Most space-efficient MQTT topic generation. This one skips the tree building step entirely and goes straight to emitting. Compare with V1 and V2 gists

mqtt_generate_topics_V3.go

package main

import (
	"fmt"
	"strings"
)

func generateTopicStrings(tokens []string) (generatedTopics []string) {
	// the expected no. of generated topics. See either of the previous
	// gists for explanation as to why this formula is used
	noTopics := (1 << len(tokens)) + (2 << len(tokens)) - 1

	generatedTopics = make([]string, 0, noTopics)

	// temp
	temp := make([]string, 0, len(tokens)+1)

	// string builder len
	strBLen := 0
	for _, t := range tokens {
		strBLen += len(t)
	}
	strBLen += (len(tokens) + 1)

	// 'traverse' while emitting
	traverse(tokens, temp, func(ts []string) {
		var strB strings.Builder
		strB.Grow(strBLen)

		strB.WriteString(ts[0])
		for _, t := range ts[1:] {
			strB.WriteByte('/')
			strB.WriteString(t)
		}
		generatedTopics = append(generatedTopics, strB.String())
	})

	return generatedTopics
}

func traverse(tokens []string, temp []string, emitTokens func([]string)) {
	if len(tokens) == 0 {
		emitTokens(temp)
	} else {
		traverse(tokens[1:], append(temp, tokens[0]), emitTokens)
		traverse(tokens[1:], append(temp, "+"), emitTokens)
	}
	emitTokens(append(temp, "#"))
}

func main() {
	// assume a given topic 'foo/bar/quz' is already split
	// into its constituent tokens
	tokens := []string{"foo", "bar", "quz"}
	ts := generateTopicStrings(tokens)
	for _, t := range ts {
		fmt.Println(t)
	}
}