Kotlin Unsigned Shift Right

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Syntax

val result = value ushr bitCount

// Alternatively, using standard method invocation:
val result = value.ushr(bitCount)

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Technical Mechanics

• Zero Extension: Unlike the shr (shift right) operator, which performs an arithmetic shift and preserves the sign bit (sign extension), ushr strictly performs a zero extension. This means a negative number shifted using ushr will become a positive number, as the MSB (the sign bit) is overwritten with a 0.
• Supported Types and the Sign-Extension Trap: The ushr operator is defined exclusively for Int (32-bit) and Long (64-bit) types. It is not defined on smaller types like Byte or Short. Attempting to use ushr directly on these types results in a compiler error (Unresolved reference: ushr). While the operand must be explicitly converted first using .toInt() or .toLong(), doing so on a negative number introduces a semantic bug due to sign extension. Calling .toInt() on a negative Byte pads the new 24 upper bits with 1s. To achieve a true 8-bit or 16-bit logical shift, you must apply a bitwise mask (e.g., and 0xFF) to clear the sign-extended bits before shifting:

val myByte: Byte = -8 // 8-bit binary: 11111000

// val error = myByte ushr 2 // Compiler error

// SEMANTIC BUG: .toInt() sign-extends to 32 bits (11111111 11111111 11111111 11111000)
// Shifting right by 2 yields a massive positive integer instead of an 8-bit shift.
val flawed = myByte.toInt() ushr 2 // Result: 1073741822 (00111111 11111111 11111111 11111110)

// CORRECT: Masking with 0xFF clears the sign-extended bits before the shift.
// 1. myByte.toInt() and 0xFF -> 00000000 00000000 00000000 11111000
// 2. ushr 2                  -> 00000000 00000000 00000000 00111110
val correct = (myByte.toInt() and 0xFF) ushr 2 // Result: 62

• Shift Distance Masking: Kotlin masks the right operand (bitCount) to prevent shifting beyond the bit-width of the left operand.
  • For an Int, the shift distance is masked with 31 (binary 11111), meaning only the lowest 5 bits of the right operand are used. x ushr 32 is equivalent to x ushr 0.
  • For a Long, the shift distance is masked with 63 (binary 111111), using the lowest 6 bits.

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Behavior Visualization

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Example 1: Positive Integer

val a = 8          // Binary: 00000000 00000000 00000000 00001000
val b = a ushr 2   // Binary: 00000000 00000000 00000000 00000010
// Result: b = 2

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Example 2: Negative Integer

val x = -2         // Binary: 11111111 11111111 11111111 11111110
val y = x ushr 1   // Binary: 01111111 11111111 11111111 11111111
// Result: y = 2147483647 (Int.MAX_VALUE)