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A mapping of keys
(cell values) and values (cell
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identifiers) is
a general way of storing grid data.
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Efficient in
terms of memory use where a default
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value can be
set, and if there are only a small number
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of non-default
mappings in the chunk (compared to
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the number of
cells in the chunk)
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Offer the means
to generating some statistics about a
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chunk very
efficiently
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the diversity
(number of different values)
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mode
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