LibImage: Add image format probing

Instead of determining the image type in Image.cpp call image probing
functions for each supported image type

LibImage/Image.cpp

@@ -41,12 +41,6 @@ namespace LibImage
 			return BAN::Error::from_errno(errno);
 		}
 
-		if (st.st_size < 2)
-		{
-			fprintf(stddbg, "invalid image (too small)\n");
-			return BAN::Error::from_errno(EINVAL);
-		}
-
 		void* addr = mmap(nullptr, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
 		if (addr == MAP_FAILED)
 		{
@@ -58,21 +52,10 @@ namespace LibImage
 
 		auto image_data_span = BAN::ConstByteSpan(reinterpret_cast<uint8_t*>(addr), st.st_size);
 
-		uint16_t u16_signature = image_data_span.as<const uint16_t>();
-		switch (u16_signature)
-		{
-			case 0x3650:
-			case 0x3550:
-			case 0x3450:
-			case 0x3350:
-			case 0x3250:
-			case 0x3150:
-				return TRY(load_netbpm(image_data_span));
-			default:
-				fprintf(stderr, "unrecognized image format\n");
-				break;
-		}
+		if (probe_netbpm(image_data_span))
+			return TRY(load_netbpm(image_data_span));
 
+		fprintf(stderr, "unrecognized image format\n");
 		return BAN::Error::from_errno(ENOTSUP);
 	}
 

LibImage/Netbpm.cpp

@@ -34,6 +34,26 @@ namespace LibImage
 		return result;
 	}
 
+
+	bool probe_netbpm(BAN::ConstByteSpan image_data)
+	{
+		if (image_data.size() < 2)
+			return false;
+		uint16_t u16_signature = image_data.as<const uint16_t>();
+		switch (u16_signature)
+		{
+			case 0x3650:
+			case 0x3550:
+			case 0x3450:
+			case 0x3350:
+			case 0x3250:
+			case 0x3150:
+				return true;
+			default:
+				return false;
+		}
+	}
+
 	BAN::ErrorOr<BAN::UniqPtr<Image>> load_netbpm(BAN::ConstByteSpan image_data)
 	{
 		if (image_data.size() < 11)

LibImage/include/LibImage/Netbpm.h

@@ -5,6 +5,7 @@
 namespace LibImage
 {
 
+	bool probe_netbpm(BAN::ConstByteSpan);
 	BAN::ErrorOr<BAN::UniqPtr<Image>> load_netbpm(BAN::ConstByteSpan);
 
 }