day 7

day-7/main.go

@@ -0,0 +1,149 @@
+/* Day 7: The Treachery of Whales */
+package main
+
+import (
+	"AoC2021/utils/log"
+	"AoC2021/utils/timer"
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func check(e error) {
+	if e != nil {
+		log.Println(log.DEBUG, "Oh snap!")
+		panic(e)
+	}
+}
+
+func main() {
+	log.Println(log.NORMAL, "--- Day 7: The Treachery of Whales ---")
+	timer.Start()
+	part1()
+	timer.Tick()
+	log.Println(log.NORMAL)
+
+	part2()
+	timer.Tick()
+	log.Println(log.NORMAL)
+}
+
+func part1() {
+	log.Println(log.NORMAL, "* Part 1 *")
+	log.Println(log.NORMAL, " Goal: Determine the horizontal position that the crabs can align to using the least fuel possible")
+	log.Println(log.NORMAL, " Answer: How much fuel must they spend to align to that position?")
+
+	doTheNeedful(calcLinearFuelCost)
+}
+
+func part2() {
+	log.Println(log.NORMAL, "* Part 2 *")
+	log.Println(log.NORMAL, " Goal: Determine the horizontal position that the crabs can align to using the least fuel possible so they can make you an escape route!")
+	log.Println(log.NORMAL, " Answer: How much fuel must they spend to align to that position?")
+
+	doTheNeedful(calcSpecialFuelCost)
+}
+
+func doTheNeedful(calcFuelCost func([]int, int) int) {
+	input, err := os.Open("./input.txt")
+	check(err)
+	defer input.Close()
+
+	scanner := bufio.NewScanner(input)
+	lines := 0
+
+	var crabsPos []int
+
+	// Parse the crab positions
+	for scanner.Scan() {
+		crabStrs := strings.Split(scanner.Text(), ",")
+		for _, crabStr := range crabStrs {
+			value, err := strconv.Atoi(crabStr)
+			check(err)
+			crabsPos = append(crabsPos, value)
+
+		}
+		lines++
+	}
+
+	// Find the optimal horizontal position
+	// We're gonna brute force it for now and try to come up with a better method later
+	// Basically do an integral manually. Starting from the lowest hPos calculate the cost
+	// and move up until we are no longer reducing the cost
+	minPos := minVal(crabsPos)
+	maxPos := maxVal(crabsPos)
+
+	bestPos := minPos
+	bestCost := calcFuelCost(crabsPos, minPos)
+
+	for pos := bestPos + 1; pos < maxPos; pos++ {
+		curCost := calcFuelCost(crabsPos, pos)
+		// we should have started from an unoptimized position. as we move closer to the center, the cost should reduce
+		// once the cost starts increasing again we know that the *last* value was the optimial one
+		if curCost > bestCost {
+			bestPos = pos - 1
+			break
+		} else {
+			// we found a better cost
+			bestCost = curCost
+		}
+	}
+
+	log.Printf(log.NORMAL, "Read %d crabs on %d lines from input between positions %d and %d\n", len(crabsPos), lines, minPos, maxPos)
+	log.Printf(log.NORMAL, "The best position is %d with a cost of %d\n", bestPos, bestCost)
+}
+
+func calcLinearFuelCost(positions []int, hPos int) int {
+	cost := 0
+	for _, p := range positions {
+		cost += abs(p - hPos)
+	}
+	return cost
+}
+
+func calcSpecialFuelCost(positions []int, hPos int) int {
+	cost := 0
+	for _, p := range positions {
+		shift := abs(p - hPos)
+		cost += (shift * (shift + 1)) / 2
+	}
+
+	return cost
+}
+
+func abs(x int) int {
+	if x >= 0 {
+		return x
+	} else {
+		return -x
+	}
+}
+
+func minVal(a []int) int {
+	if len(a) == 0 {
+		panic("minVal Passed empty array")
+	}
+
+	min := 0
+	for idx, val := range a {
+		if idx == 0 || val < min {
+			min = val
+		}
+	}
+	return min
+}
+
+func maxVal(a []int) int {
+	if len(a) == 0 {
+		panic("minVal Passed empty array")
+	}
+
+	min := 0
+	for idx, val := range a {
+		if idx == 0 || val > min {
+			min = val
+		}
+	}
+	return min
+}