adds option to create new rc version in incrementPrerelase incrementor

* [issue #286] Create rc1 in case of no rc version in incrementPrerelase strategy
* [issue #286] initialPreReleaseIfNotOnPrerelease to create rc version, old behavior when absent

docs/configuration/version.md

@@ -195,7 +195,7 @@ number is incremented. There are other predefined rules:
 -   *incrementMinorIfNotOnRelease* - increment patch number if on
     release branch. Increment minor otherwise
 -   *incrementPrerelease* - increment pre-release suffix if possible
-    (-rc1 to -rc2). Increment patch otherwise
+    (-rc1 to -rc2). Add `initialPreReleaseIfNotOnPrerelease` to increment patch with prerelase version. Increment patch otherwise
 
 You can set one of predefined rules via `scmVersion.versionIncrementer`
 method:
@@ -253,6 +253,15 @@ it's set to `release/.+`):
         versionIncrementer 'incrementMinorIfNotOnRelease', [releaseBranchPattern: 'release.*']
     }
 
+### incrementPrerelease
+
+This rule uses additional parameter `initialPreReleaseIfNotOnPrerelease` (by default
+it's empty):
+
+    scmVersion {
+        versionIncrementer 'incrementPrerelease', [initialPreReleaseIfNotOnPrerelease: 'rc1']
+    }
+
 ## Decorating
 
 Decorating phase happens only when version is read (and deserialized).

src/main/groovy/pl/allegro/tech/build/axion/release/domain/PredefinedVersionIncrementer.groovy

@@ -43,6 +43,11 @@ enum PredefinedVersionIncrementer {
                     .build()
             }
         }
+
+        if (config.initialPreReleaseIfNotOnPrerelease !=  null) {
+            return context.currentVersion.incrementPatchVersion(String.valueOf(config.initialPreReleaseIfNotOnPrerelease))
+        }
+
         return context.currentVersion.incrementPatchVersion()
     }),
 

src/test/groovy/pl/allegro/tech/build/axion/release/domain/PredefinedVersionIncrementerTest.groovy

@@ -41,15 +41,31 @@ class PredefinedVersionIncrementerTest extends Specification {
         expect:
         versionIncrementerFor('branchSpecific', ['release.*': 'incrementPatch', 'master': 'incrementMinor'])(context) == Version.valueOf('0.2.0')
     }
-    
+
     def "should increment prerelease version when incrementPrerelease rule used"() {
         given:
         VersionIncrementerContext context = new VersionIncrementerContext(Version.valueOf('0.1.0-rc1'), scmPosition('master'))
-        
+
         expect:
         versionIncrementerFor('incrementPrerelease')(context) == Version.valueOf('0.1.0-rc2')
     }
 
+    def "should increment patch version when incrementPrerelease rule used and currentVersion is not rc"() {
+        given:
+        VersionIncrementerContext context = new VersionIncrementerContext(Version.valueOf('0.1.0'), scmPosition('master'))
+
+        expect:
+        versionIncrementerFor('incrementPrerelease')(context) == Version.valueOf('0.1.1')
+    }
+
+    def "should create prerelease version when incrementPrerelease rule used with initialPreReleaseIfNotOnPrerelease"() {
+        given:
+        VersionIncrementerContext context = new VersionIncrementerContext(Version.valueOf('0.1.0'), scmPosition('master'))
+
+        expect:
+        versionIncrementerFor('incrementPrerelease', [initialPreReleaseIfNotOnPrerelease: 'rc1'])(context) == Version.valueOf('0.1.1-rc1')
+    }
+
     def "should increment prerelease version even when it has leading zeroes when incrementPrerelease rule used"() {
         given:
         VersionIncrementerContext context = new VersionIncrementerContext(Version.valueOf('0.1.0-rc01'), scmPosition('master'))