env.go

package config

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

/*
EnvLoader implements a Loader type to parse settings from environment variables.

Environment variable names are generated by expanding the path for each setting
to a list of optional prefixes and ending with the setting's name:
	opts := struct {
		HTMLParser struct {
			ElementIDs []string
			Name       string
		}
	}{}
results in paths
	["HTMLParser", "ElementIDs"]
	["HTMLParser", "Name"]
Each element is then replaced with the result of passing it to SplitName:
	["HTML", "Parser", "Element", "IDs"]
	["HTML", "Parser", "Name"]
Any dashes or underscores in each word are removed, the words capitalized, and
finally joined with underscores to produce the final names:
	HTML_PARSER_ELEMENT_IDS
	HTML_PARSER_NAME

The zero value is ready to use.
*/
type EnvLoader struct {
	settings []Setting
}

// Name returns the name of the loader. It is always "env".
func (*EnvLoader) Name() string {
	return "env"
}

/*
Init initializes the loader with the given settings.

Init must be called before Load.
*/
func (el *EnvLoader) Init(settings []Setting) {
	names := make(map[string]struct{}, len(settings))
	for i := range el.settings {
		name := el.transformName(el.settings[i].Path)
		if _, ok := names[name]; ok {
			panic(fmt.Sprintf(
				"duplicate environment variable name %s for %s",
				name, el.settings[i].Path,
			))
		}
		names[name] = struct{}{}
	}
	el.settings = settings
}

/*
Load attempts to parse the configured settings from environment variables.

The returned error, if non-nil, will be of type Errors. Each element will be
of type *ConversionError or *ValidationError.
*/
func (el *EnvLoader) Load() error {
	var errs Errors
	for i := range el.settings {
		if val := os.Getenv(el.transformName(el.settings[i].Path)); val != "" {
			if err := el.settings[i].Setter.Set(val); err != nil {
				switch err := err.(type) {
				case *ConversionError:
					err.Path = el.settings[i].Path
				case *ValidationError:
					err.Path = el.settings[i].Path
				}
				errs.Append(err)
			}
		}
	}
	return errs.AsError()
}

/*
Usage returns a string with a list of environment variables names and their
descriptions.
*/
func (el *EnvLoader) Usage() string {
	var b strings.Builder
	b.WriteString("Environment Variables:\n")
	for i := range el.settings {
		b.WriteString("  ")
		b.WriteString(el.transformName(el.settings[i].Path))
		b.WriteString("=")
		b.WriteString(FriendlyTypeName(el.settings[i].Setter.Get()))
		usage := strings.Replace(
			el.settings[i].Tag.Get("help"), "\n", "\n    \t", -1,
		)
		z := isZeroValue(el.settings[i].Setter)
		if usage != "" || !z {
			b.WriteString("\n    \t")
			b.WriteString(usage)
			if !z {
				b.WriteString(" (default ")
				b.WriteString(el.settings[i].Setter.String())
				b.WriteString(")")
			}
		}
		b.WriteString("\n")
	}
	return b.String()
}

func (el *EnvLoader) transformName(path *Path) string {
	elements := path.Elements()
	for i := range elements {
		parts := SplitName(elements[i])
		for i := range parts {
			parts[i] = strings.Trim(strings.ToUpper(parts[i]), "-_")
		}
		elements[i] = strings.Join(parts, "_")
	}
	return strings.Join(elements, "_")
}