started to fill InMemoryPostsAPI

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/api/ReadPostsAPI.kt

@@ -7,10 +7,27 @@ import com.insanusmokrassar.postssystem.core.utils.pagination.Pagination
 import com.insanusmokrassar.postssystem.core.utils.pagination.PaginationResult
 import org.joda.time.DateTime
 
+/**
+ * Simple read API by different properties
+ */
 interface ReadPostsAPI {
+    /**
+     * @return [RegisteredPost] if it is available by [id]
+     */
     suspend fun getPostById(id: PostId): RegisteredPost?
+
+    /**
+     * @return all [RegisteredPost]s which contains content with specified content [id]
+     */
     suspend fun getPostsByContent(id: ContentId): List<RegisteredPost>
+
+    /**
+     * @return all [RegisteredPost]s between [from] and [to]. Range will be used INCLUSIVE, line \[[from], [to]\]
+     */
     suspend fun getPostsByDates(from: DateTime? = null, to: DateTime? = null): List<RegisteredPost>
 
+    /**
+     * @return all posts by pages basing on their creation date
+     */
     suspend fun getPostsByPagination(pagination: Pagination): PaginationResult<out RegisteredPost>
 }

core/src/main/kotlin/com/insanusmokrassar/postssystem/core/utils/pagination/Pagination.kt

@@ -17,4 +17,19 @@ interface Pagination {
      * Size of current page. Offset can be calculated as [page] * [size]
      */
     val size: Int
-}
+}
+
+/**
+ * First number in index of objects. It can be used as offset for databases or other data sources
+ */
+val Pagination.firstIndex: Int
+    get() = page * size
+
+/**
+ * Last number in index of objects. In fact, one [Pagination] object represent data in next range:
+ *
+ * [[firstIndex], [lastIndex]]; That means, that for [Pagination] with [Pagination.size] == 10 and [Pagination.page] == 1
+ * you will retrieve [Pagination.firstIndex] == 10 and [Pagination.lastIndex] == 19.
+ */
+val Pagination.lastIndex: Int
+    get() = firstIndex + size - 1

core/src/test/kotlin/com/insanusmokrassar/postssystem/core/api/InMemoryPostsAPI.kt

@@ -0,0 +1,44 @@
+package com.insanusmokrassar.postssystem.core.api
+
+import com.insanusmokrassar.postssystem.core.content.ContentId
+import com.insanusmokrassar.postssystem.core.content.RegisteredContent
+import com.insanusmokrassar.postssystem.core.post.PostId
+import com.insanusmokrassar.postssystem.core.post.RegisteredPost
+import com.insanusmokrassar.postssystem.core.utils.pagination.*
+import org.joda.time.DateTime
+
+class InMemoryPostsAPI(
+    initialPosts: List<RegisteredPost> = emptyList()
+) : PostsAPI {
+    private val posts: MutableMap<PostId, RegisteredPost> = initialPosts.associateBy { it.id }.toMutableMap()
+    private val content: MutableMap<ContentId, RegisteredContent> = initialPosts.asSequence().flatMap {
+        it.content.asSequence()
+    }.associateBy {
+        it.id
+    }.toMutableMap()
+
+    private val sortedByDatePosts: List<RegisteredPost>
+        get() = posts.values.sortedBy { it.creationDate }
+
+    override suspend fun getPostById(id: PostId): RegisteredPost? = posts[id]
+    override suspend fun getPostsByContent(id: ContentId): List<RegisteredPost> = posts.values.filter { post ->
+        post.content.any { it.id == id }
+    }
+    override suspend fun getPostsByDates(from: DateTime?, to: DateTime?): List<RegisteredPost> = posts.values.filter {
+        from ?.let { _ -> it.creationDate > from } ?: true && to ?.let { _ -> it.creationDate < to } ?: true
+    }
+    override suspend fun getPostsByPagination(
+        pagination: Pagination
+    ): PaginationResult<out RegisteredPost> = sortedByDatePosts.subList(
+        pagination.firstIndex,
+        pagination.lastIndex
+    ).let {
+        PaginationResult(
+            pagination.page,
+            posts.size / pagination.size,
+            it
+        )
+    }
+
+
+}