BadNote/test/ctc_decoder_test.dart

import 'package:badnote/services/ocr/ctc_decoder.dart';
import 'package:flutter_test/flutter_test.dart';

/// Build a one-hot-ish logits row of [numClasses] with the max at [maxIndex].
List<double> _row(int numClasses, int maxIndex) {
  return List<double>.generate(numClasses, (i) => i == maxIndex ? 1.0 : 0.0);
}

void main() {
  group('CtcDecoder.decode', () {
    test('collapses consecutive repeats and drops blanks (+1 shift)', () {
      // charset indices: 1->'a', 2->'b', 3->'c' (blank at 0, shifted by one).
      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
      const numClasses = 4; // blank + 3 chars
      final logits = <List<double>>[
        _row(numClasses, 1), // a
        _row(numClasses, 1), // a (collapsed)
        _row(numClasses, 0), // blank
        _row(numClasses, 2), // b
        _row(numClasses, 2), // b (collapsed)
        _row(numClasses, 3), // c
      ];
      expect(decoder.decode(logits), 'abc');
    });

    test('empty input yields empty string', () {
      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
      expect(decoder.decode(<List<double>>[]), '');
    });

    test('all-blank input yields empty string', () {
      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
      const numClasses = 4;
      final logits = <List<double>>[
        _row(numClasses, 0),
        _row(numClasses, 0),
        _row(numClasses, 0),
      ];
      expect(decoder.decode(logits), '');
    });

    test('out-of-range indices are skipped', () {
      // charset has 2 entries -> valid class indices are 1 and 2. Class index 3
      // maps to charset[2] which is out of range and must be skipped.
      final decoder = CtcDecoder(['a', 'b'], blankIndex: 0);
      const numClasses = 4;
      final logits = <List<double>>[
        _row(numClasses, 1), // a
        _row(numClasses, 3), // out of range -> skipped
        _row(numClasses, 2), // b
      ];
      expect(decoder.decode(logits), 'ab');
    });
  });

  group('CtcDecoder.decodeFlat', () {
    test('reshapes a flat row-major list and decodes it', () {
      final decoder = CtcDecoder(['a', 'b', 'c'], blankIndex: 0);
      const numClasses = 4;
      const timeSteps = 3;
      final flat = <double>[
        ..._row(numClasses, 1), // a
        ..._row(numClasses, 0), // blank
        ..._row(numClasses, 2), // b
      ];
      expect(decoder.decodeFlat(flat, timeSteps, numClasses), 'ab');
    });

    test('returns empty for non-positive dimensions', () {
      final decoder = CtcDecoder(['a'], blankIndex: 0);
      expect(decoder.decodeFlat(<double>[1, 0], 0, 2), '');
      expect(decoder.decodeFlat(<double>[1, 0], 2, 0), '');
    });
  });
}