Incremental manager work.

modules/kobalt-plugin-api/src/main/kotlin/com/beust/kobalt/internal/IncrementalManager.kt

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+package com.beust.kobalt.internal
+
+import com.beust.kobalt.misc.KFiles
+import com.google.gson.Gson
+import com.google.gson.GsonBuilder
+import java.io.File
+import java.io.FileReader
+import java.nio.charset.Charset
+import java.nio.file.Files
+import java.nio.file.Paths
+import java.util.*
+
+data class TaskInfo(val taskName: String, var inputChecksum: String? = null, var outputChecksum: String? = null)
+
+class BuildInfo(var tasks: List<TaskInfo>)
+
+/**
+ * Manage the file .kobalt/build-info.kt, which keeps track of input and output checksums to manage
+ * incremental builds.
+ */
+class IncrementalManager(val fileName: String = IncrementalManager.BUILD_INFO_FILE) {
+    companion object {
+        val BUILD_INFO_FILE = KFiles.joinDir(KFiles.KOBALT_DOT_DIR, "build-info.json")
+    }
+
+    private fun buildInfo() = File(fileName).let { file ->
+        if (file.exists()) {
+            Gson().fromJson(FileReader(file), BuildInfo::class.java) ?: BuildInfo(emptyList())
+        } else {
+            BuildInfo(emptyList())
+        }
+    }
+
+    private fun taskInfos() = hashMapOf<String, TaskInfo>().apply {
+            buildInfo().tasks.forEach {
+                put(it.taskName, it)
+            }
+        }
+
+    private fun save(map: Map<String, TaskInfo>) {
+        val bi = BuildInfo(map.values.toList())
+        val json = GsonBuilder().setPrettyPrinting().create().toJson(bi)
+        Files.write(Paths.get(fileName), json.toByteArray(Charset.defaultCharset()))
+    }
+
+    private fun taskInfoFor(taskInfos: HashMap<String, TaskInfo>, taskName: String)
+            = taskInfos.getOrPut(taskName, { -> TaskInfo(taskName) })
+
+    fun saveInputChecksum(taskName: String, inputChecksum: String) {
+        with(taskInfos()) {
+            taskInfoFor(this, taskName).inputChecksum = inputChecksum
+            save(this)
+        }
+    }
+
+    fun inputChecksumFor(taskName: String) : String? =
+            taskInfoFor(taskInfos(), taskName).inputChecksum
+
+    fun saveOutputChecksum(taskName: String, outputChecksum: String) {
+        with(taskInfos()) {
+            taskInfoFor(this, taskName).outputChecksum = outputChecksum
+            save(this)
+        }
+    }
+
+    fun outputChecksumFor(taskName: String) : String? =
+            taskInfoFor(taskInfos(), taskName).outputChecksum
+}

src/test/kotlin/com/beust/kobalt/internal/IncrementalManagerTest.kt

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+package com.beust.kobalt.internal
+
+import org.testng.Assert
+import org.testng.annotations.Test
+import java.io.File
+
+@Test
+class IncrementalManagerTest {
+    val TASK = "task"
+    val TASK2 = "task2"
+
+    fun shouldSave() {
+        val file = File.createTempFile("kobalt-", "")
+        println("File: $file")
+        val im = IncrementalManager(file.absolutePath)
+        val v = im.inputChecksumFor(TASK)
+        Assert.assertNull(v)
+        im.saveInputChecksum(TASK, "44")
+        Assert.assertEquals(im.inputChecksumFor(TASK), "44")
+        im.saveInputChecksum(TASK, "42")
+        Assert.assertEquals(im.inputChecksumFor(TASK), "42")
+
+        im.saveInputChecksum(TASK2, "45")
+        Assert.assertEquals(im.inputChecksumFor(TASK2), "45")
+
+        Assert.assertEquals(im.inputChecksumFor(TASK), "42")
+
+        im.saveOutputChecksum(TASK, "49")
+        Assert.assertEquals(im.outputChecksumFor(TASK), "49")
+    }
+}