feat: Finishes filter pressure drop calculator

Sources/ApiController/ApiController.swift

@@ -42,8 +42,7 @@ extension ApiController: DependencyKey {
 
       case let .calculateFilterPressureDrop(request):
         logger.debug("Calculating filter pressure drop: \(request)")
-        // FIX:
-        fatalError()
+        return try await request.respond(logger: logger)
 
       case let .calculateHVACSystemPerformance(request):
         logger.debug("Calculating hvac system performance: \(request)")

Sources/ApiController/Extensions/FilterPressureDrop.swift

@@ -0,0 +1,156 @@
+import Foundation
+import Logging
+import Routes
+
+public extension FilterPressureDrop.Request {
+
+  func respond(logger: Logger) async throws -> FilterPressureDrop.Response {
+    switch self {
+    case let .basic(request):
+      return try request.respond(logger: logger)
+    case let .fanLaw(request):
+      return try request.respond(logger: logger)
+    }
+  }
+
+}
+
+private extension FilterPressureDrop.Request.Basic {
+
+  func respond(logger: Logger) throws -> FilterPressureDrop.Response {
+    try validate()
+
+    let cfmPerTon = Double(climateZone.cfmPerTon)
+    let totalCFM = systemSize * cfmPerTon
+    // calculate filter area in sq. ft.
+    let filterArea = (filterWidth * filterHeight) / 144
+    let filterVelocity = totalCFM / filterArea
+    let initialPressureDrop = filterType.initialPressureDrop(velocity: filterVelocity)
+    let maxPressureDrop = 0.15
+
+    var warnings = [String]()
+
+    if filterVelocity > 350 {
+      warnings.append(
+        """
+        Face velocity exceeds 350 FPM - consider using a larger filter to reduce pressure drop and
+        improve system efficiency.
+        """
+      )
+    }
+
+    if initialPressureDrop > (maxPressureDrop / 2) {
+      warnings.append(
+        """
+        Initial pressure drop is more than 50% of maximum allowable - consider using a larger filter or different
+        filter type with lower initial pressure drop.
+        """
+      )
+    }
+
+    return .basic(.init(
+      filterArea: filterArea,
+      feetPerMinute: filterVelocity,
+      initialPressureDrop: initialPressureDrop,
+      maxPressureDrop: maxPressureDrop,
+      warnings: warnings
+    ))
+  }
+
+  func validate() throws {
+    guard systemSize > 0 else {
+      throw ValidationError(message: "System size should be greater than 0.")
+    }
+    guard filterWidth > 0 else {
+      throw ValidationError(message: "Filter width should be greater than 0.")
+    }
+    guard filterHeight > 0 else {
+      throw ValidationError(message: "Filter height should be greater than 0.")
+    }
+  }
+}
+
+private extension FilterPressureDrop.Request.FanLaw {
+
+  func respond(logger: Logger) throws -> FilterPressureDrop.Response {
+    try validate()
+
+    // calculate filter area in square ft.
+    let filterArea = (filterWidth * filterHeight) / 144
+    let faceVelocity = designAirflow / filterArea
+    let velocityRatio = designAirflow / ratedAirflow
+    let pressureRatio = pow(velocityRatio, 2)
+    let predictedPressureDrop = ratedPressureDrop * pressureRatio
+
+    return .fanLaw(.init(
+      predictedPressureDrop: predictedPressureDrop,
+      velocityRatio: velocityRatio,
+      pressureRatio: pressureRatio,
+      faceVelocity: faceVelocity
+    ))
+  }
+
+  func validate() throws {
+    guard filterWidth > 0 else {
+      throw ValidationError(message: "Filter width should be greater than 0.")
+    }
+    guard filterHeight > 0 else {
+      throw ValidationError(message: "Filter height should be greater than 0.")
+    }
+    guard filterDepth > 0 else {
+      throw ValidationError(message: "Filter depth should be greater than 0.")
+    }
+    guard ratedAirflow > 0 else {
+      throw ValidationError(message: "Rated airflow should be greater than 0.")
+    }
+    guard ratedPressureDrop > 0 else {
+      throw ValidationError(message: "Rated pressure drop should be greater than 0.")
+    }
+    guard designAirflow > 0 else {
+      throw ValidationError(message: "Design airflow should be greater than 0.")
+    }
+  }
+}
+
+extension FilterPressureDrop.FilterType {
+
+  // swiftlint:disable cyclomatic_complexity
+  func initialPressureDrop(velocity: Double) -> Double {
+    switch self {
+    case .fiberglass:
+      if velocity <= 300 {
+        return 0.05
+      } else if velocity <= 350 {
+        return 0.08
+      } else {
+        return 0.12
+      }
+    case .pleatedBasic:
+      if velocity <= 300 {
+        return 0.08
+      } else if velocity <= 350 {
+        return 0.12
+      } else {
+        return 0.15
+      }
+    case .pleatedBetter:
+      if velocity <= 300 {
+        return 0.15
+      } else if velocity <= 350 {
+        return 0.18
+      } else {
+        return 0.22
+      }
+    case .pleatedBest:
+      if velocity <= 300 {
+        return 0.20
+      } else if velocity <= 350 {
+        return 0.25
+      } else {
+        return 0.3
+      }
+    }
+  }
+
+  // swiftlint:enable cyclomatic_complexity
+}