fix: Fix UN & AT VR processing logic (#167) (#168)

* fix: Fix UN & AT VR processing logic (#167)

* Change UnknownValue (UN) to be extended from BinaryRepresentation
* Add the writeTwoUint16s method to BufferStream to handle to process AT
* Optimize the readString method of BufferStream
* Adds test case to check the equality of pre-write and post-write buffer

* adds performance test-case and removes dead code

packages/adapters/src/BufferStream.js

@@ -86,6 +86,19 @@ class BufferStream {
         return this.increment(2);
     }
 
+    writeTwoUint16s(value) {
+        this.checkSize(4);
+        const first = value >> 16;
+        const second = value & 0xffff;
+        this.view.setUint16(this.offset, toInt(first), this.isLittleEndian);
+        this.view.setUint16(
+            this.offset + 2,
+            toInt(second),
+            this.isLittleEndian
+        );
+        return this.increment(4);
+    }
+
     writeInt16(value) {
         this.checkSize(2);
         this.view.setInt16(this.offset, toInt(value), this.isLittleEndian);
@@ -212,16 +225,17 @@ class BufferStream {
     }
 
     readString(length) {
-        var string = "";
-
-        var numOfMulti = length,
-            index = 0;
-        while (index++ < numOfMulti) {
-            var charCode = this.readUint8();
-            string += String.fromCharCode(charCode);
+        var chars = [];
+        var start = this.offset;
+        var end = this.offset + length;
+        if (end >= this.buffer.byteLength) {
+            end = this.buffer.byteLength;
         }
-
-        return string;
+        for (let i = start; i < end; ++i) {
+            chars.push(String.fromCharCode(this.view.getUint8(i)));
+            this.increment(1);
+        }
+        return chars.join("");
     }
 
     readHex(length) {

packages/adapters/src/Tag.js

@@ -108,7 +108,7 @@ class Tag {
             valueLength;
         tagStream.setEndian(isLittleEndian);
 
-        if (vrType == "OW" || vrType == "OB") {
+        if (vrType == "OW" || vrType == "OB" || vrType == "UN") {
             valueLength = vr.writeBytes(
                 tagStream,
                 values,

packages/adapters/src/ValueRepresentation.js

@@ -110,7 +110,6 @@ class ValueRepresentation {
                             //byteCount++;
                         }
                         var singularArgs = [v].concat(valueArgs.slice(1));
-
                         var byteCount = func.apply(stream, singularArgs);
                         written.push(byteCount);
                     });
@@ -241,10 +240,11 @@ class BinaryRepresentation extends ValueRepresentation {
         super(type);
     }
 
-    writeBytes(stream, value, syntax, isEncapsulated, writeOptions) {
+    writeBytes(stream, value, syntax, isEncapsulated, writeOptions = {}) {
         var i;
         var binaryStream;
         var { fragmentMultiframe = true } = writeOptions;
+        value = value === null || value === undefined ? [] : value;
         if (isEncapsulated) {
             var fragmentSize = 1024 * 20,
                 frames = value.length,
@@ -459,7 +459,7 @@ class AttributeTag extends ValueRepresentation {
         return super.writeBytes(
             stream,
             value,
-            super.write(stream, "Uint32", value)
+            super.write(stream, "TwoUint16s", value)
         );
     }
 }
@@ -955,17 +955,13 @@ class UniversalResource extends StringRepresentation {
     }
 }
 
-class UnknownValue extends StringRepresentation {
+class UnknownValue extends BinaryRepresentation {
     constructor() {
         super("UN");
         this.maxLength = null;
         this.padByte = "00";
         this.noMultiple = true;
     }
-
-    readBytes(stream, length) {
-        return stream.readString(length);
-    }
 }
 
 class OtherWordString extends BinaryRepresentation {