stream: improve read() performance

PR-URL: #29337
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>

lib/internal/streams/buffer_list.js

@@ -98,67 +98,63 @@ module.exports = class BufferList {
 
   // Consumes a specified amount of characters from the buffered data.
   _getString(n) {
-    var p = this.head;
-    var c = 1;
-    var ret = p.data;
-    n -= ret.length;
-    while (p = p.next) {
+    let ret = '';
+    let p = this.head;
+    let c = 0;
+    do {
       const str = p.data;
-      const nb = (n > str.length ? str.length : n);
-      if (nb === str.length)
+      if (n > str.length) {
         ret += str;
-      else
-        ret += str.slice(0, n);
-      n -= nb;
-      if (n === 0) {
-        if (nb === str.length) {
+        n -= str.length;
+      } else {
+        if (n === str.length) {
+          ret += str;
           ++c;
           if (p.next)
             this.head = p.next;
           else
             this.head = this.tail = null;
         } else {
+          ret += str.slice(0, n);
           this.head = p;
-          p.data = str.slice(nb);
+          p.data = str.slice(n);
         }
         break;
       }
       ++c;
-    }
+    } while (p = p.next);
     this.length -= c;
     return ret;
   }
 
   // Consumes a specified amount of bytes from the buffered data.
   _getBuffer(n) {
     const ret = Buffer.allocUnsafe(n);
-    var p = this.head;
-    var c = 1;
-    p.data.copy(ret);
-    n -= p.data.length;
-    while (p = p.next) {
+    const retLen = n;
+    let p = this.head;
+    let c = 0;
+    do {
       const buf = p.data;
-      const nb = (n > buf.length ? buf.length : n);
-      if (nb === buf.length)
-        ret.set(buf, ret.length - n);
-      else
-        ret.set(new Uint8Array(buf.buffer, buf.byteOffset, nb), ret.length - n);
-      n -= nb;
-      if (n === 0) {
-        if (nb === buf.length) {
+      if (n > buf.length) {
+        ret.set(buf, retLen - n);
+        n -= buf.length;
+      } else {
+        if (n === buf.length) {
+          ret.set(buf, retLen - n);
           ++c;
           if (p.next)
             this.head = p.next;
           else
             this.head = this.tail = null;
         } else {
+          ret.set(new Uint8Array(buf.buffer, buf.byteOffset, n), retLen - n);
           this.head = p;
-          p.data = buf.slice(nb);
+          p.data = buf.slice(n);
         }
         break;
       }
       ++c;
-    }
+    } while (p = p.next);
     this.length -= c;
     return ret;
   }