Verify snapshot

redwood-dom-testing/src/commonMain/kotlin/app/cash/redwood/dom/testing/DomSnapshotter.kt

@@ -15,8 +15,18 @@
  */
 package app.cash.redwood.dom.testing
 
+import kotlin.coroutines.resume
+import kotlin.coroutines.resumeWithException
+import kotlinx.browser.document
 import kotlinx.coroutines.await
+import kotlinx.coroutines.suspendCancellableCoroutine
+import org.khronos.webgl.get
+import org.w3c.dom.CanvasRenderingContext2D
 import org.w3c.dom.Element
+import org.w3c.dom.HTMLCanvasElement
+import org.w3c.dom.HTMLImageElement
+import org.w3c.dom.url.URL
+import org.w3c.files.Blob
 
 public class DomSnapshotter @PublishedApi internal constructor(
   private val path: String,
@@ -40,9 +50,75 @@ public class DomSnapshotter @PublishedApi internal constructor(
       },
     ).await()
 
-    snapshotStore.put("$path/${name ?: "snapshot"}.png", image)
+    val fileName = "$path/${name ?: "snapshot"}.png"
+
+    snapshotStore.getBlob(fileName)?.let { existing ->
+      check(existing.contentEquals(image)) {
+        "Current snapshot does not match the existing file $fileName"
+      }
+    } ?: snapshotStore.put(fileName, image)
   }
 
+  private suspend fun Blob.contentEquals(other: Blob): Boolean {
+    if (this.size != other.size) return false
+
+    val url1 = URL.createObjectURL(this)
+    val url2 = URL.createObjectURL(other)
+
+    try {
+      val img1 = loadImage(url1)
+      val img2 = loadImage(url2)
+
+      if (img1.width != img2.width || img1.height != img2.height) {
+        return false
+      }
+
+      val canvas = document.createElement("canvas") as HTMLCanvasElement
+      val ctx = canvas.getContext("2d") as CanvasRenderingContext2D
+
+      canvas.width = img1.width
+      canvas.height = img1.height
+
+      // Get data for first image
+      ctx.drawImage(img1, 0.0, 0.0)
+      val data1 = ctx.getImageData(0.0, 0.0, canvas.width.toDouble(), canvas.height.toDouble())
+
+      // Get data for second image
+      ctx.clearRect(0.0, 0.0, canvas.width.toDouble(), canvas.height.toDouble())
+      ctx.drawImage(img2, 0.0, 0.0)
+      val data2 = ctx.getImageData(0.0, 0.0, canvas.width.toDouble(), canvas.height.toDouble())
+
+      // Compare pixel by pixel
+      val pixels1 = data1.data
+      val pixels2 = data2.data
+      for (i in 0 until pixels1.length) {
+        if (pixels1[i] != pixels2[i]) {
+          return false
+        }
+      }
+
+      return true
+    } finally {
+      URL.revokeObjectURL(url1)
+      URL.revokeObjectURL(url2)
+    }
+  }
+
+  private suspend fun loadImage(url: String): HTMLImageElement =
+    suspendCancellableCoroutine { continuation ->
+      val img = document.createElement("img") as HTMLImageElement
+
+      img.onload = { _ -> continuation.resume(img) }
+      img.onerror = { _: dynamic, _: String, _: Int, _: Int, _: Any? ->
+        continuation.resumeWithException(Exception("Failed to load image"))
+      }
+      img.src = url
+
+      continuation.invokeOnCancellation {
+        img.src = ""
+      }
+    }
+
   public companion object Companion {
     public inline operator fun invoke(): DomSnapshotter {
       return DomSnapshotter("PlaceholderTestName")

redwood-dom-testing/src/commonMain/kotlin/app/cash/redwood/dom/testing/SnapshotStore.kt

@@ -21,6 +21,7 @@ import kotlinx.coroutines.await
 import okio.ByteString
 import okio.ByteString.Companion.toByteString
 import org.w3c.fetch.RequestInit
+import org.w3c.fetch.Response
 import org.w3c.files.Blob
 
 internal class SnapshotStore {
@@ -51,7 +52,17 @@ internal class SnapshotStore {
     }
   }
 
-  suspend fun get(fileName: String): ByteString? {
+  suspend fun getBlob(fileName: String): Blob? {
+    return getInternal(fileName)?.blob()?.await()
+  }
+
+  suspend fun getByteString(fileName: String): ByteString? {
+    val response = getInternal(fileName) ?: return null
+    val bytes: Promise<ByteArray> = response.asDynamic().bytes()
+    return bytes.await().toByteString()
+  }
+
+  private suspend fun getInternal(fileName: String): Response? {
     val response = window.fetch(
       input = "/snapshots/$fileName",
     ).await()
@@ -68,9 +79,7 @@ internal class SnapshotStore {
         """.trimMargin(),
       )
     }
-
-    val bytes: Promise<ByteArray> = response.asDynamic().bytes()
-    return bytes.await().toByteString()
+    return response
   }
 }
 

redwood-dom-testing/src/commonTest/kotlin/app/cash/redwood/dom/testing/DomSnapshotterSampleTest.kt

@@ -18,10 +18,12 @@
 package app.cash.redwood.dom.testing
 
 import kotlin.test.Test
+import kotlin.test.assertFailsWith
 import kotlinx.browser.document
 import kotlinx.coroutines.test.runTest
 import kotlinx.dom.appendElement
 import kotlinx.dom.appendText
+import kotlinx.dom.clear
 
 /**
  * This isn't a proper unit test for [DomSnapshotter], it's just a sample.
@@ -38,4 +40,21 @@ internal class DomSnapshotterSampleTest {
     }
     snapshotter.snapshot(element, "helloIAmTheSnapshotTest")
   }
+
+  @Test
+  fun mismatchedSnapshot() = runTest {
+    val element = document.documentElement!!
+    element.appendElement("h1") {
+      appendText("hello world")
+    }
+    snapshotter.snapshot(element, "mismatchedSnapshotTest")
+
+    element.clear()
+    element.appendElement("h2") {
+      appendText("hello world")
+    }
+    assertFailsWith<IllegalStateException> {
+      snapshotter.snapshot(element, "mismatchedSnapshotTest")
+    }
+  }
 }

redwood-dom-testing/src/commonTest/kotlin/app/cash/redwood/dom/testing/SnapshotStoreTest.kt

@@ -30,21 +30,21 @@ internal class SnapshotStoreTest {
     val store = SnapshotStore()
     val data = "Hello World!".encodeUtf8()
     store.put("greeting.txt", data)
-    assertThat(store.get("greeting.txt")).isEqualTo(data)
+    assertThat(store.getByteString("greeting.txt")).isEqualTo(data)
   }
 
   @Test
   fun putAndGetFileWithPathHierarchy() = runTest {
     val store = SnapshotStore()
     val data = "Ahoy, Matey!".encodeUtf8()
     store.put("greetings/pirate/greeting.txt", data)
-    assertThat(store.get("greetings/pirate/greeting.txt")).isEqualTo(data)
+    assertThat(store.getByteString("greetings/pirate/greeting.txt")).isEqualTo(data)
   }
 
   @Test
   fun getDirectoryTraversalReturnsNoData() = runTest {
     val store = SnapshotStore()
-    assertThat(store.get("../README.md")).isNull() // 404 Not Found.
+    assertThat(store.getByteString("../README.md")).isNull() // 404 Not Found.
   }
 
   @Test