refactor config building into separate package for better organization

config/config.go

@@ -0,0 +1,123 @@
+package config
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strconv"
+	"time"
+)
+
+type configOption[T any] struct {
+	flagName         string
+	flagDefaultValue T
+	flagHelpBase     string
+	envVarName       string
+}
+
+type configOptions struct {
+	userID                       configOption[string]
+	token                        configOption[string]
+	room                         configOption[string]
+	printHomeserverMemberCount   configOption[bool]
+	homeserverVersionInfoTimeout configOption[time.Duration]
+}
+
+type Config struct {
+	UserID                       string
+	Token                        string
+	Room                         string
+	PrintHomeserverMemberCount   bool
+	HomeserverVersionInfoTimeout time.Duration
+}
+
+var configOpts = configOptions{
+	userID:                       configOption[string]{"user-id", "", "The Matrix user ID to use.", "MRVC_USER_ID"},
+	token:                        configOption[string]{"token", "", "The Matrix access token to use.", "MRVC_TOKEN"},
+	room:                         configOption[string]{"room", "", "The Matrix room to check.", "MRVC_ROOM"},
+	printHomeserverMemberCount:   configOption[bool]{"print-homeserver-member-count", false, "Print the member count for each homeserver.", "MRVC_PRINT_HOMESERVER_MEMBER_COUNT"},
+	homeserverVersionInfoTimeout: configOption[time.Duration]{"homeserver-version-info-timeout", time.Second * 5, "Timeout for getting the homeservers version information per homeserver.", "MRVC_HOMESERVER_VERSION_INFO_TIMEOUT"},
+}
+
+func (configOpt configOption[T]) getFlagHelp() string {
+	return fmt.Sprintf("%s (EnvVar: %s)", configOpt.flagHelpBase, configOpt.envVarName)
+}
+
+func (configOpt configOption[T]) getFlagArgs() (string, T, string) {
+	return configOpt.flagName, configOpt.flagDefaultValue, configOpt.getFlagHelp()
+}
+
+// Functions for getting config values from flags and environment variables.
+// Flags take precedence over environment variables.
+func (configOpt configOption[T]) getConfigValueWithDefault(configFlag *T, visitedFlags map[string]bool, envVarParser func(string) T) T {
+	if visitedFlags[configOpt.flagName] {
+		return *configFlag
+	} else if envVar, ok := os.LookupEnv(configOpt.envVarName); ok {
+		return envVarParser(envVar)
+	} else {
+		return configOpt.flagDefaultValue
+	}
+}
+
+// This function can be used to ensure some configuration options got explicitly set.
+func (configOpt configOption[T]) getConfigValueWithError(configFlag *T, visitedFlags map[string]bool, envVarParser func(string) T) (T, error) {
+	if visitedFlags[configOpt.flagName] {
+		return *configFlag, nil
+	} else if envVar, ok := os.LookupEnv(configOpt.envVarName); ok {
+		return envVarParser(envVar), nil
+	} else {
+		return configOpt.flagDefaultValue, errors.New("no command-line flag or environment variable set")
+	}
+}
+
+var userIdFlag = flag.String(configOpts.userID.getFlagArgs())
+var tokenFlag = flag.String(configOpts.token.getFlagArgs())
+var roomFlag = flag.String(configOpts.room.getFlagArgs())
+var printHomeserverMemberCountFlag = flag.Bool(configOpts.printHomeserverMemberCount.getFlagArgs())
+var homeserverVersionInfoTimeoutFlag = flag.Duration(configOpts.homeserverVersionInfoTimeout.getFlagArgs())
+
+func Get() Config {
+	flag.Parse()
+
+	var config Config
+
+	visitedFlags := make(map[string]bool)
+	flag.Visit(func(flag *flag.Flag) {
+		visitedFlags[flag.Name] = true
+	})
+
+	var err error
+
+	config.UserID, err = configOpts.userID.getConfigValueWithError(userIdFlag, visitedFlags, func(envVar string) string { return envVar })
+	if err != nil {
+		log.Fatal("A Matrix user ID must be provided.")
+	}
+	config.Token, err = configOpts.token.getConfigValueWithError(tokenFlag, visitedFlags, func(envVar string) string { return envVar })
+	if err != nil {
+		log.Fatal("A Matrix access token must be provided.")
+	}
+	config.Room, err = configOpts.room.getConfigValueWithError(roomFlag, visitedFlags, func(envVar string) string { return envVar })
+	if err != nil {
+		log.Fatal("A Matrix room must be provided.")
+	}
+	config.PrintHomeserverMemberCount = configOpts.printHomeserverMemberCount.getConfigValueWithDefault(printHomeserverMemberCountFlag, visitedFlags, func(envVar string) bool {
+		parsedEnvVar, err := strconv.ParseBool(envVar)
+		if err != nil {
+			log.Printf("Error parsing %s:\n", configOpts.printHomeserverMemberCount.envVarName)
+			log.Fatal(err)
+		}
+		return parsedEnvVar
+	})
+	config.HomeserverVersionInfoTimeout = configOpts.homeserverVersionInfoTimeout.getConfigValueWithDefault(homeserverVersionInfoTimeoutFlag, visitedFlags, func(envVar string) time.Duration {
+		parsedEnvVar, err := time.ParseDuration(envVar)
+		if err != nil {
+			log.Printf("Error parsing %s:\n", configOpts.homeserverVersionInfoTimeout.envVarName)
+			log.Fatal(err)
+		}
+		return parsedEnvVar
+	})
+
+	return config
+}