From bb812ba81c236e184e9abea395696e77e1b7c325 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Mon, 17 Dec 2018 12:42:50 +1100
Subject: [PATCH] io: add pwrite wrappers

---
 io.hpp | 79 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 67 insertions(+), 12 deletions(-)

diff --git a/io.hpp b/io.hpp
index f3f306dd..37f78308 100644
--- a/io.hpp
+++ b/io.hpp
@@ -18,6 +18,8 @@
 #include <filesystem>
 #include <vector>
 #include <streambuf>
+#include <functional>
+
 
 #ifdef PLATFORM_WIN32
 #include "win32/windows.hpp"
@@ -55,27 +57,80 @@ namespace cruft {
 
 
     ///////////////////////////////////////////////////////////////////////////
-    /// writes all data from the provided view into the file-like-object
+    /// Repeatedly calls FunctionT to transfer data from a supplied view to a
+    /// file-like-object.
     ///
-    /// writing will continually iterate until the entire buffer has been
-    /// dispatched so as to avoid issues with partial writes. will block until
-    /// such time as the entire buffer has written.
+    /// FunctionT must return a view of the consumed data.
     ///
-    /// an exception may be thrown in the event forward progress is impossible.
-    /// in this event the progress may not be reported to the caller. in the
-    /// future an exception member variable may expose the information.
+    /// The operation will iterate until the entire view has been consumed and
+    /// hence may block for an indefinate period.
+    ///
+    /// An exception may be throw in the event forward progress is impossible
+    /// (depending on the supplied FunctionT).
+    template <
+        typename DstT,
+        typename FunctionT,
+        typename IteratorA,
+        typename IteratorB,
+        typename ...Args
+    >
+    [[nodiscard]] decltype(auto)
+    drain (
+        DstT &&dst,
+        FunctionT &&func,
+        cruft::view<IteratorA,IteratorB> const &src,
+        Args&& ...args
+    ) {
+        static_assert (std::is_pointer_v<IteratorA>);
+        static_assert (std::is_pointer_v<IteratorB>);
+
+        auto cursor = reinterpret_cast<std::byte const*> (src.data ());
+        auto remain = src.size ();
+
+        while (remain) {
+            auto count = std::invoke (func, dst, cursor, remain, args...);
+            cursor += count;
+            remain -= count;
+        }
+
+        return src;
+    }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Drains all data from the supplied view `src` to the file-like-object
+    /// `dst` by way of repeated calls to `write`.
+    ///
+    /// Returns a view over the consumed data. This SHOULD always be identical
+    /// to `src`.
     template <
         typename DstT,
         typename IteratorA,
         typename IteratorB
     >
-    cruft::view<IteratorA,IteratorB>
+    decltype(auto)
     write (DstT &&dst, const cruft::view<IteratorA, IteratorB> &src)
     {
-        auto remain = src;
-        while (!remain.empty ())
-            remain = remain.consume (dst.write (remain));
-        return src;
+        constexpr auto func = &std::decay_t<DstT>::write;
+        return drain (std::forward<DstT> (dst), func, src);
+    }
+
+
+    ///------------------------------------------------------------------------
+    /// Drain the supplied data view into the destination object by calling
+    /// pwrite repeatedly.
+    ///
+    /// Returns a view over the consumed data. This SHOULD always be identical
+    /// to `src`.
+    template <
+        typename DstT,
+        typename IteratorA,
+        typename IteratorB
+    >
+    decltype(auto)
+    pwrite (DstT &&dst, const cruft::view<IteratorA, IteratorB> &src, size_t offset)
+    {
+        return drain (std::forward<DstT> (dst), &std::decay_t<DstT>::pwrite, src, offset);
     }