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overall project description

I'm porting Apache Lucene from Java to platform agnostic kotlin common to make multiplatform library lucene-kmp which supports Kotlin/Android, Kotlin/Native(iOS, macOS, Linux), and Kotlin/JVM(desktop and server).

java package structure

For example, in core module, the root package is: lucene/lucene/core/src/java/org/apache/lucene/ core module's unit tests are in root package: lucene/lucene/core/src/test/org/apache/lucene/ And those root contains sub packages such as analysis, codecs, document, index, internal, search, store, util and their sub packages.

kmp package structure

For example, in core module, the root package for port destination is: lucene-kmp/core/src/commonMain/kotlin/org/gnit/lucenekmp/ lucene-kmp core module for unit tests are in the root package: lucene-kmp/core/src/commonTest/kotlin/org/gnit/lucenekmp/ And it contains sub packages such as analysis, codecs, document, index, internal, search, store, util, and especially the one which is not found in java lucene: jdkport

package name convention on porting process

For classes/interfaces in sub packages which is made by lucene, which is "analysis, codecs, document, index, internal, search, store, util", we will respect exact same java lucene's sub packagename for both "code" and "unit test".

For example, in case of code, org.apache.lucene.document.BinaryDocValuesField has been ported to org.gnit.lucenekmp.document.BinaryDocValuesField in case of unit test, org.apache.lucenep.util.TestUnicodeUtil has been ported to org.gnit.lucenekmp.util.TestUnicodeUtil

about jdkport package

During the porting process there was JDK classes which is used in Java lucene but have no equivalent pair in kotlin standard library (standard library of kotlin common). To smooth the porting process I decided to port also those JDK classes into "package org.gnit.lucenekmp.jdkport" which is located in lucene-kmp/core/src/commonMain/kotlin/org/gnit/lucenekmp/jdkport

There are some edge case such as Double. jdk and kotlin standard lib both has Double class but the some jdk Double functions are not found in kotlin ones. In such case, I created a kotlin file contains extension functions to fill the functionality pair gap. The file name convention for extension function is for example "for Double class, kt file is named DoubleExt.kt"

So for classes/intefaces in jdkport sub packages, we will NOT respect jdk package names but all JDK classes/interfaces will be ported to jdkport sub package. The name of the classes/interfaces will be kept exact same.

For example, java.util.BitSet which is used in lucene java but no equivalent exsits in kotlin std lib will be ported to org.gnit.lucenekmp.jdkport.BitSet java.lang.Character has been ported to org.gnit.lucenekmp.jdkport.Character java.net.Inet4Address has been ported to org.gnit.lucenekmp.jdkport.Inet4Address

naming convention for unit tests for those classes will be ClassNameTest, which is for example: org.gnit.lucenekmp.jdkport.CharacterTest and org.gnit.lucenekmp.jdkport.Inet4AddressTest

Port of Classloader.java and ServiceLoader.java is does as skeleton with no operation functions because I still did not decide how to walk around this JVM specific feature in kotlin common. So creating unit tests for Classloader and ServiceLoader is also skipped for now.

And because jdk package structure will be lost, for only jdk port class/interface, we add @Ported annotation to the class like @Ported(from = "java.lang.Character")

Steps to Port

Step 1. Create the exact same of the class/interface by replacing .java to .kt in the target package. e.g. ClazzToPort.java to ClazzToPort.kt Step 2. First copy import statements which starts with org.apache.lucene by replacing it with org.gnit.lucenekmp do not port jdk import statements at this time. Step 2.1: create empty class with LuceneTestCase inheritance and import statements with correct org.gnit.lucenekmp packagess Step 2.2: Create inner classes with proper inheritance and property members Step 2.3: Add property members and no-op functions in inner class and main class Step 2.4: Port and fill implementations of each no-op functions and try to follow as much java lucene behavior possible, run get_file_problems tool of jetbrains mcp server for each time after porting a function and fix error Step 3. run get_file_problems tool of jetbrains mcp server to the file. unresolved reference compilation error means un-ported-dependency class/interface, leave current class as it is and start over from Step 1 for that class. Step 4. After import-statements-only class/interface file ClazzToPort.kt get no compilation error with get_file_problems, start porting the body of the code. Step 5. During porting body of the class, if you find any jdk specific class used, try to find it in jdkport package and if you find import it and use it. Step 6. If you did not find jdkport class/interface, do one of the following: Case 1 -> Functional interface: interfaces in java.util.function should be replaced with kotlin function representation, e.g. Predicate<T> to be (T) -> Boolean, IntFunction<R> to be (Int) -> R Case 2 -> Collection operation such as .stream() or map should be replaced with idiomatic kotlin equivalents. array creation should be done with Array(size){ i-> entry } style. Case 3 -> java.lang.Thread related classes are mostly ported to jdkport so use it but if not found port it using kotlin coroutine Job and related coroutine building block mimicking the behavior as much as possible. Case 4 -> Large but mostly static method utility jdk classes such as java.lang.Math are ported partially. port only specific missing static method by adding that specific kotlin function to existing jdkport class. Case 5 -> Complex reflections or java.security or other jdk specific things which can not be translated into KMP common code should be bypassed by creating empty no-op class and functions throwing UnsupportedOperationException() Case 6 -> ClassLoader related logic, should be replaced with hard coded class names and constructors. see examples of existing already ported NamedSPILoader.java and NamedSPILoader.kt Case else -> go to Step 7 which is porting new jdk port Step 7. Create a empty kotlin class/interface jdkport and try to find if all the dependencies or super class, super interface is already in jdkport package, if not found, port super class/interface recursively.
Step 8. If all the super class/interface of the jdk class ported into jdkport package, start porting the jdkport class. Step 9. If there are no missing jdkport class, and if there are no un-ported-dependency class/interface from Java Lucene, port the code of the ClazzToPort.kt Step 10. After porting logic and behaviors code, run get_file_problems tool of jetbrains mcp server and edit and iterate over until all errors resolves. Step 11. Final parity pass: compare Java and Kotlin files side-by-side. Check for missing or misordered member properties, functions, and logic blocks. Add missing pieces and reorder Kotlin members/functions/logic to match Java ordering so both files stay side-by-side with full behavior and implementation parity. Step 12. Test execution order is mandatory: run and iterate on focused jvmTest first until all tests pass on JVM. Only after JVM is green, run focused macosX64Test (or active native target) as the final verification to detect Kotlin/Native-specific problems.

Non-negotiable defaults (do this without being asked)

1. Side-by-side parity is default

   • Keep Java control flow shape, local variable order, comments, and key blocks in the same order as upstream Java.
   • For tests, keep parity for important inline blocks (e.g. custom scheduler object blocks, custom InfoStream blocks, latch/barrier choreography, and merge/thread coordination).
   • If a Java block cannot be copied 1:1 due to KMP constraints, keep the structure and intent, then add the smallest possible KMP-safe replacement.

2. KMP common safety is default

   • In commonMain/commonTest, do not use JVM-only threading APIs or kotlin.concurrent.thread.
   • Replace Java/JVM thread usage with Kotlin coroutines (Job, CoroutineScope, launch, join) or jdkport concurrency primitives when they already exist.
   • For lock/coordination behavior, prefer existing jdkport constructs (e.g. ReentrantLock, CountDownLatch) where parity benefits from them.

3. Compile-fix loop is mandatory

   • After each substantial edit chunk (or after each ported function for long files), run JetBrains get_file_problems.
   • Fix all errors immediately before continuing.
   • Keep iterating until there are zero errors in edited files.
   • Warnings can remain unless they indicate a parity/correctness issue.

4. Kotlin API correctness while preserving parity

   • Keep Java intent but use Kotlin-correct API shapes:
     • anonymous subclass creation uses object : Type(...) { ... }
     • assertEquals(expected, actual, message) ordering for Kotlin test APIs
   • Do not keep Java call signatures when they are invalid in Kotlin.

5. No prompt repetition policy

   • Assume the user always wants the above parity + KMP-safe behavior unless explicitly overridden.
   • Do not wait for reminders like “keep side by side”, “run get_file_problems”, or “replace thread usage in common”.

Code Style of Port

Style 1. Ease of side by side comparison: The property names, var/val names, method/function names, inner class names, all the names should be exactly same and appear in exact order as java lucene counter part so that those who read the class can easily compare the logic and behavior of the code when porting and debugging. However, there is exceptions on getters and setters: java's ordinal getters and setters should be replaced with get() and set() of kotlin public val/var to avoid compilation error e.g. Platform declaration clash: The following declarations have the same JVM signature (getBasicModel()Lorg/gnit/lucenekmp/search/similarities/BasicModel;): fun `<get-basicModel>`(): BasicModel defined in org.gnit.lucenekmp.search.similarities.DFRSimilarity fun getBasicModel(): BasicModel defined in org.gnit.lucenekmp.search.similarities.DFRSimilarity in such case, do not port getBasicModel() but make public val basicModel publicly accessible. Style 2. Do not add function/property/value/class/interface which is not in java lucene. This will make it difficult to compare the logic and behavior. Style 3. Comment: port also the comments. Javadoc need to be translated to Kdoc. Block comments and slash comments need to be ported exactly same order as it appear in java lucene code. Style 4. Companion object: java's static methods or static classes needs to be ported into kotlin's companion object {} and breaks the side by side style rule 1 but allowed. Style 5. Nullability: Get advantage of the kotlin language of null handling as much as possible by trying to use none-nullable var, val as much as possible. if a java lucene specifically inserts null to the val/var, then it can be nullable to mimic the behavior. If the value is kotlin-none-nullable, Objects.requireNonNull() or assert(someVariable != null) can be omitted but if it is nullable in kotlin, use top level function requireNotNull(). Style 6. Use idiomatic kotlin: Get advantage of the kotlin language to write simple, concise, smart, easy to understand code while following the exact same logic and behavior while porting. Style 6.1. String concatenation using + operator in java need to be replaced with kotlin string interpolation. e.g. "DFR " + basicModel.toString() + afterEffect.toString() + normalization.toString(); to be ported into "DFR $basicModel$afterEffect$normalization" Style 6.2. Two comparisons should be converted to a range check: e.g. assert docID >= 0 && docID < maxDoc; to be assert(docID in 0..<maxDoc) Style 7. Avoid numeric value mismatch: in Java Byte and Int or Long and Double is implicitly widened, but in Kotlin they are not; use explicit toX conversions or parentheses to preserve numeric type and operator behavior. Style 8. Junit Test method/function inheritance: in unit tests defined in Java Junit super classes are executed in sub test classes, but in Kotlin unit testing using kotlin.test, super class unit test functions needs to be explicitly inherited in sub classes to be executed. So find all fun testXXX() methods in super test class and add @Test annotation and define inherited class override fun testXXX() = super.testXXX() where XXX is the name of the test function. Style 8.1. For abstract/base test classes, do not put @Test on superclass test methods. Keep superclass methods as open, and in each concrete subclass add @Test override fun testXXX() = super.testXXX() for every inherited test that must run. Style 8.2. Place inherited test overrides at the bottom of the concrete test class, grouped by superclass with comments. Example: // tests inherited from BaseSuperClassX then contiguous @Test override fun test...() = super.test...() entries for that superclass. Style 9. Atomics: Add @OptIn(ExperimentalAtomicApi::class) to var/val/function when you use classes in kotlin.concurrent.atomics Style 9.1 fun incrementAndGet(): Int is deprecated. Use incrementAndFetch() instead. Style 9.2 do not use fun get(), use fun load() instead Style 9.3 do not use fun addAndGet(), use fun addAndFetch() instead Style 10. assert keyword in java need to be replaced with assert() function with import org.gnit.lucenekmp.jdkport.assert Style 11. In commonMain/commonTest, any Java Thread-style test or production logic must be ported with coroutine/job semantics or existing jdkport synchronization tools; never introduce JVM-only thread helpers.

Test Iteration Requirement (mandatory)

• For ported/modified tests, always iterate with focused jvmTest first because it is faster.
• Keep fixing and rerunning on JVM until the focused tests pass.
• After JVM pass, run focused macosX64Test last to detect native-only issues.