From fcdf31150bf201472f54c9852699418bc339ba97 Mon Sep 17 00:00:00 2001
From: Alex Chao <alexchaoo@gmail.com>
Date: Tue, 7 Dec 2021 00:24:41 -0500
Subject: [PATCH] 2021 day7: oof integer rounding killed me... but thank
 goodness for a bit of tdd

---
 2021/day07/main.go      | 84 +++++++++++++++++++++++++++++++++++++++++
 2021/day07/main_test.go | 81 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 165 insertions(+)
 create mode 100644 2021/day07/main.go
 create mode 100644 2021/day07/main_test.go

diff --git a/2021/day07/main.go b/2021/day07/main.go
new file mode 100644
index 0000000..d382d68
--- /dev/null
+++ b/2021/day07/main.go
@@ -0,0 +1,84 @@
+package main
+
+import (
+	_ "embed"
+	"flag"
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+//go:embed input.txt
+var input string
+
+func init() {
+	// do this in init (not main) so test file has same input
+	input = strings.TrimRight(input, "\n")
+	if len(input) == 0 {
+		panic("empty input.txt file")
+	}
+}
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := calcMinFuel(input, part)
+	util.CopyToClipboard(fmt.Sprintf("%v", ans))
+	fmt.Println("Output:", ans)
+}
+
+func calcMinFuel(input string, part int) int {
+	var startingPositions []int
+	for _, val := range strings.Split(input, ",") {
+		startingPositions = append(startingPositions, cast.ToInt(val))
+	}
+
+	// horizontal positions of each crab
+	// limited fuel, so all horizontal positions need to match
+	// part1: 1 fuel to move 1 space
+	// part2: 1 fuel to move 1st space, 2 for 2nd, 3 for 3rd, etc...
+
+	// find bounds to run loop through
+	lowest, highest := startingPositions[0], startingPositions[0]
+	for _, v := range startingPositions {
+		if v < lowest {
+			lowest = v
+		}
+		if v > highest {
+			highest = v
+		}
+	}
+
+	bestFuelCost := math.MaxInt64
+	for finalIndex := lowest; finalIndex <= highest; finalIndex++ {
+		// calculate diffs to all
+		cost := 0
+		for _, startIndex := range startingPositions {
+			horizDiff := int(math.Abs(float64(startIndex - finalIndex)))
+			if part == 1 {
+				cost += horizDiff
+			} else {
+				cost += calcSummationFromOneToEnd(horizDiff)
+			}
+		}
+
+		if cost < bestFuelCost {
+			bestFuelCost = cost
+		}
+	}
+
+	return bestFuelCost
+}
+
+func calcSummationFromOneToEnd(end int) int {
+	// 1 2 3 4 5
+	// (1 + 5) * 2.5
+	ans := float64(end+1) * float64(end) / 2
+	return int(ans)
+}
diff --git a/2021/day07/main_test.go b/2021/day07/main_test.go
new file mode 100644
index 0000000..b134212
--- /dev/null
+++ b/2021/day07/main_test.go
@@ -0,0 +1,81 @@
+package main
+
+import (
+	_ "embed"
+	"testing"
+)
+
+var example = `16,1,2,0,4,2,7,1,2,14`
+
+func Test_calcMinFuel(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		part  int
+		want  int
+	}{
+		{
+			name:  "example",
+			input: example,
+			part:  1,
+			want:  37,
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  1,
+			want:  329389,
+		},
+		{
+			name:  "example",
+			input: example,
+			part:  2,
+			want:  168,
+		},
+		{
+			name:  "actual",
+			input: input,
+			part:  2,
+			want:  86397080,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := calcMinFuel(tt.input, tt.part); got != tt.want {
+				t.Errorf("calcMinFuel() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_calcSummationFromOneToEnd(t *testing.T) {
+	// auto generated these tests because math is rough...
+	type args struct {
+		end int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			args: args{end: 4},
+			want: 10,
+		},
+		{
+			args: args{end: 5},
+			want: 15,
+		},
+		{
+			args: args{end: 6},
+			want: 21,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := calcSummationFromOneToEnd(tt.args.end); got != tt.want {
+				t.Errorf("calcSummationFromOneToEnd() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}