Add clean_ligatures to core cleaners (#1326)

# Background


[Ligatures](https://en.wikipedia.org/wiki/Ligature_(writing)#Ligatures_in_Unicode_(Latin_alphabets))
can sometimes show up during the text extraction process when they
should not. Very common examples of this are with the Latin `f` related
ligatures which can be **very subtle** to spot by eye (see example
below), but can wreak havoc later.

```python
"ff": "ff",
"fi": "fi",
"fl": "fl",
"ffi": "ffi",
"ffl": "ffl",
```

Several libraries already do something like this. Most recently,
`pdfplumber` added this sort of capability as part of the text
extraction process, see jsvine/pdfplumber#598

Instead of incorporating any sort of breaking change to the PDF text
processing in `unstructured`, it is best to add this as another cleaner
and allow users to opt in. In turn, the `clean_ligatures` method has
been added in this PR - with accompanying tests.

# Example

Here is an example PDF that causes the issue. For example: `Benefits`,
which should be `Benefits`.


[example.pdf](https://github.com/Unstructured-IO/unstructured/files/12544344/example.pdf)

```bash
curl -X 'POST' \
    'https://api.unstructured.io/general/v0/general' \
    -H 'accept: application/json' \
    -H 'Content-Type: multipart/form-data' \
    -H 'unstructured-api-key: ${UNSTRUCTURED_API_KEY}' \
    -F '[email protected]' \
    -s | jq -C .
```

# Notes

An initial list of mappings was added with the most common ligatures.
There is some subjectivity to this, but this should be a relatively safe
starting set. Can always be expanded as needed.

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Features
 
 * Add Jira Connector to be able to pull issues from a Jira organization
+* Add `clean_ligatures` function to expand ligatures in text
 
 ### Fixes
 

test_unstructured/cleaners/test_core.py

@@ -62,6 +62,32 @@ def test_clean_ordered_bullets(text, expected):
     assert core.clean_ordered_bullets(text=text) == expected
 
 
+@pytest.mark.parametrize(
+    ("text", "expected"),
+    [
+        ("The æther is a classic element.", "The aether is a classic element."),
+        ("In old texts, Æsop's fables are", "In old texts, AEsop's fables are"),
+        ("The buffer zone is there.", "The buffer zone is there."),
+        ("The file was found in the system.", "The file was found in the system."),
+        ("She had a flower in her hair.", "She had a flower in her hair."),
+        ("The coffin was placed in the grave.", "The coffin was placed in the grave."),
+        ("The buffle zone was clearly marked.", "The buffle zone was clearly marked."),
+        ("The craſtsman worked with dedication.", "The craftsman worked with dedication."),
+        ("The symbol ʪ is very rare.", "The symbol ls is very rare."),
+        ("The word 'cœur' means 'heart' in French.", "The word 'coeur' means 'heart' in French."),
+        ("The word 'Œuvre' refers to the works", "The word 'OEuvre' refers to the works"),
+        ("The ȹ symbol is used in some contexts.", "The qp symbol is used in some contexts."),
+        ("The postman delivers mail daily.", "The postman delivers mail daily."),
+        (
+            "The symbol ʦ can be found in certain alphabets.",
+            "The symbol ts can be found in certain alphabets.",
+        ),
+    ],
+)
+def test_clean_ligatures(text, expected):
+    assert core.clean_ligatures(text=text) == expected
+
+
 @pytest.mark.parametrize(
     ("text", "expected"),
     [

unstructured/cleaners/core.py

@@ -68,6 +68,37 @@ def clean_ordered_bullets(text) -> str:
     return text_cl
 
 
+def clean_ligatures(text) -> str:
+    """Replaces ligatures with their most likely equivalent characters.
+
+    Example
+    -------
+    The benefits -> The benefits
+    High quality financial -> High quality financial
+    """
+    ligatures_map = {
+        "æ": "ae",
+        "Æ": "AE",
+        "ff": "ff",
+        "fi": "fi",
+        "fl": "fl",
+        "ffi": "ffi",
+        "ffl": "ffl",
+        "ſt": "ft",
+        "ʪ": "ls",
+        "œ": "oe",
+        "Œ": "OE",
+        "ȹ": "qp",
+        "st": "st",
+        "ʦ": "ts",
+    }
+    cleaned_text: str = text
+    for k, v in ligatures_map.items():
+        cleaned_text = cleaned_text.replace(k, v)
+
+    return cleaned_text
+
+
 def group_bullet_paragraph(paragraph: str) -> list:
     """Groups paragraphs with bullets that have line breaks for visual/formatting purposes.
     For example: