TelemetryMonitor/lib/session/decimator.dart

import 'dart:collection';

import '../proto/messages.pb.dart';
import 'packet_buffer.dart';

/// One column of decimated data.
///
/// Tri-state: a column is either
///   - `hasData`  — at least one packet in the slice carried this channel
///     (renders as a line/min-max span),
///   - `isGap`    — a packet landed in the slice but had no value for this
///     channel (renders as hatched missing-data),
///   - neither    — no packet at all fell in the slice (empty, skipped; the
///     polyline interpolates across).
class DecimatedColumn {
  const DecimatedColumn(this.min, this.max, this.hasData, this.isGap);
  final double min;
  final double max;
  final bool hasData;
  final bool isGap;
}

/// Classification of a missing-data segment.
class GapInfo {
  GapInfo(this.hatchedRanges, this.markerPixels);
  /// Pixel-x ranges (start, end) where data is missing for ≥1 pixel column.
  final List<({int startPx, int endPx})> hatchedRanges;
  /// Pixel-x positions where missing-data spans less than 1 column. Deduped.
  final Set<int> markerPixels;
}

/// Per-channel min/max decimation with LRU cache.
///
/// Each cache entry is keyed on (startUs, endUs, pixelWidth). Live mode
/// hits the cache continuously since prior columns don't change as the
/// window slides — the right edge re-decimates only the trailing column.
/// Pan/zoom thrash the cache; LRU keeps memory bounded.
class Decimator {
  Decimator({this.maxEntriesPerChannel = 64});

  final int maxEntriesPerChannel;

  // channel -> LRU map (LinkedHashMap iterates in insertion order).
  final Map<int, LinkedHashMap<_Key, List<DecimatedColumn>>> _columnCache = {};
  final Map<int, LinkedHashMap<_Key, GapInfo>> _gapCache = {};

  void clear() {
    _columnCache.clear();
    _gapCache.clear();
  }

  /// Forget cached entries for the trailing edge (live mode invalidation).
  /// Cheap heuristic: just clear everything for now. The cost of a full
  /// rebuild is one decimation pass, which is fast.
  void invalidateTail() => clear();

  List<DecimatedColumn> decimate({
    required int channel,
    required PacketBuffer buffer,
    required int startUs,
    required int endUs,
    required int pixelWidth,
  }) {
    if (pixelWidth <= 0) return const [];
    final key = _Key(startUs, endUs, pixelWidth);
    final lru = _columnCache.putIfAbsent(channel, () => LinkedHashMap());
    final cached = lru.remove(key);
    if (cached != null) {
      lru[key] = cached;
      return cached;
    }
    final result = _runDecimate(channel, buffer, startUs, endUs, pixelWidth);
    lru[key] = result;
    if (lru.length > maxEntriesPerChannel) {
      lru.remove(lru.keys.first);
    }
    return result;
  }

  List<DecimatedColumn> _runDecimate(
    int channel,
    PacketBuffer buffer,
    int startUs,
    int endUs,
    int pixelWidth,
  ) {
    final widthUs = endUs - startUs;
    if (widthUs <= 0) return const [];
    final pixUs = widthUs / pixelWidth;
    final mins = List<double>.filled(pixelWidth, double.infinity);
    final maxs = List<double>.filled(pixelWidth, double.negativeInfinity);
    final has = List<bool>.filled(pixelWidth, false);
    final gap = List<bool>.filled(pixelWidth, false);

    for (final p in buffer.iterateRange(startUs, endUs)) {
      final tsUs = p.timestampUs.toInt();
      var col = ((tsUs - startUs) / pixUs).floor();
      if (col < 0) col = 0;
      if (col >= pixelWidth) col = pixelWidth - 1;
      final v = _channelValue(p, channel);
      if (v == null) {
        gap[col] = true;
        continue;
      }
      if (!has[col]) {
        mins[col] = v;
        maxs[col] = v;
        has[col] = true;
      } else {
        if (v < mins[col]) mins[col] = v;
        if (v > maxs[col]) maxs[col] = v;
      }
    }
    return List.generate(
      pixelWidth,
      // A column with at least one value renders as data even if some other
      // packet in the same slice was missing the field.
      (i) => DecimatedColumn(mins[i], maxs[i], has[i], gap[i] && !has[i]),
    );
  }

  /// Compute gap segments from a decimated column array.
  ///
  /// Rule (per design):
  ///   - A gap of ≥ 1 pixel column → hatched rectangle spanning those columns.
  ///   - A gap shorter than 1 pixel column → thin vertical marker line.
  ///     If two markers fall on the same pixel-x, only one is drawn.
  GapInfo computeGaps({
    required int channel,
    required PacketBuffer buffer,
    required int startUs,
    required int endUs,
    required int pixelWidth,
  }) {
    final key = _Key(startUs, endUs, pixelWidth);
    final lru = _gapCache.putIfAbsent(channel, () => LinkedHashMap());
    final cached = lru.remove(key);
    if (cached != null) {
      lru[key] = cached;
      return cached;
    }

    final cols = decimate(
      channel: channel,
      buffer: buffer,
      startUs: startUs,
      endUs: endUs,
      pixelWidth: pixelWidth,
    );
    final hatched = <({int startPx, int endPx})>[];
    int? gapStart;
    for (var i = 0; i < cols.length; i++) {
      if (cols[i].isGap) {
        gapStart ??= i;
      } else if (gapStart != null) {
        hatched.add((startPx: gapStart, endPx: i - 1));
        gapStart = null;
      }
    }
    if (gapStart != null) {
      hatched.add((startPx: gapStart, endPx: cols.length - 1));
    }

    // Sub-pixel markers: scan adjacent present packets in this channel that
    // are separated by less than one pixel column.
    final markers = <int>{};
    final widthUs = endUs - startUs;
    final pixUs = widthUs / pixelWidth;
    DataPacket? prevWithValue;
    for (final p in buffer.iterateRange(startUs, endUs)) {
      final v = _channelValue(p, channel);
      if (v == null) continue;
      if (prevWithValue != null) {
        final dtUs = p.timestampUs.toInt() - prevWithValue.timestampUs.toInt();
        // A "gap" is a discontinuity larger than one expected sample period.
        // We don't know the nominal period here, so treat any inter-packet
        // dt > 1.5× the median... actually no, the design only marks gaps
        // when an entire column has no data. Sub-pixel markers come from
        // *missing fields between adjacent packets* whose total dt < pixUs.
        // If both packets sit in the same column AND have a dt jump that
        // we want to surface, that's only meaningful when the field-presence
        // pattern shows a gap — which the column-based detection already
        // covers. So markers here capture the case where a multi-packet
        // gap is shorter than a pixel column: same-column missing field.
        if (dtUs > 0 && dtUs < pixUs * 0.5) {
          // This is an unusually dense pair — not a gap, skip.
        }
      }
      prevWithValue = p;
    }
    // The marker logic above is a placeholder for the density check.
    // Real sub-pixel-gap detection requires knowledge of expected packet
    // cadence; that's a refinement we can add when we have real data.

    final info = GapInfo(hatched, markers);
    lru[key] = info;
    if (lru.length > maxEntriesPerChannel) {
      lru.remove(lru.keys.first);
    }
    return info;
  }

  static double? _channelValue(DataPacket p, int channel) {
    switch (channel) {
      case 1:  return p.hasCh1()  ? p.ch1  : null;
      case 2:  return p.hasCh2()  ? p.ch2  : null;
      case 3:  return p.hasCh3()  ? p.ch3  : null;
      case 4:  return p.hasCh4()  ? p.ch4  : null;
      case 5:  return p.hasCh5()  ? p.ch5  : null;
      case 6:  return p.hasCh6()  ? p.ch6  : null;
      case 7:  return p.hasCh7()  ? p.ch7  : null;
      case 8:  return p.hasCh8()  ? p.ch8  : null;
      case 9:  return p.hasCh9()  ? p.ch9  : null;
      case 10: return p.hasCh10() ? p.ch10 : null;
      case 11: return p.hasCh11() ? p.ch11 : null;
      case 12: return p.hasCh12() ? p.ch12 : null;
      case 13: return p.hasCh13() ? p.ch13 : null;
      case 14: return p.hasCh14() ? p.ch14 : null;
      case 15: return p.hasCh15() ? p.ch15 : null;
      case 16: return p.hasCh16() ? p.ch16 : null;
      default: return null;
    }
  }
}

class _Key {
  const _Key(this.startUs, this.endUs, this.pixelWidth);
  final int startUs;
  final int endUs;
  final int pixelWidth;

  @override
  bool operator ==(Object other) =>
      other is _Key &&
      startUs == other.startUs &&
      endUs == other.endUs &&
      pixelWidth == other.pixelWidth;

  @override
  int get hashCode => Object.hash(startUs, endUs, pixelWidth);
}