initial commit

src/core/FixedStep.lua

@@ -0,0 +1,23 @@
+-- Fixed-step update loop at the Game Boy's ~60Hz.  Game logic advances in
+-- whole steps regardless of the display refresh rate, which keeps movement,
+-- text speed and battle timing deterministic.
+
+local FixedStep = {}
+
+FixedStep.STEP = 1 / 60
+local MAX_ACCUM = 0.25 -- avoid spiral of death after a stall
+
+function FixedStep:init(callback)
+  self.accum = 0
+  self.callback = callback
+end
+
+function FixedStep:update(dt)
+  self.accum = math.min(self.accum + dt, MAX_ACCUM)
+  while self.accum >= self.STEP do
+    self.accum = self.accum - self.STEP
+    self.callback(self.STEP)
+  end
+end
+
+return FixedStep