Prevent decrease of higher position version while auto incrementing

Resolves #4

pkg/version/version.go

@@ -24,6 +24,8 @@ const positionMajor = 0
 const positionMinor = 1
 const positionPatch = 2
 
+var versionDecreaseError = fmt.Errorf("version decrease in combination with auto increment is not supported")
+
 func Next(current string, patternNext string) (string, error) {
 	_, current = splitPrefix("v", current)
 	currentCore, _ := coreAndExtension(current)
@@ -63,10 +65,16 @@ func Next(current string, patternNext string) (string, error) {
 	case positionMinor:
 		if currentMajor == nextMajor {
 			nextMinor = currentMinor + 1
+		} else if currentMajor > nextMajor {
+			return "", versionDecreaseError
 		}
 	case positionPatch:
 		if currentMajor == nextMajor && currentMinor == nextMinor {
 			nextPatch = currentPatch + 1
+		} else if currentMajor > nextMajor {
+			return "", versionDecreaseError
+		} else if currentMinor > nextMinor {
+			return "", versionDecreaseError
 		}
 	}
 

pkg/version/version_test.go

@@ -218,6 +218,33 @@ func TestNext(t *testing.T) {
 			want:    "2.0.0",
 			wantErr: false,
 		},
+		{
+			name: "reject decreasing major version when incrementing minor version",
+			args: args{
+				current:          "1.2.3",
+				incrementPattern: "0.x.0",
+			},
+			want:    "",
+			wantErr: true,
+		},
+		{
+			name: "reject decreasing major version when incrementing patch version",
+			args: args{
+				current:          "1.2.3",
+				incrementPattern: "0.2.x",
+			},
+			want:    "",
+			wantErr: true,
+		},
+		{
+			name: "reject decreasing minor version when incrementing patch version",
+			args: args{
+				current:          "1.2.3",
+				incrementPattern: "1.0.x",
+			},
+			want:    "",
+			wantErr: true,
+		},
 		{
 			name: "increment minor not knowing major and patch",
 			args: args{