🐛 Fix calculation of score_hint for edge cases, e.g. when CER is infinite

If the CER is infinite, we can't calculate a score_hint as an int. Fall back to None
in this case.

qurator/dinglehopper/align.py

@@ -1,3 +1,6 @@
+import math
+from math import ceil
+
 from .edit_distance import *
 from rapidfuzz.distance import Levenshtein
 
@@ -8,6 +11,22 @@ def align(t1, t2):
     return seq_align(s1, s2)
 
 
+def score_hint(er: float, n: int) -> int | None:
+    """Calculate RapidFuzz score hint for a given error rate and count.
+
+    Gives the score hint for the distance functions (= expected distance) or None if
+    the error rate is inf.
+    """
+    assert not math.isnan(er)
+    try:
+        score_hint = int(ceil(er * n))
+    except (OverflowError, ValueError):
+        # ceil(er * n) can be inf or NaN (for n == 0), so int() can throw an
+        # OverflowError and a ValueError.
+        score_hint = None
+    return score_hint
+
+
 def seq_align(s1, s2, score_hint=None):
     """Align general sequences."""
     s1 = list(s1)

qurator/dinglehopper/cli.py

@@ -8,7 +8,7 @@ from math import ceil
 
 from .character_error_rate import character_error_rate_n
 from .word_error_rate import word_error_rate_n, words_normalized
-from .align import seq_align
+from .align import seq_align, score_hint
 from .extracted_text import ExtractedText
 from .ocr_files import extract
 from .config import Config
@@ -110,12 +110,14 @@ def process(gt, ocr, report_prefix, *, metrics=True, textequiv_level="region"):
 
     cer, n_characters = character_error_rate_n(gt_text, ocr_text)
     char_diff_report = gen_diff_report(
-        gt_text, ocr_text, css_prefix="c", joiner="", none="·", score_hint=int(ceil(cer * n_characters))
+        gt_text, ocr_text, css_prefix="c", joiner="", none="·",
+        score_hint=score_hint(cer, n_characters)
     )
 
     wer, n_words = word_error_rate_n(gt_words, ocr_words)
     word_diff_report = gen_diff_report(
-        gt_words, ocr_words, css_prefix="w", joiner=" ", none="⋯", score_hint=int(ceil(wer * n_words))
+        gt_words, ocr_words, css_prefix="w", joiner=" ", none="⋯",
+        score_hint=score_hint(wer, n_words)
     )
 
     env = Environment(

qurator/dinglehopper/cli_line_dirs.py

@@ -75,10 +75,12 @@ def process(gt_dir, ocr_dir, report_prefix, *, metrics=True):
 
         # Generate diff reports
         char_diff_report += gen_diff_report(
-            gt_text, ocr_text, css_prefix="l{0}-c".format(k), joiner="", none="·", score_hint=int(ceil(l_cer * l_n_characters))
+            gt_text, ocr_text, css_prefix="l{0}-c".format(k), joiner="", none="·",
+            score_hint=score_hint(l_cer, l_n_characters)
         )
         word_diff_report += gen_diff_report(
-            gt_words, ocr_words, css_prefix="l{0}-w".format(k), joiner=" ", none="⋯", score_hint=int(ceil(l_wer * l_n_words))
+            gt_words, ocr_words, css_prefix="l{0}-w".format(k), joiner=" ", none="⋯",
+            score_hint=score_hint(l_wer, l_n_words))
         )
 
     env = Environment(

qurator/dinglehopper/tests/test_align.py

@@ -1,6 +1,7 @@
+import math
 import pytest
 from .util import unzip
-from .. import align, seq_align, distance
+from .. import align, seq_align, distance, score_hint
 
 
 def test_left_empty():
@@ -181,3 +182,7 @@ def test_lines_similar():
 
     # Test __eq__ (i.e. is it a substitution or a similar string?)
     assert list(left)[0] == list(right)[0]
+
+def test_score_hint():
+    assert score_hint(0.5, 23) == 12  # int(ceil())
+    assert score_hint(math.inf, 12345) is None