[multibody] Add IcfPartition, toward implementing constraint islands

[rpoyner-tri]

Toward #23755.

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This change is 

[rpoyner-tri]

+(release notes: none)
+a:@joemasterjohn for feature review, please.
FYI @vincekurtz

@rpoyner-tri made 3 comments.
Reviewable status: 2 unresolved discussions, LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers (waiting on joemasterjohn and rpoyner-tri).

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-- commits line 2 at r1:
working: need coauthor credits

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multibody/contact_solvers/icf/icf_partition.cc line 54 at r1 (raw file):

  // Compress the path so future queries are O(1).
  while (parent_[x] != root) {
    const int next = parent_[x];

minor: This loop body is not reached by unit tests. Should I worry?

[rpoyner-tri]

@rpoyner-tri resolved 1 discussion.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers (waiting on joemasterjohn).

[rpoyner-tri]

@rpoyner-tri made 1 comment and resolved 1 discussion.
Reviewable status: LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers (waiting on joemasterjohn).

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multibody/contact_solvers/icf/icf_partition.cc line 54 at r1 (raw file):

Previously, rpoyner-tri (Rick Poyner (rico)) wrote…

minor: This loop body is not reached by unit tests. Should I worry?

See added ForcePathCompression test.

[rpoyner-tri]

@rpoyner-tri made 1 comment.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers (waiting on joemasterjohn).

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multibody/contact_solvers/icf/icf_partition.cc line 21 at r3 (raw file):

  // island i, then we turn it into a prefix sum below. Items with a negative
  // island id are skipped.
  if (std::ssize(offsets_) < num_islands + 1) offsets_.resize(num_islands + 1);

nit This implementation pattern (checked resize) is kinda batty. Instead, we should just call resize() and trust the std::vector capactiy logic.

Here and throughout.

[rpoyner-tri]

@rpoyner-tri resolved 1 discussion.
Reviewable status: LGTM missing from assignee joemasterjohn, needs platform reviewer assigned, needs at least two assigned reviewers (waiting on joemasterjohn).

[rpoyner-tri]

@joemasterjohn should I find another reviewer?

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[joemasterjohn]

Got caught up last week, but will take a look today.

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