Add nifty utility to write data to an output buffer, extending and

releasing as necessary, instead of writing it over and over again
locally. Still needs tests, and docs haven't been updated.

source/iopipe/bufpipe.d

@@ -12,6 +12,7 @@ import iopipe.traits;
 import std.traits : isDynamicArray, hasMember;
 import std.range.primitives;
 
+
 /**
  * An example processor. This demonstrates the required items for implementing
  * an iopipe.
@@ -815,3 +816,54 @@ template iosrc(alias fun, Chain)
         return IOSource(c);
     }
 }
+
+// TODO: need to deal with general ranges.
+/**
+ * Write data from a random access range or character array into the given
+ * iopipe, and release all the data that was written. Note that this really
+ * only does anything of interest on iopipes that are writing on release (i.e.
+ * ones that have a push component).
+ *
+ * Params: c = The iopipe chain to write to.
+ *         data = The range to write to the chain.
+ *
+ * Returns: The number of elements written. This should match the elements of
+ * the range, but could potentially be less if there wasn't a way to extend
+ * more space and more space was needed.
+ */
+size_t writeBuf(Chain, Range)(ref Chain c, Range data) if (isIopipe!Chain &&
+      __traits(compiles, (c.window[0 .. 0] = data[0 .. 0])))
+{
+    // trivial case
+    if(data.length == 0)
+        return 0;
+
+    size_t result = data.length;
+
+    if(c.window.length == 0)
+        c.extend(0);
+
+    while(true)
+    {
+        const dlen = data.length;
+        const wlen = c.window.length;
+        if(wlen == 0)
+            return result - dlen;
+        if(wlen >= dlen)
+        {
+            c.window[0 .. dlen] = data[];
+            c.release(dlen);
+            return result;
+        }
+        else
+        {
+            c.window[] = data[0 .. wlen];
+            data = data[wlen .. $];
+            c.release(wlen);
+            // window is now empty, extend to get more space for writing
+            c.extend(0);
+        }
+    }
+}
+
+// TODO: need unittests for writeBuf