Add utility function to compute/check ZIM descriptions based on defau…

…lt and user provided values

src/zimscraperlib/inputs.py

@@ -5,9 +5,12 @@
 import pathlib
 import shutil
 import tempfile
+from dataclasses import dataclass
 from typing import Optional, Union
 
 from . import logger
+from .constants import MAXIMUM_DESCRIPTION_METADATA_LENGTH as MAX_DESC_LENGTH
+from .constants import MAXIMUM_LONG_DESCRIPTION_METADATA_LENGTH as MAX_LONG_DESC_LENGTH
 from .download import stream_file
 
 
@@ -49,3 +52,62 @@ def handle_user_provided_file(
         shutil.copy(source, dest)
 
     return dest
+
+
+@dataclass
+class Descriptions:
+    description: str
+    long_description: Optional[str]
+
+
+def compute_descriptions(
+    default_description: str,
+    user_description: Optional[str],
+    user_long_description: Optional[str],
+) -> Descriptions:
+    """Computes short and long descriptions compliant with ZIM standard.
+
+    Based on provided parameters, the function computes a short and a long description
+    which are compliant with the ZIM standard (in terms of length).
+
+    User description(s) are used if set. They are checked to not exceed ZIM standard
+    maximum length ; an error is thrown otherwise ; if ok, they are returned.
+
+    If user_description is not set, the description is computed based on the default
+    description, truncated if needed.
+
+    If user_long_description is not set and default description is too long for the
+    description field, the long_description is computed based on the default description
+    (truncated if needed), otherwise no long description is returned.
+
+    args:
+        default_description:   the description which will be used if user descriptions
+                               are not set (typically fetched online)
+        user_description:      the description set by the user (typically set by a
+                               CLI argument)
+        user_long_description: the long description set by the user (typically set by a
+                               CLI argument)
+    """
+
+    if user_description and len(user_description) > MAX_DESC_LENGTH:
+        raise ValueError(
+            f"Description too long ({len(user_description)}>{MAX_DESC_LENGTH})"
+        )
+    if user_long_description and len(user_long_description) > MAX_LONG_DESC_LENGTH:
+        raise ValueError(
+            f"LongDescription too long ({len(user_long_description)}"
+            f">{MAX_LONG_DESC_LENGTH})"
+        )
+
+    if not user_long_description and len(default_description) > MAX_DESC_LENGTH:
+        user_long_description = default_description[0:MAX_LONG_DESC_LENGTH]
+        if len(default_description) > MAX_LONG_DESC_LENGTH:
+            user_long_description = user_long_description[:-1] + "…"
+    if not user_description:
+        user_description = default_description[0:MAX_DESC_LENGTH]
+        if len(user_description) > MAX_DESC_LENGTH:
+            user_description = user_description[:-1] + "…"
+
+    return Descriptions(
+        description=user_description, long_description=user_long_description
+    )