feat: add constant-case strategy in building env key names

fig.go

@@ -10,6 +10,7 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unicode"
 
 	"github.com/mitchellh/mapstructure"
 	"github.com/pelletier/go-toml/v2"
@@ -112,14 +113,15 @@ func defaultFig() *fig {
 }
 
 type fig struct {
-	filename   string
-	dirs       []string
-	tag        string
-	timeLayout string
-	useEnv     bool
-	useStrict  bool
-	ignoreFile bool
-	envPrefix  string
+	filename        string
+	dirs            []string
+	tag             string
+	timeLayout      string
+	useEnv          bool
+	useConstantCase bool
+	useStrict       bool
+	ignoreFile      bool
+	envPrefix       string
 }
 
 func (f *fig) Load(cfg interface{}) error {
@@ -312,12 +314,48 @@ func (f *fig) setFromEnv(fv reflect.Value, key string) error {
 func (f *fig) formatEnvKey(key string) string {
 	// loggers[0].level --> loggers_0_level
 	key = strings.NewReplacer(".", "_", "[", "_", "]", "").Replace(key)
+	if f.useConstantCase {
+		key = f.toConstantCase(key)
+	}
 	if f.envPrefix != "" {
 		key = fmt.Sprintf("%s_%s", f.envPrefix, key)
 	}
 	return strings.ToUpper(key)
 }
 
+func (f *fig) toConstantCase(key string) string {
+	var b strings.Builder
+	runes := []rune(key)
+
+	for i := 0; i < len(runes); i++ {
+		if !unicode.IsUpper(runes[i]) {
+			b.WriteRune(runes[i])
+			continue
+		}
+
+		if i == 0 {
+			b.WriteRune(runes[i])
+			continue
+		}
+
+		if unicode.IsUpper(runes[i-1]) || !unicode.IsLetter(runes[i-1]) {
+			if len(runes) == i+1 {
+				b.WriteRune(runes[i])
+			} else if unicode.IsLower(runes[i+1]) {
+				b.WriteString("_")
+				b.WriteRune(runes[i])
+			} else {
+				b.WriteRune(runes[i])
+			}
+		} else {
+			b.WriteString("_")
+			b.WriteRune(runes[i])
+		}
+	}
+
+	return b.String()
+}
+
 // setDefaultValue calls setValue but disallows booleans from
 // being set.
 func (f *fig) setDefaultValue(fv reflect.Value, val string) error {

fig_test.go

@@ -1289,6 +1289,35 @@ func Test_fig_setSlice(t *testing.T) {
 	})
 }
 
+func Test_fig_toConstantCase(t *testing.T) {
+	fig := defaultFig()
+
+	testCases := []struct {
+		key      string
+		expected string
+	}{
+		{key: "fig_serviceName", expected: "fig_service_Name"},
+		{key: "fig_USA_test", expected: "fig_USA_test"},
+		{key: "fig_runT", expected: "fig_run_T"},
+		{key: "fig_0_test", expected: "fig_0_test"},
+		{key: "USACountry", expected: "USA_Country"},
+		{key: "helloWORLD", expected: "hello_WORLD"},
+		{key: "USA", expected: "USA"},
+		{key: "___", expected: "___"},
+		{key: "a__b__c", expected: "a__b__c"},
+		{key: "a__B__c", expected: "a__B__c"},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.key, func(t *testing.T) {
+			res := fig.toConstantCase(tc.key)
+			if res != tc.expected {
+				t.Errorf("expected %s, got %s", tc.expected, res)
+			}
+		})
+	}
+}
+
 func setenv(t *testing.T, key, value string) {
 	t.Helper()
 	t.Setenv(key, value)

option.go

@@ -118,3 +118,16 @@ func UseStrict() Option {
 		f.useStrict = true
 	}
 }
+
+// EnvConstantCaseStrategy returns an option that configures fig to
+// convert env key names to constant-case naming convention.
+// e.g. converts `app_serverPort` to `APP_SERVER_PORT`.
+//
+//	fig.Load(&cfg, fig.EnvConstantCaseStrategy())
+//
+// If this option is not used then fig builds env key names as explained in UseEnv option.
+func EnvConstantCaseStrategy() Option {
+	return func(f *fig) {
+		f.useConstantCase = true
+	}
+}