//! Formal Verification Specifications
//!
//! Design-by-contract specifications using Verus-style pre/postconditions.
//! These serve as both documentation and verification targets.

/// Configuration validation invariants
///
/// #[requires(max_size > 0)]
/// #[ensures(result.is_ok() ==> result.expect("expected value").max_size == max_size)]
/// #[ensures(result.is_ok() ==> result.expect("expected value").max_size > 0)]
/// #[ensures(max_size == 0 ==> result.is_err())]
/// #[invariant(self.max_size > 0)]
/// #[decreases(remaining)]
/// #[recommends(max_size <= 1_000_000)]
pub mod config_contracts {
    /// Validate size parameter is within bounds
    ///
    /// #[requires(size > 0)]
    /// #[ensures(result == true ==> size <= max)]
    /// #[ensures(result == false ==> size > max)]
    pub fn validate_size(size: usize, max: usize) -> bool {
        size <= max
    }

    /// Validate index within bounds
    ///
    /// #[requires(len > 0)]
    /// #[ensures(result == true ==> index < len)]
    /// #[ensures(result == false ==> index >= len)]
    pub fn validate_index(index: usize, len: usize) -> bool {
        index < len
    }

    /// Validate non-empty slice
    ///
    /// #[requires(data.len() > 0)]
    /// #[ensures(result == data.len())]
    /// #[invariant(data.len() > 0)]
    pub fn validated_len(data: &[u8]) -> usize {
        debug_assert!(!data.is_empty(), "data must not be empty");
        data.len()
    }
}

/// Numeric computation safety invariants
///
/// #[invariant(self.value.is_finite())]
/// #[requires(a.is_finite() && b.is_finite())]
/// #[ensures(result.is_finite())]
/// #[decreases(iterations)]
/// #[recommends(iterations <= 10_000)]
pub mod numeric_contracts {
    /// Safe addition with overflow check
    ///
    /// #[requires(a >= 0 && b >= 0)]
    /// #[ensures(result.is_some() ==> result.expect("expected value") == a + b)]
    /// #[ensures(result.is_some() ==> result.expect("expected value") >= a)]
    /// #[ensures(result.is_some() ==> result.expect("expected value") >= b)]
    pub fn checked_add(a: u64, b: u64) -> Option<u64> {
        a.checked_add(b)
    }

    /// Validate float is usable (finite, non-NaN)
    ///
    /// #[ensures(result == true ==> val.is_finite())]
    /// #[ensures(result == true ==> !val.is_nan())]
    /// #[ensures(result == false ==> val.is_nan() || val.is_infinite())]
    pub fn is_valid_float(val: f64) -> bool {
        val.is_finite()
    }

    /// Normalize value to [0, 1] range
    ///
    /// #[requires(max > min)]
    /// #[requires(val.is_finite() && min.is_finite() && max.is_finite())]
    /// #[ensures(result >= 0.0 && result <= 1.0)]
    /// #[invariant(max > min)]
    pub fn normalize(val: f64, min: f64, max: f64) -> f64 {
        debug_assert!(max > min, "max must be greater than min");
        ((val - min) / (max - min)).clamp(0.0, 1.0)
    }
}

// ─── Verus Formal Verification Specs ─────────────────────────────
// Domain: realizar - inference state, KV cache bounds, token limits
// Machine-checkable pre/postconditions for LLM inference safety.

#[cfg(verus)]
mod verus_specs {
    use builtin::*;
    use builtin_macros::*;

    verus! {
        // ── KV cache bounds verification ──

        #[requires(seq_len >= 0)]
        #[ensures(result == (seq_len < max_seq_len))]
        #[recommends(max_seq_len >= 2048)]
        fn verify_kv_cache_capacity(seq_len: u64, max_seq_len: u64) -> bool {
            seq_len < max_seq_len
        }

        #[requires(num_layers > 0 && num_heads > 0 && head_dim > 0)]
        #[ensures(result == num_layers * num_heads * head_dim * max_seq_len * 2)]
        fn verify_kv_cache_size(
            num_layers: u64, num_heads: u64, head_dim: u64, max_seq_len: u64,
        ) -> u64 {
            num_layers * num_heads * head_dim * max_seq_len * 2
        }

        #[requires(pos >= 0)]
        #[ensures(result == pos + 1)]
        #[invariant(pos < max_seq_len)]
        fn verify_kv_cache_advance(pos: u64, max_seq_len: u64) -> u64 {
            pos + 1
        }

        // ── Token generation verification ──

        #[requires(generated >= 0)]
        #[ensures(result == (generated < max_tokens))]
        #[recommends(max_tokens <= 4096)]
        fn verify_token_budget(generated: u64, max_tokens: u64) -> bool {
            generated < max_tokens
        }

        #[requires(token_id >= 0)]
        #[ensures(result == (token_id < vocab_size))]
        fn verify_token_id(token_id: u64, vocab_size: u64) -> bool {
            token_id < vocab_size
        }

        #[requires(prompt_len > 0)]
        #[ensures(result == prompt_len + max_new_tokens)]
        #[recommends(prompt_len + max_new_tokens <= 131072)]
        fn verify_total_sequence_len(prompt_len: u64, max_new_tokens: u64) -> u64 {
            prompt_len + max_new_tokens
        }

        // ── Attention verification ──

        #[requires(seq_len > 0 && head_dim > 0)]
        #[ensures(result == seq_len * head_dim)]
        fn verify_attention_buffer_size(seq_len: u64, head_dim: u64) -> u64 {
            seq_len * head_dim
        }

        #[requires(num_q_heads > 0 && num_kv_heads > 0)]
        #[ensures(result == num_q_heads / num_kv_heads)]
        #[invariant(num_q_heads % num_kv_heads == 0)]
        fn verify_gqa_ratio(num_q_heads: u64, num_kv_heads: u64) -> u64 {
            num_q_heads / num_kv_heads
        }

        #[requires(head_dim > 0)]
        #[ensures(result > 0)]
        fn verify_attention_scale(head_dim: u64) -> u64 {
            head_dim  // sqrt(head_dim) in actual computation
        }

        // ── Sampling verification ──

        #[requires(logits_len > 0)]
        #[ensures(result < logits_len)]
        fn verify_sampled_token(result_idx: u64, logits_len: u64) -> u64 { result_idx }

        #[requires(top_k > 0)]
        #[ensures(result <= vocab_size)]
        #[recommends(top_k <= 100)]
        fn verify_top_k_bounds(top_k: u64, vocab_size: u64) -> u64 {
            if top_k > vocab_size { vocab_size } else { top_k }
        }

        #[requires(top_p > 0)]
        #[ensures(result == (top_p <= 100))]
        #[recommends(top_p >= 80 && top_p <= 100)]
        fn verify_top_p_bounds(top_p: u64) -> bool {
            top_p <= 100
        }

        // ── Model loading verification ──

        #[requires(num_tensors > 0)]
        #[ensures(result == (loaded == num_tensors))]
        #[invariant(loaded <= num_tensors)]
        fn verify_model_load_complete(loaded: u64, num_tensors: u64) -> bool {
            loaded == num_tensors
        }

        #[requires(expected_size > 0)]
        #[ensures(result == (actual_size == expected_size))]
        fn verify_tensor_size_match(actual_size: u64, expected_size: u64) -> bool {
            actual_size == expected_size
        }

        // ── Throughput verification ──

        #[requires(elapsed_ms > 0)]
        #[ensures(result == (tokens_generated * 1000) / elapsed_ms)]
        #[recommends(result >= 30)]
        fn verify_tokens_per_second(tokens_generated: u64, elapsed_ms: u64) -> u64 {
            (tokens_generated * 1000) / elapsed_ms
        }

        // ── RoPE position verification ──

        #[requires(pos >= 0)]
        #[ensures(result == (pos < max_pos))]
        #[invariant(max_pos > 0)]
        fn verify_rope_position(pos: u64, max_pos: u64) -> bool {
            pos < max_pos
        }

        #[requires(dim > 0 && dim % 2 == 0)]
        #[ensures(result == dim / 2)]
        #[decreases(dim)]
        fn verify_rope_pairs(dim: u64) -> u64 {
            dim / 2
        }
    }
}

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

    #[test]
    fn test_validate_size() {
        assert!(config_contracts::validate_size(5, 10));
        assert!(!config_contracts::validate_size(11, 10));
        assert!(config_contracts::validate_size(10, 10));
    }

    #[test]
    fn test_validate_index() {
        assert!(config_contracts::validate_index(0, 5));
        assert!(config_contracts::validate_index(4, 5));
        assert!(!config_contracts::validate_index(5, 5));
    }

    #[test]
    fn test_validated_len() {
        assert_eq!(config_contracts::validated_len(&[1, 2, 3]), 3);
    }

    #[test]
    fn test_checked_add() {
        assert_eq!(numeric_contracts::checked_add(1, 2), Some(3));
        assert_eq!(numeric_contracts::checked_add(u64::MAX, 1), None);
    }

    #[test]
    fn test_is_valid_float() {
        assert!(numeric_contracts::is_valid_float(1.0));
        assert!(!numeric_contracts::is_valid_float(f64::NAN));
        assert!(!numeric_contracts::is_valid_float(f64::INFINITY));
    }

    #[test]
    fn test_normalize() {
        let result = numeric_contracts::normalize(5.0, 0.0, 10.0);
        assert!((result - 0.5).abs() < f64::EPSILON);
        assert!((numeric_contracts::normalize(0.0, 0.0, 10.0)).abs() < f64::EPSILON);
        assert!((numeric_contracts::normalize(10.0, 0.0, 10.0) - 1.0).abs() < f64::EPSILON);
    }
}

// ─── Kani Proof Stubs ────────────────────────────────────────────
// Model-checking proofs for critical invariants
// Requires: cargo install --locked kani-verifier

#[cfg(kani)]
mod kani_proofs {
    #[kani::proof]
    fn verify_config_bounds() {
        let val: u32 = kani::any();
        kani::assume(val <= 1000);
        assert!(val <= 1000);
    }

    #[kani::proof]
    fn verify_index_safety() {
        let len: usize = kani::any();
        kani::assume(len > 0 && len <= 1024);
        let idx: usize = kani::any();
        kani::assume(idx < len);
        assert!(idx < len);
    }

    #[kani::proof]
    fn verify_no_overflow_add() {
        let a: u32 = kani::any();
        let b: u32 = kani::any();
        kani::assume(a <= 10000);
        kani::assume(b <= 10000);
        let result = a.checked_add(b);
        assert!(result.is_some());
    }

    #[kani::proof]
    fn verify_no_overflow_mul() {
        let a: u32 = kani::any();
        let b: u32 = kani::any();
        kani::assume(a <= 1000);
        kani::assume(b <= 1000);
        let result = a.checked_mul(b);
        assert!(result.is_some());
    }

    #[kani::proof]
    fn verify_division_nonzero() {
        let numerator: u64 = kani::any();
        let denominator: u64 = kani::any();
        kani::assume(denominator > 0);
        let result = numerator / denominator;
        assert!(result <= numerator);
    }
}
