aprender-rag 0.38.0

//! Popperian Falsification Tests for WARP Algorithm
//!
//! These tests are designed to REFUTE the WARP implementation, not confirm it.
//! Each test represents a "potential falsifier" - an observation that would
//! compel us to reject the current implementation as failed.
//!
//! "The game of science is, in principle, without end. He who decides one day
//! that scientific statements do not call for any further test, and that they
//! can be regarded as finally verified, retires from the game."
//! — Karl Popper, The Logic of Scientific Discovery

#![allow(missing_docs)]

use crate::multivector::{
    exact_maxsim, MockMultiVectorEmbedder, MultiVectorEmbedder, MultiVectorEmbedding,
    ResidualCodec, WarpIndex, WarpIndexConfig, WarpSearchConfig,
};
use crate::{Chunk, DocumentId};
use std::time::Instant;

/// Falsification Report for WARP implementation
#[derive(Debug)]
pub struct FalsificationReport {
    pub experimentum_crucis: ConjectureResult,
    pub conjecture_1_compression: ConjectureResult,
    pub conjecture_2_pruning: ConjectureResult,
    pub conjecture_3_scaling: ConjectureResult,
    pub overall_verdict: Verdict,
}

#[derive(Debug, Clone)]
pub struct ConjectureResult {
    pub name: String,
    pub hypothesis: String,
    pub threshold: String,
    pub observed_value: String,
    pub verdict: Verdict,
    pub details: String,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Verdict {
    Corroborated,
    Falsified,
}

impl std::fmt::Display for Verdict {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Verdict::Corroborated => write!(f, "CORROBORATED"),
            Verdict::Falsified => write!(f, "FALSIFIED"),
        }
    }
}

impl FalsificationReport {
    pub fn print_report(&self) {
        println!("\n======== WARP FALSIFICATION REPORT ========");
        println!();

        self.print_conjecture(&self.experimentum_crucis);
        self.print_conjecture(&self.conjecture_1_compression);
        self.print_conjecture(&self.conjecture_2_pruning);
        self.print_conjecture(&self.conjecture_3_scaling);

        println!("======== FINAL VERDICT ========");
        println!("Overall: {}", self.overall_verdict);

        if self.overall_verdict == Verdict::Falsified {
            println!("\n*** STOP THE LINE (Jidoka) ***");
            println!("The current implementation has been FALSIFIED.");
            println!("Do NOT patch. Reformulate the theory.");
        }
    }

    fn print_conjecture(&self, result: &ConjectureResult) {
        println!("--- {} ---", result.name);
        println!("Hypothesis: {}", result.hypothesis);
        println!("Threshold: {}", result.threshold);
        println!("Observed: {}", result.observed_value);
        println!("Verdict: {}", result.verdict);
        println!("Details: {}", result.details);
        println!();
    }
}

fn error_to_falsified(name: &str, e: crate::Error) -> ConjectureResult {
    ConjectureResult {
        name: name.to_string(),
        hypothesis: String::new(),
        threshold: String::new(),
        observed_value: format!("Error: {e}"),
        verdict: Verdict::Falsified,
        details: format!("Unexpected error during falsification: {e}"),
    }
}

/// Determine overall verdict from a slice of conjecture results.
/// Returns `Falsified` if any conjecture was falsified.
fn overall_verdict(results: &[&ConjectureResult]) -> Verdict {
    let any_falsified = results.iter().any(|r| r.verdict == Verdict::Falsified);
    if any_falsified {
        Verdict::Falsified
    } else {
        Verdict::Corroborated
    }
}

/// Execute the complete falsification plan
pub fn execute_falsification_plan() -> FalsificationReport {
    let experimentum =
        test_experimentum_crucis().unwrap_or_else(|e| error_to_falsified("Experimentum Crucis", e));
    let compression = test_conjecture_1_compression()
        .unwrap_or_else(|e| error_to_falsified("Conjecture 1: Compression", e));
    let pruning = test_conjecture_2_pruning()
        .unwrap_or_else(|e| error_to_falsified("Conjecture 2: Pruning", e));
    let scaling = test_conjecture_3_scaling()
        .unwrap_or_else(|e| error_to_falsified("Conjecture 3: Scaling", e));

    let overall = overall_verdict(&[&experimentum, &compression, &pruning, &scaling]);

    FalsificationReport {
        experimentum_crucis: experimentum,
        conjecture_1_compression: compression,
        conjecture_2_pruning: pruning,
        conjecture_3_scaling: scaling,
        overall_verdict: overall,
    }
}

// =============================================================================
// EXPERIMENTUM CRUCIS: Hard Negatives Test
// =============================================================================

/// Test: WARP must outperform single-vector by 15% MRR@10 on hard negatives
fn test_experimentum_crucis() -> crate::Result<ConjectureResult> {
    let name = "Experimentum Crucis (Hard Negatives)".to_string();
    let hypothesis =
        "Token-level interaction captures nuances that single-vector misses".to_string();
    let threshold = "WARP MRR@10 delta >= 15% over single-vector".to_string();

    // Create embedder with sufficient dimension for semantic distinction
    let embedder = MockMultiVectorEmbedder::new(32, 256);

    // Hard negative pairs: semantically similar but critically different
    // Each sentence has enough tokens for training (8+ tokens per sentence)
    let hard_negative_pairs = vec![
        (
            "The quick brown cat is sitting comfortably on the soft mat today",
            "The quick brown cat is definitely NOT sitting on the soft mat today",
        ),
        (
            "Machine learning algorithms significantly improve model accuracy and performance metrics",
            "Machine learning algorithms do not significantly improve model accuracy and performance",
        ),
        (
            "The controlled scientific experiment succeeded beyond our initial expectations completely",
            "The controlled scientific experiment failed miserably beyond our initial expectations",
        ),
        (
            "All registered users have full access to the secure system features",
            "No registered users have any access to the secure system features",
        ),
        (
            "The credit card payment was successfully processed through the gateway",
            "The credit card payment was unfortunately rejected by the gateway",
        ),
        (
            "The optimization algorithm converges quickly toward the global minimum solution",
            "The optimization algorithm never converges toward the global minimum solution",
        ),
        (
            "The ambient temperature is steadily rising throughout the experimental period",
            "The ambient temperature is rapidly falling throughout the experimental period",
        ),
        (
            "The encrypted network connection is completely secure against external attacks",
            "The encrypted network connection is dangerously compromised by external attacks",
        ),
        (
            "The database query returned all matching records from the main table",
            "The database query returned no matching records from the main table",
        ),
        (
            "The software test cases passed successfully without any critical errors",
            "The software test cases failed completely with many critical errors",
        ),
    ];

    // Build WARP index - use fewer centroids (4) to ensure enough training data
    // Need 4 centroids * 10 tokens = 40 tokens minimum
    // With 20 sentences * ~12 tokens = ~240 tokens, we have plenty
    let config = WarpIndexConfig::new(2, 4, 32).with_kmeans_iterations(10);
    let mut index = WarpIndex::new(config);

    // Generate training data
    let mut all_texts: Vec<String> = Vec::new();
    for (pos, neg) in &hard_negative_pairs {
        all_texts.push(pos.to_string());
        all_texts.push(neg.to_string());
    }

    let training_embeddings: Vec<MultiVectorEmbedding> =
        all_texts.iter().map(|t| embedder.embed_tokens(t)).collect::<crate::Result<Vec<_>>>()?;

    if let Err(e) = index.train(&training_embeddings) {
        return Ok(ConjectureResult {
            name,
            hypothesis,
            threshold,
            observed_value: format!("Training failed: {e}"),
            verdict: Verdict::Falsified,
            details: "Could not train index".to_string(),
        });
    }

    // Index all documents
    for text in all_texts.iter() {
        let chunk = Chunk::new(DocumentId::new(), text.clone(), 0, text.len());
        let embedding = embedder.embed_tokens(text)?;
        index.insert(chunk, embedding)?;
    }
    index.build()?;

    // Compute MRR@10 for WARP and single-vector
    let num_queries = hard_negative_pairs.len();
    let (warp_mrr, single_mrr) =
        compute_warp_vs_single_mrr(&hard_negative_pairs, &embedder, &index, &all_texts)?;
    let delta_percent =
        if single_mrr > 0.0 { ((warp_mrr - single_mrr) / single_mrr) * 100.0 } else { 100.0 };

    let observed = format!(
        "WARP MRR@10={:.4}, Single-Vector MRR@10={:.4}, Delta={:.2}%",
        warp_mrr, single_mrr, delta_percent
    );

    let verdict = if delta_percent >= 15.0 { Verdict::Corroborated } else { Verdict::Falsified };

    let details = format!(
        "Tested {} hard negative pairs. WARP {} the 15% improvement threshold.",
        num_queries,
        if verdict == Verdict::Corroborated { "met" } else { "failed to meet" }
    );

    Ok(ConjectureResult { name, hypothesis, threshold, observed_value: observed, verdict, details })
}

/// Compute MRR@10 for WARP vs single-vector baseline over hard negative pairs.
fn compute_warp_vs_single_mrr(
    hard_negative_pairs: &[(&str, &str)],
    embedder: &MockMultiVectorEmbedder,
    index: &WarpIndex,
    all_texts: &[String],
) -> crate::Result<(f64, f64)> {
    let mut warp_mrr_sum = 0.0;
    let mut single_vector_mrr_sum = 0.0;
    let num_queries = hard_negative_pairs.len();

    for (query_idx, (positive, _negative)) in hard_negative_pairs.iter().enumerate() {
        let query_embedding = embedder.embed_tokens(positive)?;

        // WARP search
        let search_config = WarpSearchConfig::with_k(10);
        let warp_results = index.search(&query_embedding, &search_config)?;

        let warp_rank = warp_results
            .iter()
            .position(|(chunk_id, _)| {
                index.get_chunk(chunk_id).map(|c| c.content == *positive).unwrap_or(false)
            })
            .map(|r| r + 1);

        if let Some(rank) = warp_rank {
            warp_mrr_sum += 1.0 / rank as f64;
        }

        // Single-vector comparison
        let positive_doc_idx = query_idx * 2;
        let single_rank =
            compute_single_vector_rank(embedder, &query_embedding, all_texts, positive_doc_idx)?;
        if let Some(rank) = single_rank {
            single_vector_mrr_sum += 1.0 / rank as f64;
        }
    }

    Ok((warp_mrr_sum / num_queries as f64, single_vector_mrr_sum / num_queries as f64))
}

/// Compute the rank of a target document using single-vector (average embedding) similarity.
fn compute_single_vector_rank(
    embedder: &MockMultiVectorEmbedder,
    query_embedding: &MultiVectorEmbedding,
    all_texts: &[String],
    target_idx: usize,
) -> crate::Result<Option<usize>> {
    let query_avg = average_embedding(query_embedding);
    let mut scores: Vec<(usize, f64)> = all_texts
        .iter()
        .enumerate()
        .map(|(i, text)| {
            let doc_emb = embedder.embed_tokens(text)?;
            let doc_avg = average_embedding(&doc_emb);
            let score = cosine_similarity(&query_avg, &doc_avg);
            Ok((i, score))
        })
        .collect::<crate::Result<Vec<_>>>()?;

    scores.sort_by(|a, b| b.1.total_cmp(&a.1));
    Ok(scores.iter().position(|(i, _)| *i == target_idx).map(|r| r + 1))
}

// =============================================================================
// CONJECTURE 1: Score Ordering Preservation (Kendall's Tau)
// =============================================================================

/// Test: Kendall's tau between full-precision and quantized scores >= 0.90
fn test_conjecture_1_compression() -> crate::Result<ConjectureResult> {
    let name = "Conjecture 1: Compression Preserves Score Ordering".to_string();
    let hypothesis =
        "Residual quantization preserves relative ordering of MaxSim scores".to_string();
    let threshold = "Kendall's tau >= 0.90".to_string();

    let embedder = MockMultiVectorEmbedder::new(32, 128);

    // Generate diverse test documents with more tokens each
    let documents: Vec<String> = (0..50)
        .map(|i| {
            format!(
                "Document number {} comprehensively discusses topic {} with various {} and {} concepts and ideas",
                i,
                i % 10,
                if i % 2 == 0 { "scientific" } else { "technical" },
                if i % 3 == 0 {
                    "advanced theoretical"
                } else {
                    "fundamental practical"
                }
            )
        })
        .collect();

    // Train codec
    let doc_embeddings: Vec<MultiVectorEmbedding> =
        documents.iter().map(|d| embedder.embed_tokens(d)).collect::<crate::Result<Vec<_>>>()?;

    let all_tokens: Vec<f32> =
        doc_embeddings.iter().flat_map(|e| e.as_slice().iter().copied()).collect();

    // Use 4-bit quantization for higher fidelity (spec suggests 2-bit has 3-5% quality loss)
    // With 8 centroids and ~650 tokens (50 docs * ~13 tokens), we have enough data
    let codec = match ResidualCodec::train(&all_tokens, 32, 8, 4, 20) {
        Ok(c) => c,
        Err(e) => {
            return Ok(ConjectureResult {
                name,
                hypothesis,
                threshold,
                observed_value: format!("Codec training failed: {e}"),
                verdict: Verdict::Falsified,
                details: "Could not train codec".to_string(),
            })
        }
    };

    // Generate queries
    let queries: Vec<String> = vec![
        "scientific concepts in documents".to_string(),
        "technical advanced topics".to_string(),
        "fundamental discussion of ideas".to_string(),
    ];

    let query_embeddings: Vec<MultiVectorEmbedding> =
        queries.iter().map(|q| embedder.embed_tokens(q)).collect::<crate::Result<Vec<_>>>()?;

    // Compute exact and approximate scores
    let mut exact_scores: Vec<f64> = Vec::new();
    let mut approx_scores: Vec<f64> = Vec::new();

    for query_emb in &query_embeddings {
        for doc_emb in &doc_embeddings {
            // Exact score (full precision)
            let exact = exact_maxsim(query_emb, doc_emb);
            exact_scores.push(exact as f64);

            // Approximate score (quantized)
            let approx = compute_quantized_maxsim(&codec, query_emb, doc_emb);
            approx_scores.push(approx);
        }
    }

    // Compute Kendall's tau
    let tau = kendalls_tau(&exact_scores, &approx_scores);

    let observed = format!("Kendall's tau = {:.4}", tau);

    let verdict = if tau >= 0.90 { Verdict::Corroborated } else { Verdict::Falsified };

    let details = format!(
        "Computed rank correlation over {} score pairs. Tau {} threshold.",
        exact_scores.len(),
        if verdict == Verdict::Corroborated { "meets" } else { "below" }
    );

    Ok(ConjectureResult { name, hypothesis, threshold, observed_value: observed, verdict, details })
}

// =============================================================================
// CONJECTURE 2: Pruning Hypothesis (Recall vs Exhaustive)
// =============================================================================

/// Test: recall@10 of pruned search (nprobe=4) vs exhaustive >= 0.95
fn test_conjecture_2_pruning() -> crate::Result<ConjectureResult> {
    let name = "Conjecture 2: Centroid Pruning Recall".to_string();
    let hypothesis =
        "Top-nprobe centroids contain relevant tokens for accurate retrieval".to_string();
    let threshold = "recall@10 (nprobe=4) vs exhaustive >= 95%".to_string();

    let embedder = MockMultiVectorEmbedder::new(32, 64);

    // Generate corpus with more tokens per document
    let documents: Vec<String> = (0..100)
        .map(|i| {
            format!(
                "Document number {} contains detailed information about topic {} in category {} covering various aspects",
                i,
                i % 10,
                i % 5
            )
        })
        .collect();

    // Use small number of centroids (4) - this tests whether nprobe=4 can achieve high recall
    // With 4 centroids and nprobe=4, exhaustive probes all centroids
    let config = WarpIndexConfig::new(2, 4, 32).with_kmeans_iterations(10);
    let mut index = WarpIndex::new(config);

    let embeddings: Vec<MultiVectorEmbedding> =
        documents.iter().map(|d| embedder.embed_tokens(d)).collect::<crate::Result<Vec<_>>>()?;

    if let Err(e) = index.train(&embeddings) {
        return Ok(ConjectureResult {
            name,
            hypothesis,
            threshold,
            observed_value: format!("Training failed: {e}"),
            verdict: Verdict::Falsified,
            details: "Could not train index".to_string(),
        });
    }

    for (i, doc) in documents.iter().enumerate() {
        let chunk = Chunk::new(DocumentId::new(), doc.clone(), 0, doc.len());
        index.insert(chunk, embeddings[i].clone())?;
    }
    index.build()?;

    // Test queries
    let queries =
        vec!["information about topic 5", "document in category 2", "number contains information"];

    let mut total_recall = 0.0;
    let num_queries = queries.len();

    for query in &queries {
        let query_emb = embedder.embed_tokens(query)?;

        // Exhaustive search (high nprobe)
        let exhaustive_config = WarpSearchConfig::with_k(10).nprobe(8).bound(1000);
        let exhaustive_results = index.search(&query_emb, &exhaustive_config)?;
        let exhaustive_ids: std::collections::HashSet<_> =
            exhaustive_results.iter().map(|(id, _)| id.clone()).collect();

        // Pruned search (nprobe=4)
        let pruned_config = WarpSearchConfig::with_k(10).nprobe(4).bound(128);
        let pruned_results = index.search(&query_emb, &pruned_config)?;
        let pruned_ids: std::collections::HashSet<_> =
            pruned_results.iter().map(|(id, _)| id.clone()).collect();

        // Recall: how many of exhaustive top-10 are in pruned top-10
        let intersection = pruned_ids.intersection(&exhaustive_ids).count();
        let recall = if exhaustive_ids.is_empty() {
            1.0
        } else {
            intersection as f64 / exhaustive_ids.len() as f64
        };
        total_recall += recall;
    }

    let avg_recall = total_recall / num_queries as f64;
    let observed = format!("recall@10 = {:.4} ({:.2}%)", avg_recall, avg_recall * 100.0);

    let verdict = if avg_recall >= 0.95 { Verdict::Corroborated } else { Verdict::Falsified };

    let details = format!(
        "Tested {} queries. Recall {} 95% threshold.",
        num_queries,
        if verdict == Verdict::Corroborated { "meets" } else { "below" }
    );

    Ok(ConjectureResult { name, hypothesis, threshold, observed_value: observed, verdict, details })
}

// =============================================================================
// CONJECTURE 3: Scaling Laws
// =============================================================================

/// Test: Memory and latency scaling properties
fn test_conjecture_3_scaling() -> crate::Result<ConjectureResult> {
    let name = "Conjecture 3: Scaling Laws".to_string();
    let hypothesis = "Memory scales with N*T*bits, latency scales with nprobe not N".to_string();
    let threshold = "Memory < theoretical*1.2, latency O(nprobe) not O(N)".to_string();

    let embedder = MockMultiVectorEmbedder::new(32, 64);

    // Test with increasing corpus sizes - smaller sizes for faster testing
    let corpus_sizes = [500, 1000, 2000];
    let mut memory_results: Vec<(usize, usize)> = Vec::new();
    let mut latency_results: Vec<(usize, f64)> = Vec::new();

    for &n in &corpus_sizes {
        // More tokens per document
        let documents: Vec<String> = (0..n)
            .map(|i| {
                format!(
                    "Document {} about topic {} in field {} with additional context and details",
                    i,
                    i % 50,
                    i % 10
                )
            })
            .collect();

        // Use appropriate number of centroids for corpus size
        let num_centroids = 8.max(n / 100);
        let config = WarpIndexConfig::new(2, num_centroids, 32).with_kmeans_iterations(5);
        let mut index = WarpIndex::new(config);

        let embeddings: Vec<MultiVectorEmbedding> = documents
            .iter()
            .map(|d| embedder.embed_tokens(d))
            .collect::<crate::Result<Vec<_>>>()?;

        if index.train(&embeddings).is_err() {
            continue;
        }

        for (i, doc) in documents.iter().enumerate() {
            let chunk = Chunk::new(DocumentId::new(), doc.clone(), 0, doc.len());
            let _ = index.insert(chunk, embeddings[i].clone());
        }
        let _ = index.build();

        // Measure memory
        let memory = index.memory_usage();
        memory_results.push((n, memory));

        // Measure latency (average over multiple queries)
        let query_emb = embedder.embed_tokens("topic document field")?;
        let search_config = WarpSearchConfig::with_k(10).nprobe(4);

        let start = Instant::now();
        let num_queries = 50;
        for _ in 0..num_queries {
            let _ = index.search(&query_emb, &search_config);
        }
        let elapsed = start.elapsed();
        let avg_latency_us = elapsed.as_micros() as f64 / num_queries as f64;
        latency_results.push((n, avg_latency_us));
    }

    // Analyze memory scaling
    // Theoretical: N * T * dim * nbits / 8 bytes (compressed tokens only)
    // With overhead for metadata
    let memory_ok = if memory_results.len() >= 2 {
        let (n1, m1) = memory_results[0];
        let (n2, m2) = memory_results[1];

        // Memory should scale roughly linearly with N
        // Check if ratio is reasonable (allow 50% variance)
        let ratio = m2 as f64 / m1 as f64;
        let expected_ratio = n2 as f64 / n1 as f64;
        (ratio / expected_ratio).abs() < 1.5 && (ratio / expected_ratio).abs() > 0.5
    } else {
        true
    };

    // Analyze latency scaling
    // Latency should NOT scale linearly with N (that would mean no pruning benefit)
    let latency_ok = if latency_results.len() >= 2 {
        let (n1, l1) = latency_results[0];
        let (n2, l2) = latency_results[1];

        // If latency scaled linearly with N, l2/l1 would equal n2/n1
        // We want sub-linear scaling: l2/l1 < n2/n1
        let latency_ratio = l2 / l1;
        let n_ratio = n2 as f64 / n1 as f64;

        // Allow some increase but not proportional to N
        latency_ratio < n_ratio * 0.8 // Must be at least 20% better than linear
    } else {
        true
    };

    let observed = format!(
        "Memory: {:?}, Latency: {:?}",
        memory_results
            .iter()
            .map(|(n, m)| format!("N={}: {}KB", n, m / 1024))
            .collect::<Vec<_>>()
            .join(", "),
        latency_results
            .iter()
            .map(|(n, l)| format!("N={}: {:.0}us", n, l))
            .collect::<Vec<_>>()
            .join(", ")
    );

    let verdict = if memory_ok && latency_ok { Verdict::Corroborated } else { Verdict::Falsified };

    let details = format!(
        "Memory scaling: {}, Latency scaling: {}",
        if memory_ok { "OK" } else { "FAILED" },
        if latency_ok { "sub-linear (OK)" } else { "linear with N (FAILED)" }
    );

    Ok(ConjectureResult { name, hypothesis, threshold, observed_value: observed, verdict, details })
}

// =============================================================================
// Helper Functions
// =============================================================================

/// Sum all token vectors element-wise into an accumulator.
fn sum_token_vectors(mv: &MultiVectorEmbedding) -> Vec<f32> {
    let mut acc = vec![0.0f32; mv.dim()];
    for token in mv.tokens() {
        for (i, &v) in token.iter().enumerate() {
            acc[i] += v;
        }
    }
    acc
}

/// Divide each element of a vector by a scalar in place.
fn scale_vector(vec: &mut [f32], divisor: f32) {
    for v in vec.iter_mut() {
        *v /= divisor;
    }
}

/// L2-normalize a vector in place. Zero-norm vectors are left unchanged.
fn normalize_vector(vec: &mut [f32]) {
    let norm: f32 = vec.iter().map(|x| x * x).sum::<f32>().sqrt();
    scale_vector_if_nonzero(vec, norm);
}

/// Divide each element by `divisor` only when divisor is positive.
fn scale_vector_if_nonzero(vec: &mut [f32], divisor: f32) {
    if divisor > 0.0 {
        scale_vector(vec, divisor);
    }
}

/// Compute average embedding from multi-vector (for single-vector comparison)
fn average_embedding(mv: &MultiVectorEmbedding) -> Vec<f32> {
    if mv.num_tokens() == 0 {
        return vec![0.0; mv.dim()];
    }

    let mut avg = sum_token_vectors(mv);
    scale_vector(&mut avg, mv.num_tokens() as f32);
    normalize_vector(&mut avg);
    avg
}

/// Dot product of two f32 slices in f64 precision.
fn dot_product_f64(a: &[f32], b: &[f32]) -> f64 {
    a.iter().zip(b.iter()).map(|(x, y)| (*x as f64) * (*y as f64)).sum()
}

/// L2 norm of an f32 slice computed in f64 precision.
fn l2_norm_f64(v: &[f32]) -> f64 {
    v.iter().map(|x| (*x as f64) * (*x as f64)).sum::<f64>().sqrt()
}

/// Safe division: returns 0.0 when divisor is zero.
fn safe_div(numerator: f64, denominator: f64) -> f64 {
    if denominator > 0.0 {
        numerator / denominator
    } else {
        0.0
    }
}

/// Cosine similarity between two vectors
fn cosine_similarity(a: &[f32], b: &[f32]) -> f64 {
    let dot = dot_product_f64(a, b);
    let denom = l2_norm_f64(a) * l2_norm_f64(b);
    safe_div(dot, denom)
}

/// Compute the quantized score between a single query token and a single doc token.
fn quantized_token_score(codec: &ResidualCodec, query_token: &[f32], doc_token: &[f32]) -> f32 {
    let (centroid_id, residual) = codec.compress(doc_token);
    let centroid_score = codec.centroid_score(query_token, centroid_id);
    codec.decompress_score(query_token, centroid_id, centroid_score, &residual)
}

/// Find the maximum quantized score of a query token against all doc tokens.
fn max_quantized_score_for_query_token(
    codec: &ResidualCodec,
    query_token: &[f32],
    doc: &MultiVectorEmbedding,
) -> f32 {
    doc.tokens()
        .map(|doc_token| quantized_token_score(codec, query_token, doc_token))
        .fold(f32::NEG_INFINITY, f32::max)
}

/// Accumulate a single token's max score into the running total,
/// skipping tokens that had no valid match.
fn accumulate_finite(total: f64, score: f32) -> f64 {
    if score > f32::NEG_INFINITY {
        total + score as f64
    } else {
        total
    }
}

/// Compute quantized MaxSim score
fn compute_quantized_maxsim(
    codec: &ResidualCodec,
    query: &MultiVectorEmbedding,
    doc: &MultiVectorEmbedding,
) -> f64 {
    query
        .tokens()
        .map(|qt| max_quantized_score_for_query_token(codec, qt, doc))
        .fold(0.0, accumulate_finite)
}

/// Classify a pair as concordant (+1), discordant (-1), or tied (0).
fn classify_pair(x_diff: f64, y_diff: f64) -> i64 {
    let product = x_diff * y_diff;
    // f64::signum returns 1.0, -1.0, or 0.0
    product.signum() as i64
}

/// Count concordant and discordant pairs across all (i, j) combinations.
fn count_concordant_discordant(x: &[f64], y: &[f64]) -> (i64, i64) {
    let mut concordant = 0i64;
    let mut discordant = 0i64;

    for i in 0..x.len() {
        for j in (i + 1)..x.len() {
            let sign = classify_pair(x[i] - x[j], y[i] - y[j]);
            concordant += i64::from(sign > 0);
            discordant += i64::from(sign < 0);
        }
    }

    (concordant, discordant)
}

/// Compute Kendall's tau rank correlation coefficient
fn kendalls_tau(x: &[f64], y: &[f64]) -> f64 {
    if x.len() < 2 {
        return 1.0;
    }

    let (concordant, discordant) = count_concordant_discordant(x, y);
    let total = concordant + discordant;
    safe_div_i64(concordant - discordant, total)
}

/// Safe integer division returning f64, returns 1.0 when denominator is zero.
fn safe_div_i64(numerator: i64, denominator: i64) -> f64 {
    if denominator == 0 {
        1.0
    } else {
        numerator as f64 / denominator as f64
    }
}

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

    #[test]
    fn test_kendalls_tau_perfect_correlation() {
        let x = vec![1.0, 2.0, 3.0, 4.0, 5.0];
        let y = vec![1.0, 2.0, 3.0, 4.0, 5.0];
        let tau = kendalls_tau(&x, &y);
        assert!((tau - 1.0).abs() < 0.001);
    }

    #[test]
    fn test_kendalls_tau_perfect_inverse() {
        let x = vec![1.0, 2.0, 3.0, 4.0, 5.0];
        let y = vec![5.0, 4.0, 3.0, 2.0, 1.0];
        let tau = kendalls_tau(&x, &y);
        assert!((tau - (-1.0)).abs() < 0.001);
    }

    #[test]
    fn test_cosine_similarity_identical() {
        let a = vec![1.0, 0.0, 0.0];
        let b = vec![1.0, 0.0, 0.0];
        let sim = cosine_similarity(&a, &b);
        assert!((sim - 1.0).abs() < 0.001);
    }

    #[test]
    fn test_cosine_similarity_orthogonal() {
        let a = vec![1.0, 0.0, 0.0];
        let b = vec![0.0, 1.0, 0.0];
        let sim = cosine_similarity(&a, &b);
        assert!(sim.abs() < 0.001);
    }

    /// Integration test: Run full falsification plan
    /// This is the main test that executes all falsification tests
    #[test]
    fn test_falsification_plan() {
        let report = execute_falsification_plan();

        println!("\n");
        report.print_report();

        // We don't assert on Corroborated because the goal is to try to falsify
        // But we track the results
        println!("\nTest completed. Overall verdict: {}", report.overall_verdict);
    }

    /// Specific test for Experimentum Crucis
    #[test]
    fn test_experimentum_crucis_standalone() {
        let result = test_experimentum_crucis().unwrap();
        println!("\nExperimentum Crucis Result:");
        println!("  Observed: {}", result.observed_value);
        println!("  Verdict: {}", result.verdict);
    }

    /// Specific test for Compression Conjecture
    #[test]
    fn test_compression_conjecture_standalone() {
        let result = test_conjecture_1_compression().unwrap();
        println!("\nCompression Conjecture Result:");
        println!("  Observed: {}", result.observed_value);
        println!("  Verdict: {}", result.verdict);
    }

    /// Specific test for Pruning Conjecture
    #[test]
    fn test_pruning_conjecture_standalone() {
        let result = test_conjecture_2_pruning().unwrap();
        println!("\nPruning Conjecture Result:");
        println!("  Observed: {}", result.observed_value);
        println!("  Verdict: {}", result.verdict);
    }

    /// Specific test for Scaling Conjecture
    #[test]
    fn test_scaling_conjecture_standalone() {
        let result = test_conjecture_3_scaling().unwrap();
        println!("\nScaling Conjecture Result:");
        println!("  Observed: {}", result.observed_value);
        println!("  Verdict: {}", result.verdict);
    }
}