MultiArray.swift

struct MultiArray<Element> {
  
  private var data: [Element]  // the actual data
  private var counts: [Int]    // the size of each dimension
  private var offsets: [Int]   // multipliers for accessing each dimension quickly
  
  private func computeIndex(indices: [Int]) -> Int {
    precondition(indices.count == counts.count)
    for (index, count) in zip(indices, counts) {
      precondition(index >= 0 && index < count)
    }
    return zip(indices, offsets).reduce(0) { $0 + $1.0 * $1.1 }
  }
  
  var dimensions: Int {
    return counts.count
  }
  
  var count: Int {
    return data.count
  }
  
  subscript(indicies: Int...) -> Element {
    get {
      let index = computeIndex(indicies)
      return data[index]
    }
    set {
      let index = computeIndex(indicies)
      data[index] = newValue
    }
  }
  
  init(counts: [Int], repeatedValue: Element) {
    self.counts = counts
    
    var offset = 1
    offsets = counts.map { input in
      let output = offset
      offset *= input
      return output
    }
    
    let totalCount = counts.reduce(1) { $0 * $1 }
    
    data = Array(count: totalCount, repeatedValue: repeatedValue)
  }
}

// Usage:

var md = MultiArray(counts: [10,20,20], repeatedValue: 0)
md[1,1,1] = 10
md[0,1,0]
md[1,1,1]

md.count