Simplify the PDFDocument.fingerprint method slightly

The way that this method handles documents without an `ID` entry in the Trailer dictionary feels overly complicated to me. Hence this patch adds `getByteRange` methods to the various Stream implementations[1], and utilize that rather than manually calling `ensureRange` when computing a fallback `fingerprint`.

---
[1] Note that `PDFDocument` is only ever initialized with either a `Stream` or a `ChunkedStream`, hence why the `DecodeStream.getByteRange` method isn't implemented.

src/core/chunked_stream.js

@@ -218,6 +218,12 @@ class ChunkedStream {
   }
 
   getByteRange(begin, end) {
+    if (begin < 0) {
+      begin = 0;
+    }
+    if (end > this.end) {
+      end = this.end;
+    }
     this.ensureRange(begin, end);
     return this.bytes.subarray(begin, end);
   }

src/core/document.js

@@ -598,20 +598,16 @@ class PDFDocument {
         idArray[0] !== EMPTY_FINGERPRINT) {
       hash = stringToBytes(idArray[0]);
     } else {
-      if (this.stream.ensureRange) {
+      hash = calculateMD5(this.stream.getByteRange(0, FINGERPRINT_FIRST_BYTES),
-        this.stream.ensureRange(0,
+                          0, FINGERPRINT_FIRST_BYTES);
-          Math.min(FINGERPRINT_FIRST_BYTES, this.stream.end));
-      }
-      hash = calculateMD5(this.stream.bytes.subarray(0,
-        FINGERPRINT_FIRST_BYTES), 0, FINGERPRINT_FIRST_BYTES);
     }
 
-    let fingerprint = '';
+    const fingerprintBuf = [];
     for (let i = 0, ii = hash.length; i < ii; i++) {
       const hex = hash[i].toString(16);
-      fingerprint += (hex.length === 1 ? '0' + hex : hex);
+      fingerprintBuf.push(hex.padStart(2, '0'));
     }
-    return shadow(this, 'fingerprint', fingerprint);
+    return shadow(this, 'fingerprint', fingerprintBuf.join(''));
   }
 
   _getLinearizationPage(pageIndex) {

src/core/stream.js

@@ -19,7 +19,9 @@
  * license.
  */
 
-import { FormatError, isSpace, stringToBytes } from '../shared/util';
+import {
+  FormatError, isSpace, stringToBytes, unreachable
+} from '../shared/util';
 import { isDict } from './primitives';
 
 var Stream = (function StreamClosure() {
@@ -92,6 +94,17 @@ var Stream = (function StreamClosure() {
       this.pos -= bytes.length;
       return bytes;
     },
+
+    getByteRange(begin, end) {
+      if (begin < 0) {
+        begin = 0;
+      }
+      if (end > this.end) {
+        end = this.end;
+      }
+      return this.bytes.subarray(begin, end);
+    },
+
     skip: function Stream_skip(n) {
       if (!n) {
         n = 1;
@@ -236,6 +249,11 @@ var DecodeStream = (function DecodeStreamClosure() {
       }
       return new Stream(this.buffer, start, length, dict);
     },
+
+    getByteRange(begin, end) {
+      unreachable('Should not call DecodeStream.getByteRange');
+    },
+
     skip: function DecodeStream_skip(n) {
       if (!n) {
         n = 1;