feat: width table module with kerning-aware font metrics

Co-Authored-By: Claude Opus 4.6 <[email protected]>

tests/test_width_table.py

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+import numpy as np
+import pytest
+from PIL import ImageFont
+
+from unredact.pipeline.width_table import build_width_table, CHARSETS
+
+
+def _get_test_font() -> ImageFont.FreeTypeFont:
+    """Get any available system font for testing."""
+    import subprocess
+    result = subprocess.run(
+        ["fc-match", "--format=%{file}", "Liberation Serif"],
+        capture_output=True, text=True,
+    )
+    path = result.stdout.strip()
+    return ImageFont.truetype(path, 40)
+
+
+class TestCharsets:
+    def test_lowercase_has_26_chars(self):
+        assert len(CHARSETS["lowercase"]) == 26
+
+    def test_uppercase_has_26_chars(self):
+        assert len(CHARSETS["uppercase"]) == 26
+
+    def test_alpha_has_52_chars(self):
+        assert len(CHARSETS["alpha"]) == 52
+
+    def test_alphanumeric_has_62_chars(self):
+        assert len(CHARSETS["alphanumeric"]) == 62
+
+
+class TestBuildWidthTable:
+    def test_returns_correct_shape(self):
+        font = _get_test_font()
+        charset = "abc"
+        table = build_width_table(font, charset)
+        assert table.width_table.shape == (3, 3)
+        assert table.left_edge.shape == (3,)
+        assert table.right_edge.shape == (3,)
+        assert table.min_advance.shape == (3,)
+        assert table.max_advance.shape == (3,)
+
+    def test_widths_are_positive(self):
+        font = _get_test_font()
+        charset = "abcdefghij"
+        table = build_width_table(font, charset)
+        assert np.all(table.width_table > 0)
+
+    def test_min_max_bounds_correct(self):
+        font = _get_test_font()
+        charset = "abcdefghijklmnopqrstuvwxyz"
+        table = build_width_table(font, charset)
+        for i in range(len(charset)):
+            row = table.width_table[i]
+            assert table.min_advance[i] == pytest.approx(row.min())
+            assert table.max_advance[i] == pytest.approx(row.max())
+
+    def test_kerning_captured(self):
+        """Different char pairs should produce different advance widths."""
+        font = _get_test_font()
+        charset = "AVX"
+        table = build_width_table(font, charset)
+        assert table.width_table.shape == (3, 3)
+        # Advance of V after A should differ from advance of X after A (kerning)
+        assert table.width_table[0][1] != table.width_table[0][2]
+
+    def test_left_edge_uses_context(self):
+        font = _get_test_font()
+        charset = "abcde"
+        table = build_width_table(font, charset, left_context="T")
+        table_no_ctx = build_width_table(font, charset, left_context="")
+        assert table.left_edge.shape == (5,)
+        # Left context should change at least some advance widths
+        assert not np.allclose(table.left_edge, table_no_ctx.left_edge)
+
+    def test_right_edge_uses_context(self):
+        font = _get_test_font()
+        # Use uppercase chars that kern with lowercase context
+        charset = "AVT"
+        table = build_width_table(font, charset, right_context="o")
+        assert table.right_edge.shape == (3,)
+        # Right edge with context should differ from right edge without context
+        table_no_ctx = build_width_table(font, charset, right_context="")
+        assert not np.array_equal(table.right_edge, table_no_ctx.right_edge)