Initial library implementations and tests (#1)

- [x] Implements library according to instructions, with tests.
- [x] GitHub Actions workflow to lint and test library.

Reviewed-on: #1

src/main/kotlin/tinyvm/Commit.kt

@@ -0,0 +1,34 @@
+package tinyvm
+
+import java.security.MessageDigest
+import java.time.Instant
+
+abstract class Object(
+    val type: String,
+) {
+    abstract val data: String
+
+    fun hash(): String =
+        MessageDigest
+            .getInstance("SHA-1")
+            .digest("$type ${data.length}\u0000$data".toByteArray())
+            .toHex()
+}
+
+class Commit(
+    val tree: Tree,
+    val author: Author,
+    val message: String,
+    val timestamp: Instant,
+) : Object("commit") {
+    // Use \n\n for end of header in-case additional metadata is implemented in the future.
+    override val data: String
+        get() = "tree ${tree.hash()}\nauthor $author\ntimestamp ${timestamp.epochSecond}\n\n$message"
+}
+
+data class Author(
+    val name: String,
+    val email: String,
+) {
+    override fun toString(): String = "$name <$email>"
+}

src/main/kotlin/tinyvm/Extensions.kt

@@ -0,0 +1,5 @@
+package tinyvm
+
+import java.util.HexFormat
+
+fun ByteArray.toHex(): String = HexFormat.of().formatHex(this)

src/main/kotlin/tinyvm/Repository.kt

@@ -0,0 +1,92 @@
+package tinyvm
+
+class HashCollisionException(hash: String) : Exception("Different object types with identical hash '$hash'")
+
+class CommitTimeComparator : Comparator<Commit> {
+    override fun compare(
+        o1: Commit,
+        o2: Commit,
+    ): Int = (o1.timestamp.epochSecond - o2.timestamp.epochSecond).toInt()
+}
+
+class Repository {
+    private val commits = sortedSetOf(CommitTimeComparator())
+
+    // Store all objects in one map like git does. This would simplify the data persistence implementation (if there was
+    // one) and allows for other objects to be added in the future without modifying the data persistence implementation
+    // at all.
+    private val objects = mutableMapOf<String, Object>()
+
+    /**
+     * (Deep) get or put a commit object into the repository.
+     * This will also get or put all child trees and blobs.
+     */
+    fun commit(commit: Commit): Commit {
+        val hash = commit.hash()
+        val obj = findObject<Commit>(hash)
+        if (obj != null) return obj
+        val newCommit =
+            Commit(
+                tree = addTree(commit.tree),
+                author = commit.author,
+                message = commit.message,
+                timestamp = commit.timestamp,
+            )
+        objects[hash] = newCommit
+        commits.add(newCommit)
+        return newCommit
+    }
+
+    /**
+     * Get a commit by its hash.
+     */
+    fun getCommit(hash: String): Commit? = findObject<Commit>(hash)
+
+    /**
+     * List all commits.
+     */
+    fun listCommits(): List<Commit> = commits.toList()
+
+    /**
+     * Find commit.
+     */
+    fun findCommit(predicate: (Commit) -> Boolean): Commit? = commits.find(predicate)
+
+    /**
+     * Dump repository objects
+     */
+    fun dumpObjects(): Map<String, Object> = objects
+
+    /**
+     * (Deep) get or put a tree object into the repository.
+     * This will also get or put all child trees and blobs.
+     */
+    private fun addTree(tree: Tree): Tree {
+        val hash = tree.hash()
+        val obj = findObject<Tree>(hash)
+        if (obj != null) return obj
+        val newTree =
+            Tree(
+                tree.nodes.map { (name, node) ->
+                    when (node) {
+                        is Tree -> name to addTree(node)
+                        is Blob -> name to addObject(node)
+                    }
+                }.toMap(),
+            )
+        objects[hash] = newTree
+        return tree
+    }
+
+    /**
+     * (Shallow) get or put an object into the repository.
+     */
+    private inline fun <reified T : Object> addObject(obj: T): T =
+        objects.getOrPut(obj.hash()) { obj } as? T ?: throw HashCollisionException(obj.hash())
+
+    /**
+     * Find an object in the repository by its hash.
+     */
+    private inline fun <reified T : Object> findObject(hash: String): T? =
+        objects[hash]?.let { it as? T ?: throw HashCollisionException(hash) }
+}

src/main/kotlin/tinyvm/Tree.kt

@@ -0,0 +1,14 @@
+package tinyvm
+
+sealed class Node(type: String) : Object(type)
+
+class Tree(val nodes: Map<String, Node>) : Node("tree") {
+    // For simplicity just use the hex-formatted hash, not the actual value like git does.
+    override val data: String
+        get() =
+            nodes.map { (name, node) ->
+                "${node.type} $name\u0000${node.hash()}"
+            }.sorted().joinToString()
+}
+
+class Blob(override val data: String) : Node("blob")

src/test/kotlin/tinyvm/RepositoryTest.kt

@@ -0,0 +1,120 @@
+package tinyvm
+
+import java.time.Instant
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+internal class RepositoryTest {
+    private val repository = Repository()
+    private val commits =
+        listOf(
+            Commit(
+                tree = Tree(mapOf("test1.txt" to Blob("Hello World!"))),
+                author = Author("Gleb Koval", "gleb@koval.net"),
+                message = "Add test1.txt",
+                timestamp = Instant.ofEpochSecond(0),
+            ),
+            Commit(
+                tree =
+                    Tree(
+                        mapOf(
+                            "dir1" to
+                                Tree(
+                                    mapOf(
+                                        "test1.txt" to Blob("Hello World!"),
+                                    ),
+                                ),
+                            "dir2" to
+                                Tree(
+                                    mapOf(
+                                        "test2.txt" to Blob("This is a second file"),
+                                    ),
+                                ),
+                        ),
+                    ),
+                author = Author("Gleb Koval", "gleb@koval.net"),
+                message = "Move test1.txt and add dir2/test2.txt",
+                timestamp = Instant.ofEpochSecond(50),
+            ),
+            Commit(
+                tree =
+                    Tree(
+                        mapOf(
+                            "dir1" to
+                                Tree(
+                                    mapOf(
+                                        "test1.txt" to Blob("Hello World!"),
+                                    ),
+                                ),
+                            "dir2" to
+                                Tree(
+                                    mapOf(
+                                        "test2.txt" to Blob("This is a second file"),
+                                    ),
+                                ),
+                            "README.md" to Blob("# This is a test repo!"),
+                        ),
+                    ),
+                author = Author("Gleb Koval", "gleb@koval.net"),
+                message = "Add README.md",
+                timestamp = Instant.ofEpochSecond(100),
+            ),
+        )
+
+    @Test
+    fun `can commit`() {
+        val committed = repository.commit(commits[0])
+        assertEquals(commits[0].hash(), committed.hash())
+        assertEquals(1, repository.listCommits().size)
+        assertEquals(3, repository.dumpObjects().size)
+    }
+
+    @Test
+    fun `can deduplicate commit`() {
+        repository.commit(commits[0])
+        assertEquals(1, repository.listCommits().size)
+        val committed = repository.commit(commits[0])
+        assertEquals(1, repository.listCommits().size)
+        assertEquals(3, repository.dumpObjects().size)
+        assertEquals(commits[0].hash(), committed.hash())
+    }
+
+    @Test
+    fun `can commit twice and deduplicate objects`() {
+        repository.commit(commits[0])
+        val committed = repository.commit(commits[1])
+        assertEquals(2, repository.listCommits().size)
+        assertEquals(7, repository.dumpObjects().size)
+        assertEquals(commits[1].hash(), committed.hash())
+        assertEquals(commits.take(2).map { it.hash() }, repository.listCommits().map { it.hash() })
+    }
+
+    @Test
+    fun `can commit in-between existing commits`() {
+        repository.commit(commits[0])
+        repository.commit(commits[2])
+        val committed = repository.commit(commits[1])
+        assertEquals(3, repository.listCommits().size)
+        assertEquals(10, repository.dumpObjects().size)
+        assertEquals(commits[1].hash(), committed.hash())
+        assertEquals(commits.map { it.hash() }, repository.listCommits().map { it.hash() })
+    }
+
+    @Test
+    fun `can get commit by hash`() {
+        repository.commit(commits[0])
+        val committed = repository.commit(commits[1])
+        repository.commit(commits[2])
+        assertEquals(committed, repository.getCommit(commits[1].hash()))
+        assertEquals(null, repository.getCommit("00000000000000000000"))
+    }
+
+    @Test
+    fun `can find commit by predicate`() {
+        repository.commit(commits[0])
+        val committed = repository.commit(commits[1])
+        repository.commit(commits[2])
+        assertEquals(committed, repository.findCommit { it.message.matches("Move.*".toRegex()) })
+        assertEquals(null, repository.getCommit("00000000000000000000"))
+    }
+}