add git extension

src/main/kotlin/ltd/lulz/plugin/CorePlugin.kt

@@ -1,7 +1,9 @@
 package ltd.lulz.plugin
 
+import ltd.lulz.plugin.extension.GitExtension
 import org.gradle.api.Plugin
 import org.gradle.api.Project
+import ltd.lulz.plugin.extension.GitExtension.Companion.PLUGIN_NAME as GIT_PLUGIN_NAME
 
 @Suppress("unused")
 class CorePlugin : Plugin<Project> {
@@ -12,6 +14,11 @@ class CorePlugin : Plugin<Project> {
     override fun apply(
         project: Project,
     ) {
-        TODO("Not yet implemented")
+        gitExtension(project)
     }
+
+    private fun gitExtension(
+        project: Project,
+    ): GitExtension = project.extensions
+        .create(GIT_PLUGIN_NAME, GitExtension::class.java, project)
 }

src/main/kotlin/ltd/lulz/plugin/extension/GitExtension.kt

@@ -0,0 +1,37 @@
+package ltd.lulz.plugin.extension
+
+import org.eclipse.jgit.api.Git.open
+import org.eclipse.jgit.lib.Constants.HEAD
+import org.gradle.api.Project
+
+abstract class GitExtension(private val project: Project) {
+
+    companion object {
+        const val PLUGIN_NAME = "git"
+
+        private const val HASH_LENGTH = 8
+        private const val SNAPSHOT = "-SNAPSHOT"
+        private const val UNAVAILABLE = "n/a"
+        private val PRIMARY_BRANCHES = setOf("master", "develop")
+    }
+
+    fun version(): String = when {
+        isHead() || currentBranch() in PRIMARY_BRANCHES -> project.version.toString()
+        else -> makeVersion(project.version.toString(), currentShortHash())
+    }
+
+    fun currentShortHash(): String = open(project.projectDir)
+        .use { it.repository.exactRef(HEAD)?.objectId?.name?.take(HASH_LENGTH) ?: UNAVAILABLE }
+
+    fun currentBranch(): String = open(project.projectDir)
+        .use { it.repository.branch ?: UNAVAILABLE }
+
+    fun isHead(): Boolean = open(project.projectDir)
+        .use { it.repository.exactRef(HEAD)?.target?.name == HEAD }
+
+    private fun makeVersion(version: String, shortHash: String): String = when {
+        shortHash == UNAVAILABLE -> version.also { println("Failed to get data from GIT") }
+        version.endsWith(SNAPSHOT) -> version.replace(SNAPSHOT, ".$shortHash$SNAPSHOT")
+        else -> version.also { println("Failed version missing suffix $SNAPSHOT") }
+    }
+}