Conform Bytes to Comparable

[dimitribouniol]

Low level enough to be good to implemented here, especially useful when comparing byte arrays that represent lexicographical keys.

Naive implementation:

extension Array: Comparable where Element: Comparable {
    public static func < (lhs: [Element], rhs: [Element]) -> Bool {
        for (leftElement, rightElement) in zip(lhs, rhs) {
            if leftElement < rightElement {
                return true
            } else if leftElement > rightElement {
                return false
            }
        }
        return lhs.count < rhs.count
    }
}

Sneakier solution:
https://github.com/apple/swift/blob/aa3e5904f8ba8bf9ae06d96946774d171074f6e5/stdlib/public/core/StringComparison.swift#L280-L295

// Perform a binary comparison of bytes in memory. Return value is negative if
// less, 0 if equal, positive if greater.
@_effects(readonly)
internal func _binaryCompare<UInt8>(
  _ lhs: UnsafeBufferPointer<UInt8>, _ rhs: UnsafeBufferPointer<UInt8>
) -> Int {
  var cmp = Int(truncatingIfNeeded:
    _swift_stdlib_memcmp(
      lhs.baseAddress._unsafelyUnwrappedUnchecked,
      rhs.baseAddress._unsafelyUnwrappedUnchecked,
      Swift.min(lhs.count, rhs.count)))
  if cmp == 0 {
    cmp = lhs.count &- rhs.count
  }
  return cmp
}

https://github.com/apple/swift/blob/aa3e5904f8ba8bf9ae06d96946774d171074f6e5/stdlib/public/SwiftShims/LibcShims.h#L70-L75

SWIFT_READONLY
static inline int _swift_stdlib_memcmp(const void *s1, const void *s2,
                                       __swift_size_t n) {
  extern int memcmp(const void *, const void *, __swift_size_t);
  return memcmp(s1, s2, n);
}