Prevent sub-sampling of single-frame GIFs

Fixes #83

zoomable-image/coil/src/main/kotlin/me/saket/telephoto/zoomable/coil/svg.kt → zoomable-image/coil/src/main/kotlin/me/saket/telephoto/zoomable/coil/imageFormats.kt

@@ -8,16 +8,35 @@ import okio.ByteString.Companion.encodeUtf8
 private val SVG_TAG: ByteString = "<svg".encodeUtf8()
 private val LEFT_ANGLE_BRACKET: ByteString = "<".encodeUtf8()
 
+// https://www.matthewflickinger.com/lab/whatsinagif/bits_and_bytes.asp
+private val GIF_HEADER_87A = "GIF87a".encodeUtf8()
+private val GIF_HEADER_89A = "GIF89a".encodeUtf8()
+
 /**
  * Copied from coil-svg.
- * TODO: remove this if https://github.com/coil-kt/coil/issues/1811 is accepted.
- */
+ *
+ * Return 'true' if the [source] contains an SVG image. The [source] is not consumed.
+ *
+ * NOTE: There's no guaranteed method to determine if a byte stream is an SVG without attempting
+ * to decode it. This method uses heuristics.
+*/
 @Suppress("UnusedReceiverParameter")
 internal fun DecodeUtils.isSvg(source: BufferedSource): Boolean {
   return source.rangeEquals(0, LEFT_ANGLE_BRACKET) &&
     source.indexOf(SVG_TAG, 0, 1024) != -1L
 }
 
+/**
+ * Copied from coil-gif.
+ *
+ * Return 'true' if the [source] contains a GIF image. The [source] is not consumed.
+ */
+@Suppress("UnusedReceiverParameter")
+internal fun DecodeUtils.isGif(source: BufferedSource): Boolean {
+  return source.rangeEquals(0, GIF_HEADER_89A) ||
+    source.rangeEquals(0, GIF_HEADER_87A)
+}
+
 /** Copied from coil-svg. */
 internal fun BufferedSource.indexOf(bytes: ByteString, fromIndex: Long, toIndex: Long): Long {
   require(bytes.size > 0) { "bytes is empty" }

zoomable-image/coil/src/main/kotlin/me/saket/telephoto/zoomable/coil/subSamplingEligibility.kt

@@ -22,41 +22,33 @@ context(Resolver)
 internal suspend fun SubSamplingImageSource.canBeSubSampled(): Boolean {
   val preventSubSampling = when (this) {
     is ResourceImageSource -> isVectorDrawable()
-    is AssetImageSource -> isSvgDecoderPresent() && isSvg()
-    is UriImageSource -> isSvgDecoderPresent() && isSvg()
-    is FileImageSource -> isSvgDecoderPresent() && isSvg(FileSystem.SYSTEM.source(path))
-    is RawImageSource -> isSvgDecoderPresent() && isSvg(source.invoke())
+    is AssetImageSource -> canBeSubSampled()
+    is UriImageSource -> canBeSubSampled()
+    is FileImageSource -> canBeSubSampled(FileSystem.SYSTEM.source(path))
+    is RawImageSource -> canBeSubSampled(source.invoke())
   }
   return !preventSubSampling
 }
 
-context(Resolver)
-private fun isSvgDecoderPresent(): Boolean {
-  // Searching for coil's SvgDecoder by name isn't the best idea,
-  // but it'll prevent opening of bitmap sources and inspecting
-  // them for SVGs for projects that don't need SVGs.
-  return imageLoader.components.decoderFactories.any {
-    it::class.qualifiedName?.contains("svg", ignoreCase = true) == true
-  }
-}
-
 context(Resolver)
 private fun ResourceImageSource.isVectorDrawable(): Boolean =
   TypedValue().apply {
     request.context.resources.getValue(id, this, /* resolveRefs = */ true)
   }.string.endsWith(".xml")
 
 context(Resolver)
-private suspend fun AssetImageSource.isSvg(): Boolean =
-  isSvg(peek(request.context).source())
+private suspend fun AssetImageSource.canBeSubSampled(): Boolean =
+  canBeSubSampled(peek(request.context).source())
 
 context(Resolver)
 @SuppressLint("Recycle")
-private suspend fun UriImageSource.isSvg(): Boolean =
-  isSvg(peek(request.context).source())
+private suspend fun UriImageSource.canBeSubSampled(): Boolean =
+  canBeSubSampled(peek(request.context).source())
 
-private suspend fun isSvg(source: Source?): Boolean {
+private suspend fun canBeSubSampled(source: Source): Boolean {
   return withContext(Dispatchers.IO) {
-    source?.buffer()?.use(DecodeUtils::isSvg) == true
+    source.buffer().use {
+      DecodeUtils.isSvg(it) || DecodeUtils.isGif(it)
+    }
   }
 }