Rewrite source when downloading image

app/src/main/java/coil/decode/DecodeUtils.kt

@@ -0,0 +1,66 @@
+@file:JvmName("GifDecodeUtils")
+
+package coil.decode
+
+import kotlin.experimental.and
+import okio.BufferedSource
+import okio.ByteString.Companion.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()
+
+// https://developers.google.com/speed/webp/docs/riff_container
+private val WEBP_HEADER_RIFF = "RIFF".encodeUtf8()
+private val WEBP_HEADER_WEBP = "WEBP".encodeUtf8()
+private val WEBP_HEADER_VPX8 = "VP8X".encodeUtf8()
+
+// https://nokiatech.github.io/heif/technical.html
+private val HEIF_HEADER_FTYP = "ftyp".encodeUtf8()
+private val HEIF_HEADER_MSF1 = "msf1".encodeUtf8()
+private val HEIF_HEADER_HEVC = "hevc".encodeUtf8()
+private val HEIF_HEADER_HEVX = "hevx".encodeUtf8()
+
+/**
+ * Return 'true' if the [source] contains a GIF image. The [source] is not consumed.
+ */
+fun DecodeUtils.isGif(source: BufferedSource): Boolean {
+    return source.rangeEquals(0, GIF_HEADER_89A) ||
+        source.rangeEquals(0, GIF_HEADER_87A)
+}
+
+/**
+ * Return 'true' if the [source] contains a WebP image. The [source] is not consumed.
+ */
+fun DecodeUtils.isWebP(source: BufferedSource): Boolean {
+    return source.rangeEquals(0, WEBP_HEADER_RIFF) &&
+        source.rangeEquals(8, WEBP_HEADER_WEBP)
+}
+
+/**
+ * Return 'true' if the [source] contains an animated WebP image. The [source] is not consumed.
+ */
+fun DecodeUtils.isAnimatedWebP(source: BufferedSource): Boolean {
+    return isWebP(source) &&
+        source.rangeEquals(12, WEBP_HEADER_VPX8) &&
+        source.request(17) &&
+        (source.buffer[16] and 0b00000010) > 0
+}
+
+/**
+ * Return 'true' if the [source] contains an HEIF image. The [source] is not consumed.
+ */
+fun DecodeUtils.isHeif(source: BufferedSource): Boolean {
+    return source.rangeEquals(4, HEIF_HEADER_FTYP)
+}
+
+/**
+ * Return 'true' if the [source] contains an animated HEIF image sequence. The [source] is not
+ * consumed.
+ */
+fun DecodeUtils.isAnimatedHeif(source: BufferedSource): Boolean {
+    return isHeif(source) &&
+        (source.rangeEquals(8, HEIF_HEADER_MSF1) ||
+            source.rangeEquals(8, HEIF_HEADER_HEVC) ||
+            source.rangeEquals(8, HEIF_HEADER_HEVX))
+}

app/src/main/java/coil/decode/FrameDelayRewritingSource.kt

@@ -0,0 +1,88 @@
+@file:Suppress("SameParameterValue")
+
+package coil.decode
+
+import okio.Buffer
+import okio.ByteString
+import okio.ByteString.Companion.decodeHex
+import okio.ForwardingSource
+import okio.Source
+
+/**
+ * A [ForwardingSource] that rewrites the GIF frame delay in every graphics control block if it's
+ * below a threshold.
+ */
+internal class FrameDelayRewritingSource(delegate: Source) : ForwardingSource(delegate) {
+
+    // An intermediary buffer so we can read and alter the data before it's written to the destination.
+    private val buffer = Buffer()
+
+    override fun read(sink: Buffer, byteCount: Long): Long {
+        // Ensure our buffer has enough bytes to satisfy this read.
+        request(byteCount)
+
+        // Short circuit if there are no bytes in the buffer.
+        if (buffer.size == 0L) {
+            return if (byteCount == 0L) 0L else -1L
+        }
+
+        // Search through the buffer and rewrite any frame delays below the threshold.
+        var bytesWritten = 0L
+        while (true) {
+            val index = indexOf(FRAME_DELAY_START_MARKER)
+            if (index == -1L) break
+
+            // Write up until the end of the frame delay start marker.
+            bytesWritten += write(sink, index + FRAME_DELAY_START_MARKER_SIZE_BYTES)
+
+            // Check for the end of the graphics control extension block.
+            if (!request(5) || buffer[4] != 0.toByte()) continue
+
+            // Rewrite the frame delay if it is below the threshold.
+            // The frame delay is stored as two unsigned bytes in reverse order
+            // (i.e. the most significant bits are in the byte that's read second).
+            val frameDelay = (buffer[2].toUByte().toInt() shl 8) or buffer[1].toUByte().toInt()
+            if (frameDelay < MINIMUM_FRAME_DELAY) {
+                sink.writeByte(buffer[0].toInt())
+                sink.writeByte(DEFAULT_FRAME_DELAY)
+                sink.writeByte(0)
+                buffer.skip(3)
+            }
+        }
+
+        // Write anything left in the source.
+        if (bytesWritten < byteCount) {
+            bytesWritten += write(sink, byteCount - bytesWritten)
+        }
+        return if (bytesWritten == 0L) -1 else bytesWritten
+    }
+
+    private fun indexOf(bytes: ByteString): Long {
+        var index = -1L
+        while (true) {
+            index = buffer.indexOf(bytes[0], index + 1)
+            if (index == -1L) break
+            if (request(bytes.size.toLong()) && buffer.rangeEquals(index, bytes)) break
+        }
+        return index
+    }
+
+    private fun write(sink: Buffer, byteCount: Long): Long {
+        return buffer.read(sink, byteCount).coerceAtLeast(0)
+    }
+
+    private fun request(byteCount: Long): Boolean {
+        if (buffer.size >= byteCount) return true
+        val toRead = byteCount - buffer.size
+        return super.read(buffer, toRead) == toRead
+    }
+
+    private companion object {
+        // https://www.matthewflickinger.com/lab/whatsinagif/bits_and_bytes.asp
+        // See: "Graphics Control Extension"
+        private val FRAME_DELAY_START_MARKER = "0021F904".decodeHex()
+        private const val FRAME_DELAY_START_MARKER_SIZE_BYTES = 4
+        private const val MINIMUM_FRAME_DELAY = 2
+        private const val DEFAULT_FRAME_DELAY = 10
+    }
+}

app/src/main/java/com/hippo/ehviewer/spider/SpiderDen.kt

@@ -15,6 +15,9 @@
  */
 package com.hippo.ehviewer.spider
 
+import coil.decode.DecodeUtils
+import coil.decode.FrameDelayRewritingSource
+import coil.decode.isGif
 import coil.disk.DiskCache
 import com.hippo.Native.mapFd
 import com.hippo.Native.unmapDirectByteBuffer
@@ -35,6 +38,7 @@ import com.hippo.yorozuya.MathUtils
 import kotlinx.coroutines.currentCoroutineContext
 import kotlinx.coroutines.ensureActive
 import okhttp3.executeAsync
+import okio.buffer
 import java.io.File
 import java.io.FileInputStream
 import java.io.FileOutputStream
@@ -175,13 +179,16 @@ class SpiderDen(private val mGalleryInfo: GalleryInfo) {
 
             suspend fun doSave(chan: FileChannel): Long {
                 var receivedSize: Long = 0
-                body.source().use { source ->
-                    while (true) {
-                        currentCoroutineContext().ensureActive()
-                        val bytesRead = chan.transferFrom(source, receivedSize, TRANSFER_BLOCK)
-                        receivedSize += bytesRead
-                        notifyProgress(body.contentLength(), receivedSize, bytesRead.toInt())
-                        if (bytesRead.toInt() == 0) break
+                body.source().use { responseSource ->
+                    val source = if (DecodeUtils.isGif(responseSource)) FrameDelayRewritingSource(responseSource).buffer() else responseSource
+                    source.use {
+                        while (true) {
+                            currentCoroutineContext().ensureActive()
+                            val bytesRead = chan.transferFrom(it, receivedSize, TRANSFER_BLOCK)
+                            receivedSize += bytesRead
+                            notifyProgress(body.contentLength(), receivedSize, bytesRead.toInt())
+                            if (bytesRead.toInt() == 0) break
+                        }
                     }
                 }
                 return receivedSize