How Not To Write a Flight Route Planner

1. Do not use real data
2. Do not use DFS
3. Do not use BFS
4. Do not use Dijkstra's (naively)
5. Lat/Lon are not coordinates

1. Do not use real data

Why?

- It is impossible to track down bugs,
  especially when you get into a loop
  that repeatedly prints:
  [293, 23836, 202, 223, 2235]
- It can take a long time to load.
- It may be old data (it was)

2. Do not use DFS

Why?

- It may take a long time to complete
  - It needs to check all possibilities
    down possibly useless rabbit
    holes before checking nearer paths
- It may find a long connection
  before a short one

Consider the following
depth-first-search operations:

YYC -> YYZ
YYZ -> YVR
YYC -> YVR

3. Do not use BFS

Why?

- Does not take into account any weights:
  - cost of flight
  - distance of airports

Consider the following
breadth-first-search operations:

YYC -> AMS
YYC -> YVR
YYC -> YYZ
AMS -> TPE
YVR -> TPE

4. Do not use Dijkstra's (naively)

Why?

- What do you use for a weight?
  - cost of flight
  - number of connections
  - distance between airports
- Selecting one of these,
  without taking into account the others,
  makes for a terrible flight planner
- You may want to have additional bounds
  for example: you may want to confirm
  that the schedule for the flight lines up
  I could not find data for this, however

Q: What happens when you only take into
   account the distance between airports?
A: A flight from YYC to TPE
   would look like the following:

- YYC
- MSP
- GRR
- DTW
- EWR
- MAD
- CAI
- TUU
- DMM
- BAH
- DXB
- TPE

Q: Okay, so how do you implement a flight planner?
A: Use Dijkstra's, with multiple weights.

For mine, I used:
- The distance between airports
- + a fixed cost for connections

The distance could be swapped out with cost of flight.
I couldn't find a database of fare costs.

5. Lat/Lon are not coordinates

- You can not simply use (p1.lat - p2.lat).
- THIS WILL NOT WORK!

- Use a library, or learn a bunch of math.
- I used latlon3

Libraries:

- latlon3 (Python)
- nalgebra, nalgabra_sparse,
  serde, rust_decimal (Rust)

Demo