feat: Adds duct sizing calculations.

Sources/ManualDClient/Helpers.swift

@@ -10,3 +10,60 @@ extension Array where Element == EffectiveLengthGroup {
     reduce(0) { $0 + $1.effectiveLength }
   }
 }
+
+func roundSize(_ size: Double) throws -> Int {
+  guard size > 0 else {
+    throw ManualDError(message: "Size should be greater than 0.")
+  }
+  guard size <= 24 else {
+    throw ManualDError(message: "Size should be less than 24.")
+  }
+
+  switch size {
+  case 0..<4:
+    return 4
+  case 4..<5:
+    return 5
+  case 5..<6:
+    return 6
+  case 6..<7:
+    return 7
+  case 7..<8:
+    return 8
+  case 8..<9:
+    return 9
+  case 9..<10:
+    return 10
+  case 10..<12:
+    return 12
+  case 12..<14:
+    return 14
+  case 14..<16:
+    return 16
+  case 16..<18:
+    return 18
+  case 18..<20:
+    return 20
+  case 20..<22:
+    return 2
+  case 22..<24:
+    return 24
+  default:
+    throw ManualDError(message: "Size '\(size)' not in range.")
+
+  }
+}
+
+func velocity(cfm: Int, roundSize: Int) -> Int {
+  let cfm = Double(cfm)
+  let roundSize = Double(roundSize)
+  let velocity = cfm / (pow(roundSize / 24, 2) * 3.14)
+  return Int(round(velocity))
+}
+
+func flexSize(_ request: ManualDClient.DuctSizeRequest) throws -> Int {
+  let cfm = pow(Double(request.designCFM), 0.4)
+  let fr = pow(request.frictionRate / 1.76, 0.2)
+  let size = 0.55 * (cfm / fr)
+  return try roundSize(size)
+}