copy_n to copy_from_before

Change-Id: I9322af2674ab583f3cdb286ea06385587bacf670

src/decompress.cpp

@@ -130,19 +130,19 @@ auto decompress_block_huffman(
     if (dst.size() - static_cast<std::size_t>(dst_written) < len) {
       return DecompressStatus::DstTooSmall;
     }
-    starflate::detail::copy_n(
-        dst.begin() + (dst_written - distance), len, dst.begin() + dst_written);
+    starflate::detail::copy_from_before(
+        dst.begin() + dst_written, distance, len);
     dst_written += len;
   }
   return DecompressStatus::Success;
 }
 
-void copy_n(
-    std::span<const std::byte>::iterator src,
-    std::uint16_t n,
-    std::span<std::byte>::iterator dst)
+/// Copy n bytes from distance bytes before dst to dst.
+void copy_from_before(
+    std::span<std::byte>::iterator dst, std::uint16_t distance, std::uint16_t n)
 {
   std::ptrdiff_t n_signed{n};
+  const auto src = dst - distance;
   while (n_signed > 0) {
     const auto n_to_copy = std::min(n_signed, dst - src);
     dst = std::copy_n(src, n_to_copy, dst);

src/decompress.hpp

@@ -53,17 +53,18 @@ constexpr auto lit_or_len_end_of_block = std::uint16_t{256};
 constexpr auto lit_or_len_max = std::uint16_t{285};
 constexpr auto lit_or_len_max_decoded = std::uint16_t{258};
 
-/// Copies n bytes from src to dst, repeating the source data if necessary.
+/// Copies n bytes from (dst - distance) to dst, handling overlap by repeating.
 ///
 /// From the standard section 3.2.3:
-/// "Note also that the referenced string may overlap the current
-///  position; for example, if the last 2 bytes decoded have values
-///  X and Y, a string reference with <length = 5, distance = 2>
-///  adds X,Y,X,Y,X to the output stream."
-void copy_n(
-    std::span<const std::byte>::iterator src,
-    std::uint16_t n,
-    std::span<std::byte>::iterator dst);
+/// the referenced string may overlap the current position; for example, if the
+/// last 2 bytes decoded have values X and Y, a string reference with
+/// <length = 5, distance = 2> adds X,Y,X,Y,X to the output stream.
+///
+/// @pre dst - distance is valid.
+void copy_from_before(
+    std::span<std::byte>::iterator dst,
+    std::uint16_t distance,
+    std::uint16_t n);
 }  // namespace detail
 
 /// Decompresses the given source data into the destination buffer.

src/test/decompress_test.cpp

@@ -162,11 +162,10 @@ auto main(int, char* argv[]) -> int
     expect(header->type == detail::BlockType::DynamicHuffman);
   };
 
-  test("copy_n") = [] {
+  test("copy_from_before") = [] {
     auto src_and_dst = huffman::byte_array(1, 2, 0, 0, 0, 0);
-    const auto src_span = std::span<const std::byte>{src_and_dst};
     const auto dst_span = std::span<std::byte>{src_and_dst}.subspan(2);
-    detail::copy_n(src_span.begin(), 3, dst_span.begin());
+    detail::copy_from_before(dst_span.begin(), 2, 3);
     expect(eq(src_and_dst, huffman::byte_array(1, 2, 1, 2, 1, 0)));
   };
 };