feat(day): implement an abstract class for days

Signed-off-by: Matej Focko <me@mfocko.xyz>

src/Day.kt

@@ -0,0 +1,66 @@
+abstract class Day<Part1Answer, Part2Answer> {
+    /**
+     * Precomputes partial or final results that are used by both parts of the
+     * challenge.
+     *
+     * This function is optional, it needs to be implemented, but empty override
+     * is acceptable.
+     */
+    abstract fun precompute()
+
+    /**
+     * Tests the first part of the challenge.
+     *
+     * @param part1Answer Expected answer to the 1º part of the challenge
+     */
+    fun test(part1Answer: Part1Answer) {
+        precompute()
+
+        print("Checking part 1:\t")
+        check(part1() == part1Answer)
+        println("[OK]")
+    }
+
+    /**
+     * Tests both parts of the challenge.
+     *
+     * @param part1Answer Expected answer to the 1º part of the challenge;
+     * also can be `null`, in such case, the 1º part is not being tested
+     * @param part2Answer Expected answer to the 2º part of the challenge
+     */
+    fun test(
+        part1Answer: Part1Answer?,
+        part2Answer: Part2Answer,
+    ) {
+        part1Answer?.let { test(it) }
+
+        print("Checking part 2:\t")
+        check(part2() == part2Answer) { "Given answer: ${part2()}" }
+        println("[OK]")
+    }
+
+    /**
+     * Runs the `precompute()` and then each part of the challenge.
+     */
+    fun run() {
+        precompute()
+
+        print("Part 1:\t")
+        part1().println()
+
+        print("Part 2:\t")
+        part2().println()
+    }
+
+    /**
+     * Computes the 1º part of the challenge.
+     * @return Answer to the 1º part of the challenge
+     */
+    abstract fun part1(): Part1Answer
+
+    /**
+     * Computes the 2º part of the challenge.
+     * @return Answer to the 2º part of the challenge
+     */
+    abstract fun part2(): Part2Answer
+}