Taylor Swift to Calvin Harris: Get it Straight ... The Melody is Mine

Taylor Swift is swinging back ... calling BS on Calvin Harris' Twitter tirade in which he claims he was the architect behind the hit song, "This Is What You Came For."

Sources close to Taylor tell TMZ ... Calvin's intentionally downplaying her involvement, essentially saying he did all the music. But Taylor says not only did she write the lyrics ... she also created the melody, and those 2 elements are the essence of any song.

Our Calvin sources concede Taylor indeed wrote the melody ... but his musical genius with beats, chords and instrumentals made the song a hit.

A Calvin source scoffs, "She's acting like he's only got one hit song. LOL."