A drop-in replacement for java-diff-utils that is ~10× faster.

Fast Kotlin Diff Utils started as the change engine for ChangeDetection since java-diff-utils was causing UI freezes. By rewriting the core in Kotlin and applying profiling-driven optimizations (with help from Claude Opus 4.5, GPT-5.2, and Gemini 3 Pro), we achieved massive performance gains without changing the API.

On typical file diffs (2,000 lines, 10% edits), Fast Kotlin Diff Utils operates in microseconds where the original took milliseconds.

We don't just run the same algorithm in proper Kotlin; we improved it:

• Hash-based snake detection: We verify hash equality before expensive equals() calls, skipping ~50% of comparisons.
• Stable edge trimming: Real files often have unchanged headers/footers (imports, licenses). We detect and trim them before running the O(ND) algorithm, delivering 4-18× speedups for common cases.
• Patience-style anchors: For large files, we use unique lines as anchors to split the problem into smaller, independent chunks.
• Primitive collections: Our custom IntIntMap stores raw integers directly, avoiding the heavy memory overhead of allocating millions of Integer objects.

dependencies {
    implementation("io.github.bernaferrari:fast-kotlin-diff-core:1.0.0")

    // Optional extensions:
    // implementation("io.github.bernaferrari:fast-kotlin-diff-jvm-io:1.0.0")  // Unified diff parsing/writing
    // implementation("io.github.bernaferrari:fast-kotlin-diff-jgit:1.0.0")    // Histogram diff via JGit
}

The API is identical to java-diff-utils, so you can likely just change your imports.

Basic diff:

import com.bernaferrari.difflib.DiffUtils

val patch = DiffUtils.diff(original, revised)
val result = patch.applyTo(original)

Side-by-side diff rows:

val generator = DiffRowGenerator.create()
    .showInlineDiffs(true)
    .ignoreWhiteSpaces(true)
    .build()

val rows = generator.generateDiffRows(originalLines, revisedLines)

Complex comparison (Custom Equalizer):

// Compare objects by ID only, ignoring other fields
val patch = DiffUtils.diff(
    users,
    updatedUsers,
    MyersDiff { a, b -> a.id == b.id }
)

• fast-kotlin-diff-core: The main library. Myers diff, patch apply/restore, diff-row generation. 100% Kotlin, Multiplatform-ready.
• fast-kotlin-diff-jvm-io: Unified diff parser/writer and Java serialization helpers. (JVM only).
• fast-kotlin-diff-jgit: Histogram diff adapter via Eclipse JGit.

We welcome contributions!

• Run tests: ./gradlew test
• Run benchmarks: ./gradlew :fast-kotlin-diff-bench:jmh
• Format code: ./gradlew spotlessCheck

There are other Kotlin ports of java-diff-utils:

• petertrr/kotlin-multiplatform-diff
• GitLiveApp/kotlin-diff-utils

These are straight ports—they convert the Java code to Kotlin but don't change the underlying algorithm. Fast Kotlin Diff Utils went further: we profiled real workloads, identified bottlenecks, and rewrote the core. The result is ~10× faster performance on typical diffs. If you're migrating from one of these, just update your imports to com.bernaferrari.difflib.*.

MIT — see LICENSE.