Generate all Ramanujan numbers: (a^3) + (b^3) = (c^3) + (d^3), where 0 < a, b, c, d < n

ramanujan.swift

// Generate all Ramanujan numbers: (a^3) + (b^3) = (c^3) + (d^3)
// Where 0 < a, b, c, d < n

let n = 1000

let cubes = (0..<n).map({$0*$0*$0})
let cubeCount = cubes.count

var sumsOfPairs = [Int: (Int, Int)]()
var ramanujans = [Int: [(Int, Int)]]()

(1..<(cubeCount - 1)).forEach({a in
  ((a + 1)..<cubeCount).forEach({b in
    let (a3, b3) = (cubes[a], cubes[b])
    let sum = a3 + b3
    if let (c, d) = sumsOfPairs[sum] {
      if var r = ramanujans[sum] {
        r.append((a, b))
      } else {
        ramanujans[sum] = [(a, b), (c, d)]
      }
    } else {
      sumsOfPairs[sum] = (a, b)
    }
  })
})

let sortedRamanujans = ramanujans.sorted(isOrderedBefore: {$0.0 < $1.0})
for (ramanujanNumber, cubePairs) in sortedRamanujans {
  print("Ramanujan Number: \(ramanujanNumber), cube pairs: \(cubePairs)")
}