lean-ctx-compress 0.1.0

// SPDX-License-Identifier: Apache-2.0

//! Deterministic tabular (CSV/TSV) crusher — columnar redundancy factoring for
//! delimited data (#982, Headroom tabular-compressor port, GitLab Epic #973).
//!
//! Real-world CSV/TSV dumps (DB exports, `psql -A -F,`, analytics extracts) are
//! dominated by columns that repeat one value on every row (a `status`, `region`,
//! `tenant` column) and by near-unique noise columns (UUIDs, timestamps). This
//! module factors that out, mirroring [`lean_ctx_patterns::json_crush`] but for tables:
//!
//! - **Lossless**: every *constant* column (exactly one distinct value across all
//!   data rows) is hoisted once into `_const`; the remaining columns are kept
//!   positionally in `_rows`, so a value never repeats more than it must. The
//!   transform is exactly reversible via [`reconstruct`].
//! - **Lossy**: additionally *drops* near-unique high-entropy columns (recorded
//!   in `_dropped`); the exact original is recovered out-of-band via CCR, never
//!   from the text.
//!
//! The compact form is serialized with `serde_json` (robust quoting, escapes and
//! Unicode — no hand-rolled format that could desync the reader), and the output
//! is a pure function of the input (columns walked in header order, `_const`
//! keyed deterministically), so identical input yields byte-identical output
//! (#498). The crusher never inflates: callers gate on the shared
//! [`lean_ctx_patterns::json_crush::KEEP_DATA_DIVISOR`] threshold and a no-op input returns [`None`].

use std::collections::{BTreeMap, BTreeSet};

use serde_json::{Map, Value};

use crate::csv;
use lean_ctx_patterns::json_crush::CrushResult;

/// Marks a crushed-table document. Vanishingly unlikely as a real top-level CSV
/// shape (the input is delimited text, never a JSON object), so [`reconstruct`]
/// can identify our own output unambiguously.
const MARKER: &str = "_lc_tbl";
/// Full header, in original column order — the source of truth for reconstructing
/// each row's column sequence.
const ORDER_KEY: &str = "_order";
/// Columns hoisted to their single repeated value (lossless).
const CONST_KEY: &str = "_const";
/// Columns dropped as high-entropy noise (lossy; recover via CCR).
const DROPPED_KEY: &str = "_dropped";
/// Per-row values for the *varying, kept* columns, positional in header order.
const ROWS_KEY: &str = "_rows";

/// Below this data-row count, columnar factoring rarely beats its own overhead.
const MIN_ROWS: usize = 3;

/// Lossless columnar crush of delimited `text`, returning the compact JSON form
/// only when it clears the `beneficial` reduction gate. `None` for non-tabular,
/// ragged, or low-redundancy input — the caller keeps its own path.
pub fn crush_text_if_beneficial(text: &str, delimiter: char) -> Option<String> {
    let res = crush(text, delimiter, 1.0)?;
    (res.lossless && beneficial(&res.text, text)).then_some(res.text)
}

/// Lossy columnar crush of `text`: drops near-unique high-entropy columns whose
/// distinct-value ratio is `>= drop_entropy`. Returns the [`CrushResult`] only
/// when a column was **actually dropped** (`!lossless`) AND the compact form at
/// least halves the input ([`lean_ctx_patterns::json_crush::KEEP_DATA_DIVISOR`]). Because data is then lost, the
/// caller MUST persist the verbatim original out-of-band (CCR) before emitting —
/// the dropped columns are never reconstructible from the text. `None` for
/// non-tabular, low-redundancy, or all-lossless input.
pub fn crush_text_lossy_if_beneficial(
    text: &str,
    delimiter: char,
    drop_entropy: f64,
) -> Option<CrushResult> {
    let res = crush(text, delimiter, drop_entropy.clamp(0.0, 1.0))?;
    (!res.lossless && beneficial(&res.text, text)).then_some(res)
}

/// Tabular reshaping into the columnar JSON form carries per-cell quoting
/// overhead the JSON crusher's array-of-objects form does not, so the columnar
/// win (eliminating constant columns + dropping noise) is gated on a 1/4 byte
/// reduction rather than the JSON crusher's stricter halving — still a clear,
/// never-inflating win whose exact data stays reconstructible via [`reconstruct`].
const MIN_SAVE_RATIO_NUM: usize = 3;
const MIN_SAVE_RATIO_DEN: usize = 4;

fn beneficial(compact: &str, raw: &str) -> bool {
    compact.len().saturating_mul(MIN_SAVE_RATIO_DEN) <= raw.len().saturating_mul(MIN_SAVE_RATIO_NUM)
}

/// Core crush. `drop_entropy < 1.0` enables lossy column dropping. Returns `None`
/// unless the input is a well-formed table with at least one factorable column.
fn crush(text: &str, delimiter: char, drop_entropy: f64) -> Option<CrushResult> {
    // JSON is the JSON crusher's job; never treat it as a degenerate one-column
    // table (and our own output starts with `{`, so this is also re-entry-safe).
    let head = text.trim_start();
    if head.starts_with('{') || head.starts_with('[') {
        return None;
    }

    let rows = csv::parse(text, delimiter);
    if rows.len() < MIN_ROWS + 1 {
        return None; // need a header + >= MIN_ROWS data rows
    }
    let header = &rows[0];
    let ncols = header.len();
    if ncols < 2 {
        return None; // a single column has nothing to factor across
    }
    let data = &rows[1..];

    // Only rectangular tables: a ragged row makes positional reconstruction
    // ambiguous, so fall through to the generic path.
    if data.iter().any(|r| r.len() != ncols) {
        return None;
    }
    // Distinct headers only: a duplicate name would alias in keyed `_const`.
    let mut header_seen = BTreeSet::new();
    if !header.iter().all(|h| header_seen.insert(h.as_str())) {
        return None;
    }

    let n = data.len();
    let mut const_map = Map::new();
    let mut dropped: Vec<String> = Vec::new();
    let mut kept: Vec<usize> = Vec::new();

    for (c, name) in header.iter().enumerate() {
        let distinct = data
            .iter()
            .map(|row| row[c].as_str())
            .collect::<BTreeSet<_>>()
            .len();
        if drop_entropy < 1.0 && (distinct as f64 / n as f64) >= drop_entropy {
            dropped.push(name.clone());
            continue;
        }
        if distinct == 1 {
            const_map.insert(name.clone(), Value::String(data[0][c].clone()));
            continue;
        }
        kept.push(c);
    }

    let had_drops = !dropped.is_empty();
    if const_map.is_empty() && !had_drops {
        return None; // nothing factored — leave the table to the generic path
    }

    let rows_json: Vec<Value> = data
        .iter()
        .map(|row| {
            Value::Array(
                kept.iter()
                    .map(|&c| Value::String(row[c].clone()))
                    .collect(),
            )
        })
        .collect();

    let mut out = Map::new();
    out.insert(MARKER.to_string(), Value::from(1u8));
    out.insert(
        ORDER_KEY.to_string(),
        Value::Array(header.iter().cloned().map(Value::String).collect()),
    );
    if !const_map.is_empty() {
        out.insert(CONST_KEY.to_string(), Value::Object(const_map));
    }
    if had_drops {
        out.insert(
            DROPPED_KEY.to_string(),
            Value::Array(dropped.into_iter().map(Value::String).collect()),
        );
    }
    out.insert(ROWS_KEY.to_string(), Value::Array(rows_json));

    let text_out = serde_json::to_string(&Value::Object(out)).ok()?;
    Some(CrushResult {
        text: text_out,
        lossless: !had_drops,
    })
}

/// Rebuild the parsed rows (`[header, ..data]`) from crushed `text`. Exact for
/// lossless forms; for lossy forms the `_dropped` columns are simply absent from
/// the header and every row (recover them via CCR). `None` if `text` is not a
/// tabular-crush document or is internally inconsistent.
pub fn reconstruct(text: &str) -> Option<Vec<Vec<String>>> {
    let v: Value = serde_json::from_str(text).ok()?;
    let obj = v.as_object()?;
    obj.get(MARKER)?;

    let order: Vec<String> = obj
        .get(ORDER_KEY)?
        .as_array()?
        .iter()
        .map(|x| x.as_str().unwrap_or_default().to_string())
        .collect();
    let empty = Map::new();
    let const_map = obj
        .get(CONST_KEY)
        .and_then(Value::as_object)
        .unwrap_or(&empty);
    let dropped: BTreeSet<&str> = obj
        .get(DROPPED_KEY)
        .and_then(Value::as_array)
        .map(|a| a.iter().filter_map(Value::as_str).collect())
        .unwrap_or_default();
    let rows = obj.get(ROWS_KEY)?.as_array()?;

    // Output columns keep original order minus dropped; the positional `_rows`
    // columns are output columns minus the hoisted constants — exactly the
    // `kept` order the crush emitted.
    let out_cols: Vec<&str> = order
        .iter()
        .map(String::as_str)
        .filter(|c| !dropped.contains(c))
        .collect();
    let varying: Vec<&str> = out_cols
        .iter()
        .copied()
        .filter(|c| !const_map.contains_key(*c))
        .collect();

    let mut result: Vec<Vec<String>> = Vec::with_capacity(rows.len() + 1);
    result.push(out_cols.iter().map(|c| (*c).to_string()).collect());

    for row in rows {
        let arr = row.as_array()?;
        if arr.len() != varying.len() {
            return None;
        }
        let vary_vals: BTreeMap<&str, &str> = varying
            .iter()
            .copied()
            .zip(arr.iter().map(|x| x.as_str().unwrap_or_default()))
            .collect();
        let mut full = Vec::with_capacity(out_cols.len());
        for col in &out_cols {
            let cell = const_map
                .get(*col)
                .and_then(Value::as_str)
                .or_else(|| vary_vals.get(col).copied())
                .unwrap_or_default();
            full.push(cell.to_string());
        }
        result.push(full);
    }
    Some(result)
}

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

    /// A redundant roster: two constant columns (`status`, `region`) and two
    /// varying ones (`id`, `name`) over enough rows to clearly pay.
    fn roster_csv() -> String {
        let mut s = String::from("id,name,status,region\n");
        for i in 0..16 {
            s.push_str(&format!("{i},user{i},active,eu-central-1\n"));
        }
        s
    }

    #[test]
    fn lossless_factors_constant_columns() {
        let csv = roster_csv();
        let crushed = crush_text_if_beneficial(&csv, ',').expect("should crush");
        // Each constant value appears once, not on every row.
        assert_eq!(crushed.matches("eu-central-1").count(), 1);
        assert_eq!(crushed.matches("active").count(), 1);
        assert!(crushed.contains("_const"));
    }

    #[test]
    fn lossless_roundtrips_exactly() {
        let csv = roster_csv();
        let crushed = crush_text_if_beneficial(&csv, ',').unwrap();
        let restored = reconstruct(&crushed).unwrap();
        let original = csv::parse(&csv, ',');
        assert_eq!(restored, original);
    }

    #[test]
    fn output_is_byte_stable_across_calls() {
        let csv = roster_csv();
        let run = || crush_text_if_beneficial(&csv, ',').unwrap();
        assert_eq!(run(), run(), "crush output must be deterministic (#498)");
    }

    #[test]
    fn never_inflates_and_clears_the_gate() {
        let csv = roster_csv();
        let crushed = crush_text_if_beneficial(&csv, ',').unwrap();
        assert!(crushed.len() < csv.len(), "must never inflate");
        assert!(beneficial(&crushed, &csv), "must clear the reduction gate");
    }

    #[test]
    fn tsv_delimiter_is_honoured() {
        let mut s = String::from("id\tname\tstatus\tregion\n");
        for i in 0..16 {
            s.push_str(&format!("{i}\tuser{i}\tactive\teu-central-1\n"));
        }
        let crushed = crush_text_if_beneficial(&s, '\t').expect("tsv crushes");
        assert_eq!(reconstruct(&crushed).unwrap(), csv::parse(&s, '\t'));
    }

    #[test]
    fn quoted_fields_with_embedded_delimiters_roundtrip() {
        // `note` varies and carries embedded commas + escaped quotes, so it stays
        // a varying cell in `_rows`; the constant columns make the crush pay.
        let mut s = String::from("id,note,status,region\n");
        for i in 0..16 {
            s.push_str(&format!("{i},\"a, {i} \"\"q\"\"\",active,eu-central-1\n"));
        }
        let crushed = crush_text_if_beneficial(&s, ',').unwrap();
        let restored = reconstruct(&crushed).unwrap();
        assert_eq!(restored, csv::parse(&s, ','));
        // The embedded-delimiter, escaped-quote value survived verbatim.
        assert_eq!(restored[1][1], "a, 0 \"q\"");
    }

    #[test]
    fn skips_tables_with_nothing_to_factor() {
        // Every column varies on every row -> no constant to hoist.
        let mut s = String::from("a,b,c\n");
        for i in 0..10 {
            s.push_str(&format!("{i},{},{}\n", i + 100, i + 200));
        }
        assert!(crush_text_if_beneficial(&s, ',').is_none());
    }

    #[test]
    fn skips_ragged_too_small_and_json_input() {
        assert!(crush_text_if_beneficial("a,b\n1,ok\n2,ok", ',').is_none()); // < MIN_ROWS
        assert!(crush_text_if_beneficial("a,b\n1,ok\n2\n3,ok\n4,ok", ',').is_none()); // ragged
        assert!(crush_text_if_beneficial("col\n1\n2\n3\n4", ',').is_none()); // single column
        assert!(crush_text_if_beneficial("[{\"a\":1}]", ',').is_none()); // JSON
        assert!(crush_text_if_beneficial("", ',').is_none());
    }

    #[test]
    fn lossy_drops_high_entropy_columns_and_flags_lossy() {
        // A near-unique `uuid` column alongside a constant `status` column.
        let mut s = String::from("status,uuid\n");
        for i in 0..40 {
            s.push_str(&format!("ok,uuid-{i:08}\n"));
        }
        let res = crush_text_lossy_if_beneficial(&s, ',', 0.9).expect("lossy gate fires");
        assert!(!res.lossless, "dropping a column must report lossy");
        assert!(res.text.contains("_dropped"));
        assert!(
            !res.text.contains("uuid-00000000"),
            "dropped values are gone"
        );
        assert!(beneficial(&res.text, &s));

        // The kept columns still reconstruct; the dropped one is simply absent.
        let restored = reconstruct(&res.text).unwrap();
        assert_eq!(restored[0], vec!["status"]);
        assert_eq!(restored[1], vec!["ok"]);

        // drop_entropy = 1.0 disables dropping -> nothing lossy -> None.
        assert!(crush_text_lossy_if_beneficial(&s, ',', 1.0).is_none());
    }

    #[test]
    fn lossy_is_byte_stable() {
        let mut s = String::from("status,uuid\n");
        for i in 0..40 {
            s.push_str(&format!("ok,uuid-{i:08}\n"));
        }
        let run = || crush_text_lossy_if_beneficial(&s, ',', 0.9).unwrap().text;
        assert_eq!(run(), run());
    }
}