From c08b0f906b39a1fbc978eeeb262840f9acd735ff Mon Sep 17 00:00:00 2001
From: Carlos Alexandro Becker <caarlos0@users.noreply.github.com>
Date: Wed, 1 Mar 2023 10:54:41 -0300
Subject: [PATCH] feat: use field name by default (#253)

Signed-off-by: Carlos A Becker <caarlos0@users.noreply.github.com>
---
 env.go      | 29 ++++++++++++++++++++++++++---
 env_test.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+), 3 deletions(-)

diff --git a/env.go b/env.go
index 6bcdd90..ce84c62 100644
--- a/env.go
+++ b/env.go
@@ -9,6 +9,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 )
 
 // nolint: gochecknoglobals
@@ -102,15 +103,20 @@ type Options struct {
 	// TagName specifies another tagname to use rather than the default env.
 	TagName string
 
-	// RequiredIfNoDef automatically sets all env as required if they do not declare 'envDefault'
+	// RequiredIfNoDef automatically sets all env as required if they do not
+	// declare 'envDefault'.
 	RequiredIfNoDef bool
 
-	// OnSet allows to run a function when a value is set
+	// OnSet allows to run a function when a value is set.
 	OnSet OnSetFn
 
-	// Prefix define a prefix for each key
+	// Prefix define a prefix for each key.
 	Prefix string
 
+	// UseFieldNameByDefault defines whether or not env should use the field
+	// name by default if the `env` key is missing.
+	UseFieldNameByDefault bool
+
 	// Sets to true if we have already configured once.
 	configured bool
 }
@@ -145,6 +151,7 @@ func configure(opts []Options) []Options {
 		if item.Prefix != "" {
 			opt.Prefix = item.Prefix
 		}
+		opt.UseFieldNameByDefault = item.UseFieldNameByDefault
 		opt.RequiredIfNoDef = item.RequiredIfNoDef
 	}
 
@@ -243,6 +250,19 @@ func doParseField(refField reflect.Value, refTypeField reflect.StructField, func
 	return nil
 }
 
+const underscore rune = '_'
+
+func toEnvName(input string) string {
+	var output []rune
+	for i, c := range input {
+		if i > 0 && output[i-1] != underscore && c != underscore && unicode.ToUpper(c) == c {
+			output = append(output, underscore)
+		}
+		output = append(output, unicode.ToUpper(c))
+	}
+	return string(output)
+}
+
 func get(field reflect.StructField, opts []Options) (val string, err error) {
 	var exists bool
 	var isDefault bool
@@ -253,6 +273,9 @@ func get(field reflect.StructField, opts []Options) (val string, err error) {
 	required := opts[0].RequiredIfNoDef
 	prefix := opts[0].Prefix
 	ownKey, tags := parseKeyForOption(field.Tag.Get(getTagName(opts)))
+	if ownKey == "" && opts[0].UseFieldNameByDefault {
+		ownKey = toEnvName(field.Name)
+	}
 	key := prefix + ownKey
 	for _, tag := range tags {
 		switch tag {
diff --git a/env_test.go b/env_test.go
index 9b601c7..c23f72d 100644
--- a/env_test.go
+++ b/env_test.go
@@ -1691,6 +1691,37 @@ func TestComplePrefix(t *testing.T) {
 	isEqual(t, "blahhh", cfg.Blah)
 }
 
+func TestNoEnvKey(t *testing.T) {
+	type Config struct {
+		Foo    string
+		FooBar string
+		bar    string
+	}
+	var cfg Config
+	isNoErr(t, Parse(&cfg, Options{
+		UseFieldNameByDefault: true,
+		Environment: map[string]string{
+			"FOO":     "fooval",
+			"FOO_BAR": "foobarval",
+		},
+	}))
+	isEqual(t, "fooval", cfg.Foo)
+	isEqual(t, "foobarval", cfg.FooBar)
+	isEqual(t, "", cfg.bar)
+}
+
+func TestToEnv(t *testing.T) {
+	for in, out := range map[string]string{
+		"Foo":      "FOO",
+		"FooBar":   "FOO_BAR",
+		"fooBar":   "FOO_BAR",
+		"Foo_Bar":  "FOO_BAR",
+		"Foo__Bar": "FOO__BAR",
+	} {
+		isEqual(t, out, toEnvName(in))
+	}
+}
+
 func isTrue(tb testing.TB, b bool) {
 	tb.Helper()