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@usableFromInline
enum LEBError: Swift.Error, Equatable {
case overflow
case integerRepresentationTooLong
case insufficientBytes
}
@inlinable
func decodeLEB128<IntType, Stream>(
stream: Stream
) throws -> IntType where IntType: FixedWidthInteger, IntType: UnsignedInteger, Stream: ByteStream {
let firstByte = try stream.consumeAny()
var result: IntType = IntType(firstByte & 0b0111_1111)
if _fastPath(firstByte & 0b1000_0000 == 0) {
return result
}
var shift: UInt = 7
while true {
let byte = try stream.consumeAny()
let slice = IntType(byte & 0b0111_1111)
let nextShift = shift + 7
if nextShift >= IntType.bitWidth, (byte >> (UInt(IntType.bitWidth) - shift)) != 0 {
throw LEBError.integerRepresentationTooLong
}
result |= slice << shift
shift = nextShift
guard byte & 0b1000_0000 != 0 else { break }
}
return result
}
@inlinable
func decodeLEB128<IntType, Stream>(
stream: Stream, bitWidth: Int = IntType.bitWidth
) throws -> IntType where IntType: FixedWidthInteger, IntType: RawSignedInteger, Stream: ByteStream {
let firstByte = try stream.consumeAny()
var result = IntType.Unsigned(firstByte & 0b0111_1111)
if _fastPath(firstByte & 0b1000_0000 == 0) {
// Interpret Int${Self.bitWidth-1} as Int${Self.bitWidth}
return (IntType(bitPattern: result) << (IntType.bitWidth - 7)) >> (IntType.bitWidth - 7)
}
var shift: IntType = 7
var byte: UInt8
repeat {
byte = try stream.consumeAny()
let slice = IntType.Unsigned(byte & 0b0111_1111)
result |= slice << shift
// When we don't have enough bit width
if shift > (bitWidth - 7) {
let remainingBitWidth = bitWidth - Int(shift)
let continuationBit = (byte & 0b1000_0000) != 0
// When a next byte is expected
if continuationBit {
throw LEBError.integerRepresentationTooLong
}
let signAndDiscardingBits = Int8(bitPattern: byte << 1) >> remainingBitWidth
// When meaningful bits are discarded
if signAndDiscardingBits != 0 && signAndDiscardingBits != -1 {
throw LEBError.overflow
}
return IntType(bitPattern: result)
}
shift += 7
} while byte & 0b1000_0000 != 0
// Sign flag is second high-order bit
if byte & 0b0100_0000 != 0 {
// Sign extend
result |= IntType.Unsigned(bitPattern: ~0) << shift
}
return IntType(bitPattern: result)
}
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