A fast and flexible generic sparse 2 dimensional array.

## Features

• Exposes graph such that clients may easily walk directly between neighbouring elements to allow fast array traversal.
• Easy conversion between dense and sparse formats.

## Usage

Example:

```final sparse = SparseArray2D<String>();

sparse.put(2, 0, 'a');
sparse.put(2, 1, 'b');
sparse.put(2, 5, 'c');
sparse.put(2, 8, 'd');
sparse.put(0, 5, 'e');

print('Sparse form elements');
print(sparse.rows.join('\n'));

print('Dense form values');
print(sparse.denseRows.join('\n'));

print('Dense form with default value "_"');

print('Extract row as sparse elements');
print(sparse.rowAt(2));

print('Extract row as sparse values with default value "*"');

print('Extract column as sparse elements');
print(sparse.columnAt(5)!.join('\n'));

print('Extract column as sparse values with default value "*"');
```

Output:

``````Sparse form elements
(0)[(0, 5)[e]]
(2)[(2, 0)[a],(2, 1)[b],(2, 5)[c],(2, 8)[d]]

Dense form values
(null, null, null, null, null, e, null, null, null)
(null, null, null, null, null, null, null, null, null)
(a, b, null, null, null, c, null, null, d)

Dense form with default value "_"
(-, -, -, -, -, e, -, -, -)
(-, -, -, -, -, -, -, -, -)
(a, b, -, -, -, c, -, -, d)

Extract row as sparse elements
(2)[(2, 0)[a],(2, 1)[b],(2, 5)[c],(2, 8)[d]]

Extract row as sparse values with default value "*"
(a, b, *, *, *, c, *, *, d)

Extract column as sparse elements
(0, 5)[e]
(2, 5)[c]

Extract column as sparse values with default value "*"
e
*
c
``````

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