From 4e10039bfb84e74e6f7f3871692281eea6c2ca1d Mon Sep 17 00:00:00 2001
From: Anna Wiggins <annabunches@gmail.com>
Date: Fri, 11 Dec 2020 10:00:29 +0000
Subject: [PATCH] Day 9 solution.

---
 2020/day09.go                   | 121 ++++++++++++++++++++++++++++++++
 2020/lib/fileutils/fileutils.go |  21 ++++++
 2 files changed, 142 insertions(+)
 create mode 100644 2020/day09.go

diff --git a/2020/day09.go b/2020/day09.go
new file mode 100644
index 0000000..7e0308b
--- /dev/null
+++ b/2020/day09.go
@@ -0,0 +1,121 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"git.annabunch.es/annabunches/adventofcode/2020/lib/fileutils"
+)
+
+type SumTracker map[int][]Tuple
+
+type Tuple struct {
+	First, Second int
+}
+
+func checkValue(sumMap SumTracker, value int) bool {
+	_, ok := sumMap[value]
+	return ok
+}
+
+func addSums(sumMap SumTracker, values []int, index int) {
+	for i := index - 25; i < index; i++ {
+		sum := values[i] + values[index]
+		sumMap[sum] = append(sumMap[sum], Tuple{First: i, Second: index})
+	}
+}
+
+func removeSums(sumMap SumTracker, index int) {
+	for sum, tupleList := range sumMap {
+		newList := make([]Tuple, 0)
+
+		for _, tuple := range tupleList {
+			if !(tuple.First == index || tuple.Second == index) {
+				newList = append(newList, tuple)
+			}
+		}
+		sumMap[sum] = newList
+		if len(sumMap[sum]) == 0 {
+			delete(sumMap, sum)
+		}
+	}
+}
+
+// find and return the slice of values that sums to sum
+func findListSum(values []int, sum int) []int {
+	i := 0
+	j := 0
+	total := 0
+	for total != sum {
+		for total < sum {
+			if j >= len(values) {
+				log.Panicf("Couldn't find a match.")
+			}
+
+			total += values[j]
+			j++
+		}
+
+		for total > sum {
+			total -= values[i]
+			i++
+		}
+	}
+
+	return values[i:j]
+}
+
+func sumMinMax(values []int) int {
+	min := values[0]
+	max := values[0]
+
+	for _, x := range values {
+		if x < min {
+			min = x
+		}
+
+		if x > max {
+			max = x
+		}
+	}
+
+	return min + max
+}
+
+func main() {
+	step := os.Args[1]
+	values := fileutils.InputParserInts(os.Args[2])
+
+	var badValue int
+	sumMap := make(SumTracker)
+
+	// build up the preamble
+	for i := 0; i < 25; i++ {
+		for j := i + 1; j < 25; j++ {
+			sum := values[i] + values[j]
+			if _, ok := sumMap[sum]; !ok {
+				sumMap[sum] = make([]Tuple, 0)
+			}
+
+			sumMap[sum] = append(sumMap[sum], Tuple{First: i, Second: j})
+		}
+	}
+
+	for i := 25; i < len(values); i++ {
+		if !checkValue(sumMap, values[i]) {
+			badValue = values[i]
+			break
+		}
+		addSums(sumMap, values, i)
+		removeSums(sumMap, i-25)
+	}
+
+	switch step {
+	case "1":
+		fmt.Println(badValue)
+	case "2":
+		sumValues := findListSum(values, badValue)
+		fmt.Println(sumMinMax(sumValues))
+	}
+}
diff --git a/2020/lib/fileutils/fileutils.go b/2020/lib/fileutils/fileutils.go
index 6a56caa..8da287d 100644
--- a/2020/lib/fileutils/fileutils.go
+++ b/2020/lib/fileutils/fileutils.go
@@ -9,6 +9,27 @@ import (
 	"strings"
 )
 
+func InputParserInts(filename string) []int {
+	file, err := os.Open(filename)
+	defer file.Close()
+
+	if err != nil {
+		log.Panicf(err.Error())
+	}
+
+	values := make([]int, 0)
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		x, err := strconv.Atoi(scanner.Text())
+		if err != nil {
+			log.Panicf(err.Error())
+		}
+		values = append(values, x)
+	}
+
+	return values
+}
+
 func InputParserIntMap(filename string) map[int]bool {
 	file, err := os.Open(filename)
 	defer file.Close()