tmp/tmpuj19sa87/{from.md → to.md}
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###### Exposition-only helpers <a id="mdspan.layout.stride.expo">[[mdspan.layout.stride.expo]]</a>
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Let `REQUIRED-SPAN-SIZE(e, strides)` be:
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- `1`, if `e.rank() == 0` is `true`,
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- otherwise `0`, if the size of the multidimensional index space `e` is
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0,
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- otherwise `1` plus the sum of products of `(e.extent(r) - 1)` and
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`strides[r]` for all r in the range [0, `e.rank()`).
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Let `OFFSET(m)` be:
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- `m()`, if `e.rank() == 0` is `true`,
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- otherwise `0`, if the size of the multidimensional index space `e` is
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0,
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- otherwise `m(z...)` for a pack of integers `z` that is a
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multidimensional index in `m.extents()` and each element of `z` equals
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0.
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Let *`is-extents`* be the exposition-only variable template defined as
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follows:
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``` cpp
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template<class T>
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constexpr bool is-extents = false; // exposition only
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template<class IndexType, size_t... Args>
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constexpr bool is-extents<extents<IndexType, Args...>> = true; // exposition only
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```
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Let `layout-mapping-alike` be the exposition-only concept defined as
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follows:
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``` cpp
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template<class M>
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concept layout-mapping-alike = requires { // exposition only
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requires is-extents<typename M::extents_type>;
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{ M::is_always_strided() } -> same_as<bool>;
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{ M::is_always_exhaustive() } -> same_as<bool>;
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{ M::is_always_unique() } -> same_as<bool>;
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bool_constant<M::is_always_strided()>::value;
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bool_constant<M::is_always_exhaustive()>::value;
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bool_constant<M::is_always_unique()>::value;
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};
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```
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[*Note 2*: This concept checks that the functions
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`M::is_always_strided()`, `M::is_always_exhaustive()`, and
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`M::is_always_unique()` exist, are constant expressions, and have a
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return type of `bool`. — *end note*]
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