DictOfArraysToArrayOfDicts.swift

// DictOfArraysToArrayOfDicts benchmark
//
// Description: Convert a dictionary of [key: [values]] to an array of
//              dictionaries [[key: value]] using zipWith.
// Source: https://gist.github.com/airspeedswift/3675952127ee775551b0

import TestsUtils

public let benchmarks = [
  BenchmarkInfo(
    name: "DictOfArraysToArrayOfDicts",
    runFunction: run_DictOfArraysToArrayOfDicts,
    tags: [.algorithm, .Dictionary]
  ),
]

@inline(never)
public func run_DictOfArraysToArrayOfDicts(_ n: Int) {
  let returnedFromServer = [
    "title": ["abc",  "def", "ghi"],
    "time": ["1234", "5678", "0123"],
    "content": ["qwerty", "asdfg", "zxcvb"],
  ]
  var pairs: [[(String, String)]] = []
  var inverted: [[(String, String)]] = []
  var arrayOfDicts: [[String: String]] = [[:]]

  for _ in 1...100*n {
    pairs = returnedFromServer.map {
      (key, value) in value.map { (key, $0) }
    }
    inverted = zipWith(pairs[0], pairs[1], pairs[2]) {
      [$0] + [$1] + [$2]
    }
    arrayOfDicts = inverted
    .map { $0.map { (key: $0.0, value: $0.1) } }
    .map { Dictionary($0) }

    if !(arrayOfDicts.count == 3) {
      break
    }
  }

  check(arrayOfDicts.count == 3)
}

// Given [
//  "title" : ["abc",  "def"],
//  "time"  : ["1234", "5678", "0123"],
//  "content":["qwerty", "asdfg", "zxcvb"]
// ]
//
// how do you get to this:
//
// [
//  ["title" : "abc",
//   "time"  : "1234",
//   "content": "qwerty"],
// ["title" : "def",
//  "time"  : "5678",
//  "content": "asdfg"],
// ["title" : "ghi",
//  "time"  : "0123",
//  "content": "zxcvb"]]

public func zip3 <
  A: Sequence,B: Sequence,C: Sequence
> (_ a: A, _ b: B, _ c: C) -> ZipSequence3<A, B, C> {
  return ZipSequence3(a, b, c)
}

// Sequence of tuples created from values from three other sequences
public struct ZipSequence3<A: Sequence,B: Sequence,C: Sequence> {
  private var a: A
  private var b: B
  private var c: C

  public init (_ a: A, _ b: B, _ c: C) {
    self.a = a
    self.b = b
    self.c = c
  }  
}

extension ZipSequence3 {
  public struct Iterator {
    private var a: A.Iterator
    private var b: B.Iterator
    private var c: C.Iterator

    public init(_ a: A, _ b: B, _ c: C) {
      self.a = a.makeIterator()
      self.b = b.makeIterator()
      self.c = c.makeIterator()
    }
  }
}

extension ZipSequence3.Iterator: IteratorProtocol {
  public typealias Element = (A.Element,B.Element,C.Element)
  
  public mutating func next() -> Element? {
    switch (a.next(), b.next(), c.next()) {
    case let (aValue?, bValue?, cValue?):
      return (aValue, bValue, cValue)
    default:
      return nil
    }
  }
}

extension ZipSequence3: Sequence {
  public typealias Element = (A.Element,B.Element,C.Element)
  public typealias SubSequence = AnySequence<Element>

  public func makeIterator() -> Iterator {
    return Iterator(a, b, c)
  }
}

// Iterator that creates tuples of values from three other generators
func zipWith<
  A: Sequence, B: Sequence, C: Sequence, T
>(
  _ a: A, _ b: B, _ c: C, 
  _ combine: (A.Element,B.Element,C.Element) -> T
) -> [T] {
  return zip3(a,b,c).map(combine)
}

extension Dictionary {
  // Construct from an arbitrary sequence with elements of the tupe
  // `(Key,Value)`
  init<S: Sequence> (_ seq: S) where S.Iterator.Element == Element {
    self.init()
    self.merge(seq)
  }

  // Merge a sequence of `(Key,Value)` tuples into the dictionary
  mutating func merge<S: Sequence> (_ seq: S) where S.Element == Element {
    var gen = seq.makeIterator()
    while let (k, v) = gen.next() {
      self[k] = v
    }
  }
}