From 5aec70de2b400425f6398e20781f004e4d03d965 Mon Sep 17 00:00:00 2001
From: Brad Van Vugt <1531419+bvanvugt@users.noreply.github.com>
Date: Tue, 21 Jul 2020 17:11:12 -0700
Subject: [PATCH] Prioritize self-collisions over other collisions. Fixes #16.

---
 squad_test.go    | 51 ++++++++++++++++++++++++++++++++++++++++++++++++
 standard.go      | 20 ++++++++++++-------
 standard_test.go | 39 ++++++++++++++++++++++++++++++++++++
 3 files changed, 103 insertions(+), 7 deletions(-)

diff --git a/squad_test.go b/squad_test.go
index 7b9bfd4..5b51b80 100644
--- a/squad_test.go
+++ b/squad_test.go
@@ -338,3 +338,54 @@ func TestSquadIsGameOver(t *testing.T) {
 		require.Equal(t, test.Expected, actual)
 	}
 }
+
+func TestIssue16Regression(t *testing.T) {
+	// This is a specific test case to detect this issue:
+	// https://github.com/BattlesnakeOfficial/rules/issues/16
+	boardState := &BoardState{
+		Width:  11,
+		Height: 11,
+		Snakes: []Snake{
+			{ID: "teamBoi", Health: 10, Body: []Point{{1, 4}, {1, 3}, {0, 3}, {0, 2}, {1, 2}, {2, 2}}},
+			{ID: "Node-Red-Bellied-Black-Snake", Health: 10, Body: []Point{{1, 8}, {2, 8}, {2, 9}, {3, 9}, {4, 9}, {4, 10}}},
+			{ID: "Crash Override", Health: 10, Body: []Point{{2, 7}, {2, 6}, {3, 6}, {4, 6}, {4, 5}, {5, 5}, {6, 5}}},
+			{ID: "Zero Cool", Health: 10, Body: []Point{{6, 5}, {5, 5}, {5, 4}, {5, 3}, {4, 3}, {3, 3}, {3, 4}}},
+		},
+	}
+	squadMap := map[string]string{
+		"teamBoi":                      "BirdSnakers",
+		"Node-Red-Bellied-Black-Snake": "BirdSnakers",
+		"Crash Override":               "Hackers",
+		"Zero Cool":                    "Hackers",
+	}
+	snakeMoves := []SnakeMove{
+		{ID: "teamBoi", Move: "down"},
+		{ID: "Node-Red-Bellied-Black-Snake", Move: "left"},
+		{ID: "Crash Override", Move: "left"},
+		{ID: "Zero Cool", Move: "left"},
+	}
+
+	require.Equal(t, len(squadMap), len(boardState.Snakes), "squad map is wrong size, error in test setup")
+
+	r := SquadRuleset{
+		AllowBodyCollisions: true,
+		SquadMap:            squadMap,
+	}
+
+	nextBoardState, err := r.CreateNextBoardState(boardState, snakeMoves)
+	require.NoError(t, err)
+	require.Equal(t, len(boardState.Snakes), len(nextBoardState.Snakes))
+
+	expectedSnakes := []Snake{
+		{ID: "teamBoi", Body: []Point{{1, 5}, {1, 4}, {1, 3}, {0, 3}, {0, 2}, {1, 2}}},
+		{ID: "Node-Red-Bellied-Black-Snake", Body: []Point{{0, 8}, {1, 8}, {2, 8}, {2, 9}, {3, 9}, {4, 9}}},
+		{ID: "Crash Override", Body: []Point{{1, 7}, {2, 7}, {2, 6}, {3, 6}, {4, 6}, {4, 5}, {5, 5}}},
+		{ID: "Zero Cool", Body: []Point{{5, 5}, {6, 5}, {5, 5}, {5, 4}, {5, 3}, {4, 3}, {3, 3}}, EliminatedCause: EliminatedBySelfCollision, EliminatedBy: "Zero Cool"},
+	}
+	for i, snake := range nextBoardState.Snakes {
+		require.Equal(t, expectedSnakes[i].ID, snake.ID, snake.ID)
+		require.Equal(t, expectedSnakes[i].Body, snake.Body, snake.ID)
+		require.Equal(t, expectedSnakes[i].EliminatedCause, snake.EliminatedCause, snake.ID)
+		require.Equal(t, expectedSnakes[i].EliminatedBy, snake.EliminatedBy, snake.ID)
+	}
+}
diff --git a/standard.go b/standard.go
index 9bb84d6..a3fe9ef 100644
--- a/standard.go
+++ b/standard.go
@@ -349,15 +349,21 @@ func (r *StandardRuleset) maybeEliminateSnakes(b *BoardState) error {
 			continue
 		}
 
-		// Always check body collisions before head-to-heads
+		// Check for self-collisions first
+		if r.snakeHasBodyCollided(snake, snake) {
+			snake.EliminatedCause = EliminatedBySelfCollision
+			snake.EliminatedBy = snake.ID
+			continue
+		}
+
+		// Check for body collisions with other snakes second
 		for _, otherIndex := range snakeIndicesByLength {
 			other := &b.Snakes[otherIndex]
+			if snake.ID == other.ID {
+				continue
+			}
 			if r.snakeHasBodyCollided(snake, other) {
-				if snake.ID == other.ID {
-					snake.EliminatedCause = EliminatedBySelfCollision
-				} else {
-					snake.EliminatedCause = EliminatedByCollision
-				}
+				snake.EliminatedCause = EliminatedByCollision
 				snake.EliminatedBy = other.ID
 				break
 			}
@@ -366,7 +372,7 @@ func (r *StandardRuleset) maybeEliminateSnakes(b *BoardState) error {
 			continue
 		}
 
-		// Always check head-to-heads after body collisions
+		// Check for head-to-heads last
 		for _, otherIndex := range snakeIndicesByLength {
 			other := &b.Snakes[otherIndex]
 			if snake.ID != other.ID && r.snakeHasLostHeadToHead(snake, other) {
diff --git a/standard_test.go b/standard_test.go
index 2da82ce..dab6a5e 100644
--- a/standard_test.go
+++ b/standard_test.go
@@ -1266,6 +1266,45 @@ func TestMaybeEliminateSnakes(t *testing.T) {
 	}
 }
 
+func TestMaybeEliminateSnakesPriority(t *testing.T) {
+	tests := []struct {
+		Snakes                   []Snake
+		ExpectedEliminatedCauses []string
+		ExpectedEliminatedBy     []string
+	}{
+		{
+			[]Snake{
+				{ID: "1", Health: 0, Body: []Point{{-1, 0}, {0, 0}, {1, 0}}},
+				{ID: "2", Health: 1, Body: []Point{{-1, 0}, {0, 0}, {1, 0}}},
+				{ID: "3", Health: 1, Body: []Point{{1, 0}, {0, 0}, {1, 0}}},
+				{ID: "4", Health: 1, Body: []Point{{1, 0}, {1, 1}, {1, 2}}},
+				{ID: "5", Health: 1, Body: []Point{{2, 2}, {2, 1}, {2, 0}}},
+				{ID: "6", Health: 1, Body: []Point{{2, 2}, {2, 3}, {2, 4}, {2, 5}}},
+			},
+			[]string{
+				EliminatedByStarvation,
+				EliminatedByOutOfBounds,
+				EliminatedBySelfCollision,
+				EliminatedByCollision,
+				EliminatedByHeadToHeadCollision,
+				NotEliminated,
+			},
+			[]string{"", "", "3", "1", "6", ""},
+		},
+	}
+
+	r := StandardRuleset{}
+	for _, test := range tests {
+		b := &BoardState{Width: 10, Height: 10, Snakes: test.Snakes}
+		err := r.maybeEliminateSnakes(b)
+		require.NoError(t, err)
+		for i, snake := range b.Snakes {
+			require.Equal(t, test.ExpectedEliminatedCauses[i], snake.EliminatedCause)
+			require.Equal(t, test.ExpectedEliminatedBy[i], snake.EliminatedBy)
+		}
+	}
+}
+
 func TestMaybeFeedSnakes(t *testing.T) {
 	tests := []struct {
 		Name           string