Support more cases for underscore casing conversion (#52)

* `created_at DESC` -> `created_at_desc`
* de-duplication of underscore characters.

Author: rdner

pkg/strings/casing.go

@@ -37,19 +37,33 @@ func ToPascalCase(value string) string {
 func ToUnderscoreCase(value string) string {
 	b := strings.Builder{}
 
+	var previousUnderscore, previousUppercase bool
+
 	for _, rune := range value {
-		// Always upper the character after non-letter/non-digit skipping the character
+
+		// if it's not allowed character we replace it with underscore without
+		// duplication of underscores
 		if !unicode.IsLetter(rune) && !unicode.IsDigit(rune) {
-			b.WriteByte('_')
+			if !previousUnderscore {
+				b.WriteByte('_')
+				previousUnderscore = true
+			}
+			previousUppercase = false
 			continue
 		}
 
-		// convert pascal or camel case into underscore case
 		if unicode.IsUpper(rune) {
-			b.WriteByte('_')
+			if !previousUnderscore && !previousUppercase {
+				b.WriteByte('_')
+			}
+			b.WriteRune(unicode.ToLower(rune))
+			previousUppercase = true
+		} else {
+			b.WriteRune(rune)
+			previousUppercase = false
 		}
 
-		b.WriteRune(unicode.ToLower(rune))
+		previousUnderscore = false
 	}
 
 	return strings.Trim(b.String(), "_")

pkg/strings/casing_test.go

@@ -67,6 +67,16 @@ func TestToUnderscoreCase(t *testing.T) {
 			input:  "some_table_nameSomeName",
 			output: "some_table_name_some_name",
 		},
+		{
+			name:   "Does not double underscores",
+			input:  "some Table Name",
+			output: "some_table_name",
+		},
+		{
+			name:   "Respects several capital letters in a row",
+			input:  "some TABLE NAME",
+			output: "some_table_name",
+		},
 		{
 			name:   "Does not change the value if it's underscore casing",
 			input:  "some_name",
@@ -82,7 +92,16 @@ func TestToUnderscoreCase(t *testing.T) {
 			input:  "Some{{{",
 			output: "some",
 		},
-
+		{
+			name:   "Makes a separate upper case word a part of underscored identifier",
+			input:  "created_at DESC",
+			output: "created_at_desc",
+		},
+		{
+			name:   "Makes a separate upper case word a part of underscored identifier",
+			input:  "word$^%^%another _some   *&^%$A%$#%WORD",
+			output: "word_another_some_a_word",
+		},
 		{
 			name: "Handles empty string",
 		},