Swift sign extension with variable number of bits - swift

Question

I need to sign-extend an 8bit value to 12 bits. In C, I can do it this way. I read Apple's BinaryInteger protocol documentation, but it didn't explain sign extending to a variable number of bits (and i'm also pretty new at Swift). How can I do this in Swift, assuming val is UInt8 and numbits is 12?
#define MASKBITS(numbits) ((1 << numbits) - 1)
#define SIGNEXTEND_TO_16(val, numbits) \
( \
(int16_t)((val & MASKBITS(numbits)) | ( \
(val & (1 << (numbits-1))) ? ~MASKBITS(numbits) : 0) \
))

Answer 1

You can use Int8(bitPattern:) to convert the given unsigned
value to a signed value with the same binary representation,
then sign extend by converting to Int16, make unsigned again, and finally truncate
to the given number of bits:
func signExtend(val: UInt8, numBits: Int) -> UInt16 {
// Sign extend to unsigned 16-bit:
var extended = UInt16(bitPattern: Int16(Int8(bitPattern: val)))
// Truncate to given number of bits:
if numBits < 16 {
extended = extended & ((1 << numBits) - 1)
}
return extended
}
Example:
for i in 1...16 {
let x = signExtend(val: 200, numBits: i)
print(String(format: "%2d %04X", i, x))
}
Output:
1 0000
2 0000
3 0000
4 0008
5 0008
6 0008
7 0048
8 00C8
9 01C8
10 03C8
11 07C8
12 0FC8
13 1FC8
14 3FC8
15 7FC8
16 FFC8

Answer 2

I had the same question in the context of bitstream parsing. I needed code to parse n bit two's complement values into Int32. Here is my solution that works without any condition:
extension UInt32 {
func signExtension(n: Int) -> Int32 {
let signed = Int32.init(bitPattern: self << (32 - n))
let result = signed >> (32 - n)
return result
}
}
And a unit test function showing how to use that code:
func testSignExtension_N_2_3() {
let unsignedValue: UInt32 = 0b110
let signedValue: Int32 = unsignedValue.signExtension(n: 3)
XCTAssertEqual(signedValue, -2)
}