Kotlin ULong

ULong is an unsigned 64-bit integer type in Kotlin that represents values from 0 to 18,446,744,073,709,551,615 ($2^{64} - 1$). It is implemented as a value class (specifically, a @JvmInline value class) that wraps a standard signed 64-bit Long. This design provides unsigned arithmetic and bitwise semantics while avoiding object allocation overhead at runtime, compiling down to a primitive long on the JVM in unboxed contexts.

Instantiation and Syntax

ULong literals are defined by appending the u (or U) and L suffixes to an integer literal.

val explicitULong: ULong = 18446744073709551615uL
val inferredULong = 123UL // Type inferred as ULong
val hexULong = 0xFFFFFFFFFFFFFFFFuL
val binULong = 0b10101010uL

Standard numeric types and strings can be explicitly converted to ULong using extension functions:

val fromInt: ULong = 42.toULong()
val fromSignedLong: ULong = (-1L).toULong() // Results in ULong.MAX_VALUE due to two's complement
val fromString: ULong = "18446744073709551615".toULong()

Memory Representation and Boxing

Because ULong is an inline value class, the Kotlin compiler treats it as a primitive long during execution whenever possible. However, boxing occurs when ULong is used in generic contexts, nullable types, or as an interface implementation.

// Unboxed: Stored as a primitive 64-bit integer (long) on the JVM
val unboxed: ULong = 100UL 

// Boxed: Instantiates a wrapper object on the heap
val boxed: ULong? = 100UL 
val genericList: List<ULong> = listOf(100UL) 

Operations and Semantics

ULong supports standard arithmetic (+, -, *, /, %) and comparison (<, >, ==) operators. The Kotlin compiler translates these into unsigned operations at the bytecode level (e.g., utilizing java.lang.Long.divideUnsigned on the JVM).

val a = 10UL
val b = 3UL
val division = a / b // Evaluates to 3UL

Bitwise Operations: ULong supports standard bitwise operations (and, or, xor, inv). However, bitwise shift semantics differ from signed Long:

• shr (Shift Right): Performs a logical shift (zero-fill), whereas shr on a signed Long performs an arithmetic shift (sign-extend).
• ushr (Unsigned Shift Right): Not available on ULong, as shr already provides the logical shift behavior.
• shl (Shift Left): Functions identically to signed integers.

val value = 0xFFFFFFFFFFFFFFFFuL
val shifted = value shr 8 // Fills highest 8 bits with 0s

ULongArray

To prevent boxing overhead when working with collections of ULong data, Kotlin provides ULongArray. At the JVM level, ULongArray is backed by a primitive long[], ensuring contiguous memory allocation and zero boxing per element.

// Initialization via factory function
val array = ULongArray(5) { index -> (index * 2).toULong() }

// Initialization via vararg function
val literalArray = ulongArrayOf(1UL, 2UL, 3UL)

// Access and mutation
array[0] = 99UL
val firstElement = array[0]

Type Constraints and Interoperability

• Widening Conversions: Kotlin does not implicitly widen types. A UInt cannot be passed to a function expecting a ULong without an explicit .toULong() cast.
• Java Interoperability: Because Java lacks native unsigned types, ULong is exposed to Java code as a signed long. Functions accepting ULong in Kotlin will undergo name mangling in the generated JVM bytecode to prevent signature collisions with functions accepting signed Long.

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