VIGRA - Multi-dimensional Array Iterators

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| Multi-dimensional Array Iterators |
--- | --- | ---

General iterators for arrays of arbitrary dimension.

vigra::MultiIterator
  Iterator for unstrided vigra::MultiArrayView
vigra::MultiIteratorBase::type
  Inner class implementing most of the functionality of vigra::MultiIterator
vigra::StridedMultiIterator
  Iterator for strided vigra::MultiArrayView
vigra::StridedMultiIteratorBase::type
  Inner class implementing most of the functionality of vigra::StridedMultiIterator

The Multidimensional Iterator concept allows navigation on arrays of arbitrary dimension. It provides two modes of iteration: direct traveral, and hierarchical traversal. In general, hierarchical traversal will be faster, while only direct traversal allows for true random access in all dimensions. Via the `dim<K>()` function, operations applying to a particular dimension can be used in the direct traversal mode. In contrast, direct traversal functions should not be used in the hierarchical mode because the hierarchical functions are only well-defined if the iterator points to element 0 in all dimensions below its current dimension. The current dimension of a `MultiIterator<N, ..>` is `N-1`.

### Gerneral Requirements for MultiIterator

Local Types | Meaning
--- | ---
`MultiIterator::value_type` | the underlying arrays's pixel type
`MultiIterator::reference` | the iterator's reference type (return type of `*iter`). Will be `value_type &` for a mutable iterator, and convertible to `value_type const &` for a const iterator.
`MultiIterator::pointer` | the iterator's pointer type (return type of `iter.operator->()`). Will be `value_type *` for a mutable iterator, and convertible to `value_type const *` for a const iterator.
`MultiIterator::iterator_category` | the iterator tag (`vigra::multi_dimensional_traverser_tag`)
Operation | Result | Semantics
`MultiIterator k;` | default constructor
`MultiIterator k(i);` | copy constructor
`k = i` | `MultiIterator &` | assignment
`i == j` | `bool` | equality (iterators point to the same element)
`i != j` | `bool` | inequality (iterators don't point to the same element)
`*i` | `MultiIterator::reference` | access the current element
`i->member()` | depends on operation | call member function of underlying pixel type via `operator->` of iterator

### Requirements for Direct Traversal

Local Types | Meaning
--- | ---
`MultiIterator::multi_difference_type` | the iterator's multi-dimensional difference type (`TinyVector<ptrdiff_t, N>`)
Operation | Result | Semantics
`i += diff` | `MultiIterator &` | add offset to current position
`i -= diff` | `MultiIterator &` | subtract offset from current position
`i + diff` | `MultiIterator` | create traverser by adding offset
`i - diff` | `MultiIterator` | create traverser by subtracting offset
`i[diff]` | `MultiIterator::reference` | access element at offset `diff`
`i.dim<K>()` | `MultiIterator<K+1, T, ...>` | Access the traverser with the current dimension set to K. Typically used to call navigation functions referring to a particular dimension. Example (assuming `i, j` are 3-dimensional):
            i.dim<0>()++;   // increment dimension 0
            i.dim<1>()++;   // increment dimension 1
            i.dim<2>()++;   // increment dimension 2

            j += MultiIterator::multi_difference_type(1,1,1);    // same effect
`i, j` are of type `MultiIterator` `diff` is of type `MultiIterator::multi_difference_type` `K` is an integer compile-time constant

Note that it is impossible to support an `operator-` between two iterators which returns a `MultiIterator::multi_difference_type` because it is impossible to decide to which dimension a difference applies. Consider for example, a 2-dimensional iterator `i`, and let `j = i + multi_difference_type(width, 0)`, `k = i + multi_difference_type(0,1)`, where `width` is the array's total width. In general, `j` and `k` point to the same memory location, so that the two cases cannot easily be distinguished (it is possible, but iterator performance will suffer significantly, as is experienced with vigra::ImageIterator where differencing is allowed).

### Requirements for Hierarchical Traversal

Local Types | Meaning
--- | ---
`MultiIterator::difference_type` | the iterator's difference type (`ptrdiff_t`)
`MultiIterator::next_type` | type of the next iterator (referring to the next lower dimension) in the hierarchy
Operation | Result | Semantics
`++i` | `MultiIterator &` | pre-increment iterator in its current dimension
`i++` | `MultiIterator` | post-increment iterator in its current dimension
`--i` | `MultiIterator &` | pre-decrement iterator in its current dimension
`i--` | `MultiIterator` | post-decrement iterator in its current dimension
`i += d` | `MultiIterator &` | add `d` in current dimension
`i -= d` | `MultiIterator &` | subtract `d` in from dimension
`i + d` | `MultiIterator` | create new iterator by adding `d` in current dimension
`i - d` | `MultiIterator` | create new iterator by subtracting `d` in current dimension
`i - j` | `difference_type` | difference of `i` and `j` in the current dimension Note: The result of this operation is undefined if the iterator doesn't point to element 0 in all dimensions below its current dimension.
`i < j` | `bool` | `i - j < 0` Note: The result of this operation is undefined if the iterator doesn't point to element 0 in all dimensions below its current dimension.
`i[d]` | `MultiIterator::reference` | access element by adding offset `d` in current dimension
`i.begin()` | `next_type` | create the hierarchical iterator poiting to the first element in the next lower dimension. Note: The result of this operation is undefined if the iterator doesn't point to element 0 in all dimensions below its current dimension. Usage:
        MultiIterator<3, int> i3 = ..., end3 = ...;
        for(; i3 != end3; ++i3)
        {
            MultiIterator<3, int>::next_type i2 = i3.begin(), end2 = i3.end();
            for(; i2 != end2; ++i2)
            {
                MultiIterator<3, int>::next_type::next_type i1 = i2.begin(), end1 = i2.end();
                for(; i1 != end1; ++i1)
                {
                    ... // do something with the current element
                }
            }
        }
`i.end()` | `next_type` | create the hierarchical iterator poiting to the past-the-end location in the next lower dimension. Note: The result of this operation is undefined if the iterator doesn't point to element 0 in all dimensions below its current dimension.
`i, j` are of type `MultiIterator` `d` is of type `MultiIterator::difference_type`

© Ullrich Köthe (koethe@informatik.uni-hamburg.de) Cognitive Systems Group, University of Hamburg, Germany | html generated using doxygen and Python VIGRA 1.5.0 (7 Dec 2006)