simpliy _copyActual, add clarifying comments

Author: duncpro

lib/buffer.js

@@ -210,6 +210,7 @@ function copyImpl(source, target, targetStart, sourceStart, sourceEnd) {
     throw new ERR_INVALID_ARG_TYPE('source', ['Buffer', 'Uint8Array'], source);
   if (!ArrayBufferIsView(target))
     throw new ERR_INVALID_ARG_TYPE('target', ['Buffer', 'Uint8Array'], target);
+  // Guaranteed real buffers
 
   if (targetStart === undefined) {
     targetStart = 0;
@@ -218,6 +219,7 @@ function copyImpl(source, target, targetStart, sourceStart, sourceEnd) {
     if (targetStart < 0)
       throw new ERR_OUT_OF_RANGE('targetStart', '>= 0', targetStart);
   }
+  // Guaranteed targetStart >= 0
 
   if (sourceStart === undefined) {
     sourceStart = 0;
@@ -226,6 +228,7 @@ function copyImpl(source, target, targetStart, sourceStart, sourceEnd) {
     if (sourceStart < 0 || sourceStart > source.byteLength)
       throw new ERR_OUT_OF_RANGE('sourceStart', `>= 0 && <= ${source.byteLength}`, sourceStart);
   }
+  // Guaranteed 0 <= sourceStart <= source.byteLength
 
   if (sourceEnd === undefined) {
     sourceEnd = source.byteLength;
@@ -234,34 +237,29 @@ function copyImpl(source, target, targetStart, sourceStart, sourceEnd) {
     if (sourceEnd < 0)
       throw new ERR_OUT_OF_RANGE('sourceEnd', '>= 0', sourceEnd);
   }
+  // Guaranteed 0 <= sourceEnd
 
   if (targetStart >= target.byteLength || sourceStart >= sourceEnd)
     return 0;
 
+  // Guaranteed 0 <= sourceStart < sourceEnd (NO GUARANTEE <= source.byteLnegth)
+  // Guaranteed 0 <= targetStart < target.byteLength here
+
   return _copyActual(source, target, targetStart, sourceStart, sourceEnd);
 }
 
+// Assumes `source`, `target` are real buffers.
+// Assumes 0 <= sourceStart <= sourceEnd
+// Assumes 0 <= targetStart <= target.byteLength
+//
+// This function clamps sourceEnd such that the source subarray will not exceed the length
+// of the target subarray.
 function _copyActual(source, target, targetStart, sourceStart, sourceEnd) {
-  // Enforce: 0 <= sourceStart <= sourceEnd
-  if (!(0 <= sourceStart && sourceStart <= sourceEnd)) {
-    throw new ERR_BUFFER_OUT_OF_BOUNDS('expected 0 <= sourceStart <= sourceEnd' +
-      ' but got 0 <= ' + sourceStart + ' <= ' + sourceEnd);
-  }
-
   sourceEnd = MathMin(sourceEnd, source.byteLength);
-
-  // Enforce: 0 <= targetStart <= target.byteLength;
-  if (!(0 <= targetStart && targetStart <= target.byteLength)) {
-    throw new ERR_BUFFER_OUT_OF_BOUNDS('expected 0 <= targetStart <= target.byteLength' +
-      ' but got 0 <= ' + targetStart + ' <= ' + target.byteLength);
-  }
-
   const sourceLength = sourceEnd - sourceStart;
   const targetLength = target.byteLength - targetStart;
   const copyLength = MathMin(sourceLength, targetLength);
-
   _copy(source, target, targetStart, sourceStart, copyLength);
-
   return copyLength;
 }
 
@@ -609,6 +607,7 @@ Buffer.concat = function concat(list, length) {
       throw new ERR_INVALID_ARG_TYPE(
         `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
     }
+    //                (src, target, t_beg, s_beg, s_end)
     pos += _copyActual(buf, buffer, pos, 0, buf.length);
   }