would have
woulda
/ˈwʊdə/Reduction
In real speech
What happens to the sounds
Unstressed have loses its h and v and becomes /ə/ → “WOULD-a”. That's why native speakers sometimes misspell it “would of”.
When to use it
I woulda called you. Spoken English; in writing use would have.
Casual → formal
Slang
woulda
Neutral
would've
Formal
would have
Sound notes
Reduction: unstressed words get squeezed, and their vowels turn into the neutral sound /ə/.