Simplification tutes

_toc.yml

@@ -19,6 +19,8 @@ parts:
   chapters:
   - file: tables_and_editing
   - file: simplification
+    sections:
+    - file: advanced_simplification
   - file: viz
   - file: metadata
   - file: args

advanced_simplification.md

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+---
+jupytext:
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.12
+    jupytext_version: 1.9.1
+kernelspec:
+  display_name: Python 3
+  language: python
+  name: python3
+---
+
+```{currentmodule} tskit
+```
+
+(sec_advanced_simplification)=
+
+# _Advanced simplification_
+% remove underscores in title when tutorial is complete or near-complete
+
+
+This is a companion to the basic {ref}`sec_simplification` tutorial.
+It focuses on details of `simplify` behavior that are useful when you need precise
+control over node IDs, retained ancestry structure, and sample flags.
+
+If you are primarily subsetting samples or reducing tree sequence size, start with
+{ref}`sec_simplification`.
+
+
+```{code-cell} ipython3
+:"tags": ["hide-input"]
+# Build a full ARG for demonstrations
+import msprime
+import numpy as np
+import tskit
+import tskit_arg_visualizer as argviz
+
+arg = msprime.sim_ancestry(
+    samples=10,
+    sequence_length=1e4,
+    recombination_rate=1e-8,
+    population_size=1e4,
+    record_full_arg=True,
+    random_seed=123,
+)
+arg = msprime.sim_mutations(arg, rate=1e-8, random_seed=124)
+```
+
+:::{note}
+You can both simplify an immutable {class}`TreeSequence` (to return a new one),
+or run {meth}`~TableCollection.simplify` on a {class}`TableCollection` (which
+modifies in place, and requires {ref}`sec_table_indexes` to be built as well as the
+tables to be {meth}`sorted <TableCollection.sort>`). Simplifying tables in place
+is often useful for {ref}`forward-time simulations <sec_tskit_forward_simulations>`.
+:::
+
+## 1) Tracking node ID changes
+
+With default settings, simplification compacts tables and therefore reassigns node
+IDs. If downstream code makes use of stored node IDs, request a map:
+
+```{code-cell} ipython3
+focal = arg.samples()[-6:]  # pick last 6 samples (3 diploids)
+simp, node_map = arg.simplify(focal, map_nodes=True)
+
+print(f"Original ARG: {arg.num_individuals} individuals, simplified to {simp.num_individuals} individuals")
+print(f"Original ARG: {arg.num_nodes} nodes,  simplified to {simp.num_nodes} nodes")
+print("Dropped nodes:", int(np.sum(node_map == tskit.NULL)))
+print("Old sample ID", int(focal[0]), "maps to new ID", int(node_map[focal[0]]))
+```
+
+Note that when simplifying tables in-place using {meth}`TableCollection.simplify` a map
+is always returned. To avoid compacting the node table, and leave node IDs unchanged, use
+`filter_nodes=False`.
+
+### Obtaining the reverse map
+
+Often you might want a reverse map, mapping the new node IDs to the old ones. Here's
+a simple way to do this:
+
+```{code-cell} ipython3
+def invert_map(node_mapping):
+    kept = node_mapping != tskit.NULL
+    indexes = node_mapping[kept]  # indexes are guaranteed 0..N-1
+    rev_map = np.full_like(indexes, tskit.NULL)
+    rev_map[indexes] = np.flatnonzero(kept)
+    return rev_map
+
+reverse_map = invert_map(node_map)
+print("New sample ID 0", "maps to old ID", int(reverse_map[0]))
+```
+
+## 2) Keeping input roots
+
+:::{todo}
+This is easy to illustrate, and useful for forward sims / census approaches
+:::
+
+## 3) Setting sample flags
+
+Normally the nodes that are provided to the `simplify()` function are marked as sample
+nodes in the output (by setting the `NODE_IS_SAMPLE` flag), and other nodes have that flag unset.
+If you provide the `update_sample_flags=False` option, all node flags are left unchanged.
+Here are some cases where that can be useful.
+
+### Parallel simplification
+
+One use for the `update_sample_flags=False` option combines it with `filter_nodes=False`,
+to ensure that the node table remains untouched during simplification.
+This is primarily a use-case targetted at developers of forward simulators, and allows
+logically disjunct parts of the edge table to be simplified in parallel, without
+risking two parallel processes trying to alter the same data.
+
+:::{todo}
+Should we provide an example? However, this tends to be done at a lower level, e.g.
+using the C API, and is likely to be only useful for developers. 
+:::
+
+### Retaining non-coalescent nodes
+
+The `keep_unary=True` option retains *all* ancestral nodes after simplification, even
+if they don't represent coalescences. Sometimes you might want to retain *some* but not
+all such nodes, which can be done by adding them to the focal node list but using
+`update_sample_flags=False` to ensure they are not marked as samples in the final output.
+This is usually followed by an additional simplification pass to remove any topology
+above the additional nodes which is not ancestral to the true samples. Here are some examples:
+
+#### Keeping non-coalescent regions of coalescent nodes
+
+By default, `simplify()` deletes not just non-coalescent nodes, but also
+removes from the ancestry those regions of coalescent nodes which do not represent
+a coalescence in the local tree (i.e. are "locally unary"). We can identify coalescent
+nodes using an initial round of simplification:
+
+```{code-cell}
+simp, node_map = arg.simplify(focal, map_nodes=True)
+keep_nodes = np.where(node_map != tskit.NULL)[0]  #includes sample and coalescent nodes
+
+# Retain lots of nodes by treating them as focal, but don't change sample flags
+tmp_ts = arg.simplify(keep_nodes, update_sample_flags=False)
+# Now re-simplify, in case any coalescent nodes have unwanted ancestry above
+part_simp_ts = tmp_ts.simplify(keep_unary=True)
+```
+
+Often, leaving in the non-coalescent regions of coalescent nodes can lead to a reduction
+in the number of edges. This is one reason that the {meth}`TreeSequence.extend_haplotypes` exists
+(see the documentation of that method for more detail).
+
+```{code-cell} 
+print(f"Full simplification to samples {focal} leads to {simp.num_edges} edges")
+print(
+  f"Similar simplification leaving partially-coalescent nodes leads to {part_simp_ts.num_edges} edges"
+)
+part_simp_ts.draw_svg(title=(
+    f"ARG simplified to samples {focal}, leaving part-coalescent nodes: "
+    f"this has {part_simp_ts.num_edges} edges"
+))
+```
+
+
+#### Subsetting a full ARG
+
+The {ref}`sec_args` tutorial discusses the idea of a "full ARG", containing recombination and 
+non-coalescent common ancestor nodes, which do not correspond to coalescent events anywhere in 
+the genealogy. We can use `simplify()` to reduce a full ARG to a subset of samples, while retaining those non-coalescent nodes. Again, we need to figure out
+which nodes to keep, then add them to the focal nodes, simplifying with `update_sample_flags=False`.
+
+We can detect the common ancestor nodes by finding those which still have 2 unique children after
+simplifying with `keep_unary=True` and the recombination nodes
+by finding which still come in pairs after such simplification.
+
+```{code-cell} ipython3
+def arg_num_children(ts):  # see the ARG tutorial which explains this code
+    same_parent = np.concatenate((ts.edges_parent[1:] == ts.edges_parent[:-1], [False]))
+    same_child  = np.concatenate((ts.edges_child[1:] == ts.edges_child[:-1], [False]))
+    is_last_unique = ~same_parent | ~same_child  # last occurrence of each unique child per parent
+    return np.bincount(ts.edges_parent[is_last_unique], minlength=ts.num_nodes)
+
+re_node_pairs = np.where(arg.nodes_flags & msprime.NODE_IS_RE_EVENT)[0].reshape(-1, 2)
+print("RE nodes before simplification\n", re_node_pairs)
+```
+
+Identifying the _msprime_ recombination nodes that stay as pairs after simplification requires
+a little work:
+
+:::{todo}
+Currently the code below doesn't quite work, because `keep_unary` forces the nodes above the local
+roots to be kept, see https://github.com/tskit-dev/tskit/issues/3450. This means that some RE
+(and possibly CA) nodes are kept when they should be discarded. 
+:::
+
+```{code-cell} ipython3
+simp, node_map = arg.simplify(focal, keep_unary=True, map_nodes=True)
+reverse_map = invert_map(node_map)
+simp_re_nodes = reverse_map[np.where(simp.nodes_flags & msprime.NODE_IS_RE_EVENT)[0]]
+valid_pairs = np.isin(re_node_pairs, simp_re_nodes).all(axis=1)
+keep_re_nodes = re_node_pairs[valid_pairs, :]
+print("paired RE nodes after simplification\n", keep_re_nodes)
+
+keep_RE_nodes = keep_re_nodes.flatten()
+keep_CA_nodes = reverse_map[arg_num_children(simp) > 1]
+```
+
+Now that we have defined which nodes to keep, we can use the same trick as before,
+passing these nodes as focal, but simplifying twice, once with `update_sample_flags=False`
+then again with `keep_unary=True`:
+
+```{code-cell} ipython3
+keep = np.concatenate((focal, keep_CA_nodes, keep_RE_nodes))
+
+tmp_arg = arg.simplify(keep, update_sample_flags=False)
+subset_arg = tmp_arg.simplify(keep_unary=True)  # Defaults to focal nodes = existing samples
+```
+
+Here's what it looks like in graph form:
+
+```{code-cell} ipython3
+d3arg = argviz.D3ARG.from_ts(ts=subset_arg)
+d3arg.draw(title=f"A full ARG, subset to {subset_arg.num_samples} samples");
+```
+
+## 4) Keeping individuals
+
+In some cases, a tree sequence might contain historical individuals which are associated
+with nodes that are not samples, and you wish to retain information on individuals which are
+ancestral to the sample nodes. For example a forward-time simulation could
+define individuals for all nodes in the past, including the pedigree links between parents
+and children (see also discussion in the SLiM manual, and at
+https://github.com/MesserLab/SLiM/issues/139).
+
+To keep all the individuals associated with genetic ancestry, you can use
+`keep_unary_in_individuals=True`. 
+
+:::{todo}
+Should we have a demonstration here? {ref}`sec_tskit_forward_simulations` could be used to
+create a simulator that saves pedigree information into each individual, and we could distill
+some of the discussion from https://github.com/MesserLab/SLiM/issues/139 into that.
+:::
+
+## 5) reduce_to_site_topology
+
+:::{todo}
+Explain why you might use `reduce_to_site_topology`, e.g. if there is a lot of information
+that is not inferable from sites. Note that this does not change the topology at each site,
+although some of the topology at a site might not actually be needed to represent the site
+variation (to see how to condense topology into "polytomies", see
+https://github.com/tskit-dev/tskit/discussions/2926).
+:::