Functional Swift vs non-functional Swift

gistfile1.swift

///////////////////////////////////////
// - Non-functional
///////////////////////////////////////
var buffer: [Match] = []
var bufferDate: Int = 0
var sections: [FeedSection] = []

for match in matches {
  if bufferDate != match.createdAt.dateInt {
    if let firstMatch = buffer.first {
      sections.append(FeedSection(title: firstMatch.createdAt.prettyString(), matches: buffer))
      buffer.removeAll()
    }

    bufferDate = match.createdAt.dateInt
  }
  buffer.append(match)
}
if let firstMatch = buffer.first {
  sections.append(FeedSection(title: firstMatch.createdAt.prettyString(), matches: buffer))
}

self.sections = sections

///////////////////////////////////////
// - Functional
///////////////////////////////////////
self.sections = matches
  .categorise {
    $0.createdAt.prettyString()
  }
  .map {
    FeedSection(title: $0.key.uppercased(), matches: $0.value)
  }
  .sorted {
    $0.0.matches.first!.createdAt.dateInt > $0.1.matches.first!.createdAt.dateInt
  }