trace path we can (move right + jump) using quad tree, as it is easier to implement than a pure math based approach

fcore/trace-jump.sml

@@ -0,0 +1,68 @@
+structure TraceJump =
+struct
+  structure Trace = MakeQuadTreeFold (struct
+    type env = int
+    type state = bool
+
+    fun fold (foldPlatID, nextPlatID, hasFoundNextPlatID) =
+      hasFoundNextPlatID orelse foldPlatID = nextPlatID
+  end)
+
+  fun traceRightJumpDescent (x, y, nextPlatID, platTree) =
+    if x >= Constants.worldWidth orelse y >= Constants.worldHeight then
+      (* we hit bounds of screen and saw that there was 
+       * no way to jump to next nextPlatID *)
+      false
+    else
+      let
+        val width = Constants.moveEnemyBy
+        val height = Constants.worldHeight - y
+        val shouldJumpRight =
+          Trace.foldRegion (x, y, width, height, nextPlatID, false, platTree)
+
+        val nextX = x + Constants.moveEnemyBy
+        val nextY = y + Constants.moveEnemyBy
+      in
+        shouldJumpRight orelse 
+        traceRightJumpDescent (nextX, nextY, nextPlatID, platTree)
+      end
+
+  fun traceRightJumpAscent (x, y, remainingJump, nextPlatID, platTree, nextPlatform) =
+    if remainingJump >= Constants.jumpLimit then
+      false
+    else
+      let
+        val width = Constants.moveEnemyBy
+        val height = Constants.worldHeight - y
+        val shouldJumpRight =
+          Trace.foldRegion (x, y, width, height, nextPlatID, false, platTree)
+
+        val nextX = x + Constants.moveEnemyBy
+        val nextY = y - Constants.moveEnemyBy
+        val nextJump = remainingJump + Constants.moveEnemyBy
+      in
+        shouldJumpRight orelse
+        traceRightJumpAscent (nextX, nextY, nextJump, nextPlatID, platTree, nextPlatform)
+      end
+
+  fun traceRightJump (enemy, nextPlatID, platTree, nextPlat) =
+    let
+      open GameType
+      val {x, y, ...}: enemy = enemy
+    in
+      if EnemyPhysics.standingOnArea (x, y, platTree) then
+        traceRightJumpAscent (x, y, 0, nextPlatID, platTree, nextPlat)
+      else
+        case #yAxis enemy of
+          JUMPING amt =>
+            traceRightJumpAscent (x, y, amt, nextPlatID, platTree, nextPlat)
+        | ON_GROUND =>
+            traceRightJumpAscent (x, y, 0, nextPlatID, platTree, nextPlat)
+        | FALLING =>
+            traceRightJumpDescent (x, y, nextPlatID, platTree)
+        | DROP_BELOW_PLATFORM =>
+            traceRightJumpDescent (x, y, nextPlatID, platTree)
+        | FLOATING _ =>
+            traceRightJumpDescent (x, y, nextPlatID, platTree)
+    end
+end