// Package envflag is an extremely small bit of code to make configuration
// via environment variables a little bit less of a hassle when using
// the flag package in the standard library.
//
// Example:
//
//	package main
//
//	import (
//		"flag"
//		"fmt"
//		"os"
//
//		"riedstra.dev/go/envflag"
//	)
//
//	func main() {
//		fl := flag.NewFlagSet("envflag", flag.ExitOnError)
//
//		var aFlag string = "some default"
//		fmt.Println("aFlag:", aFlag)
//
//		envflag.String(fl, &aFlag, "a", "FLAG_A", "Set's the value of -a")
//
//		// By default, command line flags override environment vars.
//		fl.Parse(os.Args[1:])
//
//		fmt.Println("aFlag:", aFlag)
//	}
//
// Known caveat, variables set in the environment will override what the flag
// package sees as the "default" value.
package envflag

import (
	"flag"
	"fmt"
	"os"
	"strconv"
)

// String is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func String(fl *flag.FlagSet, p *string, name, envvar, usage string) {
	if v := os.Getenv(envvar); v != "" {
		*p = v
	}

	fl.StringVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))
}

// Bool is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func Bool(fl *flag.FlagSet, p *bool, name, envvar, usage string) error {
	if v := os.Getenv(envvar); v != "" {
		res, err := strconv.ParseBool(v)

		if err != nil {
			return fmt.Errorf("Bool: cannot parse '%s=%s', %w",
				envvar, v, err)
		}

		*p = res
	}

	fl.BoolVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))

	return nil
}

// Int is a convent way to set value from environment variable,
// and allow override when a command line flag is set. It's assumed `p` is
// not nil.
func Int(fl *flag.FlagSet, p *int, name, envvar, usage string) error {
	if v := os.Getenv(envvar); v != "" {
		res, err := strconv.ParseInt(v, 10, 32)
		if err != nil {
			return fmt.Errorf("Int: cannot parse '%s=%s', %w",
				envvar, v, err)
		}

		*p = int(res)
	}

	fl.IntVar(p, name, *p, fmt.Sprintf("%s (Environ: '%s')", usage, envvar))

	return nil
}