Day 19.2 solved.

2020/day19.go

@@ -9,16 +9,16 @@ import (
 	"git.annabunch.es/annabunches/adventofcode/2020/lib/util"
 )
 
-type Node struct {
-	name     string
-	children [][]*Node
-}
-
 const (
 	STATE_RULES = iota
 	STATE_DATA
 )
 
+type Node struct {
+	name     string
+	children [][]*Node
+}
+
 func NewRule(name string) *Node {
 	return &Node{
 		name:     name,
@@ -26,8 +26,6 @@ func NewRule(name string) *Node {
 	}
 }
 
-var expandRe = regexp.MustCompile("[0-9]+")
-
 func parseRule(name string, rules map[string]string) *Node {
 	ruleString := rules[name]
 	rule := NewRule(name)
@@ -50,12 +48,11 @@ func parseRule(name string, rules map[string]string) *Node {
 	return rule
 }
 
-func parseInput(input []string) (map[string]bool, []string) {
+func parseInput(input []string) (map[string]string, []string) {
 	stateRe := regexp.MustCompile("^([0-9]+): (.*)$")
 	state := STATE_RULES
 	data := make([]string, 0)
 
-	var root *Node
 	rules := make(map[string]string) // unparsed rules
 
 	for _, line := range input {
@@ -75,8 +72,11 @@ func parseInput(input []string) (map[string]bool, []string) {
 		}
 	}
 
-	// now parse the rules we stored
+	return rules, data
-	root = parseRule("0", rules)
+}
+
+func createLanguage(rules map[string]string, rootName string) map[string]bool {
+	root := parseRule(rootName, rules)
 
 	// now we expand the grammar - generate all possible strings in the language
 	rawLanguage := expand(root)
@@ -85,7 +85,7 @@ func parseInput(input []string) (map[string]bool, []string) {
 		language[term] = true
 	}
 
-	return language, data
+	return language
 }
 
 // Expand the list of all possible substrings starting with node
@@ -133,14 +133,62 @@ func combine(terms [][]string, acc string) []string {
 }
 
 func main() {
-	// step := os.Args[1]
+	step := os.Args[1]
 	values := util.InputParserStrings(os.Args[2])
-	language, data := parseInput(values)
+	rules, data := parseInput(values)
 	count := 0
+
+	switch step {
+	case "1":
+		language := createLanguage(rules, "0")
 		for _, item := range data {
 			if language[item] {
 				count++
 			}
 		}
+	case "2":
+		// stub
+		// for step 2, our base rule expands to 0: 42{N} 31{M}, for N > M and N > 2
+		// we exploit this, along with the fact that all valid strings for 42 and 31 are
+		// 8 characters long, to "cheat" a little
+		matchLength := 8
+		left := createLanguage(rules, "42")
+		right := createLanguage(rules, "31")
+
+		for _, item := range data {
+			leftCount := 0
+			rightCount := 0
+			onLeft := true
+			valid := true // valid until proven otherwise
+
+			for index := 0; index < len(item); index += matchLength {
+				if len(item[index:]) < matchLength {
+					fmt.Println("Bad Length")
+					valid = false // wrong length alignment
+					break
+				}
+				subString := item[index : index+matchLength]
+				if onLeft {
+					if left[subString] {
+						leftCount++
+					} else {
+						onLeft = false
+					}
+				}
+				if !onLeft {
+					if right[subString] {
+						rightCount++
+					} else {
+						valid = false
+						break
+					}
+				}
+			}
+
+			if valid && leftCount > 1 && rightCount > 0 && leftCount > rightCount {
+				count++
+			}
+		}
+	}
 	fmt.Println(count)
 }