/lean/LeanCtxProofs/Compression/TerseEngine.lean

/-
  LeanCTX Formal Verification — Terse Compression Engine Invariants

  Proves correctness properties of the 4-layer terse compression engine:
    - Passthrough guarantee for Off level
    - Structural marker protection
    - Filler/decoration removal rules
    - Max-to-Standard fallback correctness
    - Threshold ordering across levels

  Mirrors: rust/src/core/terse/engine.rs, rust/src/core/terse/pipeline.rs
  Enterprise guarantee: these proofs ensure the compression engine
  never silently destroys information.

  Scientific basis:
    - Surprisal-based information density (Shannon 1948)
    - Structural marker preservation (analogous to Named Entity Recognition)
-/
import LeanCtxProofs.Basic

namespace LeanCtxProofs.Compression.TerseEngine

-- ============================================================================
-- Core Types (mirror rust/src/core/config.rs + engine.rs)
-- ============================================================================

/-- Compression levels, ordered by aggressiveness.
    Mirrors `CompressionLevel` in config.rs. -/
inductive CompressionLevel where
  | off
  | lite
  | standard
  | max
  deriving DecidableEq, Repr

/-- A scored text line with information density metadata. -/
structure ScoredLine where
  content : String
  score : Nat
  hasStructuralMarker : Bool
  isEmpty : Bool
  isPureDecoration : Bool
  isFiller : Bool
  deriving DecidableEq, Repr

/-- Whether a compression level is active (not Off). -/
def CompressionLevel.isActive : CompressionLevel → Bool
  | .off => false
  | _ => true

/-- Score threshold for each level (scaled ×10 for Nat precision).
    Mirrors constants in engine.rs: 2.5→25, 3.0→30, 3.5→35. -/
def scoreThreshold : CompressionLevel → Nat
  | .off => 0
  | .lite => 25
  | .standard => 30
  | .max => 35

-- ============================================================================
-- Line Filtering (mirrors engine.rs filter logic)
-- ============================================================================

/-- A line should be removed if: empty, pure decoration, unprotected filler,
    or below score threshold without structural markers. -/
def shouldRemove (level : CompressionLevel) (line : ScoredLine) : Bool :=
  line.isEmpty ||
  line.isPureDecoration ||
  (line.isFiller && !line.hasStructuralMarker) ||
  (line.score < scoreThreshold level && !line.hasStructuralMarker)

/-- The core filter: keep lines that should NOT be removed. -/
def filterLines (level : CompressionLevel) (lines : List ScoredLine) : List ScoredLine :=
  lines.filter (! shouldRemove level ·)

/-- Max-level with fallback to Standard on quality failure. -/
def compressWithFallback (lines : List ScoredLine)
    (maxQualityPassed : Bool) : List ScoredLine :=
  if maxQualityPassed then filterLines .max lines
  else filterLines .standard lines

-- ============================================================================
-- Engine Correctness Theorems
-- ============================================================================

/-- **Theorem 1 (Off Never Removes): Off level marks non-filler content for keeping.
    (Off threshold is 0, so only empty/decoration/unprotected-filler lines are removed.) -/
theorem off_never_removes (line : ScoredLine)
    (h_ne : line.isEmpty = false)
    (h_nd : line.isPureDecoration = false)
    (h_nf : line.isFiller = false) :
    shouldRemove .off line = false := by
  simp [shouldRemove, scoreThreshold, h_ne, h_nd, h_nf]

/-- **Theorem 2 (Structural Marker Safety): Lines with structural markers
    are NEVER removed, regardless of compression level.** -/
theorem structural_markers_preserved (level : CompressionLevel) (line : ScoredLine)
    (h_marker : line.hasStructuralMarker = true)
    (h_not_empty : line.isEmpty = false)
    (h_not_deco : line.isPureDecoration = false) :
    shouldRemove level line = false := by
  simp [shouldRemove, h_not_empty, h_not_deco, h_marker]

/-- **Theorem 3 (Empty Lines Always Removed): Empty lines are removed
    at any compression level.** -/
theorem empty_lines_always_removed (level : CompressionLevel) (line : ScoredLine)
    (h_empty : line.isEmpty = true) :
    shouldRemove level line = true := by
  simp [shouldRemove, h_empty]

/-- **Theorem 4 (Decoration Always Removed): Pure decoration lines are
    removed at any compression level.** -/
theorem decoration_always_removed (level : CompressionLevel) (line : ScoredLine)
    (h_deco : line.isPureDecoration = true) :
    shouldRemove level line = true := by
  simp [shouldRemove, h_deco]

/-- **Theorem 5 (Filler With Markers Kept): Filler lines that have structural
    markers are NOT removed — the marker protects them.** -/
theorem filler_with_marker_kept (level : CompressionLevel) (line : ScoredLine)
    (h_filler : line.isFiller = true)
    (h_marker : line.hasStructuralMarker = true)
    (h_not_empty : line.isEmpty = false)
    (h_not_deco : line.isPureDecoration = false) :
    shouldRemove level line = false := by
  simp [shouldRemove, h_not_empty, h_not_deco, h_filler, h_marker]

/-- **Theorem 6 (Filter is Subset): Filtered output is always a subset of input.** -/
theorem filter_subset (level : CompressionLevel) (lines : List ScoredLine) :
    ∀ l ∈ filterLines level lines, l ∈ lines := by
  intro l hl
  simp [filterLines, List.mem_filter] at hl
  exact hl.1

/-- **Theorem 7 (Threshold Monotonicity): Lite ≤ Standard threshold.** -/
theorem lite_le_standard : scoreThreshold .lite ≤ scoreThreshold .standard := by
  native_decide

/-- **Theorem 8 (Threshold Monotonicity): Standard ≤ Max threshold.** -/
theorem standard_le_max : scoreThreshold .standard ≤ scoreThreshold .max := by
  native_decide

/-- **Theorem 9 (Threshold Monotonicity): Lite ≤ Max threshold.** -/
theorem lite_le_max : scoreThreshold .lite ≤ scoreThreshold .max := by
  native_decide

/-- **Theorem 10 (Fallback Returns Standard): When Max quality fails,
    fallback returns Standard compression.** -/
theorem fallback_on_failure (lines : List ScoredLine) :
    compressWithFallback lines false = filterLines .standard lines := by
  simp [compressWithFallback]

/-- **Theorem 11 (Fallback Uses Max): When Max quality passes,
    fallback returns Max compression.** -/
theorem fallback_on_success (lines : List ScoredLine) :
    compressWithFallback lines true = filterLines .max lines := by
  simp [compressWithFallback]

/-- **Theorem 12 (Off Not Active): Off level is never active.** -/
theorem off_not_active : CompressionLevel.isActive .off = false := by rfl

/-- **Theorem 13 (Lite Active): Lite level is active.** -/
theorem lite_is_active : CompressionLevel.isActive .lite = true := by rfl

/-- **Theorem 14 (Standard Active): Standard level is active.** -/
theorem standard_is_active : CompressionLevel.isActive .standard = true := by rfl

/-- **Theorem 15 (Max Active): Max level is active.** -/
theorem max_is_active : CompressionLevel.isActive .max = true := by rfl

/-- **Theorem 16 (Filter Empty): Filtering an empty list produces an empty list.** -/
theorem filter_empty (level : CompressionLevel) :
    filterLines level [] = [] := by rfl

/-- **Theorem 17 (High Score Protected): A non-empty, non-decoration line with
    score above Max threshold is never removed at any level.** -/
theorem high_score_kept (level : CompressionLevel) (line : ScoredLine)
    (h_score : scoreThreshold .max ≤ line.score)
    (h_not_empty : line.isEmpty = false)
    (h_not_deco : line.isPureDecoration = false)
    (h_not_filler : line.isFiller = false) :
    shouldRemove level line = false := by
  simp [shouldRemove, h_not_empty, h_not_deco, h_not_filler]
  intro h_lt
  have h_le : scoreThreshold level ≤ scoreThreshold .max := by
    cases level <;> simp [scoreThreshold] <;> omega
  omega

end LeanCtxProofs.Compression.TerseEngine