indicate round and game end with errors

Fixes #8.

game/game.go

@@ -113,8 +113,8 @@ func (g *Match) Opponent() *Player {
 }
 
 // Apply uses an item by index for the current player.
-// This may cause the current game to end, but does not start the next game
+// If this causes the current game to end (a beer discharges the last shell),
-// if so.
+// returns ErrGameEnded but does not start the next game.
 // Returns ErrWrongTurn if id does not correspond to the current player.
 func (g *Match) Apply(id player.ID, item int) error {
 	cur := g.CurrentPlayer()
@@ -127,6 +127,9 @@ func (g *Match) Apply(id player.ID, item int) error {
 	if cur.items[item].Apply(g) {
 		cur.items[item] = ItemNone
 	}
+	if g.Empty() {
+		return ErrGameEnded
+	}
 	return nil
 }
 
@@ -175,7 +178,9 @@ func (g *Match) MatchWinner() *Player {
 }
 
 // Shoot fires the shotgun, at the opponent if self is false and at the current
-// player if self is true. Afterward, the round or game may be over.
+// player if self is true. Advances the turn if appropriate.
+// Returns ErrRoundEnded if this causes the round to end.
+// Returns ErrGameEnded if this causes the game but not the round to end.
 // Returns ErrWrongTurn if id does not correspond to the current player.
 func (g *Match) Shoot(id player.ID, self bool) error {
 	cur := g.CurrentPlayer()
@@ -189,27 +194,37 @@ func (g *Match) Shoot(id player.ID, self bool) error {
 	live := g.popShell()
 	if live {
 		target.hp -= g.damage
-		g.NextTurn()
+		if target.hp <= 0 {
-	} else if !self {
+			// If the target is the challenger, the match is over as well.
+			if target == &g.players[1] {
+				g.round = 3
+			}
+			return ErrRoundEnded
+		}
+	}
+	if g.Empty() {
+		return ErrGameEnded
+	}
+	if !self || live {
 		g.NextTurn()
 	}
 	return nil
 }
 
 // Concede sets the player with the given ID to zero health and ends the match.
-// The returned bool indicates whether the match has ended. (It will be false
+// The returned error is ErrRoundEnded if id corresponds to either player and
-// if id does not correspond to either player.)
+// ErrWrongTurn otherwise.
-func (g *Match) Concede(id player.ID) bool {
+func (g *Match) Concede(id player.ID) error {
 	switch id {
 	case g.players[0].id:
 		g.players[0].hp = 0
 	case g.players[1].id:
 		g.players[1].hp = 0
 	default:
-		return false
+		return ErrWrongTurn
 	}
 	g.round = 3
-	return true
+	return ErrRoundEnded
 }
 
 // DTO returns the current match state as viewed by the given player.
@@ -239,4 +254,8 @@ func (g *Match) ShellCounts() serve.ShellCounts {
 	return counts
 }
 
-var ErrWrongTurn = errors.New("not your turn")
+var (
+	ErrWrongTurn  = errors.New("not your turn")
+	ErrGameEnded  = errors.New("the shotgun is empty")
+	ErrRoundEnded = errors.New("someone h*ckin died")
+)