# fakes A Go library and CLI for generating fake data, built for **internationalization**. It exists because the existing Go fake libraries lacked the locale coverage and format control we needed. - **Standards first** — formats, names and structures follow international and local standards first and foremost. - **Locale-aware** — names, addresses, postal codes and phone numbers follow per-locale data and formats. The shipped data is organised by full locale tag (`sv_SE`), but the engine treats folders as plain namespaces — name yours anything. - **Data lives in JSON** — all source data is recursive JSON on disk, read when you create a faker and then served from memory. Add or change data without touching the library. Behavior belongs in data too: the engine grows a built-in function only for what data can't express (a checksum, a time-based id), never for what character classes and choices already do. - **Composable** — templates nest without limit: weighted choices, character classes and sub-templates combine to model any format. - **Reproducible** — seed a faker and it emits the same sequence every time, for a given version of the data: changing how a value is composed shifts the stream for that value and for everything drawn after it in the same faker. Every built-in draws only from that seed — no wall-clock, no `crypto/rand` — so determinism holds end to end. - **Zero dependencies** — standard library only. ## CLI Install the `fakes` command, then give it one or more `-data-path` directories and a path — it prints one value to stdout. Each dot segment descends one level: folders, then the category (a JSON file), then fields inside it. ```sh go install github.com/Timewave-AB/fakes/cmd/fakes@latest fakes -data-path ./data/sv_SE person # Sara Eriksson fakes -data-path ./data/sv_SE person.last # Eriksson (dotted path into a category) fakes -data-path ./data sv_SE.person # point at the tree; the folder is a segment fakes -data-path ./data/sv_SE -data-path ./mydata word # layer dirs; the last wins a name clash fakes -seed 42 -data-path ./data/sv_SE address fakes -repeat 3 -data-path ./data/sv_SE person # three values, one per line fakes -repeat 3 -separator ', ' -data-path ./data/sv_SE word # nät, barn, sol fakes -data-path ./data/sv_SE -list # every path this data offers ``` `-data-path` is repeatable (last wins a name clash) and the path comes last — all flags must precede it. `-repeat N` renders the path N times — each an independent draw — joined by `-separator` (default a newline, so values land one per line). Not sure what a data set offers? `-list` prints every path you can ask for; `-version` prints the build version. Without installing, run it from a checkout with `go run ./cmd/fakes …`. Exit codes: `0` success (including `-list`, `-version`, `-h`), `1` runtime error (missing dir, unknown path), `2` misuse. ### Generating a file from a custom template A category is just a JSON file in a data directory, so you can drop in your own and render it — no code change. Save this as `data/sv_SE/sql.json`: ```json { "format": "INSERT INTO users V#ALUES({sql-username});", "sql-username": { "format": "'{username}'", "repeat": 3, "separator": "),(", "username": ["pixelfox", "snork", "turbohund", "blip", "zoom", "wahoo"] } } ``` `sql-username` renders `'{username}'` `repeat` times and joins the results with the `),(` separator; the outer `V#ALUES(…)` wraps that into one valid row list. (`#A` escapes the literal `A`, which a format string would otherwise read as a letter token — see [Data format](#data-format).) ```sh fakes -seed 1 -data-path ./data/sv_SE sql # INSERT INTO users VALUES('zoom'),('wahoo'),('blip'); ``` Raise the template's `repeat` for more rows per statement; use the CLI's `-repeat` for more statements — together they build a whole seed file: ```sh fakes -repeat 100 -data-path ./data/sv_SE sql > seed.sql ``` ## Library ```sh go get github.com/Timewave-AB/fakes # requires Go 1.22+ (for math/rand/v2) ``` Point `New` at one or more data directories, then generate values by path with `Fake`. Each dot segment descends one level: folders, then the category (a JSON file), then fields inside it. ```go package main import ( "fmt" "log" "github.com/Timewave-AB/fakes" ) func main() { f, err := fakes.New([]string{"./data/sv_SE"}) if err != nil { log.Fatal(err) } for _, path := range []string{"person", "address", "phone", "address.locality"} { v, err := f.Fake(path) if err != nil { log.Fatal(err) } fmt.Printf("%-18s %s\n", path, v) } } ``` ``` person Sara Eriksson address Kungsvägen 68 379 17 Stockholm phone 072-402 91 67 address.locality Linköping ``` Seed a faker for reproducible output — same seed + locale yields an identical sequence, handy for stable tests: ```go a, _ := fakes.New([]string{"./data/sv_SE"}, fakes.WithSeed(42)) b, _ := fakes.New([]string{"./data/sv_SE"}, fakes.WithSeed(42)) av, _ := a.Fake("person") bv, _ := b.Fake("person") av == bv // true ``` `f.List()` returns the sorted paths the loaded data offers — the categories, their dotted fields and folder segments (what the CLI's `-list` prints). Every path it lists renders. A `*Fakes` is **not** safe for concurrent use — create one per goroutine. ## Data The library ships a ready-to-use set under [`data/`](data): one folder per locale (`en_US`, `sv_SE`) plus a locale-neutral `misc` folder. Point either tool at the whole tree, a single folder, a copy, or your own directory — anywhere on disk; no naming rules. A directory is just a namespace. Each JSON file is a category named after the file; each subdirectory is a dot-path segment — folders nest exactly like JSON objects do. So `data/sv_SE/person.json` is `Fake("person")` when you point at `data/sv_SE`, or `Fake("sv_SE.person")` when you point at `data`. Pass several directories and they merge, left to right: matching folders combine by their children, and any other clash is won by the last directory loaded. That lets you layer your own data over the built-ins without copying them: ```go fakes.New([]string{"./data/sv_SE", "./mydata"}) // mydata overrides on a clash ``` Each shipped locale carries these categories, formatted per locale (e.g. `date` is `MM/DD/YYYY` in `en_US`, `YYYY-MM-DD` in `sv_SE`; `ssn` is a US SSN vs a Swedish personnummer): `address`, `color`, `company`, `date`, `email`, `ip`, `person`, `phone`, `price`, `sentence`, `ssn`, `time`, `url`, `username`, `version`, `word`. `data/misc` carries locale-neutral categories — universal data that isn't tied to a language or region: - ids & networking: `uuid` (a proper random v4), `mac`, `creditcard` (per-network numbers ending in a valid `{luhn()}` digit) - reference codes: `currency` (ISO 4217), `country` (ISO 3166), `language` (ISO 639), `timezone` (IANA) - web & systems: `mimetype`, `httpstatus`, `useragent` - misc: `coordinate` (a lat/long point), `emoji`, `car` Many carry dotted sub-fields — `currency.symbol`, `country.alpha2`, `mimetype.ext`, `httpstatus.code`, `car.maker`. A time-ordered v7 UUID can't be expressed as data, so it's the `{uuid()}` builtin instead (see **Functions**). ## Data format Each JSON file in a data directory is a **category** named after the file (`address.json` → `address`), rendered by `Fake("address")`. Drop in a new file or folder — no code change, no recompile. Every value is a **node**, one of three shapes, nestable without limit: | Node | JSON | Meaning | |------|------|---------| | literal | `"Malmö"` | emitted verbatim — never formatted | | choice | `["a", "b", …]` | one element, picked at random | | template | `{"format": "…", …}` | a format string plus the named sub-nodes it references | **Weight.** A template node may carry a `weight` (default `1`) to skew its odds within a choice: ```json [ { "format": "#070-000 00 00", "weight": 10 }, { "format": "#01-000 00 00" }, { "format": "#010-000 00 00" } ] ``` Only template (object) nodes carry `weight` — a bare string or nested array in a choice always counts as `1`. Weights are checked when you create the faker: a negative, non-numeric, or all-zero set is rejected at `New`, so a typo fails fast instead of silently skewing output. **Repeat.** A template node may carry a `repeat` (default `1`) to render its `format` that many times — each render an independent pick — joined by `separator` (default `""`): ```json { "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] } ``` This yields e.g. `bar foo baz`. `repeat` must be a positive integer and `separator` a string, both checked at `New`. **Functions.** A `{name()}` token calls a built-in function instead of rendering a field. `{luhn()}` appends a Luhn check digit over the digits emitted **so far** in the current format (non-digits skipped but kept); unknown functions or wrong argument counts are rejected at `New`. This is what makes a generated Swedish personnummer valid — its last digit is a Luhn checksum over the nine before it: ```json { "format": "00{mmdd}-000{luhn()}", "mmdd": [ … ] } ``` renders e.g. `811218-987`, then `{luhn()}` appends `6` → `811218-9876`. Place it after its payload (it reads what is to its left). The buffer it reads is per-expansion, so nesting keeps fixed parts out of the sum — e.g. a 12-digit form prefixes the century outside the checksummed core: ```json { "format": "{century}{core}", "century": ["19", "20"], "core": { "format": "00{mmdd}-000{luhn()}", "mmdd": [ … ] } } ``` A function must be deterministic (no wall-clock), so a seeded faker stays reproducible. A time-based id (UUID v7, ULID) therefore draws its timestamp from the rng, not the clock — the result is a valid, reproducible value, not a real point in time. There are four kinds. **Derivations** read the digits emitted so far, so put them after their payload; **generators** read only the rng, so they stand alone; one **session counter** (`seq`) advances state held on the faker; and one **computation** (`calc`) evaluates arithmetic over sibling fields. Arguments are validated at `New` (a bad count, range, country, or expression fails fast); a length, count or decimal place beyond a sane maximum is rejected there too, so a fat-fingered `hex(2000000000)` can't try to allocate gigabytes at render. | Function | Kind | Emits | |----------|------|-------| | `{luhn()}` | derivation | Luhn check digit (mod-10) over preceding digits | | `{mod11()}` | derivation | weighted mod-11 check char (weights 2–7 from the right); `X` when it would be 10 | | `{ean()}` | derivation | EAN-13 / UPC-A / ISBN-13 / GTIN check digit | | `{uuid()}` | generator | UUID v7 (v4 ships as data — see [Data](#data)) | | `{ulid()}` | generator | ULID, 26-char Crockford base32 | | `{objectid()}` | generator | MongoDB ObjectID, 24 hex chars | | `{nanoid(n)}` | generator | URL-safe Nano ID, `n` chars | | `{hex(n)}` | generator | `n` lowercase hex digits | | `{base64(n)}` | generator | `n` random bytes, base64 | | `{int(min,max)}` | generator | uniform integer in `[min, max]` | | `{float(min,max,dp)}` | generator | number in `[min, max]` with `dp` decimals | | `{iban(CC)}` | generator | a length- and mod-97-valid IBAN for country `CC` (BE, DE, DK, ES, FI, NO, SE) | | `{seq()}`, `{seq(name)}` | session counter | next integer (from 1) in this faker's sequence; `name` selects an independent counter | | `{calc(expr)}`, `{calc(expr,dp)}` | computation | value of an arithmetic expression over number literals and sibling fields; `dp` rounds | `{ean()}` is also the ISBN-13 check (an ISBN-13 *is* an EAN-13 — build the 978/979 prefix in data and call `{ean()}`). `{iban()}` is a generator, not a derivation: an IBAN's check digits sit *before* the account number, which a left-to-right reader can't reach, so it emits the whole value (a generic numeric BBAN — valid length and checksum, not real bank routing). `{seq()}`'s counter lives on the faker, so it spans `Fake` calls (and `repeat`) and resets when you build a new faker — `seq` is reproducible by being ordered, not random. It's the natural fit for a primary-key column in the SQL example above. **Computation.** `{calc(expr)}` evaluates an arithmetic expression — `+ - * /`, parentheses and unary minus, the usual precedence — and emits the result. Operands are number literals and **sibling field names**, each rendered then read as a number; an optional second arg rounds to that many decimals (`{calc(net * qty, 2)}`), otherwise the value prints in minimal form. A hyphen is always subtraction, so a hyphenated field name can't be an operand. The expression is checked at `New` (parse, and that every name is a real field): ```json { "format": "{net} x {qty} = {calc(net * qty, 2)}", "net": ["19.99"], "qty": ["3"] } ``` renders `19.99 x 3 = 59.97`. Each name is rendered where it appears, so a field shown *and* used in a `calc` is drawn twice — keep a shared operand in a single-value field if the two must agree. A field that doesn't render to a number yields `NaN`, and a division by zero yields `Inf`; both print rather than failing the render. **References.** A `{..path}` token renders a node from the **data root** instead of a sibling field — the dot path is the one `Fake` takes, resolved across every loaded directory. One category can borrow another, even across folders or layered data dirs: ```json { "format": "Hej, {..en_US.person}!" } ``` renders e.g. `Hej, Pat Smith!`. References are bound when you create the faker, so a path that is unknown, names a folder, or steps through a multi-variant choice fails at `New`. A reference that leads back to its own value (directly, mutually, or through a chain) is a cycle that would never finish rendering, so it too is rejected at `New`. **Format string.** Every character is literal except: | Token | Expands to | |-------|-----------| | `0` | digit 0–9 | | `1` | digit 1–9 | | `A` | letter A–Z | | `a` | letter a–z | | `#` | escape — the next char is literal (`#0` → `0`, `##` → `#`) | | `{name}` | render the sibling field `name` | | `{name()}` | call a built-in function (see **Functions**) | | `{..path}` | render the node at a dot path from the data root (see **References**) | `{a|b}` renders one of the sibling fields `a` or `b`, chosen at random; an arm may be a `{..path}` reference too (`{name|..en_US.person}`). Inside a `format`, `0 1 A a` are **always** character classes — so a fixed `0`, `1`, `A` or `a` must be escaped (`#1`, `#A`) or it becomes random. A format of `100 Main St` renders e.g. `506 Mdin St` — the `1`, `0`, `0` and `a` were random, the `M`, `in` and `St` were not. A half-fixed string is the trap: `555-0000` keeps `555-` and randomises the last four digits. For a value with no tokens at all, use a bare string node (`"100 Main St"`), emitted verbatim. **Putting it together** (`person.json`): ```json [ { "format": "{prefix}{femalefirst|malefirst} {last}", "femalefirst": ["Anna", "Astrid", "Elin"], "malefirst": ["Anders", "Erik", "Gustav"], "last": [ { "format": "{first}sson", "first": ["Ander", "Erik", "Karl"] }, ["Berg", "von Flemming"] ], "prefix": [ "", { "format": "{string} ", "string": ["dr", "prof"], "weight": 0.05 } ] } ] ``` This yields e.g. `Anna Eriksson`, `Erik Berg`, or rarely `dr Astrid von Flemming`. Any field is reachable by dotted path — `Fake("person.last")` renders just a surname; choices along the path are resolved at random. A path may continue *through* a choice only where every variant carries the rest of it — every `currency` variant carries `symbol`, so `currency.symbol` resolves — which keeps a path from rendering on one call and failing on the next. ### Performance Each file is parsed, validated and weight-indexed once, in `New`. After that a `Fake` call costs about what its output costs — it scans the chosen format and renders nested tokens, independent of how large your lists are: - Picking from a list is **O(1)** whatever its length — a 10-name list and a 100 000-name list cost the same. - Giving entries a `weight` makes that list's pick **O(log n)** instead (a search over cumulative weights). Still tiny, but an unweighted list is the cheapest — only add `weight` where you actually want skew. - Long `format` strings, deep nesting and many `{tokens}` add cost in proportion to the output produced. ## Development Everything runs in Docker — **no local tooling beyond Docker is needed**. Source is bind-mounted; build caches persist in the `gocache` volume. ```sh docker compose run --rm test # run tests docker compose run --rm ci # vet + format check + tests docker compose run --rm cover # tests with coverage docker compose run --rm bench # benchmarks docker compose run --rm build # compile the library docker compose run --rm vet # go vet docker compose run --rm dev # interactive shell ``` Commands that rewrite source keep your file ownership when run with `--user`: ```sh docker compose run --rm --user "$(id -u):$(id -g)" fmt # gofmt -w . docker compose run --rm --user "$(id -u):$(id -g)" tidy # go mod tidy ``` Every pull request runs `docker compose run --rm ci` against both the latest and the lowest supported Go, and must pass before it can be merged. `docker build .` runs the same vet and tests locally. Tests run against the latest Go by default. Set `GO_VERSION` to check the lowest supported version too: ```sh GO_VERSION=1.22.12 docker compose run --rm test # lowest supported docker compose run --rm test # latest ``` ## Layout ``` fakes.go Fakes, New, List, options, seeding node.go the node model and JSON -> node compilation render.go Fake and the recursive renderer (choices, format strings, paths) template.go the {token} grammar: scanning, function tokens, validation reference.go {..path} binding across the tree, and cycle detection builtins.go the {name()} function registry and its implementations calc.go the {calc()} arithmetic evaluator: parser, eval, validation data.go data loading: folders/files -> namespace tree, multi-path merge cmd/fakes/ the `fakes` CLI (New + Fake/List over stdout) data/ shipped data (JSON): locale folders + a misc folder ``` To add a category, drop a JSON file into a data directory; to add a locale, add a subdirectory of JSON files. ## License MIT — see [LICENSE](LICENSE).