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<li class="breadcrumb-item active" aria-current="page"><span class="ellipsis">Load Algorithm Hook</span></li>
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<section id="load-algorithm-hook">
<h1>Load Algorithm Hook<a class="headerlink" href="#load-algorithm-hook" title="Link to this heading">#</a></h1>
<nav class="contents local" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#the-load-algorithm" id="id1">The Load Algorithm</a></p></li>
<li><p><a class="reference internal" href="#descriptors" id="id2">Descriptors</a></p>
<ul>
<li><p><a class="reference internal" href="#nexus" id="id3">Nexus</a></p>
<ul>
<li><p><a class="reference internal" href="#nexusdescriptorlazy" id="id4">NexusDescriptorLazy</a></p></li>
<li><p><a class="reference internal" href="#nexusdescriptor" id="id5">NexusDescriptor</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#legacynexus" id="id6">LegacyNexus</a></p>
<ul>
<li><p><a class="reference internal" href="#legacynexusdescriptor" id="id7">LegacyNexusDescriptor</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#non-hdf" id="id8">Non-HDF</a></p>
<ul>
<li><p><a class="reference internal" href="#filedescriptor" id="id9">FileDescriptor</a></p></li>
</ul>
</li>
</ul>
</li>
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</nav>
<section id="the-load-algorithm">
<h2><a class="toc-backref" href="#id1" role="doc-backlink">The Load Algorithm</a><a class="headerlink" href="#the-load-algorithm" title="Link to this heading">#</a></h2>
<p>This page describes how to register a new file loading algorithm
so that it can be used by the general-purpose <a class="reference external" href="https://docs.mantidproject.org/nightly/algorithms/Load-v1.html#algm-load" title="(in MantidProject v6.15)"><span class="xref std std-ref">Load</span></a> algorithm.
The <a class="reference external" href="https://docs.mantidproject.org/nightly/algorithms/Load-v1.html#algm-load" title="(in MantidProject v6.15)"><span class="xref std std-ref">Load</span></a> algorithm is a special algorithm
that performs very little loading itself.
It instead tries to search for the most suitable algorithm for a given file
and then uses this algorithm as a child to load the file.
An algorithm wishing to be included as part of the search
must register itself slightly differently
and not use the <code class="docutils literal notranslate"><span class="pre">DECLARE_ALGORITHM</span></code> macro.</p>
<p>The process of searching for the correct loader needs to be fairly quick
as long load times will be especially noticeable in the GUI.
To speed up the process the loaders are currently split into three categories:</p>
<ul class="simple">
<li><p>Nexus (HDF5)</p></li>
<li><p>Legacy Nexus (HDF4/5 – for ISIS muon scattering only)</p></li>
<li><p>Non-HDF</p></li>
</ul>
<p>A quick check is performed using <code class="docutils literal notranslate"><span class="pre">H5::H5File::isHdf5()</span></code> to test whether the file looks like a
<a class="reference external" href="http://www.hdfgroup.org/">HDF5</a> file.
If the file is HDF5, then only the Nexus loaders are checked.
If the file is not HDF5, then it is checked for the HDF4 signature.
If the file is HDF5, then only the Legacy Nexus loaders are checked.
If the file is neither HDF5 nor HDF4, then the non-HDF group of loaders is checked.</p>
</section>
<section id="descriptors">
<h2><a class="toc-backref" href="#id2" role="doc-backlink">Descriptors</a><a class="headerlink" href="#descriptors" title="Link to this heading">#</a></h2>
<p>To avoid opening and closing the file within each <code class="docutils literal notranslate"><span class="pre">confidence</span></code> method,
a <code class="docutils literal notranslate"><span class="pre">Descriptor</span></code> object is provided.
The actual <code class="docutils literal notranslate"><span class="pre">Descriptor</span></code> class depends on whether the file is
an HDF5 file,
an HDF4 file,
or neither.</p>
<section id="nexus">
<h3><a class="toc-backref" href="#id3" role="doc-backlink">Nexus</a><a class="headerlink" href="#nexus" title="Link to this heading">#</a></h3>
<p>To register an algorithm as a Nexus loader,
use the <code class="docutils literal notranslate"><span class="pre">IFileLoader<NexusDescriptorLazy></span></code> interface
as a base class for your algorithm.
In your cpp file, include the <code class="docutils literal notranslate"><span class="pre">MantidAPI/RegisterFileLoader.h</span></code> header
and use the <code class="docutils literal notranslate"><span class="pre">DECLARE_NEXUS_FILELOADER_ALGORITHM</span></code> macro
<em>instead</em> of the standard <code class="docutils literal notranslate"><span class="pre">DECLARE_ALGORITHM</span></code> macro.</p>
<p>The interface requires that the method
<code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">int</span> <span class="pre">confidence(Nexus::NexusDescriptorLazy</span> <span class="pre">&descriptor)</span> <span class="pre">const</span> <span class="pre">=</span> <span class="pre">0;</span></code>
be overridden in the inheriting class.
It should use the provided descriptor to check
whether the loader is able to load the wrapped file,
and return a confidence level as an integer.</p>
<section id="nexusdescriptorlazy">
<h4><a class="toc-backref" href="#id4" role="doc-backlink">NexusDescriptorLazy</a><a class="headerlink" href="#nexusdescriptorlazy" title="Link to this heading">#</a></h4>
<p>The lazy Nexus descriptor performs a shallow preload of the file,
and then only lazily checks for entries (without opening them) as loaders query them.
It owns a handle to an HDF5 file.
This is also able to load string data, if necessary for determining file loader.
Avoid calling the <code class="docutils literal notranslate"><span class="pre">getAllEntries()</span></code> method;
doing so may lead to unexpected or inefficient results.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>Do <strong>NOT</strong> open a <code class="docutils literal notranslate"><span class="pre">Nexus::File</span></code> object using the descriptor’s filename.
Doing so defeats efficiency gains of using a shallow loader.
If you need to check a data string,
use <code class="docutils literal notranslate"><span class="pre">NexusDescriptorLazy::getStrData()</span></code>,
passing the absolute dataset address within the file (e.g. <cite>/entry/instrument/definition</cite>).</p>
</div>
<p>All new algorithms for loading HDF5 data should use the lazy descriptor.</p>
</section>
<section id="nexusdescriptor">
<h4><a class="toc-backref" href="#id5" role="doc-backlink">NexusDescriptor</a><a class="headerlink" href="#nexusdescriptor" title="Link to this heading">#</a></h4>
<p>This Nexus descriptor should not be used for confidence checks in <cite>Load</cite>,
as it performs an eager load of the entire filetree.
Use the lazy descriptor instead.</p>
</section>
</section>
<section id="legacynexus">
<h3><a class="toc-backref" href="#id6" role="doc-backlink">LegacyNexus</a><a class="headerlink" href="#legacynexus" title="Link to this heading">#</a></h3>
<p>These loaders preserve the state of <code class="docutils literal notranslate"><span class="pre">Mantid::Nexus</span></code> prior to the Nexus refactor.
They still rely on <code class="docutils literal notranslate"><span class="pre">napi</span></code> for loading HDF files,
and do not include many of the improvements in efficiency and memory use from the refactor.
This type of loader should only be used for algorithms meant to load ISIS muon files,
many of which still use HDF4.
These are the only loaders in Mantid capable of loading HDF4 files.</p>
<p>LegacyNexus is deprecated.
No new loaders should be written to work in LegacyNexus.</p>
<section id="legacynexusdescriptor">
<h4><a class="toc-backref" href="#id7" role="doc-backlink">LegacyNexusDescriptor</a><a class="headerlink" href="#legacynexusdescriptor" title="Link to this heading">#</a></h4>
<p>This uses old <code class="docutils literal notranslate"><span class="pre">napi</span></code> behavior to load the entire filetree and store in a dictionary cache.
Parts of the tree may be queried using available methods.</p>
</section>
</section>
<section id="non-hdf">
<h3><a class="toc-backref" href="#id8" role="doc-backlink">Non-HDF</a><a class="headerlink" href="#non-hdf" title="Link to this heading">#</a></h3>
<p>To register an algorithm as a Non-HDF loader use the <code class="docutils literal notranslate"><span class="pre">IFileLoader</span></code> interface
as a base class for your algorithm.
In your cpp file include the <code class="docutils literal notranslate"><span class="pre">MantidAPI/RegisterFileLoader.h</span></code> header
and use the <code class="docutils literal notranslate"><span class="pre">DECLARE_FILELOADER</span></code> macro
<em>instead</em> of the standard <code class="docutils literal notranslate"><span class="pre">DECLARE_ALGORITHM</span></code> macro.</p>
<p>The interface requires that the method
<code class="docutils literal notranslate"><span class="pre">virtual</span> <span class="pre">int</span> <span class="pre">confidence(Kernel::FileDescriptor</span> <span class="pre">&</span> <span class="pre">descriptor)</span> <span class="pre">const</span> <span class="pre">=</span> <span class="pre">0;</span></code>
be overridden in the inheriting class.
It should use the provided descriptor to check
whether the loader is able to load the wrapped file,
and return a confidence level as an integer.</p>
<section id="filedescriptor">
<h4><a class="toc-backref" href="#id9" role="doc-backlink">FileDescriptor</a><a class="headerlink" href="#filedescriptor" title="Link to this heading">#</a></h4>
<p>This provides methods to query some characteristics of the current file
and also access the open stream to avoid repeated opening/closing of the file.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p><em>Avoid</em> closing the stream. The code calling the <code class="docutils literal notranslate"><span class="pre">confidence</span></code> method
ensures that the stream is back at the start of the file before checking the next loader
so simply use the stream as necessary and leave it as it is.</p>
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