offline GTFS connection from A to B - sql

Question

The project I am working on involves static offline GTFS data in a mobile app. All the GTFS data is available inside realm-objects (or SQLite if needed).
Now, I would like to establish all train- or bus-connections from A to B (starting after a certain departure-time).
How do I query the GTFS-data in order to get a connection from A to B ???
I reealized to get all trips leaving from A.
I realized to get all station-names along that trip including times.
But I find it very hard to get the connection information between two locations A and B. What SQL queries do i have to set up in order to get that information ?
Any help appreciated !

Answer 1

If you just want to dynamically calculate shortest travel routes between static hubs, in an offline application, determined like the following image, you can use the following formula:
   (Source)
Here's the pseudo code:
1 function Dijkstra(Graph, source):
2
3 create vertex set Q
4
5 for each vertex v in Graph: // Initialization
6 dist[v] ← INFINITY // Unknown distance from source to v
7 prev[v] ← UNDEFINED // Previous node in optimal path from source
8 add v to Q // All nodes initially in Q (unvisited nodes)
9
10 dist[source] ← 0 // Distance from source to source
11
12 while Q is not empty:
13 u ← vertex in Q with min dist[u] // Node with the least distance
14 // will be selected first
15 remove u from Q
16
17 for each neighbor v of u: // where v is still in Q.
18 alt ← dist[u] + length(u, v)
19 if alt < dist[v]: // A shorter path to v has been found
20 dist[v] ← alt
21 prev[v] ← u
22
23 return dist[], prev[]
(Source)
Alright, I'll admit so far my answer was a tad facetious...
I suspect you don't realize just how complicated it is to do what you're asking, especially in an offline environment. Companies like Google and Microsoft have spent millions on research with huge teams of data scientists.
If this is something you are serious about, I'd encourage you to start with a 10×10 grid and work on the logic of getting from "Point A → Point B" when you start adding barriers in random places (this simulating roads beginning & ending). Recently I was surprised how complicated a seemingly-simple, somewhat-related Stack Overflow "pipe sizes conversion" question that I answered had become.
If you didn't have the "offline" condition, I would've suggested looking into getting a [free] API Key for Google Web Services Directions API. Basically you tell it a where Points A & B are, and it gives you detailed route information, via transit or other methods.
Another of Google's many API's that could be helpful is the Snap to Roads API, which turns partial or error-ridden paths into driveable ones.
The study Route Logic and Shortest Path Logic is actually really fascinating stuff, and there are some amazing resources to learn about the related theories (see below), including a video from Google explaining how they went about it.
...but unfortunately this isn't something that's going to be accomplished with a simple SQL Query.
Actual Resources:
YouTube : Google Tech Talk: Fast Route Planning
Wikipedia : Shortest path problem
Wikipedia : Dijkstra's algorithm
...and some slightly Lighter Reading:
Wikipedia : Travelling Salesman Problem
Why UPS drivers don’t turn left and you probably shouldn’t either
Google Web Services : Directions API Developer's Guide
Stack Overflow : Shortest Route of converters between two different pipe sizes?
Wikipedia : Seven Bridges of Königsberg