Day 9: Part 1 and 2

Signed-off-by: James Griffin <james@unsupervised.ca>

nine/day_nine.go

@@ -0,0 +1,135 @@
+package nine
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+)
+
+type xmas struct {
+	preamble int
+	values   []int
+}
+
+func newXmas(filename string, preamble int) (*xmas, error) {
+	x := &xmas{}
+	if err := x.load(filename, preamble); err != nil {
+		return nil, err
+	}
+
+	return x, nil
+}
+
+func (x *xmas) load(filename string, preamble int) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		v, err := strconv.Atoi(scanner.Text())
+		if err != nil {
+			return err
+		}
+		x.values = append(x.values, v)
+	}
+
+	x.preamble = preamble
+
+	return nil
+}
+
+func (x *xmas) detectVulnerability() (int, error) {
+	index := x.preamble
+
+	if index > len(x.values) {
+		return 0, fmt.Errorf("Preamble is larger than the range of inputs")
+	}
+
+	var match bool
+
+	for index < len(x.values) {
+		searchable := x.values[index-x.preamble : index]
+		match = false
+
+		for i := 0; i < len(searchable); i++ {
+			for j := i + 1; j < len(searchable); j++ {
+				if searchable[i] == searchable[j] {
+					continue
+				}
+
+				if searchable[i]+searchable[j] == x.values[index] {
+					match = true
+				}
+			}
+		}
+
+		if !match {
+			return x.values[index], nil
+		}
+		index++
+	}
+
+	return 0, fmt.Errorf("Did not find a vulnerable value")
+}
+
+func (x *xmas) computeWeakness() (int, error) {
+	v, err := x.detectVulnerability()
+	if err != nil {
+		return 0, nil
+	}
+
+	for i := 0; i < len(x.values); i++ {
+		sum := x.values[i]
+		min := x.values[i]
+		max := x.values[i]
+
+		if x.values[i] == v {
+			return 0, fmt.Errorf("Did not find encryption weakness before %d", v)
+		}
+
+		for j := i + 1; j < len(x.values); j++ {
+			sum += x.values[j]
+			if x.values[j] < min {
+				min = x.values[j]
+			}
+
+			if x.values[j] > max {
+				max = x.values[j]
+			}
+
+			// We found the contiguous range of values!
+			if sum == v {
+				return min + max, nil
+			}
+
+			// This means that the contiguous run starting at i was a bust
+			if sum > v {
+				break
+			}
+		}
+	}
+
+	return 0, nil
+}
+
+// PartOne Find the first vulnerable number in the xmas chain
+func PartOne() string {
+	x, _ := newXmas("nine/input.txt", 25)
+
+	v, _ := x.detectVulnerability()
+
+	return fmt.Sprintf("The first vulnerable value in the XMAS input is %d", v)
+}
+
+// PartTwo Find the encryption weaknes in the xmas chain
+func PartTwo() string {
+	x, _ := newXmas("nine/input.txt", 25)
+
+	w, _ := x.computeWeakness()
+
+	return fmt.Sprintf("The encryption weakness in the XMAS input is %d", w)
+}