feat: update Fairhold formula with new linear one

app/models/Fairhold.test.ts

@@ -10,19 +10,18 @@ it("can be instantiated", () => {
 
 it("correctly calculates the fairhold formula below the threshold", () => {
   const fairhold = new Fairhold({
-    affordability: 0.05,
+    affordability: 0.15,
     landPriceOrRent: 100,
   });
-  expect(fairhold.discountLand).toBeCloseTo(0.6877641290737884);
+  expect(fairhold.discountLand).toBeCloseTo(0.535);
 });
 
 it("correctly calculates the fairhold formula above the threshold", () => {
   const fairhold = new Fairhold({
-    affordability: 0.05,
+    affordability: 0.5,
     landPriceOrRent: 100,
-    plateau: 3,
   });
-  expect(fairhold.discountLand).toBeCloseTo(0.68776);
+  expect(fairhold.discountLand).toBeCloseTo(0.15);
 });
 
 it("correctly deals with negative values of landPriceOrRent", () => {

app/models/Fairhold.ts

@@ -1,50 +1,22 @@
 import * as math from "mathjs";
 
-const DEFAULT_AMPLITUDE = 0.25;
-const DEFAULT_LENGTH = 1;
-const DEFAULT_POSITION = 0.45;
-const DEFAULT_PLATEAU = 0.15;
-const DEFAULT_THRESHOLD = 0.5;
-
-type ConstructorParams = Pick<Fairhold, "affordability" | "landPriceOrRent"> &
-  Partial<
-    Pick<
-      Fairhold,
-      "amplitude" | "length" | "position" | "plateau" | "threshold"
-    >
-  >;
+type ConstructorParams = Pick<Fairhold, "affordability" | "landPriceOrRent">;
 
 export class Fairhold {
   public affordability: number;
   public landPriceOrRent: number;
-  public amplitude: number;
-  public length: number;
-  public position: number;
-  public plateau: number;
-  public threshold: number;
   public discountLand: number;
   public discountedLandPriceOrRent: number;
 
   constructor(params: ConstructorParams) {
     this.affordability = params.affordability;
     this.landPriceOrRent = params.landPriceOrRent;
-    this.amplitude = params.amplitude || DEFAULT_AMPLITUDE;
-    this.length = params.length || DEFAULT_LENGTH;
-    this.position = params.position || DEFAULT_POSITION;
-    this.plateau = params.plateau || DEFAULT_PLATEAU;
-    this.threshold = params.threshold || DEFAULT_THRESHOLD;
     this.discountLand = this.calculateFairholdDiscount();
     this.discountedLandPriceOrRent = this.calculateDiscountedPriceOrRent();
   }
 
   private calculateFairholdDiscount() {
-    if (this.affordability > this.threshold) return this.plateau;
-
-    const discountLand =
-      this.amplitude *
-        math.cos((2 * math.pi * this.affordability) / this.length) +
-      this.position;
-
+    const discountLand = math.max(-1.1 * this.affordability + 0.7, 0.15)
     return discountLand;
   }