Hashmap (O(1)) supporting joker/match-all keys-CodePudding

The title is not so clear, because I cannot put my problem in a sentence (If you have a better title for this question, please suggest). I'll try to clarify my requirement with an example:

Suppose I have a table like this:

| Origin | Destination | Airline   | Free Baggage |
===================================================
| NYC    | London      | American  | 20KG         |
---------------------------------------------------
| NYC    | *           | Southwest | 30KG         |
---------------------------------------------------
| *      | *           | Southwest | 25KG         |
---------------------------------------------------
| *      | LA          | *         | 20KG         |
---------------------------------------------------
| *      | *           | *         | 15KG         |
---------------------------------------------------
and so on ...

This table describes free baggage amount that the airlines provide in different routes. You can see that some rows have * value, meaning that they match all possible values (those values are not known necessarily).

So we have a large list of baggage rules (like the table above) and a large list of flights (which their origin, destination and airline is known), and we intend to find the baggage amount for each one of flights in the most efficient way (iterating the list is not an efficient way, obviously, as it will cost an O(N) computation). It is possible to exist more than one result for each flight, but we will assume that in this case either the first matching or the most specific one will be preferred (whichever is simpler for you to continue with).

If there was not * signs in the table, the problem would be easy, and we could use a Hashmap or Dictionary with a Tuple of values as a key. But with presence of those * (lets say match-all) keys, it is not so straight forward to provide a general solution for that.

Please note that the above example was just an example, and I need a solution that can be used for any number of keys, not just three.

Do you have any idea or implementation for this problem, with a lookup method having time complexity equal or close to O(1) like a regular hashmap (memory will not be an issue)? What would be the best possible solution?

CodePudding user response：

Regarding the comments, the more I think about it, and the more it looks like a relational database with indexes rather than an hashmap...

A trivial, quite easy solution could be something like an In-memory SQlite database. But it would probably be something in O(log2(n)), and not O(1). The main advantage is that it's easy to set up, and IF performances are good enough, it could be the final solution. Here, key is to use proper indexes, the LIKE operator, and of course well-defined JOIN clauses.

From scratch, I can't think about any solution that, having N rows and M columns, isn't at least in O(M)... But usually, you'll have way less columns than rows. Quickly - I may have skipped a detail, I write that on-the-fly - I can propose you this algorithm / container:

Data must be stored in a vector-like container VECDATA, accessed by a simple index in O(1). Think about this as a primary key in databases, and we'll call it PK. Knowing PK gives you instantly, in O(1), the required data. You'll have N rows grand total.
For each row NOT containing any *, you'll insert in a real hashmap called MAINHASH the pair (<tuple>, PK). This is your primary index, for exact results. It will be in O(1), BUT what you requested may not be within... Obviously, you must maintain consistency between MAINHASH and VECDATA, with whatever is needed (mutexes, locks, don't care as long as both are consistents). This hash contains at most N entries. Without any joker, it will act near as a standard hashmap, but for the indirection to VECDATA. It's still O(1) in this case.
For each searchable column, you'll build a specific index, dedicated to this column. The index has N entries. It will be a standard hashmap, but it MUST allow multiple values for a given key. That's quite a common container, so it shouldn't be an issue. For each row, the index entry will be: ( <VECDATA value>, PK ). The container is stored in a vector of indexes, INDEX[i] (with 0<=i<M). Same as MAINHASH, consistency must be enforced.

Obviously, all these indexes / subcontainers should be constructed when an entry is inserted into VECDATA, and saved on disk across sessions if needed - you don't want to reconstruct all this each time you start the application...

Searching a row

So, user search for a given tuple.

Search it in MAINHASH. If found, return it, search done. Upgrade (see below): search also in CACHE before going to step #2.
For each tuple element tuple[0<=i<M], search in INDEX[i] for both tuple[i] (returns a vector of PK, EXACT[i]) AND for * (returns another vector of PK, FUZZY[i]).
With these two vectors, build another (temporary) hash TMPHASH, associating ( PK, integer COUNT ). It quite simple: COUNT is initialized to 1 if entry comes from EXACT, and 0 if it comes from FUZZY.
For next column, build EXACT and FUZZY (see #2). But instead of making a new TMPHASH, you'll MERGE the results into rather than creating a new temporary hash. Method is: if TMPHASH doesn't have this PK entry, trash this entry: it can't match at all. Otherwise, read the COUNT value, add 1 or 0 to it according to where it comes from, reinject it in TMPHASH.
Once all columns are done, you'll have to analyze TMPHASH.

Analyzing `TMPHASH`

First, if TMPHASH is empty, then you don't have any suitable answer. Return that to user. If it contains only one entry, same: return to user directly. For more than one element in TMPHASH:

Parse the whole TMPHASH container, searching for the maximum COUNT. Maintain in memory the PK associated to the current maximum for COUNT.
Developper's choice: in case of multiple COUNT at the same maximum value, you can either return them all, return the first one, or the last one.
COUNT if obviously always stricly lower than M - otherwise, you would have found the tuple in MAINHASH. This value, compared to M, can give a confidence mark to your result (=100*COUNT/M% of confidence).
You can also now store the original tuple searched, and the corresponding PK, in another hashmap called CACHE. Since it would be way too complicated to update properly CACHE when adding/modifying something in VECDATA, simply purge CACHE when it occurs. It's only a cache, after all...

This is quite complex to implement if the language doesn't help you, in particular by allowing to redefine operators and having all base containers available, but it should work.

Exact matches / cached matches are in O(1). Fuzzy search is in O(n.M), n being the number of matching rows (and 0<=n<N, of course).

Without further researchs, I can't see anything better than that. It will consume an obscene amount of memory, but you said that it won't be an issue.

CodePudding user response：

I would recommend doing this with Tries that have a little data decorated. For routes, you want to know the lowest route ID so we can match to the first available route. For flights you want to track how many flights there are left to match.

What this will allow you to do, for instance, is partway through the match ONLY ONCE realize that flights from city1 to city2 might be matching routes that start off city1, city2, or city1, * or *, city2, or *, * without having to repeat that logic for each route or flight.

Here is a proof of concept in Python:

import heapq
import weakref

class Flight:
    def __init__(self, fields, flight_no):
        self.fields = fields
        self.flight_no = flight_no

class Route:
    def __init__(self, route_id, fields, baggage):
        self.route_id = route_id
        self.fields = fields
        self.baggage = baggage

class SearchTrie:
    def __init__(self, value=0, item=None, parent=None):
        # value = # unmatched flights for flights
        # value = lowest route id for routes.
        self.value = value
        self.item = item
        self.trie = {}
        self.parent = None
        if parent:
            self.parent = weakref.ref(parent)

    def add_flight (self, flight, i=0):
        self.value  = 1
        fields = flight.fields
        if i < len(fields):
            if fields[i] not in self.trie:
                self.trie[fields[i]] = SearchTrie(0, None, self)
            self.trie[fields[i]].add_flight(flight, i 1)
        else:
            self.item = flight

    def remove_flight(self):
        self.value -= 1
        if self.parent and self.parent():
            self.parent().remove_flight()

    def add_route (self, route, i=0):
        route_id = route.route_id
        fields = route.fields
        if i < len(fields):
            if fields[i] not in self.trie:
                self.trie[fields[i]] = SearchTrie(route_id)
            self.trie[fields[i]].add_route(route, i 1)
        else:
            self.item = route

    def match_flight_baggage(route_search, flight_search):
        # Construct a heap of one search to do.
        tmp_id = 0
        todo = [((0, tmp_id), route_search, flight_search)]
        # This will hold by flight number, baggage.
        matched = {}

        while 0 < len(todo):
            priority, route_search, flight_search = heapq.heappop(todo)
            if 0 == flight_search.value: # There are no flights left to match
                # Already matched all flights.
                pass
            elif flight_search.item is not None:
                # We found a match!
                matched[flight_search.item.flight_no] = route_search.item.baggage
                flight_search.remove_flight()
            else:
                for key, r_search in route_search.trie.items():
                    if key == '*': # Found wildcard.
                        for a_search in flight_search.trie.values():
                            if 0 < a_search.value:
                                heapq.heappush(todo, ((r_search.value, tmp_id), r_search, a_search))
                                tmp_id  = 1
                    elif key in flight_search.trie and 0 < flight_search.trie[key].value:
                        heapq.heappush(todo, ((r_search.value, tmp_id), r_search, flight_search.trie[key]))
                        tmp_id  = 1

        return matched

# Sample data - the id is the position.
route_data = [
    ["NYC", "London", "American", "20KG"],
    ["NYC", "*", "Southwest", "30KG"],
    ["*", "*", "Southwest", "25KG"],
    ["*", "LA", "*", "20KG"],
    ["*", "*", "*", "15KG"],
]
routes = []
for i in range(len(route_data)):
    data = route_data[i]
    routes.append(Route(i, [data[0], data[1], data[2]], data[3]))

flight_data = [
    ["NYC", "London", "American"],
    ["NYC", "Dallas", "Southwest"],
    ["Dallas", "Houston", "Southwest"],
    ["Denver", "LA", "American"],
    ["Denver", "Houston", "American"],
]
flights = []
for i in range(len(flight_data)):
    data = flight_data[i]
    flights.append(Flight([data[0], data[1], data[2]], i))

# Convert to searches.
flight_search = SearchTrie()
for flight in flights:
    flight_search.add_flight(flight)

route_search = SearchTrie()
for route in routes:
    route_search.add_route(route)


print(route_search.match_flight_baggage(flight_search))

Searching a row

Analyzing TMPHASH

Analyzing `TMPHASH`