Architecture

The source of truth for why this package is shaped the way it is. Read it before changing anything under src/compile/ or src/resolve/ — several shapes there look simplifiable and are not.

For why the package exists at all — the drop-in stance towards tailwind-variants, the two departures from it, and the trade this design makes — see DECISIONS.md.

The one idea

A variant configuration is fixed the moment tv is called. A resolver is then called once per render, forever. So everything that depends only on the configuration is settled once, and resolution reads the result. The once is the resolver's second call, not tv itself: the first call reads the configuration directly, so a component defined and rendered a single time — a cold start, a page that shows it once — never pays for a plan it would not have used.

tv(config) ──▶ resolver ──1st call──▶ cold lane ──2nd call──▶ VariantPlan ──resolve──▶ "px-4 py-2 bg-primary"
  a wrapper                reads the configuration      compiled once           every call after

The same idea applied a second time: within one configuration, the answer depends only on the selection, and a list renders the same few selections thousands of times. So a resolution is settled once per selection, and a repeat call is a lookup.

props ──encode──▶ selection key ──▶ cache hit ──▶ the same answer, no walk and no merge

That split is the directory layout:

Testing a compound condition lives in compile/ rather than resolve/ because the test and the encoding it reads are one contract. Splitting them by phase would put the two halves of that contract out of each other's sight.

What the plan settles, and why each one matters

Default classes are looked up at compile time. A resolver call reads entry.defaultClasses, a monomorphic field, instead of defaults[name] then group[value] — two dictionary lookups on the path taken by every variant the caller did not pass, which is most of them.

Compound conditions are flat lists. Compiling them removes an Object.keys allocation per compound variant per call, and resolves each condition's configured fallback once.

Every class value is flattened to a string. This is what lets resolution be nothing but string concatenation — no intermediate array, no spread, no flattening pass at runtime. It is safe because flattening joins each argument's contributions in order and drops the empty ones, so flattening each value separately gives the same answer as flattening them together.

Slot maps carry slot positions, not slot names. See below.

The slot lane is inverted, and that is the point

A slot map is sparse: a variant value typically names one to seven of a component's ten or twelve slots. The obvious implementation — each slot scans every variant and asks "do you have anything for me?" — therefore spends most of its lookups missing.

So the loop runs the other way. Each compiled value carries the indices of the slots it targets, and one pass distributes classes into a per-slot buffer shared by every resolver of that call. A slot called without its own props then just reads its entry.

Per-slot props are the exception: they can select different variant values and flip a compound's conditions, so a slot called with props re-resolves from scratch (resolveSlotWithOverrides). That lane is deliberately the slow one.

base is always slot position zero. The plan synthesises it whether or not the configuration declares it, which is why a plain string class value can be assigned to texts[0] without a lookup.

The cold lane answers exactly what the plan answers

The first call has no plan, so resolve/cold.ts walks the configuration itself. It is a second statement of the resolution rules, and every rule the plan settles at compile time it applies inline instead: a missing selection takes the configured default, or false for a group keyed by true/false; the slot lane alone treats a falsy key as no selection; a boolean compound condition reads an absent value as false in the flat lane and in compound slots, never in a slot configuration's compound variants; compound slots read the call's props and ignore a slot's own overrides. Where the plan copies each group onto a prototype-less object once, the cold lane guards each read with Object.hasOwn, which is the same answer paid per read instead of per definition — and the cold lane reads each group once.

Two things follow. The cold lane's answer is not remembered: the store belongs to the plan, so identity of a slot object holds from the second call on, and a caller who wants it warm renders twice. And the cold lane is held to the plan by the behaviour sweep, which drives the whole corpus through a fresh resolver per call and requires every outcome to equal the compiled one — the only proof this file accepts that the two statements of the rules agree.

What the selection key may and may not collapse

A key is a mixed-radix number, one digit per variant. Two calls sharing a key must be indistinguishable to everything downstream, and there are two different notions of "same value":

• Resolution only reads entry.group[key], so two values with the same group key select the same classes and can share a digit. A digit is therefore the group key's id — cheap, because it is an object read.
• A compound compares against the value the caller passed. true and "true" share the group key "true" and compare differently, so for any variant a compound tests, a digit is instead the id of the raw value, held in a Map (which distinguishes them, and 0 from "0"). That costs a Map.get on those axes only.

Everything else follows from having to make that distinction hold:

• A digit is reserved for "the call omitted this variant", distinct from every value, because an omitted variant takes the compiled default and a compound reads its configured fallback.
• Every value the group does not answer shares one id, because they all resolve to no classes.
• A compound may test a name no variant declares. That name still decides the outcome, so it gets an axis of its own.
• A raw-value axis is capped. Past the cap the call reports itself unencodable and resolves the long way, which is also what a configuration too large to address in one safe integer does.
• The store's generation limit is sized against what a design system actually asks for, not a guess. A key is one selection plus the caller's own class string, so the entries a component can fill is the number of distinct call sites it has. Across this repository's 800 .tsx files the busiest component reaches fifty, and none exceeds the limit. Two generations then mean nothing in a real page is ever evicted, and the caller's class needs no tier of its own.
• Ids are handed out as values turn up rather than up front, so a group of two hundred values costs nothing to compile and only what a caller actually selects to run.
• valueIds is a null-prototype object, and so is every copied variant group and the slot index map. A plain object answers group["toString"] with a function rather than undefined, and a caller chooses that key by passing it as a variant value; "__proto__" hands the slot lane Object.prototype to read slot positions off. The slot lane therefore always reads a copy. The flat lane accepts only a string it reads, so a group whose values are all strings on a plain prototype is read in place, and the encoder asks Object.hasOwn before it hands a value an id; every other group is copied key by key, which measured about a quarter of either bulk form onto a prototype-less object.

Shapes that are load-bearing

• selectForSlot stays out of line. Inlining it by hand measured slower on every slot scenario — the engine already inlines it, and the larger caller falls out of the shape it optimises.
• The flat lane accepts only a string it reads. A configuration without slots compiles every class value to a string, so the per-slot form cannot reach that code — and a string-only group is the caller's own object, whose inherited members the type check is what keeps out. Loosening it to a truthiness check reintroduces both.
• toClassText is not a wrapper around the flattener. Its string check is what keeps the common case out of it entirely, at compile time and for the runtime class/className prop.
• The flattener in class-names.ts reproduces clsx exactly, corners included — a bigint contributes nothing though the type admits one, and object keys are read with for…in. It replaced the dependency, so matching it is the contract; the behaviour sweep run against the build that still had clsx is what holds it there.
• A slot call context keeps conditionValues, not the caller's props. A cached resolver outlives its call, and the props object a component passes carries children — most of a tree. Only the names a compound tests are copied out, and a configuration without compounds shares one empty object.
• The per-slot memo holds undefined and a sentinel apart. A slot that resolves to nothing and a slot nobody has read yet are both absent from an array of strings, and they are not the same.
• A resolver returns the same slot object for a repeated selection. That is the point — the merge per slot happens once — and it makes the object shared, so nothing may mutate it.

The trade this design makes

The plan is the expensive end, and it is paid on the second call rather than in tv. define-only-* prices tv itself and first-render-* a definition plus the cold lane; both are controls off the aggregates, and both now read against upstream rather than behind it. The resolution rows price the plan, and a component that renders more than a handful of times earns its plan back on the repeat-* shape — read those rows, not a figure written here, for what any of this costs today.

Three things keep the second call from paying more than it must, and all are worth preserving: the selection encoder hands out ids as values turn up rather than up front, so a group of two hundred values costs nothing to compile and only what a caller actually selects to run; a flat-lane group whose values are all strings is read in place rather than copied, behind the type check the flat lane already makes; and every other group is copied onto a prototype-less object exactly once, which is what lets the slot lane index it by whatever a caller passed without a guard per read.

If the second call ever needs to come down, compileVariantPlan is where the time goes — the slot-lane copies most of all — and the difference between a fresh resolver's second and first call is how to see it.

How much of this a page can actually feel

Every figure above is a microbenchmark, and one measurement bounds what any of them is worth. Rendering apps/web's registry demos through renderToString — sixty-eight components, the densest page this repository can assemble — takes about 15 ms and makes 266 resolver calls. At the ~95 ns a call the cache saves on those configurations, the whole cache is worth ~25 µs, or 0.17% of the render. Three runs of the identical build spread 1.15 ms, so the effect sits roughly four times below the noise it would have to clear to be seen at all.

That is not an argument against the work — the library is measurably faster, and a consumer with a heavier variant load than this one gets more of it. It is an argument about where to spend next: resolution is no longer what a page waits on, so a further micro-optimisation here buys a fraction of a fraction of a percent. Anyone about to open this file to shave nanoseconds should read this paragraph first and go find a bigger number somewhere else.

cacheResolutions: false may not be free: the slot lane still allocates a per-slot memo, and uncached-slots-with-merge has read 0.89–0.99 against the build before the cache existed. That is also the suite's least stable row, and the cost was never isolated from it — treat the allocation as a suspect, not a finding. Nothing in this repository passes the option, which is why it stayed one. The memo itself is not wasted under the option: the propless path reads it on every slot call, so it still pays for a caller that reads one slot twice.

Changing any of this

Resolution is a hot path and it is layout-sensitive: reshaping a function has moved rows the change could not reach. Treat every edit under resolve/, and any reshape of compileVariantPlan, as a performance change requiring a paired A/B. The method is in benchmarks/di/BENCH_GUIDE.md; both of the mechanisms it describes work here, and pairing them wrong fails silently:

• Swap the source — check out or stash packages/tailwind-variants/src per side, then run bench. That runner rebuilds the package before sampling, which is exactly what makes the swap take effect.

• Swap the build — copy a prebuilt dist over packages/tailwind-variants/dist per side, then drive the child entry directly, one scenario at a time:

  BENCH_ONLY=slots-without-merge node --import tsx/esm src/codefast-benches.ts

  Copy

  The child entry is mandatory here. bench and bench:fast both run src/harness/run.ts, whose unconditional rebuild would overwrite the swapped dist from src — both sides then measure the same build and every row reports parity.

Scenario ids come from BENCH_LIST=1.

Read the right row for what you changed. Every prop fixture repeats its selections, so with the cache on almost every row now measures a lookup rather than the resolver:

• uncached-* runs with cacheResolutions: false and is the row to read for the plan walk. A change under resolve/ that does not move it did not do what you think. It has no counterpart in a library without the same switch, so its ratio column is meaningless — it is a control, and it is off the aggregates for that reason.
• repeat-* is the shape a UI actually has: three selections, fresh props objects, over and over.
• define-only-* prices tv alone and first-render-* a definition plus its first render, which is the cold lane; both are per component definition where everything else is per render, so both are off the aggregates too. Neither touches the plan — a change under compile/ shows up between a fresh resolver's second call and its first.

Rows here are batched loops, so the noise floor is tighter than the DI suite's: an A/A run put every median within ±0.6%. Treat a ratio at or above 1.03× as signal, and re-measure anything smaller with more passes before believing it — a flat-lane row with no causal path to a slot change once read 0.979 over three passes and 1.013 over five. The slot rows carry the widest per-trial IQR and have swung 0.91 to 0.99 across runs on unchanged code; do not read a single one closely.

Correctness is not checked by the hand-written tests alone. They assert the behaviour someone thought to write down, and the changes that hurt here are the ones nobody considered.

tests/unit/support/behaviour-sweep.ts generates the corpus instead — every variant value, every pair, every slot with and without per-slot props, every odd value a caller can pass including the inherited keys of a plain object — and runs the whole thing twice, because a resolver that remembers must answer the second pass exactly as it answered the first. Over fifty thousand outcomes, in well under a second.

tests/unit/common/behaviour-sweep.test.ts then holds the corpus to a property that needs no stored baseline: a resolver that remembers must answer exactly what one that does not answers. That is what caught the selection cache collapsing toString, constructor and hasOwnProperty onto one entry while every other test stayed green, and it now runs on every pnpm test:unit.

For a change the property cannot see — one that moves both lanes together, such as a reshape of resolveVariantClasses — run collectSweepOutcomes under each build and diff the two, which is what the property does across an option rather than across a version.

License

Released under the MIT License.