luzer: introduce oneof() in FuzzedDataProvider

```
> fdp:oneof({'clickhouse', 'ydb', 'tarantool'})
---
- tarantool
...

>
```

Author: ligurio

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Examples with tests.
 - Documentation with usecases, API etc.
 - Support command-line options.
+- Method `oneof()` in FuzzedDataProvider.
 
 ### Changed
 

docs/api.md

@@ -68,6 +68,7 @@ The `FuzzedDataProvider` then supports the following functions:
   If there's no input data left, always returns 0.
 - `remaining_bytes()` - returns the number of unconsumed bytes in the fuzzer
   input.
+- `oneof()` - returns a random element in specified Lua array.
 
 Examples:
 

luzer/fuzzed_data_provider.cc

@@ -208,6 +208,33 @@ luaL_remaining_bytes(lua_State *L)
 	return 1;
 }
 
+/* Returns a random element of specified array. */
+static int
+luaL_oneof(lua_State *L)
+{
+	lua_userdata_t *lfdp;
+	lfdp = (lua_userdata_t *)luaL_checkudata(L, 1, FDP_LUA_UDATA_NAME);
+	luaL_checktype(L, 2, LUA_TTABLE);
+
+	int len = 0;
+	/* Push starting `nil` for iterations. */
+	lua_pushnil(L);
+	while (lua_next(L, 2) != 0) {
+		/*
+		 * Remove `value` from the stack. Keeps `key` for
+		 * the next iteration.
+		 */
+		lua_pop(L, 1);
+		len++;
+	}
+
+	int idx = lfdp->fdp->ConsumeIntegralInRange(1, len);
+	lua_pushinteger(L, idx);
+	lua_gettable(L, -2);
+
+	return 1;
+}
+
 static int close(lua_State *L) {
 	lua_userdata_t *lfdp;
 	lfdp = (lua_userdata_t *)luaL_checkudata(L, 1, FDP_LUA_UDATA_NAME);
@@ -233,6 +260,7 @@ const luaL_Reg methods[] =
 	{ "consume_integers", luaL_consume_integers },
 	{ "consume_probability", luaL_consume_probability },
 	{ "remaining_bytes", luaL_remaining_bytes },
+	{ "oneof", luaL_oneof },
 	{ "__gc", close },
 	{ "__tostring", tostring },
 	{ NULL, NULL }

luzer/tests/test_unit.lua

@@ -174,6 +174,13 @@ assert(p1 >= 0 and p2 >= 0)
 assert(p1 <= 1 and p2 <= 1)
 assert(p1 ~= p2)
 
+-- luzer.FuzzedDataProvider.oneof()
+fdp = luzer.FuzzedDataProvider("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
+assert(type(fdp.oneof) == "function")
+
+local n = fdp:oneof({1, 2})
+assert(type(n) == "number" and (n == 1 or n == 2))
+
 local function custom_mutator(data, size, max_size, seed)
     assert(type(data) == "string")
     assert(type(size) == "number")