Error with inconsistent vehicle capacity arrays

[jcoupey]

My intent when parsing json what to ensure all vehicles either have no capacity key, or have all the same length for the capacity array. In particular I expect a missing capacity array to trigger an error. This happens for example when using:

{
  "vehicles": [
    {
      "id":0,
      ...
    },
    {
      "id":1,
      "capacity": [1],
      ...
    ],
    ...

resulting in an error with the message "Inconsistent capacity length: 1 and 0."

So far so good, but I just found out that reversing the order of vehicles above changes the behavior: we no longer get an error, instead the capacity for vehicle with id 0 is considered as zero and we move on with solving.
In this situation the vehicle won't get a route assigned if all tasks have a non-zero demand, so this is not such a big problem if considering that no capacity key means zero capacity.

Yet I think we should change this because:

• it would be more consistent to enforce always explicitly providing a capacity array as initially intended;
• it is clearer to get an error in the first place than to have to debug why those vehicles do not get routes (the reason I stumbled upon this);
• getting either a "usual" solution or an error just by changing vehicle order in input is not really acceptable.

[jcoupey]

In practice, this happens because we use the length of the capacity for the first vehicle as the reference for amount lengths, assuming consistency but not enforcing it if the key is absent.

All amounts are parsed using the same function:

vroom/src/utils/input_parser.cpp

Lines 51 to 55 in 176f57a

	inline Amount get_amount(const rapidjson::Value& object,
	const char* key,
	unsigned amount_size) {
	// Default to zero amount with provided size.
	Amount amount(amount_size);

We default to an empty amount because it makes sense for shipment.amount, job.pickup or job.delivery keys.