2017-day15: some bitwise stuff to compare binary numbers

2017/day15/main.go

@@ -0,0 +1,79 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/mathutil"
+	"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 := duelingGenerators(util.ReadFile("./input.txt"), part)
+	fmt.Println("Output:", ans)
+}
+
+func duelingGenerators(input string, part int) int {
+	values := parseInput(input)
+
+	factors := []int{16807, 48271}
+	divisor := 2147483647
+
+	// set criteria for passing a value to judge, and rounds based on part num
+	criteria := []int{1, 1}
+	rounds := 40000000
+	if part == 2 {
+		rounds = 5000000
+		criteria = []int{4, 8}
+	}
+
+	var judgeCount int
+	for i := 0; i < rounds; i++ {
+		for i, v := range values {
+			values[i] = getNextValue(v, factors[i], divisor, criteria[i])
+		}
+
+		// do the back 16 line up ?
+		// XOR them together, the last 16 should not have bits because they should
+		// should be zero'ed out.
+		// (XOR result) % (2^16) will be zero (i.e. no remainder) if back 16 match
+		compareVal := values[0] ^ values[1]
+		twoPow16 := 1 << 16
+		if (compareVal % twoPow16) == 0 {
+			judgeCount++
+		}
+	}
+
+	return judgeCount
+}
+
+// iterate until a value that can be passed to the judge is reached
+// for part 1 the criteria will be 1, so only one iteration is made
+// for part 2 it will be 4 or 8, so the loop will run until a good value is reached
+func getNextValue(value, factor, divisor, criteria int) int {
+	// run at least once
+	value *= factor
+	value %= divisor
+
+	for value%criteria != 0 {
+		value *= factor
+		value %= divisor
+	}
+
+	return value
+}
+
+func parseInput(input string) (ans []int) {
+	lines := strings.Split(input, "\n")
+	for _, l := range lines {
+		split := strings.Split(l, " starts with ")
+		ans = append(ans, mathutil.StrToInt(split[1]))
+	}
+	return ans
+}