Extracting reusable libraries from Xapiand

Xapiand is full of small, well-made components that have nothing to do with search and everything to do with "things you need when writing serious C++." Three are already out in their own repos — base-x, uinteger_t, and fantasyname — and this document is the plan to do the same for the rest.

It's a complete inventory of the codebase's own (non-vendored) code, grouped into extraction waves by return-on-effort, with a concrete split-out plan for each piece, a suggested set of repos to create, and the licensing flags to carry. For the deep technical detail behind any entry, see ARCHITECTURE.md; the scheduling stack has its own study in SCHEDULER.md.

How to read this

• Difficulty = how much Xapiand coupling you must cut: Easy (header-only or a couple of macro stubs), Moderate (swap a few helpers / a thread abstraction), Hard (peel domain glue off a good core).
• Value = how broadly useful outside a search engine.
• License = MIT unless flagged. A few files carry other authors' notices that must travel with them; see the provenance map. Anything under src/xapian/ is GPL and out of scope here.

The shared decoupling toolkit

Most of the catalog is unlocked by the same five cuts. Do these once as a pattern and the per-component work mostly evaporates:

1. Stub the logging macros. L_CALL, L_ERR, L_DEBUG_HOOK, L_EXC, … all compile to L_NOTHING by default already (log.h). Replace with no-ops or a single injectable trace callback. This is the single most common coupling and it's mechanical.
2. Swap THROW(Type, …) for standard exceptions. Many leaf files only touch Xapiand exceptions through this macro; std::runtime_error/invalid_argument substitute cleanly. (Or extract the exception type itself — it's Wave 1.)
3. Replace strings::format with fmt::format / std::format. The internal strings helper is just an fmt wrapper.
4. Delete ThreadPolicyType. It's threaded through the whole thread/scheduler stack as a template parameter but thread.cc ignores it at runtime (only sets the thread name). Replace with a name string.
5. Drop opts reads. A few components read the global CLI-options struct; replace those with constructor parameters or a small config struct.

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Wave 1 — header-only, (near-)zero coupling

Quick wins. Mostly copy the file(s), stub a macro or two, done.

Proposed lib	Source	What it is	Diff	Value	Split plan
perfect-hash	phf.hh	Fully constexpr minimal perfect hash for integer keys	Easy	High	Copy as-is; fix the inverted empty() (phf.hh:1410). Zero deps.
lru-cache	lru.h	Intrusive LRU + optional TTL with policy-callback eviction	Easy	High	Copy as-is; pure STL.
callable-traits	callable_traits.hh	Full C++17 callable introspection (all qualifier combos)	Easy	High	Copy as-is; already namespaced, zero deps.
errno-names	error.hh + errnos.h	errno → symbolic name and description, cross-platform X-macro	Easy	High	Copy both verbatim; zero coupling.
located-exception	exception.h + exception.cc	Exception that captures func/file/line + lazy fmt, with a WITHOUT_FMT fallback	Easy	High	Drop the unused config.h include; keep THROW/RETHROW. (Note: captures location, not a stack trace.)
bloom-filter	bloom_filter.hh	Fixed-size double-hashed Bloom filter	Easy	Med	Carries hashes.hh (or template the hash).
split	split.h	Lazy string_view splitter (full-delimiter + find-first-of)	Easy	High	Copy as-is, or fold into the string toolkit below.
msgpack-xchange	xchange/rapidjson.hpp, xchange/string_view.hpp, xchange/chaiscript.hpp	msgpack-c adapters for RapidJSON, string_view, and ChaiScript	Easy	High (json/sv), Low-Med (chai)	Ship the three as one pack; fix one THROW→throw in string_view; keep xpol's co-credit on the rapidjson header. ChaiScript adapter is an optional module.

Wave 2 — small decoupling (stub macros / swap exceptions / fmt)

A short, mechanical decoupling each, following the toolkit above.

Proposed lib	Source	What it is	Diff	Value	Split plan
term-color	ansi_color.hh, colors.h, color_tools.hh	Compile-time truecolor+256+16 ANSI escapes, graceful downgrade, hsv2rgb	Easy	High	Split the L_* logging shortcuts out of colors.h; inline ESC; strings::format→fmt.
constexpr-strings	static_string.hh, hashes.hh, chars.hh, strict_stox.hh	Compile-time strings, xxHash/FNV/djb2 + string-switch UDLs, table-driven char classification, strict numeric parsing	Easy–Mod	High	Bundle as one "compile-time string toolkit"; swap THROW for std exceptions in strict_stox.
string-metrics	metrics/ (+ string_metric.h)	CRTP string-distance kit: Levenshtein, Jaro, Jaro–Winkler, Jaccard, Sørensen–Dice, LCS, soundex-metric	Easy	High	Drop the 3 serialise helpers in basic_string_metric.h.
phonetic	phonetic/	Multilingual soundex (EN/FR/DE/ES)	Easy	High	Same serialise-helper cut; fix the Spanish first-letter bug (spanish_soundex.h:113). Could merge into string-metrics.
memory-stats	memory_stats.*	Portable RAM/swap/disk/inode + per-proc RSS/VSZ	Easy	High	Swap ~10 L_*/error::* calls for a callback or fprintf. Bundle into sysinfo with system.*.
lz4-stream	compressor_lz4.*	LZ4 block-streaming wrappers (file + data, ring-buffered)	Easy	High	Depends only on vendored lz4 + io; note the unused-digest bug (wire up the XXH32 it already computes).
urlcodec	url_parser.* (the urldecode + QueryParser half)	Configurable percent-decoder + zero-copy query-string parser	Easy–Mod	High	Inline the ~10-line hexdec; leave PathParser (Xapiand URL dialect).
mime-types	server/mime_types.* (+ the nginx table)	Load mime.types, map extension → content type	Easy–Mod	Low–Med	Replace ct_type_t with std::pair, make the path a parameter, drop logging.
alloc-kit	allocators.*	STL allocator over pluggable backend + pool allocator + optional malloc tracking	Easy	Med	Split the global operator new/delete override into an opt-in TU; keep moya-lang's BSD-3 notice.

Wave 3 — coherent subsystems (bundle & lift)

Bigger pieces that stand on their own once a thread abstraction or a few helpers come along. Several are best shipped as a small dependency tree.

Proposed lib	Source	What it is	Diff	Value	Split plan
stash	stash.h	Lock-free hierarchical timer-wheel slot store (the crown jewel)	Easy	High	Stub trace macros; delete the vestigial _Ring/_CurrentKey params. See SCHEDULER.md.
thread-pool	thread.hh, threadpool.hh	Blocking-queue thread pool + CRTP Thread (promise-based join)	Easy–Mod	Med	Delete ThreadPolicyType; deps blocking_concurrent_queue.h + likely.h; strings::format→fmt.
timer-scheduler	scheduler.h (+ stash + thread-pool)	24h multi-resolution timer wheel + thread; cancellable tasks; inline or pool dispatch	Mod	High	Build on stash + thread-pool; stub macros. See SCHEDULER.md.
debouncer	debouncer.h	Per-key throttle/debounce with randomized force-window	Easy*	High	Rides on timer-scheduler; vendor callable_traits + 3 random helpers. (*after the scheduler is out.)
posix-io	io.cc, io.hh	EINTR-retry / partial-IO / fsync / fallocate wrappers + fault injection	Mod	High	Cut log/error/exception; gate or drop the opts-driven fault injection; likely.h→[[likely]]. SQLite-derived fallocate fallback is public-domain.
sysinfo	system.* + memory_stats.*	Open/max fd counts, OS/arch probes, RAM/disk/swap/inode stats	Mod	Med–High	Drop the stray STATE_* defines in system.cc; carry posix-io; swap logging.
timeutil	times.*, nanosleep.h, epoch.hh, time_point.hh	timespec arithmetic + macOS clock_gettime shim + chrono one-liners	Easy	Med	De-dup the doubled nanosleep; keep MM Weiss's BSD-3 on the macOS shim (guard it behind legacy-macOS).
traceback	traceback.*	Symbolized backtraces, cross-thread stack dump (signal), __cxa_throw capture	Mod	Med	The backtrace/symbolize core lifts cleanly; leave the global allocator interposition out (it's invasive).
libev-reactor	worker.*	shared_ptr lifetime tree + cross-loop async control + cooperative GC of finished workers	Mod	High	Stub macros; depends only on libev otherwise.
condensed-uuid	cuuid/	UUID generation + variable-length condenser for v1 time-UUIDs	Mod	Med	Decouple the node-salt from Node::get_local_node (inject the node id).

Wave 4 — keystones & hard cases

High value, but each needs a good core peeled away from real domain glue.

Proposed lib	Source	What it is	Diff	Value	Split plan
htm-geo	geospatial/, cartesian.*	Geometry → contiguous integer trixel ranges on a sphere (HTM), full shape support	Mod	High	Cut THROW/strings::format; drop the ~940 lines of matplotlib debug writers (htm.cc:838+). Fix the Cartesian operator^=/operator* bugs first (see ARCHITECTURE.md).
haystack-store	storage.h (+ lz4-stream)	Single-file append-only blob store templated on format structs	Mod	High	Replace opts.* with constructor params; stub logging. Wire up the inert footer/header validation before relying on it.
iso8601 / datemath	datetime.* (string/tm_t/clk_t/calendar core)	ISO-8601 + Elasticsearch-style date-math parser/formatter	Hard	High	Peel off every const MsgPack& overload + phf/reserved dispatch; vendor strict_stox; reparent exceptions onto std::runtime_error. The pure parser/formatter is excellent.
sortable-serialise ⚠️	sortable_serialise.*	Order-preserving (memcmp-sortable) float/double encoding	Easy	High	Two files, <cmath>/<cstring> only — but GPL (Xapian origin). License-incompatible with the MIT libs; keep separate or reimplement.
msgpack-patch	msgpack_patcher.*	RFC 6902 JSON-Patch over a msgpack DOM	Mod	Med	Travels with a future MsgPack extraction; collapse ClientError/Error into one patch_error; keep RapidJSON only for the RFC 6901 tokenizer (or hand-roll it).

The MsgPack keystone. msgpack.h (the COW wrapper with the custom UNDEFINED ext type) is the hub several Wave-4 pieces hang off (msgpack-patch, and the data-interchange side of cast). It's a Hard extraction in its own right (entangled with config.h, exceptions, RapidJSON, ChaiScript), but if you ever lift it, the msgpack-xchange adapters and msgpack-patch become a natural family around it.

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Suggested bundles (the repos to actually create)

Many entries above are tiny and should ship together. A sensible repo set:

• Foundational singletons: perfect-hash, lru-cache, callable-traits, errno-names, located-exception, bloom-filter, sortable-serialise (GPL, separate).
• string-kit — constexpr-strings + split + string-metrics + phonetic (or split metrics/phonetic into their own repo if you want them findable by name).
• term-color — the ANSI/color trio.
• scheduler — stash + thread-pool + timer-scheduler + debouncer as one repo with internal layering (or stash standalone + scheduler depending on it). See SCHEDULER.md.
• sysutil — posix-io + sysinfo + timeutil + alloc-kit + traceback (the "systems-programming odds and ends" most projects re-implement).
• msgpack-x — msgpack-xchange now; msgpack-patch later if MsgPack is lifted.
• Standalone heavies: htm-geo, haystack-store, libev-reactor, iso8601, urlcodec, mime-types, condensed-uuid.

Provenance & license map

Carry these notices on extraction; most of the tree is Dubalu LLC MIT.

Source	Origin / license	Action
sortable_serialise.*	Xapian — GPL	Keep in a GPL-licensed repo, or reimplement clean-room. Do not mix into the MIT libs.
allocators.h (pool allocator)	moya-lang.org — BSD-3	Keep the BSD-3 notice in alloc-kit.
times.cc (macOS clock_gettime)	MM Weiss — BSD-3	Keep the BSD-3 notice in timeutil.
xchange/rapidjson.hpp	co-credited to xpol	Keep xpol's attribution.
io.cc (fallocate fallback)	SQLite — public domain	Fine; note it.
y2j/	Mapzen — MIT (vendored)	Not Xapiand's to republish; point users upstream.
http_parser.h	Joyent/Node.js (vendored)	Not ours; use upstream.
everything else	Dubalu LLC — MIT	Standard.

Not worth extracting

Domain glue or too-coupled / too-thin to package:

• Schema & query glue: cast.*, script.*, reserved/*, query_dsl.*, aggregations/*, multivalue/*, database/*, serialise* (Xapiand schema model).
• Cluster/transport: server/discovery.*, server/*_protocol*, manager.*, node.*, endpoint.* (a good reference Raft-over-UDP, but not a library).
• Vendored or vendored-glue: metrics.* (your metric catalog over upstream prometheus-cpp), chaipp/* + module_* (ChaiScript↔document binding), exception_xapian.h, cmdoutput.h (TCLAP + Package), readable_revents.hh (libev-only, tiny), y2j/, http_parser.h.
• Superseded / trivial: fs.* (mostly std::filesystem now; only normalize_path + quarantine_files are value-add), concurrent_queue.h / atomic_shared_ptr.h (thin shims; the former has a locking bug), check_size.* (debug-only), PathParser (bespoke URL dialect).

Suggested sequencing

1. Wave 1 first — they're nearly free and several (errno-names, located-exception, the msgpack adapters, perfect-hash) are immediately useful elsewhere.
2. scheduler next — it's the highest-interest piece, and stash is the most novel thing in the whole codebase. SCHEDULER.md is already its blueprint.
3. string-kit and term-color — high value, all Wave-1/2 effort.
4. sysutil — bundles a lot of Wave-2/3 systems plumbing into one useful repo.
5. The heavies last — htm-geo, haystack-store, iso8601 reward the effort but each needs real decoupling and a bug fix or two on the way out.

Before extracting any component flagged with a bug in ARCHITECTURE.md, fix the bug on the way out — a freshly-published library is the worst place to ship a known operator^= corruption or an inverted empty().