//! Audit trail for solver invocations.
//!
//! Every solve operation can produce a [`SolverAuditEntry`] that captures a
//! tamper-evident fingerprint of the input, output, convergence metrics, and
//! timing. Entries are cheap to produce and can be streamed to any log sink
//! (structured logging, event store, or external SIEM).
//!
//! # Hashing
//!
//! We use [`std::hash::DefaultHasher`] (SipHash-2-4 on most platforms) rather
//! than a cryptographic hash. This is sufficient for audit deduplication and
//! integrity detection but is **not** suitable for security-critical tamper
//! proofing. If cryptographic guarantees are needed, swap in a SHA-256
//! implementation behind a feature gate.

use std::hash::{DefaultHasher, Hash, Hasher};
use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};

use serde::{Deserialize, Serialize};

use crate::types::{Algorithm, CsrMatrix, SolverResult};

// ---------------------------------------------------------------------------
// Audit entry
// ---------------------------------------------------------------------------

/// A single audit trail record for one solver invocation.
///
/// Captures a deterministic fingerprint of the problem (input hash), the
/// solution (output hash), performance counters, and a monotonic timestamp.
///
/// # Serialization
///
/// Derives `Serialize` / `Deserialize` so entries can be persisted as JSON,
/// MessagePack, or any serde-compatible format.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SolverAuditEntry {
    /// Unique identifier for this solve request.
    pub request_id: String,

    /// Algorithm that produced the result.
    pub algorithm: Algorithm,

    /// 8-byte hash of the input (matrix + rhs). Produced by
    /// [`hash_input`].
    pub input_hash: [u8; 8],

    /// 8-byte hash of the output solution vector. Produced by
    /// [`hash_output`].
    pub output_hash: [u8; 8],

    /// Number of iterations the solver executed.
    pub iterations: usize,

    /// Wall-clock time in microseconds.
    pub wall_time_us: u64,

    /// Whether the solver converged within tolerance.
    pub converged: bool,

    /// Final residual L2 norm.
    pub residual: f64,

    /// Timestamp as nanoseconds since the Unix epoch.
    pub timestamp_ns: u128,

    /// Number of rows in the input matrix.
    pub matrix_rows: usize,

    /// Number of non-zero entries in the input matrix.
    pub matrix_nnz: usize,
}

// ---------------------------------------------------------------------------
// Hash helpers
// ---------------------------------------------------------------------------

/// Compute a deterministic 8-byte fingerprint of the solver input.
///
/// Hashes the matrix dimensions, structural arrays (`row_ptr`, `col_indices`),
/// value bytes, and the right-hand-side vector.
pub fn hash_input(matrix: &CsrMatrix<f32>, rhs: &[f32]) -> [u8; 8] {
    let mut h = DefaultHasher::new();

    // Matrix structure
    matrix.rows.hash(&mut h);
    matrix.cols.hash(&mut h);
    matrix.row_ptr.hash(&mut h);
    matrix.col_indices.hash(&mut h);

    // Values as raw bytes (avoids floating-point hashing issues)
    for &v in &matrix.values {
        v.to_bits().hash(&mut h);
    }

    // RHS
    for &v in rhs {
        v.to_bits().hash(&mut h);
    }

    h.finish().to_le_bytes()
}

/// Compute a deterministic 8-byte fingerprint of the solution vector.
pub fn hash_output(solution: &[f32]) -> [u8; 8] {
    let mut h = DefaultHasher::new();
    for &v in solution {
        v.to_bits().hash(&mut h);
    }
    h.finish().to_le_bytes()
}

// ---------------------------------------------------------------------------
// Builder
// ---------------------------------------------------------------------------

/// Convenience builder for [`SolverAuditEntry`].
///
/// Start a timer at the beginning of a solve, then call [`finish`] with the
/// result to produce a complete audit record.
///
/// # Example
///
/// ```ignore
/// let audit = AuditBuilder::start("req-42", &matrix, &rhs);
/// let result = solver.solve(&matrix, &rhs)?;
/// let entry = audit.finish(&result, tolerance);
/// tracing::info!(?entry, "solve completed");
/// ```
pub struct AuditBuilder {
    request_id: String,
    input_hash: [u8; 8],
    matrix_rows: usize,
    matrix_nnz: usize,
    start: Instant,
    timestamp_ns: u128,
}

impl AuditBuilder {
    /// Begin an audit trace for a new solve request.
    ///
    /// Records the wall-clock start time and computes the input hash eagerly
    /// so that the hash is taken before any mutation.
    pub fn start(request_id: impl Into<String>, matrix: &CsrMatrix<f32>, rhs: &[f32]) -> Self {
        let timestamp_ns = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap_or(Duration::ZERO)
            .as_nanos();

        Self {
            request_id: request_id.into(),
            input_hash: hash_input(matrix, rhs),
            matrix_rows: matrix.rows,
            matrix_nnz: matrix.values.len(),
            start: Instant::now(),
            timestamp_ns,
        }
    }

    /// Finalize the audit entry after the solver returns.
    ///
    /// `tolerance` is the target tolerance that was requested so that
    /// `converged` can be computed from the residual.
    pub fn finish(self, result: &SolverResult, tolerance: f64) -> SolverAuditEntry {
        let elapsed = self.start.elapsed();

        SolverAuditEntry {
            request_id: self.request_id,
            algorithm: result.algorithm,
            input_hash: self.input_hash,
            output_hash: hash_output(&result.solution),
            iterations: result.iterations,
            wall_time_us: elapsed.as_micros() as u64,
            converged: result.residual_norm <= tolerance,
            residual: result.residual_norm,
            timestamp_ns: self.timestamp_ns,
            matrix_rows: self.matrix_rows,
            matrix_nnz: self.matrix_nnz,
        }
    }
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use super::*;
    use crate::types::{Algorithm, ConvergenceInfo, SolverResult};
    use std::time::Duration;

    fn sample_matrix() -> CsrMatrix<f32> {
        CsrMatrix::<f32>::from_coo(
            2,
            2,
            vec![(0, 0, 2.0), (0, 1, -0.5), (1, 0, -0.5), (1, 1, 2.0)],
        )
    }

    fn sample_result() -> SolverResult {
        SolverResult {
            solution: vec![0.5, 0.5],
            iterations: 10,
            residual_norm: 1e-9,
            wall_time: Duration::from_millis(2),
            convergence_history: vec![ConvergenceInfo {
                iteration: 9,
                residual_norm: 1e-9,
            }],
            algorithm: Algorithm::Neumann,
        }
    }

    #[test]
    fn hash_input_deterministic() {
        let m = sample_matrix();
        let rhs = vec![1.0f32, 1.0];
        let h1 = hash_input(&m, &rhs);
        let h2 = hash_input(&m, &rhs);
        assert_eq!(h1, h2, "same input must produce same hash");
    }

    #[test]
    fn hash_input_changes_with_values() {
        let m1 = sample_matrix();
        let mut m2 = sample_matrix();
        m2.values[0] = 3.0;
        let rhs = vec![1.0f32, 1.0];
        assert_ne!(
            hash_input(&m1, &rhs),
            hash_input(&m2, &rhs),
            "different values must produce different hashes",
        );
    }

    #[test]
    fn hash_input_changes_with_rhs() {
        let m = sample_matrix();
        let rhs1 = vec![1.0f32, 1.0];
        let rhs2 = vec![1.0f32, 2.0];
        assert_ne!(
            hash_input(&m, &rhs1),
            hash_input(&m, &rhs2),
            "different rhs must produce different hashes",
        );
    }

    #[test]
    fn hash_output_deterministic() {
        let sol = vec![0.5f32, 0.5];
        assert_eq!(hash_output(&sol), hash_output(&sol));
    }

    #[test]
    fn hash_output_changes() {
        let sol1 = vec![0.5f32, 0.5];
        let sol2 = vec![0.5f32, 0.6];
        assert_ne!(hash_output(&sol1), hash_output(&sol2));
    }

    #[test]
    fn audit_builder_produces_entry() {
        let m = sample_matrix();
        let rhs = vec![1.0f32, 1.0];
        let builder = AuditBuilder::start("test-req-1", &m, &rhs);

        let result = sample_result();
        let entry = builder.finish(&result, 1e-6);

        assert_eq!(entry.request_id, "test-req-1");
        assert_eq!(entry.algorithm, Algorithm::Neumann);
        assert_eq!(entry.iterations, 10);
        assert!(entry.converged, "residual 1e-9 < tolerance 1e-6");
        assert_eq!(entry.matrix_rows, 2);
        assert_eq!(entry.matrix_nnz, 4);
        assert!(entry.timestamp_ns > 0);
    }

    #[test]
    fn audit_builder_not_converged() {
        let m = sample_matrix();
        let rhs = vec![1.0f32, 1.0];
        let builder = AuditBuilder::start("test-req-2", &m, &rhs);

        let mut result = sample_result();
        result.residual_norm = 0.1; // Above tolerance
        let entry = builder.finish(&result, 1e-6);

        assert!(!entry.converged);
    }

    #[test]
    fn audit_entry_is_serializable() {
        // Verify that the entry can be serialized/deserialized via serde.
        // We test using bincode (available as a dev-dep) or just verify the
        // derive attributes are correct by round-tripping through Debug.
        let m = sample_matrix();
        let rhs = vec![1.0f32, 1.0];
        let builder = AuditBuilder::start("ser-test", &m, &rhs);
        let result = sample_result();
        let entry = builder.finish(&result, 1e-6);

        // At minimum, verify Debug output contains expected fields.
        let debug = format!("{:?}", entry);
        assert!(debug.contains("ser-test"), "debug: {debug}");
        assert!(debug.contains("Neumann"), "debug: {debug}");

        // Verify Clone works (which Serialize/Deserialize depend on for some codecs).
        let cloned = entry.clone();
        assert_eq!(cloned.request_id, entry.request_id);
        assert_eq!(cloned.input_hash, entry.input_hash);
        assert_eq!(cloned.output_hash, entry.output_hash);
        assert_eq!(cloned.iterations, entry.iterations);
    }
}