Pure Kotlin multiplatform systems programming library for exact C code porting — providing deterministic, bit-exact low-level primitives across Kotlin/JS and Kotlin/Native targets to enable reliable C-to-Kotlin code migration.

Source-code rule: no import java.* / import javax.* in src/. See CLAUDE.md.

KLang is not a cinterop wrapper or FFI layer. It's a pure Kotlin implementation that precisely replicates C's bitwise operations, memory model, and numeric behavior. This enables accurate porting of C code to idiomatic Kotlin multiplatform while maintaining exact bit-level compatibility.

Multiple large-scale C-to-Kotlin porting projects failed or took months to debug due to subtle behavioral differences between Kotlin and C:

• 16-bit C code had different sign extension and overflow behavior
• Double-double floating point algorithms broke due to Kotlin's multiple rounding steps
• Cryptographic implementations produced incorrect results from platform-specific bitwise operations
• High-precision computation (HPC) code couldn't be reliably ported
• Mark Adler's compression algorithms relied on specific C bit manipulation semantics

KLang solves these issues by implementing C semantics in pure, portable Kotlin.

KLang bridges the gap between Kotlin's high-level abstractions and C's low-level control, enabling:

• Exact C behavior: Bit-exact replication of C operations in pure Kotlin
• Cross-platform determinism: Identical behavior across targets with explicit little-endian semantics
• Pure Kotlin implementation: No native dependencies, GMP, or glibc required — everything is reproducible Kotlin code
• C code porting: Direct mapping of C memory models, pointer arithmetic, and type semantics
• Performance: Native-speed operations with optional arithmetic-mode for bit-exact C compatibility

All C-style memory lives in a single GlobalHeap backed by a pure Kotlin ByteArray. Pointers are Int byte offsets, providing:

• 16-byte aligned allocator (KMalloc) with coalescing, splitting, and size-class bins
• Deterministic typed I/O: All loads/stores are little-endian across platforms
• Complete libc surface: malloc/calloc/realloc/free, strlen/strcmp/memcpy/memmove, etc.

Unlike C's platform-dependent long double, KLang provides explicit control:

• CDouble: IEEE-754 binary64 (exact on all targets)
• CLongDouble: Selectable flavors — DOUBLE64, EXTENDED80, or IEEE128
• CFloat128: Double-double arithmetic (~106-bit mantissa precision)

Benchmark Results (100M summations of 1e-8, expected: 1.0):

Simple double:       1.000000082740371 (error: 8.27e-08)  
Kahan compensated:   1.000000000000000 (error: 1.11e-16, 1.8x slower)
KLang double-double: 1.000000000000000 (error: 4.44e-31, 2.1x slower)

Double-double provides ~15 orders of magnitude better precision than compensated summation while maintaining consistent behavior across all platforms.

A unified shift interface supporting both performance and C-exact determinism:

• NATIVE mode: Fast path using Kotlin's native shl/shr/ushr operations (use only after validation)
• ARITHMETIC mode: Pure multiply/divide/add operations that exactly replicate C behavior across all platforms
• Forbidden practices: Raw Kotlin bitwise operators and hard-coded masks are prohibited outside BitShiftEngine to ensure C compatibility
• Array operations: Efficient multi-limb shifts for arbitrary-precision arithmetic

Why this matters: Kotlin's shl, shr, and bitwise operators have platform-specific behavior that breaks C algorithm ports. A single misplaced and 0xFF can cause cryptographic or compression algorithms to produce different results on different platforms.

All C types have Klang equivalents with exact semantic matching:

• Standard integers: C_UInt8, C_UInt16, C_UInt32, C_UInt64 (and signed variants)
• Extended integers: C_UInt128, C_Int128 (matching GCC/Clang's __uint128 and __int128)
• SwAR128: Experimental SIMD-within-a-register 128-bit operations (Klang's own invention)
• Floating point: CFloat, CDouble, CLongDouble, CFloat128 with configurable precision profiles

All types use heap-based storage for zero-copy operations and exact C memory layout.

• JavaScript: ES2015+ with ES modules (browser environment)
• Native:
  • macOS (ARM64, x64)
  • Linux (x64, ARM64)
  • Windows (x64 via MinGW)

All platforms provide identical semantics — no endianness leaks, no platform-specific floating-point quirks.

• C code porting: Port C libraries and algorithms to pure Kotlin multiplatform with confidence
• Cross-platform numeric algorithms: Implement once in C-style, get identical results everywhere
• Legacy code migration: Migrate 16-bit or 32-bit C code without behavioral regressions
• High-precision computing: Extended precision without platform-specific dependencies or GMP
• Cryptography: Deterministic operations critical for security implementations
• Compression algorithms: Port zlib, bzip2, or similar with exact C behavior
• Systems programming in Kotlin: Memory management, pointer arithmetic, bit manipulation

Memory Management

• GlobalArrayHeap: Single expandable heap with typed load/store operations
• KMalloc: Production-ready allocator with coalescing and fragmentation control
• CPointer<T>: Type-safe pointer abstraction over Int offsets

Numerical Computing

• BitShiftEngine: Configurable shift strategies (native vs arithmetic)
• ArrayBitShifts: Efficient multi-limb operations for arbitrary-precision math
• HeapUInt128: 128-bit unsigned integer with full arithmetic support
• CFloat128: Double-double precision (~31 decimal digits)

C Library Compatibility

• String operations: strlen, strcmp, strcpy, strncpy, strchr, strstr
• Memory operations: memcpy, memmove, memset, memchr, memcmp
• All operations use word-at-a-time loops for high throughput

Detailed documentation available in docs/klang/:

• 00-index.md — Documentation overview
• 01-overview.md — Architecture and key concepts
• 02-global-heap.md — Memory model
• 03-allocator-kmalloc.md — Allocator design
• 05-bitshift-engine.md — Shift strategies
• 11-poc-benchmark.md — Performance analysis

• JDK 11 or higher
• Gradle 8.x (wrapper included)

# Build all targets
./gradlew build

# Build web/JS target
./gradlew jsWeb

# Build native executable (macOS ARM64)
./gradlew buildMac

# Run native executable
./gradlew run

# Build Docker image (Linux ARM64)
./gradlew buildDockerImage

# Run all tests
./gradlew test

# Run JS tests only
./gradlew jsTest

# Run native tests (macOS ARM64)
./gradlew macosArm64Test

src/
├── commonMain/kotlin/io/github/kotlinmania/klang/
│   ├── mem/              # Memory management (GlobalHeap, KMalloc, CLib)
│   ├── bitwise/          # BitShiftEngine, array operations
│   ├── fp/               # Floating-point types (CDouble, CLongDouble, CFloat128)
│   ├── int/hpc/          # HeapUInt128 and arbitrary-precision integers
│   └── stringshift/      # String manipulation utilities
├── jsMain/              # JavaScript-specific implementations
└── nativeMain/          # Native platform implementations

tools/                   # C reference implementations for validation
docs/klang/             # Comprehensive documentation

import io.github.kotlinmania.klang.mem.*

// Initialize heap
GlobalHeap.init(4096)

// Allocate memory
val ptr = Runtime.kmalloc(256)
ptr.store(0x12345678)
val value = ptr.load()

// C-style string operations
val str = "Hello, KLang!".toCString()
val len = CLib.strlen(str)

Runtime.kfree(ptr)

import io.github.kotlinmania.klang.fp.CFloat128

// Double-double precision
val a = CFloat128(1.0 / 3.0)  // ~106-bit mantissa
val b = CFloat128(2.0 / 3.0)
val sum = a + b
// Result is accurate to ~31 decimal digits

import io.github.kotlinmania.klang.bitwise.*

// Configure shift mode
BitShiftConfig.defaultMode = BitShiftMode.NATIVE

// Shift large limb arrays
val limbs = intArrayOf(0x1234, 0xABCD, 0x00FF, 0xC0DE)
val shifted = ArrayBitShifts.shl16LEInPlace(limbs, bits = 37)

KLang is designed for production use with performance characteristics competitive with native C implementations:

• Native mode shifts: Near-zero overhead vs Kotlin's built-in operations
• Allocator: O(1) small allocations via size-class bins, efficient coalescing
• String operations: Word-at-a-time processing (4-8 bytes per iteration)
• Double-double arithmetic: 2-3x slower than double, 15+ orders of magnitude more precise

See docs/klang/11-poc-benchmark.md for detailed performance analysis.

Contributions welcome! Areas of interest:

• Additional C library functions
• Platform-specific optimizations
• Extended precision rounding modes
• Documentation and examples

Licensed under the Apache License, Version 2.0. See LICENSE for details.