Fix README and test for throws on branch not found.

Closes #6.

Implements branch support by _only extending_ the existing functionality. Note that the existing functionality DOES NOT implement a `parent` pointer for commits, and as a result branches are also implemented differently to how git implements them.

git branches are simply a named pointer to a commit object (which then has a pointer to its parent commit), here branches store the entire sequence of pointers to commit objects.

### Why different to git?
I made a mistake in the initial implementation by using a `TreeSet` instead of manually implementing a tree for commits. This allowed the ability to insert commits in-between other commits, which isn't normally possible in `git` _without rebasing which changes commits' hashes_.

The specification for this feature stated:
> All functionality that is implemented already should be preserved.

And so since I cannot remove this functionality any more, I have to build on top of it.

Reviewed-on: #7

README.md

@@ -10,6 +10,26 @@ The package is named `tinyvm` for 'tiny version manager'.
 Since this is an internship application project, I have assumed that a minimal usage of external libraries is preferred,
 so that a greater technical understanding can be demonstrated.
 
+## Usage
+
+### Gradle
+```kotlin
+repositories {
+    // other repositories
+    maven { url "https://git.koval.net/api/packages/cyclane/maven" }
+}
+
+dependencies {
+    // other dependencies
+    implementation("net.koval.teamcity-gitea-test-task:tinyvm:0.1.1")
+}
+```
+
+### Documentation
+Use autocompletion and hover menus in your IDE, or download the
+generated HTML documentation
+from the [latest release](https://git.koval.net/cyclane/teamcity-gitea-test-task/releases).
+
 ## Instructions
 
 Create a library that implements simple Git functionality. You need to implement at least three entities:

build.gradle.kts

@@ -1,6 +1,7 @@
 plugins {
     kotlin("jvm") version "1.9.21"
     id("org.jmailen.kotlinter") version "4.1.0"
+    id("org.jetbrains.dokka") version "1.9.10"
     `maven-publish`
 }
 
@@ -23,10 +24,18 @@ kotlin {
     jvmToolchain(17)
 }
 
+val dokkaHtml by tasks.getting(org.jetbrains.dokka.gradle.DokkaTask::class)
+val javadocJar: TaskProvider<Jar> by tasks.registering(Jar::class) {
+    dependsOn(dokkaHtml)
+    archiveClassifier.set("javadoc")
+    from(dokkaHtml.outputDirectory)
+}
+
 publishing {
     publications.register<MavenPublication>("gpr") {
         artifactId = "tinyvm"
         from(components["java"])
+        artifact(javadocJar)
         pom {
             name.set("TeamCity support for Gitea - Test Task - tiny version manager")
             description.set("This is a small project to implement a subset of git's functionality in Kotlin and was " +

src/main/kotlin/tinyvm/Commit.kt

@@ -1,29 +1,20 @@
 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()
-}
-
+/**
+ * Commits are a pointer to a 'head' tree with some metadata.
+ */
 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"
+
+    override fun toString(): String = "commit ${hash()}\n$data"
 }
 
 data class Author(

src/main/kotlin/tinyvm/Extensions.kt

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

src/main/kotlin/tinyvm/Object.kt

@@ -0,0 +1,17 @@
+package tinyvm
+
+import java.security.MessageDigest
+import java.util.HexFormat
+
+/**
+ * Represents an arbitrary version manager object.
+ */
+abstract class Object(val type: String) {
+    abstract val data: String
+
+    fun hash(): String =
+        HexFormat.of().formatHex(
+            MessageDigest.getInstance("SHA-1")
+                .digest("$type ${data.length}\u0000$data".toByteArray()),
+        )
+}

src/main/kotlin/tinyvm/Repository.kt

@@ -1,22 +1,59 @@
 package tinyvm
 
+import java.util.TreeSet
+
 class HashCollisionException(hash: String) : Exception("Different object types with identical hash '$hash'")
 
-class CommitTimeComparator : Comparator<Commit> {
+private 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())
+private val commitTimeComparator = CommitTimeComparator()
+
+private class Branch(val name: String, val commits: TreeSet<Commit>)
+
+class Repository(initialBranch: String = "master") {
+    private var head = Branch(initialBranch, sortedSetOf(commitTimeComparator))
+    private val commits = mutableMapOf(initialBranch to head)
+
+    val branch: String
+        get() = head.name
+
+    val branches: Set<String>
+        get() = commits.keys
 
     // 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>()
 
+    /**
+     * Create a new branch as a copy of the current branch.
+     * @param name New branch name.
+     * @param use Whether to switch to this branch.
+     */
+    fun createBranch(
+        name: String,
+        use: Boolean = true,
+    ) {
+        commits[name] =
+            Branch(
+                name,
+                head.commits.clone() as? TreeSet<Commit> ?: throw Exception("TreeSet.clone() unexpected result"),
+            )
+        if (use) useBranch(name)
+    }
+
+    /**
+     * Use (switch to) a branch.
+     */
+    fun useBranch(name: String) {
+        head = commits[name] ?: throw BranchNotFoundException(name)
+    }
+
     /**
      * (Deep) get or put a commit object into the repository.
      * This will also get or put all child trees and blobs.
@@ -33,7 +70,7 @@ class Repository {
                 timestamp = commit.timestamp,
             )
         objects[hash] = newCommit
-        commits.add(newCommit)
+        head.commits.add(newCommit)
         return newCommit
     }
 
@@ -45,12 +82,12 @@ class Repository {
     /**
      * List all commits.
      */
-    fun listCommits(): List<Commit> = commits.toList()
+    fun listCommits(): List<Commit> = head.commits.toList()
 
     /**
      * Find commit.
      */
-    fun findCommit(predicate: (Commit) -> Boolean): Commit? = commits.find(predicate)
+    fun findCommit(predicate: (Commit) -> Boolean): Commit? = head.commits.find(predicate)
 
     /**
      * Dump repository objects
@@ -89,4 +126,6 @@ class Repository {
      */
     private inline fun <reified T : Object> findObject(hash: String): T? =
         objects[hash]?.let { it as? T ?: throw HashCollisionException(hash) }
-}
+}
+
+class BranchNotFoundException(name: String) : Exception("Branch '$name' does not exist")

src/main/kotlin/tinyvm/Tree.kt

@@ -1,7 +1,13 @@
 package tinyvm
 
+/**
+ * Tree nodes are either trees or blobs, represented by this sealed class.
+ */
 sealed class Node(type: String) : Object(type)
 
+/**
+ * A tree is a set of named nodes.
+ */
 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
@@ -9,6 +15,22 @@ class Tree(val nodes: Map<String, Node>) : Node("tree") {
             nodes.map { (name, node) ->
                 "${node.type} $name\u0000${node.hash()}"
             }.sorted().joinToString()
+
+    override fun toString(): String =
+        "tree ${hash()}\n" +
+            nodes.map { (name, node) ->
+                when (node) {
+                    is Tree -> "+$name/\n"
+                    is Blob -> "+$name\n"
+                } + node.toString().insertLeftMargin()
+            }.sorted().joinToString("\n")
 }
 
-class Blob(override val data: String) : Node("blob")
+/**
+ * A blob is a data container.
+ */
+class Blob(override val data: String) : Node("blob") {
+    override fun toString(): String = "blob ${hash()}\n${data.insertLeftMargin()}"
+}
+
+private fun String.insertLeftMargin(): String = split('\n').joinToString("\n") { "| $it" }

src/test/kotlin/tinyvm/RepositoryTest.kt

@@ -1,5 +1,6 @@
 package tinyvm
 
+import org.junit.jupiter.api.assertThrows
 import java.time.Instant
 import kotlin.test.Test
 import kotlin.test.assertEquals
@@ -18,22 +19,22 @@ internal class RepositoryTest {
                 tree =
                     Tree(
                         mapOf(
+                            "dir2" to
+                                Tree(
+                                    mapOf(
+                                        "test1.txt" to Blob("This is a second file"),
+                                    ),
+                                ),
                             "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",
+                message = "Move test1.txt to dir1 and add dir2/test1.txt",
                 timestamp = Instant.ofEpochSecond(50),
             ),
             Commit(
@@ -49,7 +50,7 @@ internal class RepositoryTest {
                             "dir2" to
                                 Tree(
                                     mapOf(
-                                        "test2.txt" to Blob("This is a second file"),
+                                        "test1.txt" to Blob("This is a second file"),
                                     ),
                                 ),
                             "README.md" to Blob("# This is a test repo!"),
@@ -117,4 +118,75 @@ internal class RepositoryTest {
         assertEquals(committed, repository.findCommit { it.message.matches("Move.*".toRegex()) })
         assertEquals(null, repository.getCommit("00000000000000000000"))
     }
+
+    @Test
+    fun `can create and use branch`() {
+        repository.commit(commits[0])
+        repository.commit(commits[1])
+        repository.commit(commits[2])
+        repository.createBranch("test-branch", use = false)
+        assertEquals("master", repository.branch)
+        repository.createBranch("test-branch-2")
+        assertEquals("test-branch-2", repository.branch)
+        assertEquals(setOf("master", "test-branch", "test-branch-2"), repository.branches)
+        assertEquals(commits.map { it.hash() }, repository.listCommits().map { it.hash() })
+        assertEquals(10, repository.dumpObjects().size)
+    }
+
+    @Test
+    fun `cannot use non-existent branch`() {
+        assertThrows<BranchNotFoundException> {
+            repository.useBranch("test-branch")
+        }
+    }
+
+    @Test
+    fun `can commit to branch`() {
+        repository.commit(commits[0])
+        repository.createBranch("test-branch")
+        repository.commit(commits[1])
+        repository.createBranch("test-branch-2")
+        repository.commit(commits[2])
+
+        assertEquals(commits.map { it.hash() }, repository.listCommits().map { it.hash() })
+        repository.useBranch("test-branch")
+        assertEquals(commits.take(2).map { it.hash() }, repository.listCommits().map { it.hash() })
+        repository.useBranch("master")
+        assertEquals(commits.take(1).map { it.hash() }, repository.listCommits().map { it.hash() })
+    }
+
+    @Test
+    fun `can display commit`() {
+        assertEquals(
+            """
+            commit 804c6f0c66da0ea0eab8ac29f23a627e03a962b2
+            tree aaebbb258bce30749fc302cbd161b78462252a32
+            author Gleb Koval <gleb@koval.net>
+            timestamp 0
+            
+            Add test1.txt
+            """.trimIndent(),
+            commits[0].toString(),
+        )
+    }
+
+    @Test
+    fun `can display tree`() {
+        assertEquals(
+            """
+            tree 048b8e267dd7a6c5d4849d59f6ff82e6ae948f13
+            +dir1/
+            | tree aaebbb258bce30749fc302cbd161b78462252a32
+            | +test1.txt
+            | | blob c57eff55ebc0c54973903af5f72bac72762cf4f4
+            | | | Hello World!
+            +dir2/
+            | tree 0e14cbdba02924cd63a58c2492fc3dd05dc682bd
+            | +test1.txt
+            | | blob da3fcc31cbc16fcbb7526d68771f77e7e7f02fb1
+            | | | This is a second file
+            """.trimIndent(),
+            commits[1].tree.toString(),
+        )
+    }
 }