Add class ArticleId

app/src/main/java/com/causa_arcana/ArticleId.kt

@@ -0,0 +1,36 @@
+package com.causa_arcana
+
+import java.lang.RuntimeException
+
+class ArticleId(val year: UInt, val month: UInt, val day: UInt, val slug: String) {
+    init {
+        if (year  !in 2000u..3000u) throw RuntimeException("Invalid article year")
+        if (month !in 1u..12u)      throw RuntimeException("Invalid article month")
+        if (day   !in 1u..31u)      throw RuntimeException("Invalid article day")
+        if (!SLUG_RE.matches(slug)) throw RuntimeException("Invalid article slug")
+    }
+
+    override fun toString(): String {
+        return "${yearPadded()}-${monthPadded()}-${dayPadded()}-$slug"
+    }
+
+    private fun yearPadded():  String = year .toString().padStart(4, '0')
+    private fun monthPadded(): String = month.toString().padStart(2, '0')
+    private fun dayPadded():   String = day  .toString().padStart(2, '0')
+
+    companion object {
+        private val ID_RE = """^(\d{4})-(\d\d)-(\d\d)-([a-z0-9]+)(-[a-z0-9]+)*$""".toRegex()
+        private val SLUG_RE = """^([a-z0-9]+)(-[a-z0-9]+)*$""".toRegex()
+
+        fun fromString(id: String): ArticleId {
+            (ID_RE.matchEntire(id) ?: throw RuntimeException("Invalid article ID")).destructured
+                .let { (year, month, day, slugHead, slugTail) -> return ArticleId(
+                        year.toUInt(),
+                        month.toUInt(),
+                        day.toUInt(),
+                        slugHead + slugTail,
+                    )
+                }
+        }
+    }
+}

app/src/test/java/com/causa_arcana/ExampleUnitTest.kt

@@ -4,14 +4,19 @@ import org.junit.Test
 
 import org.junit.Assert.*
 
-/**
- * Example local unit test, which will execute on the development machine (host).
- *
- * See [testing documentation](http://d.android.com/tools/testing).
- */
 class ExampleUnitTest {
     @Test
-    fun addition_isCorrect() {
-        assertEquals(4, 2 + 2)
+    fun articleIdFromString() {
+        val articleId = ArticleId.fromString("2021-01-01-hello-world")
+        assertEquals(2021u, articleId.year)
+        assertEquals(1u, articleId.month)
+        assertEquals(1u, articleId.day)
+        assertEquals("hello-world", articleId.slug)
+    }
+
+    @Test
+    fun articleIdToString() {
+        val articleId = ArticleId(2021u, 1u, 1u, "hello-world")
+        assertEquals("2021-01-01-hello-world", articleId.toString())
     }
 }