value_source.go

package cli

import (
	"fmt"
	"os"
	"strings"
)

// ValueSource is a source which can be used to look up a value,
// typically for use with a cli.Flag
type ValueSource interface {
	fmt.Stringer
	fmt.GoStringer

	// Lookup returns the value from the source and if it was found
	// or returns an empty string and false
	Lookup() (string, bool)
}

// ValueSourceChain contains an ordered series of ValueSource that
// allows for lookup where the first ValueSource to resolve is
// returned
type ValueSourceChain struct {
	Chain []ValueSource
}

func (vsc *ValueSourceChain) String() string {
	s := []string{}

	for _, vs := range vsc.Chain {
		s = append(s, vs.String())
	}

	return strings.Join(s, ",")
}

func (vsc *ValueSourceChain) GoString() string {
	s := []string{}

	for _, vs := range vsc.Chain {
		s = append(s, vs.GoString())
	}

	return fmt.Sprintf("&ValueSourceChain{Chain:{%[1]s}}", strings.Join(s, ","))
}

func (vsc *ValueSourceChain) Lookup() (string, bool) {
	s, _, ok := vsc.LookupWithSource()
	return s, ok
}

func (vsc *ValueSourceChain) LookupWithSource() (string, ValueSource, bool) {
	for _, src := range vsc.Chain {
		if value, found := src.Lookup(); found {
			return value, src, true
		}
	}

	return "", nil, false
}

// envVarValueSource encapsulates a ValueSource from an environment variable
type envVarValueSource struct {
	Key string
}

func (e *envVarValueSource) Lookup() (string, bool) {
	return os.LookupEnv(strings.TrimSpace(string(e.Key)))
}

func (e *envVarValueSource) String() string { return fmt.Sprintf("environment variable %[1]q", e.Key) }
func (e *envVarValueSource) GoString() string {
	return fmt.Sprintf("&envVarValueSource{Key:%[1]q}", e.Key)
}

// EnvVars is a helper function to encapsulate a number of
// envVarValueSource together as a ValueSourceChain
func EnvVars(keys ...string) ValueSourceChain {
	vsc := ValueSourceChain{Chain: []ValueSource{}}

	for _, key := range keys {
		vsc.Chain = append(vsc.Chain, &envVarValueSource{Key: key})
	}

	return vsc
}

// fileValueSource encapsulates a ValueSource from a file
type fileValueSource struct {
	Path string
}

func (f *fileValueSource) Lookup() (string, bool) {
	data, err := os.ReadFile(f.Path)
	return string(data), err == nil
}

func (f *fileValueSource) String() string { return fmt.Sprintf("file %[1]q", f.Path) }
func (f *fileValueSource) GoString() string {
	return fmt.Sprintf("&fileValueSource{Path:%[1]q}", f.Path)
}

// Files is a helper function to encapsulate a number of
// fileValueSource together as a ValueSourceChain
func Files(paths ...string) ValueSourceChain {
	vsc := ValueSourceChain{Chain: []ValueSource{}}

	for _, path := range paths {
		vsc.Chain = append(vsc.Chain, &fileValueSource{Path: path})
	}

	return vsc
}