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# Indexing and Search
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Efficient indexing system for fast querying and filtering of large meteorological datasets. Supports multi-key indexes, complex queries, and high-performance access to message collections.
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## Capabilities
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### Index Creation
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Create indexes for fast message lookup and filtering.
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```python { .api }
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def codes_index_new_from_file(filename, keys):
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    """
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    Create index from file.
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    Parameters:
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    - filename (str): Path to GRIB/BUFR file
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    - keys (list[str]): List of keys to index on
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    Returns:
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    int: Index handle ID
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    """
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def codes_index_add_file(indexid, filename):
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    """
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    Add file to existing index.
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    Parameters:
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    - indexid (int): Index handle ID
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    - filename (str): Path to additional file
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    """
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def codes_index_release(indexid):
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    """
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    Release index from memory.
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    Parameters:
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    - indexid (int): Index handle ID
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    """
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```
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### Index Queries
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Query and filter messages using index.
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```python { .api }
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def codes_index_select(indexid, key, value):
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    """
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    Select messages by key value.
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    Parameters:
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    - indexid (int): Index handle ID
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    - key (str): Key name to filter on
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    - value: Value to match
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    Returns:
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    int: Number of matching messages
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    """
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def codes_index_get(indexid, key):
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    """
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    Get unique values for key.
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    Parameters:
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    - indexid (int): Index handle ID
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    - key (str): Key name
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    Returns:
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    list: Unique values for the key
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    """
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def codes_index_get_size(indexid, key):
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    """
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    Get number of unique values for key.
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    Parameters:
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    - indexid (int): Index handle ID
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    - key (str): Key name
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    Returns:
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    int: Number of unique values
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    """
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```
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### Message Retrieval
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Retrieve messages from index queries.
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```python { .api }
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def codes_new_from_index(indexid):
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    """
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    Create message from current index selection.
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    Parameters:
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    - indexid (int): Index handle ID
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    Returns:
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    int: Message handle ID or None if no more messages
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    """
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```
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## Usage Examples
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### Creating and Using an Index
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```python
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import eccodes
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# Create index on multiple keys
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index = eccodes.codes_index_new_from_file(
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    'forecast.grib', 
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    ['paramId', 'level', 'dataDate']
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)
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# Get available values for each key
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params = eccodes.codes_index_get(index, 'paramId')
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levels = eccodes.codes_index_get(index, 'level')
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dates = eccodes.codes_index_get(index, 'dataDate')
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print(f"Parameters: {params}")
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print(f"Levels: {levels}")
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print(f"Dates: {dates}")
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# Select specific data
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eccodes.codes_index_select(index, 'paramId', 130)  # Temperature
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eccodes.codes_index_select(index, 'level', 850)    # 850 hPa
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# Iterate through matching messages
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while True:
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    msg = eccodes.codes_new_from_index(index)
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    if msg is None:
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        break
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    date = eccodes.codes_get(msg, 'dataDate')
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    time = eccodes.codes_get(msg, 'dataTime')
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    print(f"Temperature at 850 hPa: {date} {time:04d}Z")
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    eccodes.codes_release(msg)
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eccodes.codes_index_release(index)
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```