Solution

.gitignore

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+*.iml
+.gradle
+/local.properties
+/.idea/caches
+/.idea/libraries
+/.idea/modules.xml
+/.idea/workspace.xml
+/.idea/navEditor.xml
+/.idea/assetWizardSettings.xml
+.DS_Store
+/build
+/captures
+.externalNativeBuild
+.cxx
+local.properties

app/.gitignore

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+/build

app/build.gradle

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+plugins {
+    id 'com.android.application'
+    id 'org.jetbrains.kotlin.android'
+}
+
+android {
+    compileSdk 30
+
+    defaultConfig {
+        applicationId "com.sima.se1challenge"
+        minSdk 23
+        targetSdk 30
+        versionCode 1
+        versionName "1.0"
+
+        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
+    }
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+        }
+    }
+
+    buildFeatures {
+        dataBinding true
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+    kotlinOptions {
+        jvmTarget = '1.8'
+    }
+}
+
+dependencies {
+
+    implementation 'androidx.core:core-ktx:1.6.0'
+    implementation 'androidx.appcompat:appcompat:1.3.1'
+    implementation 'com.google.android.material:material:1.4.0'
+    implementation 'androidx.constraintlayout:constraintlayout:2.1.0'
+    testImplementation 'junit:junit:4.13.2'
+    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
+    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
+}

app/proguard-rules.pro

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+# Add project specific ProGuard rules here.
+# You can control the set of applied configuration files using the
+# proguardFiles setting in build.gradle.
+#
+# For more details, see
+#   http://developer.android.com/guide/developing/tools/proguard.html
+
+# If your project uses WebView with JS, uncomment the following
+# and specify the fully qualified class name to the JavaScript interface
+# class:
+#-keepclassmembers class fqcn.of.javascript.interface.for.webview {
+#   public *;
+#}
+
+# Uncomment this to preserve the line number information for
+# debugging stack traces.
+#-keepattributes SourceFile,LineNumberTable
+
+# If you keep the line number information, uncomment this to
+# hide the original source file name.
+#-renamesourcefileattribute SourceFile

app/src/androidTest/java/com/sima/se1challenge/ExampleInstrumentedTest.kt

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+package com.sima.se1challenge
+
+import androidx.test.platform.app.InstrumentationRegistry
+import androidx.test.ext.junit.runners.AndroidJUnit4
+
+import org.junit.Test
+import org.junit.runner.RunWith
+
+import org.junit.Assert.*
+
+/**
+ * Instrumented test, which will execute on an Android device.
+ *
+ * See [testing documentation](http://d.android.com/tools/testing).
+ */
+@RunWith(AndroidJUnit4::class)
+class ExampleInstrumentedTest {
+    @Test
+    fun useAppContext() {
+        // Context of the app under test.
+        val appContext = InstrumentationRegistry.getInstrumentation().targetContext
+        assertEquals("com.sima.se1challenge", appContext.packageName)
+    }
+}

app/src/main/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.sima.se1challenge">
+
+    <application
+        android:allowBackup="true"
+        android:icon="@mipmap/ic_launcher"
+        android:label="@string/app_name"
+        android:roundIcon="@mipmap/ic_launcher_round"
+        android:supportsRtl="true"
+        android:theme="@style/Theme.SE1Challenge">
+        <activity
+            android:name=".MainActivity"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+
+</manifest>

app/src/main/java/com/sima/se1challenge/MainActivity.kt

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+package com.sima.se1challenge
+
+import android.os.Bundle
+import androidx.appcompat.app.AppCompatActivity
+import androidx.databinding.DataBindingUtil
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.ViewModelProvider
+import com.sima.se1challenge.databinding.ActivityMainBinding
+import com.sima.se1challenge.di.DI
+import com.sima.se1challenge.order.loader.DispatchQueue
+
+class MainActivity : AppCompatActivity() {
+
+    private lateinit var viewModel: SEChallengeViewModel
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        val binding: ActivityMainBinding =
+            DataBindingUtil.setContentView(this, R.layout.activity_main)
+        binding.lifecycleOwner = this
+        viewModel =
+            ViewModelProvider(this, ViewModelFactory()).get(SEChallengeViewModel::class.java)
+        binding.viewModel = viewModel
+    }
+
+    override fun onDestroy() {
+        super.onDestroy()
+        DispatchQueue.shutdown()
+    }
+
+    class ViewModelFactory : ViewModelProvider.Factory {
+        override fun <T : ViewModel?> create(modelClass: Class<T>): T {
+            return when {
+                modelClass.isAssignableFrom(SEChallengeViewModel::class.java) -> DI.seChallengeViewModel() as T
+                else -> throw IllegalStateException("Unknown ViewModel")
+            }
+        }
+    }
+}

app/src/main/java/com/sima/se1challenge/SEChallengeViewModel.kt

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+package com.sima.se1challenge
+
+import androidx.databinding.ObservableField
+import androidx.lifecycle.ViewModel
+import com.sima.se1challenge.order.OrderCalculator
+
+class SEChallengeViewModel(
+    private val orderCalculator: OrderCalculator
+) : ViewModel() {
+
+
+    var resultText = ObservableField("")
+
+    fun calculate() {
+        orderCalculator.calculate {
+            it.fold({
+                resultText.set("%.2f".format(it.price))
+            }, {
+                resultText.set("Error ${it.message}")
+            })
+        }
+    }
+}

app/src/main/java/com/sima/se1challenge/di/DI.kt

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+package com.sima.se1challenge.di
+
+import com.sima.se1challenge.SEChallengeViewModel
+import com.sima.se1challenge.order.OrderCalculator
+import com.sima.se1challenge.order.cache.OrderCache
+import com.sima.se1challenge.order.discount.OrderPriceDiscount
+import com.sima.se1challenge.order.generator.OrderGenerator
+import com.sima.se1challenge.order.loader.OrderPricingLoader
+import com.sima.se1challenge.order.loader.RepetitionOrderPricingLoader
+import com.sima.se1challenge.order.logging.OrderLogger
+import com.sima.se1challenge.order.multiplier.OrderPriceMultiplier
+
+object DI {
+    fun orderCache() = OrderCache.getDefault()
+
+    fun orderPriceDiscount() = OrderPriceDiscount.getDefault()
+
+    fun orderGenerator() = OrderGenerator.getDefault()
+
+    fun orderPricingLoader() = OrderPricingLoader.getDefault()
+
+    fun orderLogger() = OrderLogger.getDefault()
+
+    fun orderPriceMultiplier() = OrderPriceMultiplier.getDefault()
+
+    fun repetitionOrderPricingLoader() = RepetitionOrderPricingLoader(orderPricingLoader())
+
+    fun orderCalculator() = OrderCalculator(
+        orderGenerator(),
+        repetitionOrderPricingLoader(),
+        orderCache(),
+        orderPriceMultiplier(),
+        orderPriceDiscount(),
+        orderLogger()
+    )
+
+    fun seChallengeViewModel() = SEChallengeViewModel(
+        orderCalculator()
+    )
+}

app/src/main/java/com/sima/se1challenge/model/Order.kt

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+package com.sima.se1challenge.model
+
+data class Order(val id: String)

app/src/main/java/com/sima/se1challenge/model/OrderPrice.kt

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+package com.sima.se1challenge.model
+
+data class OrderPrice(val price: Double)

app/src/main/java/com/sima/se1challenge/model/PriceDiscount.kt

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+package com.sima.se1challenge.model
+
+data class PriceDiscount(val value: Double)

app/src/main/java/com/sima/se1challenge/model/PriceMultiplier.kt

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+package com.sima.se1challenge.model
+
+data class PriceMultiplier(val value: Double)

app/src/main/java/com/sima/se1challenge/order/OrderCalculator.kt

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+package com.sima.se1challenge.order
+
+import com.sima.se1challenge.model.Order
+import com.sima.se1challenge.model.OrderPrice
+import com.sima.se1challenge.order.cache.OrderCache
+import com.sima.se1challenge.order.discount.OrderPriceDiscount
+import com.sima.se1challenge.order.generator.OrderGenerator
+import com.sima.se1challenge.order.loader.RepetitionOrderPricingLoader
+import com.sima.se1challenge.order.logging.OrderLogger
+import com.sima.se1challenge.order.multiplier.OrderPriceMultiplier
+
+class OrderCalculator(
+    private val orderGenerator: OrderGenerator,
+    private val repetitionOrderPricingLoader: RepetitionOrderPricingLoader,
+    private val orderCache: OrderCache,
+    private val multiplier: OrderPriceMultiplier,
+    private val discount: OrderPriceDiscount,
+    private val orderLogger: OrderLogger
+) {
+    fun calculate(completion: (Result<OrderPrice>) -> Unit) {
+        val order = orderGenerator.next()
+        orderCache.getOrderPrice(order)?.let {
+            completion(Result.success(it))
+            return@calculate
+        }
+
+        repetitionOrderPricingLoader.load(order) {
+            val result = it.map { price ->
+                price.applyAction(order, multiplier)
+                    .applyAction(order, discount)
+            }
+
+            result.onSuccess {
+                orderCache.putOrderPrice(order, it)
+            }
+
+            completion(result)
+        }
+    }
+
+    private fun OrderPrice.applyAction(
+        order: Order,
+        orderPriceAction: OrderPriceAction,
+        logging: Boolean = true
+    ): OrderPrice {
+        fun logOrder(orderPrice: OrderPrice) {
+            if (logging) orderLogger.log(order, orderPrice)
+        }
+
+        logOrder(this)
+        return orderPriceAction.apply(this).also {
+            logOrder(it)
+        }
+    }
+}