Decompose chase.go (refactor step 7)

chase splits into chaseBestSpot/chaseTry/scareScrollAt with a
chaseSearch state struct; chaseStep gains chaseRooms, chaseGoal,
dragonBreath/dragonShoots, and chaseTakeObject; runners gains
runnerTurn; findDest gains objectClaimed. chase.go is complexity-clean.
Behavior and RNG call order unchanged.

game/chase.go

@@ -9,17 +9,32 @@ const dragonShot = 5
 func (g *RogueGame) runners(int) {
 	list := append([]*Monster(nil), g.Level.Monsters...)
 	for _, tp := range list {
-		if !tp.On(Held) && tp.On(Awake) {
+		g.runnerTurn(tp)
+	}
+
+	if g.HasHit {
+		g.endmsg()
+		g.HasHit = false
+	}
+}
+
+// runnerTurn gives one monster its motion for the turn; flying monsters
+// far from the hero move twice (the loop body of chase.c runners).
+func (g *RogueGame) runnerTurn(tp *Monster) {
+	if tp.On(Held) || !tp.On(Awake) {
+		return
+	}
+
 	origPos := tp.Pos
 
 	wastarget := tp.On(Targeted)
 	if removed := g.moveMonster(tp); removed {
-				continue
+		return
 	}
 
 	if tp.On(Flying) && distCp(g.Player.Pos, tp.Pos) >= 3 {
 		if removed := g.moveMonster(tp); removed {
-					continue
+			return
 		}
 	}
 
@@ -29,13 +44,6 @@ func (g *RogueGame) runners(int) {
 		g.ToDeath = false
 	}
 }
-	}
-
-	if g.HasHit {
-		g.endmsg()
-		g.HasHit = false
-	}
-}
 
 // moveMonster executes a single turn of running for a monster (chase.c
 // move_monst). The result reports that the monster died or left the
@@ -87,13 +95,48 @@ func (g *RogueGame) relocate(th *Monster, newLoc Coord) {
 	}
 }
 
-// chaseStep makes one thing chase another (chase.c do_chase). removed
+// chaseStep makes one thing chase another (chase.c do_chase). The
-// reports that the chaser died or left the level in the attempt (the C
+// result reports that the chaser died or left the level in the attempt
-// -1 return).
+// (the C -1 return).
-func (g *RogueGame) chaseStep(th *Monster) (removed bool) {
+func (g *RogueGame) chaseStep(th *Monster) bool {
-	p := &g.Player
 	stoprun := false // true means we are there
-	mindist := 32767
+
+	rer, ree, door := g.chaseRooms(th)
+
+	this, shot := g.chaseGoal(th, rer, ree, door, 32767)
+	if shot {
+		return false
+	}
+	// This now contains what we want to run to this time so we run to it.
+	// If we hit it we either want to fight it or stop running
+	if g.chase(th, this) {
+		if th.Type == 'F' {
+			return false
+		}
+	} else {
+		switch this {
+		case g.Player.Pos:
+			return g.attack(th)
+		case *th.Dest:
+			g.chaseTakeObject(th)
+
+			stoprun = th.Type != 'F'
+		}
+	}
+
+	g.relocate(th, g.chRet)
+	// And stop running if need be
+	if stoprun && th.Pos == *th.Dest {
+		th.Flags.Clear(Awake)
+	}
+
+	return false
+}
+
+// chaseRooms finds the rooms of the chaser and its desire; doors do not
+// count as inside rooms here (the setup of chase.c do_chase).
+func (g *RogueGame) chaseRooms(th *Monster) (*Room, *Room, bool) {
+	p := &g.Player
 
 	rer := th.Room // find room of chaser
 	if th.On(Greedy) && rer.GoldVal == 0 {
@@ -106,15 +149,26 @@ func (g *RogueGame) chaseStep(th *Monster) (removed bool) {
 	} else {
 		ree = g.roomIn(*th.Dest)
 	}
-	// We don't count doors as inside rooms for this routine
-	door := g.Level.Char(th.Pos.Y, th.Pos.X) == Door
 
+	return rer, ree, g.Level.Char(th.Pos.Y, th.Pos.X) == Door
+}
+
+// chaseGoal picks the spot the chaser runs toward this turn: the
+// nearest exit toward its desire when it is in a different room, or the
+// desire itself. shot means a dragon breathed flame instead of moving
+// (the goal loop of chase.c do_chase).
+func (g *RogueGame) chaseGoal(th *Monster, rer, ree *Room, door bool, mindist int) (Coord, bool) {
 	var this Coord
 
 	for {
 		// If the object of our desire is in a different room, and we are
 		// not in a corridor, run to the door nearest to our goal.
-		if rer != ree {
+		if rer == ree {
+			this = *th.Dest
+
+			return this, g.dragonBreath(th)
+		}
+
 		for i := range rer.Exits {
 			curdist := distCp(*th.Dest, rer.Exits[i])
 			if curdist < mindist {
@@ -123,21 +177,25 @@ func (g *RogueGame) chaseStep(th *Monster) (removed bool) {
 			}
 		}
 
-			if door {
+		if !door {
+			return this, false
+		}
+
 		rer = &g.Level.Passages[*g.Level.FlagsAt(th.Pos.Y, th.Pos.X)&FPassNum]
 		door = false
-
+		// the C goto over: redo with the passage as room
-				continue // the C goto over: redo with the passage as room
 	}
-		} else {
+}
-			this = *th.Dest
+
-			// For dragons check and see if (a) the hero is on a straight
+// dragonBreath checks whether a dragon shoots flame at the hero instead
-			// line from it, and (b) that it is within shooting distance,
+// of moving, and shoots it (the D block of chase.c do_chase).
-			// but outside of striking range.
+func (g *RogueGame) dragonBreath(th *Monster) bool {
-			if th.Type == 'D' && (th.Pos.Y == p.Pos.Y || th.Pos.X == p.Pos.X ||
+	if th.Type != 'D' || !g.dragonShoots(th) {
-				abs(th.Pos.Y-p.Pos.Y) == abs(th.Pos.X-p.Pos.X)) &&
+		return false
-				distCp(th.Pos, p.Pos) <= BoltLength*BoltLength &&
+	}
-				!th.On(Cancelled) && g.rnd(dragonShot) == 0 {
+
+	p := &g.Player
+
 	g.Delta.Y = sign(p.Pos.Y - th.Pos.Y)
 
 	g.Delta.X = sign(p.Pos.X - th.Pos.X)
@@ -155,18 +213,27 @@ func (g *RogueGame) chaseStep(th *Monster) (removed bool) {
 		g.Kamikaze = false
 	}
 
-				return false
+	return true
-			}
 }
 
-		break
+// dragonShoots decides whether the dragon takes the shot: the hero is
+// on a straight line from it, within shooting distance but outside
+// striking range, it is not cancelled, and the shot roll comes up
+// (chase.c do_chase).
+func (g *RogueGame) dragonShoots(th *Monster) bool {
+	p := &g.Player
+	if th.Pos.Y != p.Pos.Y && th.Pos.X != p.Pos.X &&
+		abs(th.Pos.Y-p.Pos.Y) != abs(th.Pos.X-p.Pos.X) {
+		return false
 	}
-	// This now contains what we want to run to this time so we run to it.
+
-	// If we hit it we either want to fight it or stop running
+	return distCp(th.Pos, p.Pos) <= BoltLength*BoltLength &&
-	if !g.chase(th, this) {
+		!th.On(Cancelled) && g.rnd(dragonShot) == 0
-		if this == p.Pos {
+}
-			return g.attack(th)
+
-		} else if this == *th.Dest {
+// chaseTakeObject has the monster pick up the object it was running to
+// (the dest arm of chase.c do_chase).
+func (g *RogueGame) chaseTakeObject(th *Monster) {
 	for _, obj := range g.Level.Objects {
 		if th.Dest == &obj.Pos {
 			g.Level.RemoveObject(obj)
@@ -183,34 +250,12 @@ func (g *RogueGame) chaseStep(th *Monster) (removed bool) {
 			break
 		}
 	}
-
-			if th.Type != 'F' {
-				stoprun = true
-			}
-		}
-	} else {
-		if th.Type == 'F' {
-			return false
-		}
-	}
-
-	g.relocate(th, g.chRet)
-	// And stop running if need be
-	if stoprun && th.Pos == *th.Dest {
-		th.Flags.Clear(Awake)
-	}
-
-	return false
 }
 
 // chase finds the spot for the chaser to move closer to the chasee
 // (chase.c chase). Returns true if we want to keep on chasing later, false
 // if we reach the goal. The chosen spot lands in g.chRet.
 func (g *RogueGame) chase(tp *Monster, ee Coord) bool {
-	p := &g.Player
-	er := tp.Pos
-	plcnt := 1
-
 	var curdist int
 
 	// If the thing is confused, let it move randomly. Invisible Stalkers
@@ -226,11 +271,27 @@ func (g *RogueGame) chase(tp *Monster, ee Coord) bool {
 			tp.Flags.Clear(Confused)
 		}
 	} else {
-		// Otherwise, find the empty spot next to the chaser that is
+		curdist = g.chaseBestSpot(tp, ee)
-		// closest to the chasee. This will eventually hold where we move
+	}
-		// to get closer. If we can't find an empty spot, we stay where we
+
-		// are.
+	return curdist != 0 && g.chRet != g.Player.Pos
-		curdist = distCp(er, ee)
+}
+
+// chaseSearch is the scan state while chase looks for the step that
+// gets a monster closest to its chasee.
+type chaseSearch struct {
+	er      Coord // where the chaser is
+	ee      Coord // where it wants to go
+	curdist int
+	plcnt   int
+}
+
+// chaseBestSpot finds the empty spot next to the chaser that is closest
+// to the chasee, leaving it in g.chRet; if there is none, the chaser
+// stays where it is (the search half of chase.c chase).
+func (g *RogueGame) chaseBestSpot(tp *Monster, ee Coord) int {
+	er := tp.Pos
+	s := chaseSearch{er: er, ee: ee, curdist: distCp(er, ee), plcnt: 1}
 	g.chRet = er
 
 	ey := er.Y + 1
@@ -249,54 +310,59 @@ func (g *RogueGame) chase(tp *Monster, ee Coord) bool {
 		}
 
 		for y := er.Y - 1; y <= ey; y++ {
+			g.chaseTry(&s, y, x)
+		}
+	}
+
+	return s.curdist
+}
+
+// chaseTry scores one candidate square, reservoir-sampling among ties
+// (the scan body of chase.c chase).
+func (g *RogueGame) chaseTry(s *chaseSearch, y, x int) {
 	tryp := Coord{X: x, Y: y}
-				if !g.diagOk(er, tryp) {
+	if !g.diagOk(s.er, tryp) {
-					continue
+		return
 	}
 
 	ch := g.Level.VisibleChar(y, x)
-				if stepOk(ch) {
+	if !stepOk(ch) {
-					// If it is a scroll, it might be a scare monster
+		return
-					// scroll so we need to look it up to see what type
-					// it is.
-					if ch == Scroll {
-						var found *Object
-
-						for _, obj := range g.Level.Objects {
-							if y == obj.Pos.Y && x == obj.Pos.X {
-								found = obj
-
-								break
-							}
-						}
-
-						if found != nil && found.ScrollKind() == ScrollScareMonster {
-							continue
 	}
+	// If it is a scroll, it might be a scare monster scroll so we need
+	// to look it up to see what type it is.
+	if ch == Scroll && g.scareScrollAt(y, x) {
+		return
 	}
 	// It can also be a Xeroc, which we shouldn't step on
 	if m := g.Level.MonsterAt(y, x); m != nil && m.Type == 'X' {
-						continue
+		return
 	}
-					// If we didn't find any scrolls at this place or it
+	// If we didn't find any scrolls at this place or it wasn't a scare
-					// wasn't a scare scroll, then this place counts
+	// scroll, then this place counts
-					thisdist := distance(y, x, ee.Y, ee.X)
+	thisdist := distance(y, x, s.ee.Y, s.ee.X)
-					if thisdist < curdist {
+	if thisdist < s.curdist {
-						plcnt = 1
+		s.plcnt = 1
 		g.chRet = tryp
-						curdist = thisdist
+		s.curdist = thisdist
-					} else if thisdist == curdist {
+	} else if thisdist == s.curdist {
-						if plcnt++; g.rnd(plcnt) == 0 {
+		if s.plcnt++; g.rnd(s.plcnt) == 0 {
 			g.chRet = tryp
-							curdist = thisdist
+			s.curdist = thisdist
-						}
-					}
-				}
 		}
 	}
 }
 
-	return curdist != 0 && g.chRet != p.Pos
+// scareScrollAt reports whether the object lying at (y, x) is a scare
+// monster scroll (chase.c chase).
+func (g *RogueGame) scareScrollAt(y, x int) bool {
+	for _, obj := range g.Level.Objects {
+		if y == obj.Pos.Y && x == obj.Pos.X {
+			return obj.ScrollKind() == ScrollScareMonster
+		}
+	}
+
+	return false
 }
 
 // setOldChar sets the oldch character for the monster (chase.c set_oldch).
@@ -431,22 +497,23 @@ func (g *RogueGame) findDest(tp *Monster) *Coord {
 			continue
 		}
 
-		if g.roomIn(obj.Pos) == tp.Room && g.rnd(100) < prob {
+		if g.roomIn(obj.Pos) == tp.Room && g.rnd(100) < prob &&
-			claimed := false
+			!g.objectClaimed(obj) {
-
-			for _, other := range g.Level.Monsters {
-				if other.Dest == &obj.Pos {
-					claimed = true
-
-					break
-				}
-			}
-
-			if !claimed {
 			return &obj.Pos
 		}
 	}
-	}
 
 	return &g.Player.Pos
 }
+
+// objectClaimed reports whether some monster already runs toward this
+// object (chase.c find_dest).
+func (g *RogueGame) objectClaimed(obj *Object) bool {
+	for _, other := range g.Level.Monsters {
+		if other.Dest == &obj.Pos {
+			return true
+		}
+	}
+
+	return false
+}