objects/oFly/step.gml
From spelunky.gmk, Spelunky Classic 1.1's source, © 2008, 2009 Derek Yu and Mossmouth, LLC, shown whole under the Spelunky User License v1.1b. Extracted by spelunky.ai's tools, an unofficial fan project, one file per script or event; the // ==== lines are the extractor's, naming each action of the event. Some scripts are others' work that Derek Yu credits in the source's readme.txt: Martin Piecyk's platform engine, ChevyRay's view scaling scripts and tsg1zzn's Super Sound System.
oFly's step event, 5 lines of GML.
x += xVel;y += yVel;if (xVel < 0) sprite_index = sFlyLeft;else sprite_index = sFlyRight;