adventofcode/2020/day19.go

package main

import (
	"fmt"
	"os"
	"regexp"
	"strings"

	"git.annabunch.es/annabunches/adventofcode/2020/lib/util"
)

type Node struct {
	name     string
	children [][]*Node
}

const (
	STATE_RULES = iota
	STATE_DATA
)

func NewRule(name string) *Node {
	return &Node{
		name:     name,
		children: make([][]*Node, 0),
	}
}

var expandRe = regexp.MustCompile("[0-9]+")

func parseRule(name string, rules map[string]string) *Node {
	ruleString := rules[name]
	rule := NewRule(name)

	// and split into subrules
	for _, text := range strings.Split(ruleString, " | ") {
		// quoted strings are our terminals
		if strings.HasPrefix(text, "\"") {
			rule.name = strings.Trim(text, "\"") // change the rule to its "real" name
			continue
		}
		// everything else are rule indexes, which we turn into child nodes
		childSet := make([]*Node, 0)
		for _, childName := range strings.Split(text, " ") {
			childSet = append(childSet, parseRule(childName, rules))
		}
		rule.children = append(rule.children, childSet)
	}

	return rule
}

func parseInput(input []string) (map[string]bool, []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 {
		switch state {
		case STATE_RULES:
			if line == "" {
				state = STATE_DATA
				continue
			}

			reData := stateRe.FindAllStringSubmatch(line, 16)

			// rule name
			rules[reData[0][1]] = reData[0][2]
		case STATE_DATA:
			data = append(data, line)
		}
	}

	// now parse the rules we stored
	root = parseRule("0", rules)

	// now we expand the grammar - generate all possible strings in the language
	rawLanguage := expand(root)
	language := make(map[string]bool)
	for _, term := range rawLanguage {
		language[term] = true
	}

	return language, data
}

// Expand the list of all possible substrings starting with node
func expand(node *Node) []string {
	items := make([]string, 0)

	if len(node.children) == 0 {
		items = append(items, node.name)
		return items
	}

	for _, childSet := range node.children {
		newItems := make([][]string, 0)
		for _, child := range childSet {
			newItems = append(newItems, expand(child))
		}
		items = append(items, combine(newItems, "")...)
	}

	return items
}

// takes a list of strings and combines them in order, returning a list of
// strings of all possible combinations. Example:
// [["foo"] ["bar baz"] ["fnord"]] => ["foobarfnord" "foobazfnord"]
func combine(terms [][]string, acc string) []string {
	remaining := len(terms)
	results := make([]string, 0)

	if remaining == 0 {
		return results
	}

	for _, part := range terms[0] {
		newAcc := acc + part
		if remaining == 1 {
			results = append(results, newAcc)
			continue
		}

		results = append(results, combine(terms[1:], newAcc)...)
	}

	return results
}

func main() {
	// step := os.Args[1]
	values := util.InputParserStrings(os.Args[2])
	language, data := parseInput(values)
	count := 0
	for _, item := range data {
		if language[item] {
			count++
		}
	}
	fmt.Println(count)
}