Add a way to get a safe ByteBuffer representation of a VBlob (#74)

Author: lopcode

README.md

@@ -20,7 +20,7 @@ repositories {
 }
 
 dependencies {
-    implementation("app.photofox.vips-ffm:vips-ffm-core:0.5.6")
+    implementation("app.photofox.vips-ffm:vips-ffm-core:0.5.7")
 }
 ```
 

core/src/main/java/app/photofox/vipsffm/VBlob.java

@@ -1,18 +1,30 @@
 package app.photofox.vipsffm;
 
+import app.photofox.vipsffm.jextract.VipsArea;
+import app.photofox.vipsffm.jextract.VipsRaw;
+
+import java.lang.foreign.Arena;
 import java.lang.foreign.MemorySegment;
+import java.nio.ByteBuffer;
+
+import static app.photofox.vipsffm.jextract.VipsRaw.C_LONG;
 
 /**
  * Represents a VipsBlob, boxed to avoid exposing its raw MemorySegment
  */
 public final class VBlob {
 
+    private final Arena arena;
     final MemorySegment address;
 
-    VBlob(MemorySegment address) throws VipsError {
+    VBlob(
+        Arena arena,
+        MemorySegment address
+    ) throws VipsError {
         if (!VipsValidation.isValidPointer(address)) {
             throw new VipsError("invalid pointer used for creation");
         }
+        this.arena = arena;
         this.address = address;
     }
 
@@ -24,4 +36,18 @@ public final class VBlob {
     public MemorySegment getUnsafeAddress() {
         return this.address;
     }
+
+    public long byteSize() {
+        return VipsArea.length(this.address);
+    }
+
+    public ByteBuffer asByteBuffer() {
+        var lengthOutPointer = arena.allocate(C_LONG);
+        var dataPointer = VipsRaw.vips_area_get_data(this.address, lengthOutPointer, MemorySegment.NULL, MemorySegment.NULL, MemorySegment.NULL);
+        var length = lengthOutPointer.get(C_LONG, 0);
+        if (length < 0) {
+            throw new VipsError("unexpected length of vblob data " + length);
+        }
+        return dataPointer.asSlice(0, length).asByteBuffer();
+    }
 }