Is there any difference in meaning?
1. A simplified code of the A-star algorithm was implemented in C# programming language.
2. The simplified code of the A-star algorithm was implemented in C# programming language.
I've made them into sentences, because sentences make better examples to discuss.
1. means that some (any) A-star code was written.
2. means that a certain code already identified was written.
The context of where the sentence appears in relations to the code would make a difference. If you're writing a paper, and you've already presented the code, you'd say "The simplified code". If this sentence appeared in the "methods" section of a paper, it would be normal to write "A simplified code".
Otherwise the normal rules for definite and indefinite articles apply.