buffer: simplify Buffer.concat

Signed-off-by: Richard Gibson <richard.gibson@gmail.com>

Author: gibson042

lib/buffer.js

@@ -616,66 +616,51 @@ Buffer.concat = function concat(list, length) {
   if (list.length === 0)
     return new FastBuffer();
 
-  if (length === undefined) {
+  const autoLength = length === undefined;
+  if (autoLength) {
     length = 0;
-    for (let i = 0; i < list.length; i++) {
-      const buf = list[i];
-      if (!isUint8Array(buf)) {
-        // TODO(BridgeAR): This should not be of type ERR_INVALID_ARG_TYPE.
-        // Instead, find the proper error code for this.
-        throw new ERR_INVALID_ARG_TYPE(
-          `list[${i}]`, ['Buffer', 'Uint8Array'], buf);
-      }
-      length += TypedArrayPrototypeGetByteLength(buf);
-    }
-
-    const buffer = allocate(length);
-    let pos = 0;
-    for (let i = 0; i < list.length; i++) {
-      const buf = list[i];
-      const bufLength = TypedArrayPrototypeGetByteLength(buf);
-      TypedArrayPrototypeSet(buffer, buf, pos);
-      pos += bufLength;
-    }
-
-    if (pos < length) {
-      TypedArrayPrototypeFill(buffer, 0, pos, length);
-    }
-    return buffer;
+  } else {
+    validateOffset(length, 'length');
   }
 
-  validateOffset(length, 'length');
-  for (let i = 0; i < list.length; i++) {
-    if (!isUint8Array(list[i])) {
-      // TODO(BridgeAR): This should not be of type ERR_INVALID_ARG_TYPE.
-      // Instead, find the proper error code for this.
-      throw new ERR_INVALID_ARG_TYPE(
-        `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
+  let i = 0;
+  let j = 0;
+  let pos = 0;
+  const postPositions = new Array(list.length);
+  try {
+    for (; i < list.length; i++) {
+      pos += TypedArrayPrototypeGetByteLength(list[i]);
+      postPositions[i] = pos;
+      j += pos <= length;
     }
+  } catch {
+    // TODO(BridgeAR): This should not be of type ERR_INVALID_ARG_TYPE.
+    // Instead, find the proper error code for this.
+    throw new ERR_INVALID_ARG_TYPE(
+      `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
+  }
+  if (autoLength) {
+    length = pos;
+    j = list.length;
   }
 
   const buffer = allocate(length);
-  let pos = 0;
-  for (let i = 0; i < list.length; i++) {
-    const buf = list[i];
-    const bufLength = TypedArrayPrototypeGetByteLength(buf);
-    if (pos + bufLength > length) {
-      TypedArrayPrototypeSet(buffer,
-                             TypedArrayPrototypeSubarray(buf, 0, length - pos),
-                             pos);
-      pos = length;
-      break;
-    }
-    TypedArrayPrototypeSet(buffer, buf, pos);
-    pos += bufLength;
+  // Copy from every input that fits completely.
+  for (pos = i = 0; i < j; i++) {
+    TypedArrayPrototypeSet(buffer, list[i], pos);
+    pos = postPositions[i];
   }
 
-  // Note: `length` is always equal to `buffer.length` at this point
   if (pos < length) {
-    // Zero-fill the remaining bytes if the specified `length` was more than
-    // the actual total length, i.e. if we have some remaining allocated bytes
-    // there were not initialized.
-    TypedArrayPrototypeFill(buffer, 0, pos, length);
+    // Populate the remaining bytes, either by partially consuming the next
+    // input or by zero-filling.
+    if (j < list.length) {
+      TypedArrayPrototypeSet(buffer,
+                             TypedArrayPrototypeSubarray(list[j], 0, length - pos),
+                             pos);
+    } else {
+      TypedArrayPrototypeFill(buffer, 0, pos, length);
+    }
   }
 
   return buffer;