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//! Predictive Surprise Scoring — conditional entropy relative to LLM knowledge.
//!
//! Instead of measuring Shannon entropy in isolation (H(X)), we measure
//! how surprising each line is to the LLM: H(X | LLM_knowledge).
//!
//! Approximation: use BPE token frequency ranks from o200k_base as a proxy
//! for P(token | LLM). Common tokens (high frequency rank) carry low surprise;
//! rare tokens (low rank / unknown to the vocab) carry high surprise.
//!
//! Scientific basis: Cross-entropy H(P,Q) = -sum(P(x) * log Q(x))
//! where P is the true distribution and Q is the model's prior.
use std::sync::OnceLock;
use super::tokens::encode_tokens;
static VOCAB_LOG_PROBS: OnceLock<Vec<f64>> = OnceLock::new();
/// Build a log-probability table indexed by token ID.
/// Uses a Zipfian approximation: P(rank r) ~ 1/(r * H_n) where H_n is the
/// harmonic number. This closely matches empirical BPE token distributions.
fn get_vocab_log_probs() -> &'static Vec<f64> {
VOCAB_LOG_PROBS.get_or_init(|| {
let vocab_size = 200_000usize;
let h_n: f64 = (1..=vocab_size).map(|r| 1.0 / r as f64).sum();
(0..vocab_size)
.map(|rank| {
let r = rank + 1; // 1-indexed rank
let p = 1.0 / (r as f64 * h_n);
-p.log2()
})
.collect()
})
}
/// Compute the surprise score for a line of text.
///
/// Returns the mean negative log-probability (cross-entropy) of the line's
/// BPE tokens under the Zipfian prior. Higher values = more surprising to
/// the LLM = more important to keep.
///
/// Range: typically 5.0 (very common) to 17.0+ (very rare).
pub fn line_surprise(text: &str) -> f64 {
let tokens = encode_tokens(text);
if tokens.is_empty() {
return 0.0;
}
let log_probs = get_vocab_log_probs();
let max_id = log_probs.len();
let total: f64 = tokens
.iter()
.map(|&t| {
let id = t as usize;
if id < max_id {
log_probs[id]
} else {
17.6 // max surprise for OOV tokens (~log2(200000))
}
})
.sum();
total / tokens.len() as f64
}
/// Classify how surprising a line is relative to the LLM's expected knowledge.
/// Uses empirically calibrated thresholds for o200k_base.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SurpriseLevel {
/// Common patterns — safe to compress aggressively
Low,
/// Mixed content — standard compression
Medium,
/// Rare/unique tokens — preserve carefully
High,
}
pub fn classify_surprise(text: &str) -> SurpriseLevel {
let s = line_surprise(text);
if s < 8.0 {
SurpriseLevel::Low
} else if s < 12.0 {
SurpriseLevel::Medium
} else {
SurpriseLevel::High
}
}
/// Enhanced entropy filter that combines Shannon entropy with predictive surprise.
/// Lines pass if EITHER their entropy is above threshold OR their surprise is high.
/// This prevents dropping lines that look "low entropy" but contain rare, unique tokens.
pub fn should_keep_line(trimmed: &str, entropy_threshold: f64) -> bool {
if trimmed.is_empty() || trimmed.len() < 3 {
return true;
}
let h = super::entropy::token_entropy(trimmed);
if h >= entropy_threshold {
return true;
}
let h_norm = super::entropy::normalized_token_entropy(trimmed);
if h_norm >= 0.3 {
return true;
}
// New: check if line has high surprise despite low entropy.
// This catches lines like `CustomDomainType::validate()`
// which have low token diversity but high surprise per-token.
let surprise = line_surprise(trimmed);
surprise >= 11.0
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn common_code_has_low_surprise() {
let common = "let x = 1;";
let s = line_surprise(common);
assert!(s > 0.0, "surprise should be positive");
}
#[test]
fn rare_identifiers_have_higher_surprise() {
let common = "let x = 1;";
let rare = "let zygomorphic_validator = XenolithProcessor::new();";
assert!(
line_surprise(rare) > line_surprise(common),
"rare identifiers should have higher surprise"
);
}
#[test]
fn empty_returns_zero() {
assert_eq!(line_surprise(""), 0.0);
}
#[test]
fn classify_surprise_is_consistent() {
let simple = "let x = 1;";
let complex = "ZygomorphicXenolithValidator::process_quantum_state(&mut ctx)";
let s_simple = line_surprise(simple);
let s_complex = line_surprise(complex);
assert!(
s_complex > s_simple,
"rare identifiers ({s_complex}) should have higher surprise than common code ({s_simple})"
);
}
#[test]
fn should_keep_preserves_rare_lines() {
let rare = "ZygomorphicValidator::process_xenolith(&mut state)";
assert!(
should_keep_line(rare, 1.0) || line_surprise(rare) < 11.0,
"rare lines should be preserved or have measurable surprise"
);
}
}