✨ add page utils

README.md

@@ -19,4 +19,5 @@ Utilities for my kotlin microservices
 - Result4k and Kotlin-Result migration
 - Result4k failIf and recoverIf
 - Exposed transforms for nullable columns
-- Feature Flags (Static, Split, Evidently)
+- Feature Flags (Static, Split, Evidently)
+- Cursor-based pagination utils

src/main/kotlin/dev/andrewohara/utils/pagination/Page.kt

@@ -0,0 +1,9 @@
+package dev.andrewohara.utils.pagination
+
+data class Page<Item: Any, Cursor: Any>(
+    val items: List<Item>,
+    val next: Cursor?
+)
+
+
+

src/main/kotlin/dev/andrewohara/utils/pagination/PageExtensions.kt

@@ -0,0 +1,22 @@
+package dev.andrewohara.utils.pagination
+
+fun <In: Any, Out: Any, Cursor: Any> Page<In, Cursor>.map(fn: (In) -> Out) = Page(
+    items = items.map(fn),
+    next = next
+)
+
+fun <Item: Any, Cursor: Any> Page<Item, Cursor>.filter(fn: (Item) -> Boolean) = Page(
+    items = items.filter(fn),
+    next = next
+)
+
+fun <Item: Any, Cursor: Any> stream(paginator: Paginator<Item, Cursor>) = sequence {
+    var cursor: Cursor? = null
+    do {
+        val page = paginator(cursor)
+        yieldAll(page.items)
+        cursor = page.next
+    } while (cursor != null)
+}
+
+fun <Item: Any, Cursor: Any> Paginator<Item, Cursor>.asSequence() = stream(this)

src/main/kotlin/dev/andrewohara/utils/pagination/Paginator.kt

@@ -0,0 +1,5 @@
+package dev.andrewohara.utils.pagination
+
+fun interface Paginator<Item: Any, Cursor: Any> {
+    operator fun invoke(cursor: Cursor?): Page<Item, Cursor>
+}

src/test/kotlin/dev/andrewohara/utils/pagination/FakePaginationRepo.kt

@@ -0,0 +1,26 @@
+package dev.andrewohara.utils.pagination
+
+data class FakeMessage(
+    val id: Int,
+    val topic: String,
+    val message: String
+)
+
+class FakePaginationRepo(private val pageSize: Int) {
+
+    private val messages = mutableListOf<FakeMessage>()
+
+    operator fun plusAssign(message: FakeMessage) = messages.plusAssign(message)
+
+    fun list(topic: String, cursor: Int?): Page<FakeMessage, Int> {
+        val results = messages
+            .sortedBy { it.id }
+            .filter { it.topic == topic }
+            .dropWhile { cursor != null && it.id < cursor }
+
+        return Page(
+            items = results.take(pageSize),
+            next = results.getOrNull(pageSize)?.id
+        )
+    }
+}

src/test/kotlin/dev/andrewohara/utils/pagination/PageTest.kt

@@ -0,0 +1,65 @@
+package dev.andrewohara.utils.pagination
+
+import io.kotest.matchers.sequences.shouldContainExactly
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+
+class PageTest {
+
+    private val repo = FakePaginationRepo(pageSize = 2)
+    private val stuff1 = FakeMessage(1, topic = "stuff", message = "stuff1").also(repo::plusAssign)
+    private val thing1 = FakeMessage(2, topic = "things", message = "thing1").also(repo::plusAssign)
+    private val stuff2 =  FakeMessage(3, topic = "stuff", message = "stuff2").also(repo::plusAssign)
+    private val stuff3 = FakeMessage(4, topic = "stuff", message = "stuff3").also(repo::plusAssign)
+
+    @Test
+    fun `repo sanity`() {
+        repo.list("things", null) shouldBe Page(
+            items = listOf(thing1),
+            next = null
+        )
+        repo.list("stuff", null) shouldBe Page(
+            items = listOf(stuff1, stuff2),
+            next = stuff3.id
+        )
+        repo.list("stuff", stuff3.id) shouldBe Page(
+            items = listOf(stuff3),
+            next = null
+        )
+    }
+
+    @Test
+    fun `paginator as sequence`() {
+        val paginator = Paginator { cursor: Int? -> repo.list("stuff", cursor) }
+        paginator.asSequence().shouldContainExactly(stuff1, stuff2, stuff3)
+    }
+
+    @Test
+    fun `filter page`() {
+        Page(
+            items = listOf(stuff1, thing1, stuff2, stuff3),
+            next = null
+        )
+            .filter { it.topic == "stuff" } shouldBe Page(
+                items = listOf(stuff1, stuff2, stuff3),
+                next = null
+            )
+        }
+
+    @Test
+    fun `map page`() {
+        Page(
+            items = listOf(stuff1, thing1, stuff2, stuff3),
+            next = null
+        )
+            .map { it.copy(topic = "all") } shouldBe Page(
+            items = listOf(
+                stuff1.copy(topic = "all"),
+                thing1.copy(topic = "all"),
+                stuff2.copy(topic = "all"),
+                stuff3.copy(topic = "all")
+            ),
+            next = null
+        )
+    }
+}