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M. Scot Breitenfeld
305f1df3bf
H5Z__format_double_canonical() previously found the shortest round-tripping %.*g precision by brute-force search, verified with an strtod() readback. On HDFGroup/hdf5#6153 we agreed a well-tested library was the safer choice for a conversion that lands in the on-disk format, and settled on Ryu. Vendors the double-to-shortest-decimal subset of Ryu (src/ryu), following the same pattern as the existing tomlc17 vendoring: pristine upstream sources pinned to a commit, per-file SHA-256s, an HDF5-authored h5_ryu_prefix.h renaming its globals via force-include, and matching CMake treatment for symbol visibility and warnings. Ryu emits unconditional scientific notation with no padding (3.0 -> "3E0"), so H5Z__format_double_canonical() takes its digit string and decimal exponent apart and lays them out %g-style -- fixed-point in the human-scale range, scientific outside it -- which keeps the canonical form unchanged (rate = 3.5, not rate = 3.5E0) and needs no strtod() round-trip check: the re-layout only moves the decimal point. Ryu also writes ASCII digits directly rather than through locale-sensitive snprintf(), so the localeconv() decimal-separator fixup the old implementation needed is gone. Updated the get_config plugin contract, the h5repack/h5tools rationale comments, and tooling (clang-format, codespell, format_source) that exempted src/tomlc17 to exempt src/ryu the same way. Verified with a 2M-sample bit-pattern sweep plus all 2098 powers of two round-tripping bit-exact through the new formatter, and the full tfilter2 canonicalization suite passing unchanged.