From f2a0efe97cd500703f83cee2b576c019dc8fdf63 Mon Sep 17 00:00:00 2001
From: Knut Ahlers <knut@ahlers.me>
Date: Thu, 2 Aug 2018 12:02:13 +0200
Subject: [PATCH] Add AutoEnv feature

to automatically derive environment variable names from struct field
names

Signed-off-by: Knut Ahlers <knut@ahlers.me>
---
 autoenv.go      | 64 +++++++++++++++++++++++++++++++++++++++++++++++++
 autoenv_test.go | 29 ++++++++++++++++++++++
 config.go       | 17 +++++++++++--
 3 files changed, 108 insertions(+), 2 deletions(-)
 create mode 100644 autoenv.go
 create mode 100644 autoenv_test.go

diff --git a/autoenv.go b/autoenv.go
new file mode 100644
index 0000000..70ab679
--- /dev/null
+++ b/autoenv.go
@@ -0,0 +1,64 @@
+package rconfig
+
+import "strings"
+
+type characterClass [2]rune
+
+func (c characterClass) Contains(r rune) bool {
+	return c[0] <= r && c[1] >= r
+}
+
+type characterClasses []characterClass
+
+func (c characterClasses) Contains(r rune) bool {
+	for _, cc := range c {
+		if cc.Contains(r) {
+			return true
+		}
+	}
+	return false
+}
+
+var (
+	charGroupUpperLetter = characterClass{'A', 'Z'}
+	charGroupLowerLetter = characterClass{'a', 'z'}
+	charGroupNumber      = characterClass{'0', '9'}
+	charGroupLowerNumber = characterClasses{charGroupLowerLetter, charGroupNumber}
+)
+
+func deriveEnvVarName(s string) string {
+	var (
+		words []string
+		word  []rune
+	)
+
+	for _, l := range s {
+		switch {
+		case charGroupUpperLetter.Contains(l):
+			if len(word) > 0 && charGroupLowerNumber.Contains(word[len(word)-1]) {
+				words = append(words, string(word))
+				word = []rune{}
+			}
+			word = append(word, l)
+
+		case charGroupLowerLetter.Contains(l):
+			if len(word) > 1 && charGroupUpperLetter.Contains(word[len(word)-1]) {
+				words = append(words, string(word[0:len(word)-1]))
+				word = word[len(word)-1:]
+			}
+			word = append(word, l)
+
+		case charGroupNumber.Contains(l):
+			word = append(word, l)
+
+		default:
+			if len(word) > 0 {
+				words = append(words, string(word))
+			}
+			word = []rune{}
+		}
+	}
+	words = append(words, string(word))
+
+	return strings.ToUpper(strings.Join(words, "_"))
+}
diff --git a/autoenv_test.go b/autoenv_test.go
new file mode 100644
index 0000000..47db714
--- /dev/null
+++ b/autoenv_test.go
@@ -0,0 +1,29 @@
+package rconfig
+
+import (
+	"testing"
+)
+
+func TestDeriveEnvVarName(t *testing.T) {
+	for test, expect := range map[string]string{
+		"1Foobar":              "1_FOOBAR",
+		"BC1":                  "BC1",
+		"BIGCase1":             "BIG_CASE1",
+		"BIGCase":              "BIG_CASE",
+		"Camel1":               "CAMEL1",
+		"camel":                "CAMEL",
+		"Camel":                "CAMEL",
+		"CAMEL":                "CAMEL",
+		"CamelCase":            "CAMEL_CASE",
+		"_foobar":              "FOOBAR",
+		"ILoveGoAndJSONSoMuch": "I_LOVE_GO_AND_JSON_SO_MUCH",
+		"mrT":         "MR_T",
+		"my_case1":    "MY_CASE1",
+		"MyFieldName": "MY_FIELD_NAME",
+		"SmallCASE":   "SMALL_CASE",
+	} {
+		if d := deriveEnvVarName(test); d != expect {
+			t.Errorf("Derived variable %q did not match expectation %q", d, expect)
+		}
+	}
+}
diff --git a/config.go b/config.go
index ee9301e..eedde00 100644
--- a/config.go
+++ b/config.go
@@ -17,6 +17,7 @@ import (
 )
 
 var (
+	autoEnv          bool
 	fs               *pflag.FlagSet
 	variableDefaults map[string]string
 )
@@ -60,6 +61,13 @@ func Args() []string {
 	return fs.Args()
 }
 
+// AutoEnv enables or disables automated env variable guessing. If no `env` struct
+// tag was set and AutoEnv is enabled the env variable name is derived from the
+// name of the field: `MyFieldName` will get `MY_FIELD_NAME`
+func AutoEnv(enable bool) {
+	autoEnv = enable
+}
+
 // Usage prints a basic usage with the corresponding defaults for the flags to
 // os.Stdout. The defaults are derived from the `default` struct-tag and the ENV.
 func Usage() {
@@ -116,7 +124,7 @@ func execTags(in interface{}, fs *pflag.FlagSet) error {
 		}
 
 		value := varDefault(typeField.Tag.Get("vardefault"), typeField.Tag.Get("default"))
-		value = envDefault(typeField.Tag.Get("env"), value)
+		value = envDefault(typeField, value)
 		parts := strings.Split(typeField.Tag.Get("flag"), ",")
 
 		switch typeField.Type {
@@ -334,9 +342,14 @@ func registerFlagUint(t reflect.Kind, fs *pflag.FlagSet, field interface{}, part
 	}
 }
 
-func envDefault(env, def string) string {
+func envDefault(field reflect.StructField, def string) string {
 	value := def
 
+	env := field.Tag.Get("env")
+	if env == "" && autoEnv {
+		env = deriveEnvVarName(field.Name)
+	}
+
 	if env != "" {
 		if e := os.Getenv(env); e != "" {
 			value = e