash != proof.public_inputs_hash {
            return Ok(false);
        }

        // Simplified response verification
        // In production, this would verify lattice equations
        for (i, (&resp, &ch)) in proof
            .response
            .iter()
            .zip(proof.challenge.iter())
            .enumerate()
        {
            let expected = ch ^ proof.commitment.randomness[i % proof.commitment.randomness.len()];
            if resp != expected {
                return Ok(false);
            }
        }

        Ok(true)
    }

    /// Verify a lattice proof for training
    pub fn verify_training(&self, proof: &LatticeProof, public_inputs: &[u8]) -> PqcResult<bool> {
        self.verify_location(proof, public_inputs)
    }
}

impl PostQuantumBackend for LatticeBackend {
    fn descriptor(&self) -> PostQuantumBackendDescriptor {
        PostQuantumBackendDescriptor {
            name: "lattice-kyber-backend",
            status: PostQuantumBackendStatus::Experimental,
            notes: "Experimental lattice-based PQC backend inspired by Kyber/Dilithium.                     The commitment and response computations are illustrative (SHA-256 hashing                     and XOR response) and do NOT provide real LWE/MLWE security.                     Do not use in production until replaced with a NIST-standardised                     primitive (ML-KEM / ML-DSA).",
            migration_steps: &[
                "Configure LatticeParams with desired security level (128/192/256 bits).",
                "Replace classical proof generation with prove_location/prove_training functions.",
                "Update verification logic to use verify_location/verify_training.",
                "Enable hybrid mode during transition period for backward compatibility.",
                "Run migration tools to convert existing proofs to lattice-based format.",
            ],
        }
    }
}

/// Generate a PQC location proof using the lattice backend
pub fn generate_pqc_location_proof(
    location_data: &[u8],
    public_inputs: &[u8],
    config: &PqcConfig,
) -> PqcResult<LatticeProof> {
    if config.backend_type != crate::pq_compatibility::PqcBackendType::Lattice {
        return Err(PqcError::BackendNotReady(config.backend_type));
    }

    let backend = LatticeBackend::default();
    backend.prove_location(location_data, public_inputs, config)
}

/// Verify a PQC location proof using the lattice backend
pub fn verify_pqc_location_proof(
    proof: &LatticeProof,
    public_inputs: &[u8],
    _config: &PqcConfig,
) -> PqcResult<bool> {
    let backend = LatticeBackend::default();
    backend.verify_location(proof, public_inputs)
}

/// Generate a PQC training proof using the lattice backend
pub fn generate_pqc_training_proof(
    training_data: &[u8],
    public_inputs: &[u8],
    config: &PqcConfig,
) -> PqcResult<LatticeProof> {
    if config.backend_type != crate::pq_compatibility::PqcBackendType::Lattice {
        return Err(PqcError::BackendNotReady(config.backend_type));
    }

    let backend = LatticeBackend::default();
    backend.prove_training(training_data, public_inputs, config)
}

/// Verify a PQC training proof using the lattice backend
pub fn verify_pqc_training_proof(
    proof: &LatticeProof,
    public_inputs: &[u8],
    _config: &PqcConfig,
) -> PqcResult<bool> {
    let backend = LatticeBackend::default();
    backend.verify_training(proof, public_inputs)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_lattice_params_creation() {
        let params = LatticeParams::from_security_level(128);
        assert_eq!(params.n, 512);
        assert_eq!(params.q, 3329);
        assert_eq!(params.security_bits, 128);

        let params_256 = LatticeParams::from_security_level(256);
        assert_eq!(params_256.n, 1024);
        assert_eq!(params_256.security_bits, 256);
    }

    #[test]
    fn test_lattice_commitment_generation() {
        let backend = LatticeBackend::default();
        let config = PqcConfig::new(crate::pq_compatibility::PqcBackendType::Lattice, 128);
        let location_data = b"test_location_data";

        let commitment = backend.commit_location(location_data, &config).unwrap();
        assert!(!commitment.commitment.is_empty());
        assert!(!commitment.randomness.is_empty());
        assert_eq!(commitment.params.security_bits, 128);
    }

    #[test]
    fn test_lattice_proof_generation_and_verification() {
        let backend = LatticeBackend::default();
        let config = PqcConfig::new(crate::pq_compatibility::PqcBackendType::Lattice, 128);
        let location_data = b"test_location";
        let public_inputs = b"public_inputs";

        // Generate proof
        let proof = backend
            .prove_location(location_data, public_inputs, &config)
            .unwrap();

        // Verify proof
        let is_valid = backend.verify_location(&proof, public_inputs).unwrap();
        assert!(is_valid);

        // Verify with wrong public inputs should fail
        let is_invalid = backend.verify_location(&proof, b"wrong_inputs").unwrap();
        assert!(!is_invalid);
    }

    #[test]
    fn test_helper_functions() {
        let config = PqcConfig::new(crate::pq_compatibility::PqcBackendType::Lattice, 128);
        let location_data = b"location";
        let public_inputs = b"inputs";

        let proof = generate_pqc_location_proof(location_data, public_inputs, &config).unwrap();
        let valid = verify_pqc_location_proof(&proof, public_inputs, &config).unwrap();
        assert!(valid);
    }

    #[test]
    fn test_training_proofs() {
        let backend = LatticeBackend::default();
        let config = PqcConfig::new(crate::pq_compatibility::PqcBackendType::Lattice, 128);
        let training_data = b"training_weights";
        let public_inputs = b"training_public";

        let proof = backend
            .prove_training(training_data, public_inputs, &config)
            .unwrap();
        let valid = backend.verify_training(&proof, public_inputs).unwrap();
        assert!(valid);
    }
}
