Solution for 2024, day 13

Author: lucianoq

2024/13/Makefile

@@ -0,0 +1,19 @@
+input:
+	http "https://adventofcode.com/2024/day/13/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input

2024/13/common.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+type C struct{ x, y int }
+
+type Config struct {
+	A, B, P C
+}
+
+func parseConfigs() []Config {
+	scanner := bufio.NewScanner(os.Stdin)
+
+	configs := []Config{}
+	config := Config{}
+	for scanner.Scan() {
+		line := scanner.Text()
+
+		if line == "" {
+			config = Config{}
+		}
+
+		var x, y int
+		switch {
+		case strings.HasPrefix(line, "Button A"):
+			fmt.Sscanf(line, "Button A: X+%d, Y+%d", &x, &y)
+			config.A = C{x, y}
+
+		case strings.HasPrefix(line, "Button B"):
+			fmt.Sscanf(line, "Button B: X+%d, Y+%d", &x, &y)
+			config.B = C{x, y}
+
+		case strings.HasPrefix(line, "Prize"):
+			fmt.Sscanf(line, "Prize: X=%d, Y=%d", &x, &y)
+			config.P = C{x, y}
+			configs = append(configs, config)
+		}
+	}
+	return configs
+}
+
+func solve(c Config) int {
+	// { Px = a*Ax + b*Bx
+	// { Py = a*Ay + b*By
+	//-------------------------
+	// { a = (Px - b*Bx) / Ax
+	// { b = (Py - a*Ay) / By
+	//
+	// b = ( Py - ((Px-b*Bx)/Ax)*Ay ) / By
+	// b*Ay*By = Ax*Py - Ay*Px + b*Ay*Bx
+	// b = ( Ax*Py - Ay*Px ) / ( Ax*By - Ay*Bx )
+	b := (c.A.x*c.P.y - c.A.y*c.P.x) / (c.A.x*c.B.y - c.A.y*c.B.x)
+	a := (c.P.x - b*c.B.x) / c.A.x
+
+	if a*c.A.x+b*c.B.x == c.P.x && a*c.A.y+b*c.B.y == c.P.y {
+		return 3*a + b
+	}
+
+	return 0
+}

2024/13/main1.go

@@ -0,0 +1,13 @@
+package main
+
+import "fmt"
+
+func main() {
+	configs := parseConfigs()
+
+	tokens := 0
+	for _, config := range configs {
+		tokens += solve(config)
+	}
+	fmt.Println(tokens)
+}

2024/13/main2.go

@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+func main() {
+	configs := parseConfigs()
+
+	for i := range configs {
+		configs[i].P.x += 10000000000000
+		configs[i].P.y += 10000000000000
+	}
+
+	tokens := 0
+	for _, config := range configs {
+		tokens += solve(config)
+	}
+	fmt.Println(tokens)
+}