lean-ctx 3.9.8

//! Surprisal-based line scoring for deterministic compression.
//!
//! Each line receives an information density score based on:
//! - Character-level entropy (Shannon)
//! - Structural markers (paths, numbers, identifiers)
//! - Repetition detection (overlap with previous lines)

use std::collections::HashSet;

/// Score for a single line — higher means more informative.
#[derive(Debug, Clone)]
pub struct LineScore {
    pub line_idx: usize,
    pub entropy: f32,
    pub has_structural_marker: bool,
    pub repetition_ratio: f32,
    pub combined: f32,
}

const MAX_TRIGRAM_SET_SIZE: usize = 10_000;

/// Scores all lines in the input text for information density.
pub fn score_lines(text: &str) -> Vec<LineScore> {
    let lines: Vec<&str> = text.lines().collect();
    let mut seen_trigrams: HashSet<String> = HashSet::new();
    let mut trigram_saturated = false;
    let mut scores = Vec::with_capacity(lines.len());

    for (idx, line) in lines.iter().enumerate() {
        let trimmed = line.trim();

        let entropy = char_entropy(trimmed);
        let is_noise = is_encoded_blob(trimmed);
        let has_marker = !is_noise && has_structural_marker(trimmed);
        let rep_ratio = if trigram_saturated {
            0.0
        } else {
            repetition_ratio(trimmed, &seen_trigrams)
        };

        if !trigram_saturated {
            register_trigrams(trimmed, &mut seen_trigrams);
            if seen_trigrams.len() >= MAX_TRIGRAM_SET_SIZE {
                trigram_saturated = true;
            }
        }

        let combined = compute_combined(entropy, has_marker, rep_ratio, is_noise);

        scores.push(LineScore {
            line_idx: idx,
            entropy,
            has_structural_marker: has_marker,
            repetition_ratio: rep_ratio,
            combined,
        });
    }

    scores
}

fn char_entropy(line: &str) -> f32 {
    if line.is_empty() {
        return 0.0;
    }
    let mut freq = [0u32; 128];
    let mut total = 0u32;
    for b in line.bytes() {
        if (b as usize) < 128 {
            freq[b as usize] += 1;
            total += 1;
        }
    }
    if total == 0 {
        return 0.0;
    }
    let mut ent = 0.0f32;
    for &count in &freq {
        if count > 0 {
            let p = count as f32 / total as f32;
            ent -= p * p.log2();
        }
    }
    ent
}

fn has_structural_marker(line: &str) -> bool {
    if line.contains('/') && (line.contains('.') || line.contains("src")) {
        return true;
    }
    if line.chars().any(|c| c.is_ascii_digit()) {
        return true;
    }
    if line.contains("error") || line.contains("Error") || line.contains("ERROR") {
        return true;
    }
    if line.contains("warning") || line.contains("Warning") || line.contains("WARN") {
        return true;
    }
    let long_idents = line
        .split(|c: char| !c.is_alphanumeric() && c != '_')
        .filter(|w| w.len() >= 6)
        .count();
    long_idents >= 2
}

fn repetition_ratio(line: &str, seen: &HashSet<String>) -> f32 {
    let chars: Vec<char> = line.chars().collect();
    if chars.len() < 9 {
        return 0.0;
    }
    let total = chars.len().saturating_sub(2);
    if total == 0 {
        return 0.0;
    }
    let mut repeated = 0;
    for i in 0..total {
        let end = (i + 3).min(chars.len());
        let trigram: String = chars[i..end].iter().collect();
        if seen.contains(&trigram) {
            repeated += 1;
        }
    }
    repeated as f32 / total as f32
}

fn register_trigrams(line: &str, seen: &mut HashSet<String>) {
    let chars: Vec<char> = line.chars().collect();
    if chars.len() < 3 {
        return;
    }
    for i in 0..chars.len().saturating_sub(2) {
        let end = (i + 3).min(chars.len());
        let trigram: String = chars[i..end].iter().collect();
        seen.insert(trigram);
    }
}

fn compute_combined(entropy: f32, has_marker: bool, rep_ratio: f32, is_noise: bool) -> f32 {
    if is_noise {
        return 0.0;
    }
    let marker_bonus = if has_marker { 0.3 } else { 0.0 };
    let rep_penalty = rep_ratio * 0.5;
    (entropy + marker_bonus - rep_penalty).max(0.0)
}

/// True when `line` contains an encoded blob (base64 or hex) — either as the
/// whole line or as one whitespace-delimited token in a `label: <blob>`
/// shaped line (`trace id: <hex>`, `commit <sha>`, `session token: <b64>`,
/// all common in real logs) — rather than prose/code. High Shannon entropy
/// but zero semantic content, so it must not be scored as information-dense.
fn is_encoded_blob(line: &str) -> bool {
    line.split_whitespace().any(is_blob_token)
}

/// True when a single whitespace-delimited token looks like a random,
/// non-prose blob (base64 or hex) rather than an English word or code
/// identifier. `pub(super)` so `quality::check` can also treat a long blob
/// as a must-preserve identifier — a payload-shaped blob (e.g. the resolved
/// hash from `git rev-parse HEAD`) must still trip the quality gate if
/// dropped, even though it has no alphabetic characters for the ordinary
/// identifier check to key on.
pub(super) fn is_blob_token(token: &str) -> bool {
    const MIN_BLOB_LEN: usize = 24;

    if token.len() < MIN_BLOB_LEN {
        return false;
    }

    let is_hex = token.chars().all(|c| c.is_ascii_hexdigit());

    // Base64 padding/symbols are an unambiguous signal on their own. Without
    // them, require the digit+upper+lower mix typical of random tokens so a
    // long plain identifier (all-lowercase or camelCase, no digits) doesn't
    // get misclassified as noise.
    let has_b64_symbol = token.contains('+') || token.contains('/') || token.contains('=');
    let charset_ok = token
        .chars()
        .all(|c| c.is_ascii_alphanumeric() || c == '+' || c == '/' || c == '=');
    let has_digit = token.chars().any(|c| c.is_ascii_digit());
    let has_upper = token.chars().any(|c| c.is_ascii_uppercase());
    let has_lower = token.chars().any(|c| c.is_ascii_lowercase());
    let is_base64 = charset_ok && (has_b64_symbol || (has_digit && has_upper && has_lower));

    is_hex || is_base64
}

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

    #[test]
    fn empty_line_zero_entropy() {
        assert_eq!(char_entropy(""), 0.0);
    }

    #[test]
    fn uniform_string_low_entropy() {
        let e = char_entropy("aaaaaaaaaa");
        assert!(e < 0.01, "uniform string should have ~0 entropy, got {e}");
    }

    #[test]
    fn mixed_string_higher_entropy() {
        let low = char_entropy("aaaaaaaaaa");
        let high = char_entropy("abcdefghij");
        assert!(high > low, "mixed > uniform entropy");
    }

    #[test]
    fn structural_marker_path() {
        assert!(has_structural_marker("src/core/config.rs"));
    }

    #[test]
    fn structural_marker_error() {
        assert!(has_structural_marker("error[E0308]: mismatched types"));
    }

    #[test]
    fn structural_marker_missing() {
        assert!(!has_structural_marker("this is a simple line"));
    }

    #[test]
    fn encoded_blob_detected_as_noise() {
        assert!(is_encoded_blob(
            "MTIzNDU2Nzg5MGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6MDk4NzY1NDMyMQ=="
        ));
        assert!(is_encoded_blob(
            "9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08"
        ));
        assert!(!is_encoded_blob("src/core/config.rs"));
        assert!(!is_encoded_blob("this is a simple line with words"));
    }

    #[test]
    fn prefixed_blob_is_still_detected_as_noise() {
        // Real logs almost always label the blob rather than emit it bare
        // (`trace id: <hex>`, `commit <sha>`, `session token: <b64>`). Built
        // at runtime (not a literal) so a fake test token isn't itself
        // mistaken for a real secret by tooling.
        let hex64: String = "9f86d0".repeat(11);
        let b64_padded: String = format!("{}==", "aZ9".repeat(8));

        assert!(is_encoded_blob(&format!("trace id: {hex64}")));
        assert!(is_encoded_blob(&format!(
            "build session token: {b64_padded}"
        )));
        assert!(is_encoded_blob(&format!("commit {hex64}")));
    }

    #[test]
    fn prefixed_real_content_is_not_noise() {
        assert!(!is_encoded_blob(
            "error in module ConfigurationManagerFactory during init"
        ));
    }

    #[test]
    fn long_camel_case_identifier_is_not_noise() {
        assert!(!is_encoded_blob(
            "configureApplicationRuntimeEnvironmentSettings"
        ));
        assert!(!is_encoded_blob(
            "configure_premium_feature_flags_for_tenant"
        ));
    }

    #[test]
    fn encoded_blob_scores_lower_than_real_error_line() {
        let text = "error: connection refused at host during handshake attempt\nMTIzNDU2Nzg5MGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6MDk4NzY1NDMyMQ==";
        let scores = score_lines(text);
        assert!(
            scores[0].combined > scores[1].combined,
            "real error line should score above encoded blob noise: {} vs {}",
            scores[0].combined,
            scores[1].combined
        );
    }

    #[test]
    fn score_lines_returns_all_lines() {
        let text = "line one\nline two\nline three";
        let scores = score_lines(text);
        assert_eq!(scores.len(), 3);
    }

    #[test]
    fn repetitive_lines_get_lower_score() {
        let text = "exactly the same line repeated here\nexactly the same line repeated here\nunique content with different words";
        let scores = score_lines(text);
        assert!(
            scores[2].combined >= scores[1].combined,
            "unique line should score >= repeated: {} vs {}",
            scores[2].combined,
            scores[1].combined
        );
    }
}