The feeling of being relevant and contributing is a very powerful force. Its why people will take enhancers that help them win and place high in local 5K run races. Like it even matters, but to them it is everything. Worth cheating to get it they think. Something that has always helped me is knowing that cheaters can't lead, they can only follow. He'll need to steal credit from someone else to keep this "look" going.
So, for now ... I say to keep the peace - take it on the chin. Good learning experience for you. You did not properly credit yourself and made the way for this to happen. Note that you must be balanced in life too and not toot your horn too loudly. Have balance, allow others to succeed and get credit too. Maybe you saw the sample code on stack-overflow and refactored it. We all learn from others anyway.
The real problem here is the pat on the back from the boss, not the code itself which you saw as trivial anyway. Be aware of how this company operates. However, don't stop contributing! That will cause you to die inside. Your contributions are great and will help you to grow. So, no matter who gets the credit, keep doing it so that you'll be better off when you leave this place.
"Visibility" is one of the major drivers of promotion in every organization. They don't know your good if they can't see it.
If there are no mitigating circumstances here, like "actually we worked together but I did most of it". Then it should be a simple matter to talk to the boss and get to the bottom of how it was misattributed. No accusations, no assumptions but a fact finding mission. The goal here is to find out how it happened so you can avoid it in the future, and as a secondary bonus it will obviously alert your boss to the fact you are the real author.
If people are able to take credit for your work it indicates a larger problem with your visibility in the organization and it should be dealt with swiftly if you care about your career.
There wasn't collaboration other than plugging it into his code. It was not an oversight it was very clear he claimed to have written it all.You are correct though, partly my fault, why not mention it first myself, why wait for someone else to.
One final suggestion, if when bringing this up you come off as emotional as you do in your original post it won't be effective. Approach it as just a big misunderstanding. There is nothing wrong with ensuring correct attribution to work where its something big (which a library qualifies for).
So, for now ... I say to keep the peace - take it on the chin. Good learning experience for you. You did not properly credit yourself and made the way for this to happen. Note that you must be balanced in life too and not toot your horn too loudly. Have balance, allow others to succeed and get credit too. Maybe you saw the sample code on stack-overflow and refactored it. We all learn from others anyway.
The real problem here is the pat on the back from the boss, not the code itself which you saw as trivial anyway. Be aware of how this company operates. However, don't stop contributing! That will cause you to die inside. Your contributions are great and will help you to grow. So, no matter who gets the credit, keep doing it so that you'll be better off when you leave this place.