2020-day24: hex coordinates are tough. 1240/668

2020/day24/main.go

@@ -15,32 +15,129 @@ func main() {
 	flag.Parse()
 	fmt.Println("Running part", part)
 
-	if part == 1 {
-		ans := part1(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
-		fmt.Println("Output:", ans)
-	} else {
-		ans := part2(util.ReadFile("./input.txt"))
-		util.CopyToClipboard(fmt.Sprintf("%v", ans))
-		fmt.Println("Output:", ans)
+	ans := part1(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+// 1240/668
+func part1(input string, part int) int {
+	instructions := strings.Split(input, "\n")
+	tileIsBlack := map[[6]int]bool{}
+	for _, inst := range instructions {
+		// tally the steps taken in each of the hex directions
+		var tallyDirections [6]int
+
+		// iterate thorugh the NONDELIMITED instructions
+		for i := 0; i < len(inst); {
+			// set the direction to the next two characters if possible
+			dir := string(inst[i])
+			if i+2 <= len(inst) {
+				dir = inst[i : i+2]
+			}
+
+			// if the two character direction is se, sw, nw or ne, both characters
+			// must be used, because using one is invalid ("n" or "s")
+			switch dir {
+			case "se", "sw", "nw", "ne":
+				tallyDirections[dirIndices[dir]]++
+				i += 2
+			default:
+				tallyDirections[dirIndices[dir[0:1]]]++
+				i++
+			}
+			// collapse and directions that cancel out, such as sw/nw -> w
+			tallyDirections = zeroOutHexDirections(tallyDirections)
+		}
+		// flip that tile
+		tileIsBlack[tallyDirections] = !tileIsBlack[tallyDirections]
 	}
-}
 
-func part1(input string) int {
-	parsed := parseInput(input)
-	_ = parsed
+	// for part 2, play game of life 100 times
+	if part == 2 {
+		for i := 0; i < 100; i++ {
+			// flip based on neighbors
+			nextState := map[[6]int]bool{}
 
-	return 0
-}
+			// collect all coordinates to check
+			toCheck := map[[6]int]bool{}
+			for i := 0; i < 6; i++ {
+				for k := range tileIsBlack {
+					k[i]++
+					toCheck[zeroOutHexDirections(k)] = true
+				}
+			}
 
-func part2(input string) int {
-	return 0
-}
-
-func parseInput(input string) (ans []int) {
-	lines := strings.Split(input, "\n")
-	for _, l := range lines {
-		ans = append(ans, mathutil.StrToInt(l))
+			for coord := range toCheck {
+				// count neighbors
+				var neighbors int
+				for i := 0; i < 6; i++ {
+					clone := coord // don't want to modify the original coord
+					clone[i]++     // generates the six directions around coord
+					clone = zeroOutHexDirections(clone)
+					if tileIsBlack[clone] {
+						neighbors++
+					}
+				}
+				// flipping logic:
+				// back with zero or more than 2 neighbors becomes white
+				if tileIsBlack[coord] && (neighbors == 0 || neighbors > 2) {
+					nextState[coord] = false
+				} else if !tileIsBlack[coord] && neighbors == 2 {
+					// white with exactly 2 neighbors becomes black
+					nextState[coord] = true
+				} else {
+					// stays the same
+					nextState[coord] = tileIsBlack[coord]
+				}
+			}
+			tileIsBlack = nextState
+		}
 	}
-	return ans
+
+	var count int
+	for _, b := range tileIsBlack {
+		if b {
+			count++
+		}
+	}
+
+	return count
+
+}
+
+var dirIndices = map[string]int{
+	"e":  0,
+	"se": 1,
+	"sw": 2,
+	"w":  3,
+	"nw": 4,
+	"ne": 5,
+}
+
+// borrowed from my 2017 day 11 code which calculated hex coordinate manhattan distances
+func zeroOutHexDirections(tally [6]int) [6]int {
+	// zero out opposite indices
+	for i := range tally {
+		if tally[i] != 0 {
+			oppositeIndex := (i + 3) % 6
+			smaller := mathutil.MinInt(tally[oppositeIndex], tally[i])
+			tally[oppositeIndex] -= smaller
+			tally[i] -= smaller
+		}
+	}
+
+	// handle neighbors which collapse into the current direction
+	// e.g. sw,se == s
+	for i := range tally {
+		toLeft := (i + 5) % 6
+		toRight := (i + 1) % 6
+		if tally[toLeft] > 0 && tally[toRight] > 0 {
+			smaller := mathutil.MinInt(tally[toLeft], tally[toRight])
+			tally[toLeft] -= smaller
+			tally[toRight] -= smaller
+			tally[i] += smaller
+		}
+	}
+
+	return tally
 }

2020/day24/main_test.go

@@ -2,38 +2,48 @@ package main
 
 import (
 	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
 )
 
+var example = `sesenwnenenewseeswwswswwnenewsewsw
+neeenesenwnwwswnenewnwwsewnenwseswesw
+seswneswswsenwwnwse
+nwnwneseeswswnenewneswwnewseswneseene
+swweswneswnenwsewnwneneseenw
+eesenwseswswnenwswnwnwsewwnwsene
+sewnenenenesenwsewnenwwwse
+wenwwweseeeweswwwnwwe
+wsweesenenewnwwnwsenewsenwwsesesenwne
+neeswseenwwswnwswswnw
+nenwswwsewswnenenewsenwsenwnesesenew
+enewnwewneswsewnwswenweswnenwsenwsw
+sweneswneswneneenwnewenewwneswswnese
+swwesenesewenwneswnwwneseswwne
+enesenwswwswneneswsenwnewswseenwsese
+wnwnesenesenenwwnenwsewesewsesesew
+nenewswnwewswnenesenwnesewesw
+eneswnwswnwsenenwnwnwwseeswneewsenese
+neswnwewnwnwseenwseesewsenwsweewe
+wseweeenwnesenwwwswnew`
+
 func Test_part1(t *testing.T) {
 	tests := []struct {
 		name  string
 		input string
+		part  int
 		want  int
 	}{
-		// {"actual", util.ReadFile("input.txt"), ACTUAL_ANSWER},
+		{"part1_example", example, 1, 10},
+		{"part1_actual", util.ReadFile("input.txt"), 1, 277},
+		{"part2_example", example, 2, 2208},
+		{"part2_actual", util.ReadFile("input.txt"), 2, 3531},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			if got := part1(tt.input); got != tt.want {
+			if got := part1(tt.input, tt.part); got != tt.want {
 				t.Errorf("part1() = %v, want %v", got, tt.want)
 			}
 		})
 	}
 }
-
-func Test_part2(t *testing.T) {
-	tests := []struct {
-		name  string
-		input string
-		want  int
-	}{
-		// {"actual", util.ReadFile("input.txt"), ACTUAL_ANSWER},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			if got := part2(tt.input); got != tt.want {
-				t.Errorf("part2() = %v, want %v", got, tt.want)
-			}
-		})
-	}
-}