perf(matrix): fill a fixed buffer instead of a vector per cell - #827
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samueltardieu merged 1 commit intoSep 11, 2026
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`neighbours` is already lazy, but `bfs_reachable` and `dfs_reachable` collect it
into a freshly allocated `Vec` for every cell they visit, so walking a matrix
allocates and frees once per cell. A cell has at most eight neighbours, so they
fit in a fixed buffer and the allocation buys nothing.
Output is unchanged. Besides the existing tests, both traversals were compared
against the previous code over 6092 traversals across 60 random matrices,
covering both diagonal settings and every starting cell: identical fingerprint
over every cell returned.
Measured against this commit's parent, four passes with the order of the two
binaries swapped on alternate passes:
Matrix::bfs_reachable, 400x400, 12% blocked: about 26% off
Matrix::dfs_reachable, 400x400, 12% blocked: about 31% off
Measured on Windows 11, Intel Core Ultra 7 265K, 64 GB RAM.
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Closes #826.
Collects the neighbours of a cell into a fixed
[(usize, usize); 8]rather than a vector, inbfs_reachableanddfs_reachable.neighboursitself is untouched — it already returns an iterator, so only the two traversals needed changing.Checking
All tests pass, clippy and rustfmt clean.
Beyond those, both traversals were compared against the previous code over 6092 traversals across 60 random matrices, covering both diagonal settings and every starting cell. Identical fingerprint over every cell returned, so this is not "faster because it visits less".
Measured against this branch's parent, four passes with the order of the two binaries swapped on alternate passes, consistent in both directions:
Measured on Windows 11, Intel Core Ultra 7 265K, 64 GB RAM.
Same shape as #824 but independent of it: that one touches
grid.rs, this onematrix.rs.