Stocking Up

Remy Porter

Computers were a mistake, which is why I'm trying to shoot them into space. Editor-in-Chief for TDWTF.

Sometimes, you find some code that almost works, that almost makes sense. In a way, that's worse than just plain bad code. René was recently going through some legacy JavaScript code for their warehouse management system.

Like any such warehousing system, there's a problem you have to solve: sometimes, the number of units you need to pick to complete the order is larger than the stock you have available. At that point, you need to make a decision: do you hold the order until stock comes in, do you partially fill it and then follow up with a second shipment, or do you perhaps just cancel the order?

René found a line like this:

pick.qty_to_pick -= pick.qty_to_pick - stock.available

So, if I want to pick 100 units, but only have 25 in stock, I'll decrement the qty_to_pick by 75. Which vaguely makes sense, but also is a weird and awkward way of saying "make the qty_to_pick equal to the stock.available".

I assume there are guards around this line which make sure it's executed only if the qty_to_pick is greater than the stock.available. At least, I hope so, because if not, some customers are going to be surprised by the quantity when their order arrives.

In the end, this code isn't strictly wrong, it's just the weirdest most awkward way of copying one value to another variable.

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Steve_The_Cynic (nodebb) 2021-02-10
That made me think of a thing I found in our code a couple of months ago, where the intent was to see if a was greater than b. Key points: this is in C and both variables were of type int, and neither of them could be less than zero. (They are both size-related, but had long ago been given, for unknowable reasons, a signed type.)

On the face of it, sounds easy, right?
```
    if ( a > b )
```
But no. For whatever reason, the person who wrote this line of code did this:
```
    if ( (a - b) > 0 )
```
Which works approximately fine until we change (for reasons of the "get rid of signed/unsigned comparison warnings somewhere else" type) the type of both variables from int to size_t which is not signed, so now all combinations except "exactly equal" would trigger the if().

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