tools: resolve images by short references

src/test/kotlin/ImageIndexTest.kt

@@ -0,0 +1,32 @@
+package top.jie65535.mirai
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNull
+
+class ImageIndexTest {
+    @Test
+    fun `assigns short sequential indices`() {
+        val index = ImageIndex()
+
+        assertEquals(1, index.add("image-a", "https://example.com/a"))
+        assertEquals(2, index.add("image-b", "https://example.com/b"))
+        assertEquals("https://example.com/a", index.getUrl(1))
+        assertEquals("https://example.com/b", index.getUrl(2))
+    }
+
+    @Test
+    fun `reuses index for repeated image id`() {
+        val index = ImageIndex()
+
+        assertEquals(1, index.add("image-a", "https://example.com/a-old"))
+        assertEquals(1, index.add("image-a", "https://example.com/a-new"))
+        assertEquals(2, index.add("image-b", "https://example.com/b"))
+        assertEquals("https://example.com/a-new", index.getUrl(1))
+    }
+
+    @Test
+    fun `returns null for unknown index`() {
+        assertNull(ImageIndex().getUrl(1))
+    }
+}