several additions and tests for pagination reversing

CHANGELOG.md

@@ -5,6 +5,8 @@
 * `Pagination`
     * `Common`:
         * Extension `Pagination#reverse` has been added
+        * Factory `PaginationByIndexes`
+        * Shortcut `calculatePagesNumber` with reversed parameters
 
 ## 0.2.0
 

pagination/common/src/commonMain/kotlin/dev/inmo/micro_utils/pagination/Pagination.kt

@@ -43,6 +43,11 @@ val Pagination.lastIndex: Int
 fun calculatePagesNumber(datasetSize: Long, pageSize: Int): Int {
     return ceil(datasetSize.toDouble() / pageSize).toInt()
 }
+/**
+ * Calculates pages count for given [datasetSize]. As a fact, it is shortcut for [calculatePagesNumber]
+ * @return calculated page number which can be correctly used in [PaginationResult] as [PaginationResult.page] value
+ */
+fun calculatePagesNumber(pageSize: Int, datasetSize: Long): Int = calculatePagesNumber(datasetSize, pageSize)
 /**
  * Calculates pages count for given [datasetSize]
  */

pagination/common/src/commonMain/kotlin/dev/inmo/micro_utils/pagination/SimplePagination.kt

@@ -27,7 +27,21 @@ data class SimplePagination(
     override val size: Int
 ) : Pagination
 
+/**
+ * Factory for [SimplePagination]
+ */
 fun Pagination(
     page: Int,
     size: Int
 ) = SimplePagination(page, size)
+
+/**
+ * @param firstIndex Inclusive first index of pagination
+ * @param lastIndex INCLUSIVE last index of pagination (last index of object covered by result [SimplePagination])
+ */
+fun PaginationByIndexes(
+    firstIndex: Int,
+    lastIndex: Int
+) = maxOf(0, (lastIndex - firstIndex + 1)).let { size ->
+    Pagination(calculatePage(firstIndex, size), size)
+}

pagination/common/src/commonMain/kotlin/dev/inmo/micro_utils/pagination/utils/PaginationReversing.kt

@@ -11,16 +11,19 @@ import dev.inmo.micro_utils.pagination.*
  * @return Reversed version of this [Pagination]
  */
 fun Pagination.reverse(objectsCount: Long): SimplePagination {
-    val resultSize = minOf(size, objectsCount.toInt())
-    return when {
-        firstIndex > objectsCount -> Pagination(calculatePage(resultSize, resultSize), resultSize)
-        size > objectsCount -> FirstPagePagination(resultSize)
-        else -> {
-            val firstIndex = (objectsCount - firstIndex - resultSize).toInt()
-            Pagination(
-                firstIndex,
-                resultSize
-            )
+    val firstIndex = (objectsCount - (this.lastIndex + 1)).let {
+        when {
+            it < 0 -> it
+            it > objectsCount -> objectsCount
+            else -> it
         }
+    }.toInt()
+    val lastIndex = (objectsCount - (this.firstIndex + 1)).let {
+        when {
+            it < 0 -> it
+            it > objectsCount -> objectsCount
+            else -> it
         }
+    }.toInt()
+    return PaginationByIndexes(firstIndex, lastIndex)
 }

pagination/common/src/commonTest/kotlin/dev/inmo/micro_utils/pagination/utils/PaginationReversing.kt

@@ -0,0 +1,24 @@
+package dev.inmo.micro_utils.pagination.utils
+
+import dev.inmo.micro_utils.pagination.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class PaginationReversingTests {
+    @Test
+    fun testThatCommonCaseWorksOk() {
+        val pageSize = 2
+        val collectionSize = 8
+        val pages = calculatePage(collectionSize, pageSize)
+
+        doWithPagination(FirstPagePagination(pageSize)) {
+            val reversed = it.reverse(collectionSize.toLong())
+            assertEquals(Pagination(calculatePage(collectionSize - it.firstIndex - it.size, it.size), it.size), reversed)
+            if (it.page < pages) {
+                it.nextPage()
+            } else {
+                null
+            }
+        }
+    }
+}