From adb8381b85cd5591cfb65d48859e210c65807e16 Mon Sep 17 00:00:00 2001
From: alexchao26 <alexchaoo@gmail.com>
Date: Wed, 16 Dec 2020 18:11:34 -0500
Subject: [PATCH] 2017-day14: knot hash, num islands, disk defrag :)

---
 2017/day14/main.go      | 181 ++++++++++++++++++++++++++++++++++++++++
 2017/day14/main_test.go |  28 +++++++
 2 files changed, 209 insertions(+)
 create mode 100644 2017/day14/main.go
 create mode 100644 2017/day14/main_test.go

diff --git a/2017/day14/main.go b/2017/day14/main.go
new file mode 100644
index 0000000..50b4545
--- /dev/null
+++ b/2017/day14/main.go
@@ -0,0 +1,181 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := diskDefrag(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func diskDefrag(input string, part int) int {
+	// 128x128 grid, free (false) or used (true)
+	diskState := make([][]bool, 128)
+	for i := 0; i < 128; i++ {
+		// hash input = {puzzleInput}-{0 through 127}
+		hashKeyRaw := fmt.Sprintf("%s-%d", input, i)
+		hexHash := calcHexKnotHash(hashKeyRaw)
+
+		// convert hex-hash to bits
+		bitsHash := transformHexToBits(hexHash)
+
+		if len(bitsHash) != 128 {
+			panic(fmt.Sprintf("expected bit hash to be 128 chars long, got %d", len(bitsHash)))
+		}
+
+		// set disk state for this row
+		diskState[i] = make([]bool, 128)
+		for b, bit := range bitsHash {
+			switch bit {
+			case '0':
+				diskState[i][b] = false // free
+			case '1':
+				diskState[i][b] = true // used
+			}
+		}
+	}
+
+	if part == 1 {
+		// tally up used cells for part 1
+		var usedCount int
+		for _, row := range diskState {
+			for _, b := range row {
+				if b {
+					usedCount++
+				}
+			}
+		}
+
+		return usedCount
+	}
+
+	return numIslands(diskState)
+}
+
+func parseInputASCII(input string) (ans []int) {
+	for _, char := range input {
+		ans = append(ans, int(char))
+	}
+	// add default lengths to end
+	ans = append(ans, 17, 31, 73, 47, 23)
+	return ans
+}
+
+func calcHexKnotHash(input string) string {
+	lengths := parseInputASCII(input)
+	nums := make([]int, 256)
+	for i := range nums {
+		nums[i] = i
+	}
+	var position, skipSize int
+
+	// 64 rounds of hashing
+	for i := 0; i < 64; i++ {
+		for _, length := range lengths {
+			if length > 0 {
+				nums = reverse(nums, position, position+length-1)
+			}
+			position += skipSize + length
+			position %= len(nums)
+			skipSize++
+		}
+	}
+
+	var denseHash []int
+	for i := 0; i < 16; i++ {
+		var xord int
+		for j := i * 16; j < (i+1)*16; j++ {
+			xord ^= nums[j]
+		}
+		denseHash = append(denseHash, xord)
+	}
+	var hexdHash string
+	for _, dense := range denseHash {
+		// use %x to get hexadecimal version & 02 ensures leading 0 if needed
+		hexdHash += fmt.Sprintf("%02x", dense)
+	}
+
+	return hexdHash
+}
+
+// helper for knot hasher
+func reverse(nums []int, left, right int) []int {
+	right %= len(nums)
+	if right < left {
+		right += len(nums)
+	}
+
+	for left < right {
+		leftModded := left % len(nums)
+		rightModded := right % len(nums)
+		nums[leftModded], nums[rightModded] = nums[rightModded], nums[leftModded]
+		left++
+		right--
+	}
+
+	return nums
+}
+
+func transformHexToBits(hex string) string {
+	var bits string
+	for _, char := range strings.Split(hex, "") {
+		// parse hex (base 16) to an int type (always base 10)
+		baseTen, err := strconv.ParseInt(char, 16, 64)
+		if err != nil {
+			panic("strconv error " + err.Error())
+		}
+		// add to bits string in binary form, 4 characters required
+		bits += fmt.Sprintf("%04b", baseTen)
+	}
+	return bits
+}
+
+// for part 2, iterative algo for counting the number of connected islands
+func numIslands(grid [][]bool) int {
+	var count int
+
+	directions := [4][2]int{
+		{0, 1},
+		{0, -1},
+		{1, 0},
+		{-1, 0},
+	}
+	for i := 0; i < len(grid); i++ {
+		for j := 0; j < len(grid[0]); j++ {
+			if grid[i][j] {
+				// zero out connected cells (i.e. zero out this island)
+				queue := [][2]int{{i, j}}
+				for len(queue) > 0 {
+					// pop off queue
+					currentRow, currentCol := queue[0][0], queue[0][1]
+					grid[currentRow][currentCol] = false
+					queue = queue[1:]
+
+					// check neighbors, add to queue if true
+					for _, dir := range directions {
+						nextRow, nextCol := currentRow+dir[0], currentCol+dir[1]
+						if nextRow >= 0 && nextRow < len(grid) && nextCol >= 0 && nextCol < len(grid[0]) {
+							if grid[nextRow][nextCol] {
+								queue = append(queue, [2]int{nextRow, nextCol})
+							}
+						}
+					}
+				}
+				count++
+			}
+		}
+	}
+
+	return count
+}
diff --git a/2017/day14/main_test.go b/2017/day14/main_test.go
new file mode 100644
index 0000000..0e5cc5f
--- /dev/null
+++ b/2017/day14/main_test.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func Test_diskDefrag(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{"example_part1", "flqrgnkx", 1, 8108},
+		{"actual_part1", util.ReadFile("input.txt"), 1, 8204},
+		{"example_part2", "flqrgnkx", 2, 1242},
+		// {"actual_part2", util.ReadFile("input.txt"), 2, 1089},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := diskDefrag(tt.input, tt.part); got != tt.want {
+				t.Errorf("diskDefrag() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}