src/Prices/PriceCalculator.cs

@@ -16,6 +16,13 @@ public static class PriceCalculator
             throw new ArgumentOutOfRangeException(nameof(discountPercent), "A discount is between 0 and 100 percent.");
         }
 
-        return price - discountPercent;
+        var raw = price * (100 - discountPercent) / 100m;
+        return RoundToFiveRappen(raw);
     }
-}
+
+    /// <summary>Rounds a decimal to the nearest 0.05 (Swiss Rappenrundung).</summary>
+    private static decimal RoundToFiveRappen(decimal value)
+    {
+        return Math.Round(value * 20m, MidpointRounding.AwayFromZero) / 20m;
+    }
+}

tests/Prices.Tests/PriceCalculatorTests.cs

@@ -33,4 +33,25 @@ public sealed class PriceCalculatorTests
     {
         Assert.Throws<ArgumentOutOfRangeException>(() => PriceCalculator.ApplyDiscount(100m, 101m));
     }
-}
+
+    [Fact]
+    public void Swiss_rappernrundung_rounds_to_nearest_0_05()
+    {
+        // 99.90 * 0.9 = 89.91 → 89.91 * 20 = 1798.2 → round to 1798 → 1798 / 20 = 89.90
+        Assert.Equal(89.90m, PriceCalculator.ApplyDiscount(99.90m, 10m));
+    }
+
+    [Fact]
+    public void Rounding_rounds_down_when_cent_remainder_is_below_0_05()
+    {
+        // 1.23 * 0.5 = 0.615 → 0.615 * 20 = 12.3 → round to 12 → 12 / 20 = 0.60
+        Assert.Equal(0.60m, PriceCalculator.ApplyDiscount(1.23m, 50m));
+    }
+
+    [Fact]
+    public void Rounding_rounds_up_when_cent_remainder_is_at_0_05()
+    {
+        // 3.37 * 0.5 = 1.685 → 1.685 * 20 = 33.7 → round to 34 → 34 / 20 = 1.70
+        Assert.Equal(1.70m, PriceCalculator.ApplyDiscount(3.37m, 50m));
+    }
+}

web/src/app/app.html

@@ -341,4 +341,6 @@
 <!-- * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * -->
 
 
+<app-discount-calculator></app-discount-calculator>
+
 <router-outlet />

web/src/app/app.ts

@@ -1,12 +1,13 @@
 import { Component, signal } from '@angular/core';
 import { RouterOutlet } from '@angular/router';
+import { DiscountCalculator } from './discount-calculator/discount-calculator';
 
 @Component({
   selector: 'app-root',
-  imports: [RouterOutlet],
+  imports: [RouterOutlet, DiscountCalculator],
   templateUrl: './app.html',
   styleUrl: './app.scss'
 })
 export class App {
   protected readonly title = signal('web');
-}
+}

web/src/app/discount-calculator/discount-calculator.html

@@ -0,0 +1,34 @@
+<div class="calculator">
+  <h2>Rabatt-Rechner</h2>
+
+  <div class="field">
+    <label for="price">Preis (CHF)</label>
+    <input
+      id="price"
+      type="number"
+      min="0"
+      step="0.05"
+      placeholder="z.B. 100"
+      (input)="onPriceInput($event)"
+    />
+  </div>
+
+  <div class="field">
+    <label for="discount">Rabatt (%)</label>
+    <input
+      id="discount"
+      type="number"
+      min="0"
+      max="100"
+      step="0.5"
+      placeholder="z.B. 10"
+      (input)="onDiscountInput($event)"
+    />
+  </div>
+
+  @if (discountedPrice !== null) {
+    <div class="result">
+      Endpreis: <strong>{{ formatPrice(discountedPrice) }} CHF</strong>
+    </div>
+  }
+</div>

web/src/app/discount-calculator/discount-calculator.scss

@@ -0,0 +1,47 @@
+:host {
+  display: block;
+  max-width: 400px;
+  margin: 2rem auto;
+  padding: 1.5rem;
+  border: 1px solid #ddd;
+  border-radius: 8px;
+  font-family: sans-serif;
+}
+
+.calculator {
+  h2 {
+    margin: 0 0 1rem;
+    font-size: 1.25rem;
+  }
+}
+
+.field {
+  margin-bottom: 1rem;
+
+  label {
+    display: block;
+    margin-bottom: 0.25rem;
+    font-weight: 500;
+  }
+
+  input {
+    width: 100%;
+    padding: 0.5rem;
+    border: 1px solid #ccc;
+    border-radius: 4px;
+    font-size: 1rem;
+    box-sizing: border-box;
+  }
+}
+
+.result {
+  margin-top: 1rem;
+  padding: 0.75rem;
+  background: #e8f5e9;
+  border-radius: 4px;
+  font-size: 1.1rem;
+
+  strong {
+    color: #2e7d32;
+  }
+}

web/src/app/discount-calculator/discount-calculator.spec.ts

@@ -0,0 +1,67 @@
+import { TestBed, type ComponentFixture } from '@angular/core/testing';
+import { DiscountCalculator } from './discount-calculator';
+
+describe('DiscountCalculator', () => {
+  async function createFixture(): Promise<ComponentFixture<DiscountCalculator>> {
+    await TestBed.configureTestingModule({
+      imports: [DiscountCalculator],
+    }).compileComponents();
+    return TestBed.createComponent(DiscountCalculator);
+  }
+
+  it('should create the component', async () => {
+    const fixture = await createFixture();
+    const component = fixture.componentInstance;
+    expect(component).toBeTruthy();
+  });
+
+  it('should calculate 100 CHF with 10% discount to 90.00', async () => {
+    const fixture = await createFixture();
+    const component = fixture.componentInstance;
+
+    component['price'].set(100);
+    component['discountPercent'].set(10);
+    fixture.detectChanges();
+
+    const compiled = fixture.nativeElement as HTMLElement;
+    expect(compiled.querySelector('.result')?.textContent).toContain('90.00 CHF');
+  });
+
+  it('should round 99.90 CHF with 10% discount to 89.90 (Swiss 0.05 rounding)', async () => {
+    const fixture = await createFixture();
+    const component = fixture.componentInstance;
+
+    component['price'].set(99.90);
+    component['discountPercent'].set(10);
+    fixture.detectChanges();
+
+    // raw = 99.90 * 90 / 100 = 89.91
+    // Math.round(89.91 * 20) / 20 = Math.round(1798.2) / 20 = 1798 / 20 = 89.90
+    const compiled = fixture.nativeElement as HTMLElement;
+    expect(compiled.querySelector('.result')?.textContent).toContain('89.90 CHF');
+  });
+
+  it('should not show result when price is negative', async () => {
+    const fixture = await createFixture();
+    const component = fixture.componentInstance;
+
+    component['price'].set(-5);
+    component['discountPercent'].set(10);
+    fixture.detectChanges();
+
+    const compiled = fixture.nativeElement as HTMLElement;
+    expect(compiled.querySelector('.result')).toBeNull();
+  });
+
+  it('should not show result when discount is negative', async () => {
+    const fixture = await createFixture();
+    const component = fixture.componentInstance;
+
+    component['price'].set(100);
+    component['discountPercent'].set(-1);
+    fixture.detectChanges();
+
+    const compiled = fixture.nativeElement as HTMLElement;
+    expect(compiled.querySelector('.result')).toBeNull();
+  });
+});

web/src/app/discount-calculator/discount-calculator.ts

@@ -0,0 +1,36 @@
+import { Component, signal } from '@angular/core';
+
+@Component({
+  selector: 'app-discount-calculator',
+  imports: [],
+  templateUrl: './discount-calculator.html',
+  styleUrl: './discount-calculator.scss'
+})
+export class DiscountCalculator {
+  protected readonly price = signal<number | null>(null);
+  protected readonly discountPercent = signal<number | null>(null);
+
+  protected onPriceInput(event: Event): void {
+    const value = (event.target as HTMLInputElement).value;
+    this.price.set(value === '' ? null : Number(value));
+  }
+
+  protected onDiscountInput(event: Event): void {
+    const value = (event.target as HTMLInputElement).value;
+    this.discountPercent.set(value === '' ? null : Number(value));
+  }
+
+  protected get discountedPrice(): number | null {
+    const p = this.price();
+    const d = this.discountPercent();
+    if (p === null || d === null || p < 0 || d < 0) {
+      return null;
+    }
+    const raw = p * (100 - d) / 100;
+    return Math.round(raw * 20) / 20;
+  }
+
+  protected formatPrice(value: number): string {
+    return value.toFixed(2);
+  }
+}