Rewrite the GIF frame delay if it's below a threshold, reland, Resolve #641

app/src/main/cpp/gifutils.c

@@ -1,15 +1,43 @@
 #include <jni.h>
-
+#include <string.h>
+#include <stdbool.h>
 #include <android/log.h>
 
 #define LOG_TAG "gifUtils"
 
 #include "ehviewer.h"
 
+#define GIF_HEADER_87A "GIF87a"
+#define GIF_HEADER_89A "GIF89a"
+
+static int FRAME_DELAY_START_MARKER = 0x0021F904;
+
+typedef signed char byte;
+
+#define FRAME_DELAY_START_MARKER ((byte*)(&FRAME_DELAY_START_MARKER))
+#define MINIMUM_FRAME_DELAY 2
+#define DEFAULT_FRAME_DELAY 10
+
+static inline bool isGif(void *addr) {
+    return !memcmp(addr, GIF_HEADER_87A, 6) || !memcmp(addr, GIF_HEADER_89A, 6);
+}
+
 JNIEXPORT void JNICALL
 Java_com_hippo_image_Image_rewriteGifSource(JNIEnv *env, jclass clazz, jobject buffer) {
-    void *addr = (*env)->GetDirectBufferAddress(env, buffer);
+    byte *addr = (*env)->GetDirectBufferAddress(env, buffer);
     size_t size = (*env)->GetDirectBufferCapacity(env, buffer);
-    EH_UNUSED(addr);
-    EH_UNUSED(size);
+    if (size < 7 || !isGif(addr)) return;
+    for (size_t i = 0; i < size - 8; i++) {
+        if (addr[i] == FRAME_DELAY_START_MARKER[3] && addr[i + 1] == FRAME_DELAY_START_MARKER[2] &&
+            addr[i + 2] == FRAME_DELAY_START_MARKER[1] &&
+            addr[i + 3] == FRAME_DELAY_START_MARKER[0]) {
+            byte *end = addr + i + 4;
+            if (end[4] != 0) continue;
+            int frameDelay = end[2] << 8 | end[1];
+            if (frameDelay > MINIMUM_FRAME_DELAY)
+                break; // Quit if the first block looks normal, for performance
+            end[1] = DEFAULT_FRAME_DELAY;
+            end[2] = 0;
+        }
+    }
 }