1.1 "fixed point release"

This release adds fixed notation support, along with substantial float and SIMD performance improvements, a smaller binary footprint via optional size optimizations, a standalone C header, broad portability fixes (32-bit, clang, GCC/NEON, MSVC), and a rewritten verification and benchmarking toolchain. Overall, double-to-string conversion is roughly 23-28% faster than in 1.0 on Apple M5 Max and 28-38% on AMD EPYC Milan.

Time per double on Apple M5 Max (smaller is better), from the dtoa-benchmark:

New Features & API

• Added support for fixed notation, which write automatically emits (for example 123.456 rather than 1.23456e+02) for values of moderate magnitude, switching to scientific notation for large or small magnitudes, similar to the default floating-point format in Python and other languages (#64, #111, #124, thanks @dougallj).
• Added a dedicated C header (zmij-c.h) (#93).
• Extended to_decimal with a negative flag, which also enables correct handling of negative zero.
• Added the ZMIJ_OPTIMIZE_SIZE macro that enables binary size optimizations when defined to 1 (the default when compiling with __OPTIMIZE_SIZE__), reducing zmij's code and data from ~20 KB to ~4 KB (~79% reduction on Apple clang at -O2).
• Applied Dougall Johnson's pow10 compression method when compiling in binary-size-optimized mode (#97).
• Added the ZMIJ_USE_SIMD CMake option to control SIMD (#128, thanks @spinicist) and CMake Threads support for the tooling (#122, thanks @spinicist).

Performance & SIMD

• Built the float string entirely inside a SIMD register for up to a ~30% speed-up (#140, thanks @TobiSchluter).
• Added SIMD for floats and optimized the float path more broadly (#120, thanks @TobiSchluter; addresses #99, #121).
• Assembled fixed-notation output in a single SIMD register on SSE4.1 for a ~6-8% speed-up on fixed-notation benchmarks (#137, thanks @TobiSchluter).
• Improved the SSE4.1 fixed-double path (#110, thanks @Antares0982) and the NEON fixed-double path (#114, thanks @Antares0982).
• Converted BCD → ASCII after shuffling for a ~3% speed-up (#112, thanks @TobiSchluter).
• Used addition instead of OR when inserting zeros to enable LEA (#82, thanks @TobiSchluter).
• Optimized pow10 table access to avoid keeping the index live across umul192.
• Removed the Schubfach fallback for both the double and float paths.
• Applied a digit-split optimization from the xjb paper and extended it across the scalar, SSE, and NEON paths.
• Folded the leading-zero shift into the SIMD digit shuffle and reworked handling of the last digit to reduce branching.
• Made exponent output branch-free (idea by @xjb714) and optimized it for floats.
• Added an optional precomputed exponent table and micro-optimizations for GCC (e.g. forcing cmov, replacing poorly predicted branches).
• Landed additional SSE improvements and ensured scalars do not affect the SIMD memory layout (#125, #129, thanks @TobiSchluter).
• Derived length from the unshuffled value for the x86-64-v2 microarchitecture and reorganized the SSE code.
• Forced SIMD constants to be loaded from memory rather than synthesized, improving code generation (#78, thanks @TobiSchluter).
• Re-enabled SIMD in the C version and fixed several SIMD configuration corner cases, including building the tests with ZMIJ_USE_SIMD=0 (#127).

Portability & Toolchain Fixes

• Fixed clang compile errors (#104, thanks @Antares0982) and GCC NEON build issues, including a build failure on Apple Silicon (#116).
• Fixed MSVC compilation of the C version and multiple C portability issues (#83, #84, #103, thanks @AlexGuteniev, @jcol2), including MSVC ARM64 and x86/x64 warning cleanups (#126, #136).
• Avoided a disallowed conversion from unsigned to int (#113, thanks @TobiSchluter) and renamed the C version's select to zmij_select to avoid a clash with the POSIX select function (#139, thanks @MasterDuke17).
• Added a correct fallback for 32-bit systems and re-enabled SSE on 32-bit MSVC builds (#119, #135, fixes #132, thanks @IRainman).
• Guarded __builtin_is_constant_evaluated for compilers such as VS2017 (#142, fixes #141, thanks @archo5).
• Fixed various warnings in the C version (#87, #90, #92, #95, thanks @MasterDuke17, @IRainman, @himikof).
• Applied the correct test settings (#106, thanks @TobiSchluter) and removed unnecessary null termination for doubles (#107, thanks @AlexGuteniev).

Correctness & Safety

• Fixed a buffer underrun in the SSE fixed path (#86).
• Fixed incorrect NaN formatting that produced -nan (#109).
• Corrected handling of the last digit, including round-to-even behavior (#118).

Verification & Tooling

• Added a double-to-string verification script (verify.py) that verifies zmij's rounding against the shortest correctly rounded representation for all significands of every binary exponent.
• Reworked benchmarking:
  • Switched to Google Benchmark and improved reporting.
  • Added an A/B testing script (abtest.py) with float support and a "benchmark all commits" workflow.
  • Added a fixed-notation benchmark (Canada dataset), a float benchmark, and a results-plotting script.
  • Fixed the fixed-notation benchmark (#143, thanks @TobiSchluter).
• Added CI for Linux, macOS, and Windows, the last in #115 (thanks @Antares0982).

New Contributors

• @MasterDuke17: first contribution in #87
• @IRainman: first contribution in #90
• @himikof: first contribution in #95
• @jcol2: first contribution in #103
• @Antares0982: first contribution in #104
• @spinicist: first contribution in #122
• @archo5: first contribution in #142

Thanks also to @TobiSchluter, @AlexGuteniev, @dougallj, and @xjb714 for their continued contributions, and to everyone who reported issues.

Full Changelog: v1.0...v1.1