In-Memory Database

All keys, fields, and values are strings; empty strings are valid and nullopt denotes absence. No disk, network, wall time, or threading is needed.

Prototype: implement RecordStore() with set(), get(), and scan(). Setting replaces an existing field. Scan includes fields starting with the case-sensitive prefix and sorts by field name. Missing records produce nullopt or []. Returned lists are fresh snapshots; records and instances are independent.

Final: implement TemporalDatabase():

• Every public method takes a nonnegative integer ts. Valid calls, including reads and unsuccessful conditional/deletion calls, advance the accepted time. Equal timestamps are allowed; a later call at the same timestamp wins. A smaller timestamp raises std::invalid_argument without changing history or accepted time. Booleans and noninteger arguments are outside this exercise's input domain.
• set() writes a version. ttl=nullopt means no expiration; otherwise ttl is a positive integer. A write at ts is visible on [ts, ts + ttl). Replacing a field replaces its lifetime too. A nonpositive TTL raises std::invalid_argument before any time or state change.
• get() reads at ts.
• erase() appends a deletion marker only when the field is currently visible. Missing or expired fields return false.
• compare_and_set() writes the new value and lifetime only if the current visible value equals the string expected. Missing/expired fields return false. TTL validation occurs even if comparison would fail.
• scan(ts, key, prefix="") returns a fresh list of visible (field, value) tuples, filtered by case-sensitive prefix and sorted by field name.
• get_at() reads history at integer at, where 0 <= at <= ts. Invalid at raises std::invalid_argument without advancing accepted time. Reads before creation, at/after expiration, or after a deletion return nullopt. An expired newer version never reveals an older version. History reflects operations already processed; later same-timestamp writes become visible to later calls.

Rejected operations preserve history. All validation errors preserve accepted time as well. Separate instances share nothing.

The public C++ API below is authoritative for field types and return values. erase is used where another language might use a method named delete, because delete is a C++ keyword. Returned records are value snapshots with the documented named fields; mutating a record or its nested vectors cannot affect the service.

class RecordStore { public:
 RecordStore();
 void set(const std::string& key,const std::string& field,const std::string& value);
 Opt<std::string> get(const std::string& key,const std::string& field) const;
 Vec<std::pair<std::string,std::string>> scan(const std::string& key,const std::string& prefix="") const;
};
class TemporalDatabase { public:
 TemporalDatabase();
 void set(Int ts,const std::string& key,const std::string& field,const std::string& value,Opt<Int> ttl=std::nullopt);
 Opt<std::string> get(Int ts,const std::string& key,const std::string& field);
 bool erase(Int ts,const std::string& key,const std::string& field);
 bool compare_and_set(Int ts,const std::string& key,const std::string& field,const std::string& expected,const std::string& value,Opt<Int> ttl=std::nullopt);
 Vec<std::pair<std::string,std::string>> scan(Int ts,const std::string& key,const std::string& prefix="");
 Opt<std::string> get_at(Int ts,const std::string& key,const std::string& field,Int at);
};

Compile and run from this directory:

c++ -std=c++20 -O2 -Wall -Wextra -pedantic -pthread main.cpp -o practice
./practice --stage prototype
./practice --stage final

Edit exercise.hpp. Both stages start unfinished and must report failures until implemented. Keep main.cpp as the executable contract. The runner returns nonzero for a failed check, an unexpected exception, an invalid stage, or an empty suite. C++20 and only the standard library are required.