commit-reach: remove get_reachable_subset()#2141
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get_reachable_subset() and tips_reachable_from_bases() answer the same question: which to-commits are reachable from any of the from-commits. get_reachable_subset() used a graph traversal with a prio_queue until it finds all targets. tips_reachable_from_bases() uses DFS with a rising generation floor, and should be strictly faster, both due to additional pruning via generation numbers as well as O(1) DFS operations instead of O(log(n)) prio_queue operations. In practice these perform the same, and the main win is the code deduplication. The flag in remote.c changes from 1 (bit 0) to TMP_MARK (bit 4) because tips_reachable_from_bases() uses SEEN (bit 0) internally. TMP_MARK is already used for deduplication earlier in the same function and is cleared before the reachability check. Signed-off-by: Kristofer Karlsson <krka@spotify.com>
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/cc @derrickstolee |
https://gitgitgadget.github.io/``` |
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commit-reach: remove get_reachable_subset()
I have investigated what core graph algorithms exist and how
they are implemented and found that there is some overlap.
get_reachable_subset() and tips_reachable_from_bases() answer
the same question: which commits in a target set are reachable
from a set of base commits.
This patch converts the two callers of get_reachable_subset()
to use tips_reachable_from_bases() and deletes the now-unused
function.
tips_reachable_from_bases() has a minor algorithmic edge: it sorts
targets by generation and dynamically raises the pruning floor as
targets are found. In practice this makes no measurable difference
where the target set is typically small. Without a commit-graph, both
degrade identically -- all generation numbers are INFINITY, so
neither can prune. The main value is removing ~70 lines of code.
A few design choices I'd appreciate feedback on:
add_missing_tags() converts its sent_tips array to a
commit_list to match the tips_reachable_from_bases() API.
This is O(n) list-node allocations, negligible compared to
the graph walk. An array overload could avoid this, but
didn't seem worth adding for a single call site.
The flag changes from 1 (bit 0) to TMP_MARK (bit 4) because
tips_reachable_from_bases() uses SEEN (bit 0) internally.
TMP_MARK is already used for deduplication earlier in the
same function and is cleared before the reachability block.
I kept this as a single commit since the change is small.
Happy to split into convert-callers + delete-function if
that's preferred.
The test helper and test names are renamed from
get_reachable_subset to tips_reachable_from_bases to match
the function being exercised. test_all_modes already covers
both with and without commit-graph.