From 5bcb131028f6812f3c1c6e56d586648af9a21f15 Mon Sep 17 00:00:00 2001
From: Gary Miguel <garymm@garymm.org>
Date: Sun, 25 Jun 2023 17:47:07 -0700
Subject: [PATCH] implement bit_span

for easy iteration of the bits in some data

Change-Id: I6f1dee0448ff071e9b820aa5a020120717b61ab2
---
 huffman/BUILD.bazel            |  1 +
 huffman/huffman.hpp            |  1 +
 huffman/src/bit.hpp            |  1 +
 huffman/src/bit_span.hpp       | 88 ++++++++++++++++++++++++++++++++++
 huffman/test/BUILD.bazel       | 10 ++++
 huffman/test/bit_span_test.cpp | 31 ++++++++++++
 6 files changed, 132 insertions(+)
 create mode 100644 huffman/src/bit_span.hpp
 create mode 100644 huffman/test/bit_span_test.cpp

diff --git a/huffman/BUILD.bazel b/huffman/BUILD.bazel
index f7cc9a7..a0391cb 100644
--- a/huffman/BUILD.bazel
+++ b/huffman/BUILD.bazel
@@ -4,6 +4,7 @@ cc_library(
     name = "huffman",
     srcs = [
         "src/bit.hpp",
+        "src/bit_span.hpp",
         "src/code.hpp",
         "src/detail/base_view.hpp",
         "src/detail/iterator_interface.hpp",
diff --git a/huffman/huffman.hpp b/huffman/huffman.hpp
index 867d8e7..986be4b 100644
--- a/huffman/huffman.hpp
+++ b/huffman/huffman.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include "huffman/src/bit.hpp"
+#include "huffman/src/bit_span.hpp"
 #include "huffman/src/code.hpp"
 #include "huffman/src/encoding.hpp"
 #include "huffman/src/table.hpp"
diff --git a/huffman/src/bit.hpp b/huffman/src/bit.hpp
index 670cfe6..a0fc4f4 100644
--- a/huffman/src/bit.hpp
+++ b/huffman/src/bit.hpp
@@ -1,3 +1,4 @@
+
 #pragma once
 
 #include <cassert>
diff --git a/huffman/src/bit_span.hpp b/huffman/src/bit_span.hpp
new file mode 100644
index 0000000..0bfc8ea
--- /dev/null
+++ b/huffman/src/bit_span.hpp
@@ -0,0 +1,88 @@
+
+#include "huffman/src/bit.hpp"
+#include "huffman/src/detail/iterator_interface.hpp"
+
+#include <bitset>
+#include <cassert>
+#include <climits>
+#include <cstddef>
+#include <cstdint>
+#include <iterator>
+#include <limits>
+
+namespace gpu_deflate::huffman {
+class bit_span
+{
+
+  const std::byte* data_;
+  size_t bit_size_;
+
+public:
+  class iterator : public detail::iterator_interface<iterator>
+  {
+  private:
+    const bit_span* parent_;
+    size_t offset_;
+
+  public:
+    using difference_type = std::ptrdiff_t;
+    using iterator_category = std::random_access_iterator_tag;
+    using value_type = bit;
+    using reference = bit;
+    using pointer = void;
+
+    iterator() = default;
+    constexpr iterator(const bit_span& parent, size_t offset)
+        : parent_(&parent), offset_(offset)
+    {
+      assert(offset_ <= std::numeric_limits<difference_type>::max());
+    }
+
+    constexpr auto operator*() const -> reference
+    {
+      // TODO: switch to std::bitset once GCC's
+      // std::bitset<_Nb>::reference::operator bool() is constexpr
+      std::uint8_t byte{
+          // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+          static_cast<std::uint8_t>(parent_->data_[offset_ / CHAR_BIT])};
+      // most significant bit first
+      return bit{bool(byte & 1 << (CHAR_BIT - 1 - (offset_ % CHAR_BIT)))};
+    }
+
+    constexpr auto operator+=(difference_type n) -> iterator&
+    {
+      const auto newOffset = static_cast<difference_type>(offset_) + n;
+      assert(newOffset >= 0);
+      offset_ = static_cast<size_t>(newOffset);
+      return *this;
+    }
+
+    friend constexpr auto
+    operator-(const iterator& lhs, const iterator& rhs) -> difference_type
+    {
+      assert(lhs.parent_ == rhs.parent_);
+
+      return static_cast<difference_type>(lhs.offset_) -
+             static_cast<difference_type>(rhs.offset_);
+    }
+
+    friend constexpr auto
+    operator<=>(const iterator&, const iterator&) = default;
+  };
+
+  constexpr bit_span(const std::byte* data, size_t bit_size)
+      : data_(data), bit_size_(bit_size)
+  {}
+
+  [[nodiscard]]
+  constexpr auto begin() const -> iterator
+  {
+    return iterator{*this, 0};
+  };
+  [[nodiscard]]
+  constexpr auto end() const -> iterator
+  {
+    return iterator{*this, bit_size_};
+  };
+};
+}  // namespace gpu_deflate::huffman
diff --git a/huffman/test/BUILD.bazel b/huffman/test/BUILD.bazel
index 97a3092..cee9e2a 100644
--- a/huffman/test/BUILD.bazel
+++ b/huffman/test/BUILD.bazel
@@ -60,6 +60,16 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "bit_span_test",
+    timeout = "short",
+    srcs = ["bit_span_test.cpp"],
+    deps = [
+        "//huffman",
+        "@boost_ut",
+    ],
+)
+
 cc_binary(
     name = "bench",
     srcs = ["bench.cpp"],
diff --git a/huffman/test/bit_span_test.cpp b/huffman/test/bit_span_test.cpp
new file mode 100644
index 0000000..5048ee2
--- /dev/null
+++ b/huffman/test/bit_span_test.cpp
@@ -0,0 +1,31 @@
+#include "huffman/huffman.hpp"
+
+#include <boost/ut.hpp>
+
+#include <array>
+#include <climits>
+#include <cstddef>
+#include <cstdint>
+
+auto main() -> int
+{
+  using ::boost::ut::aborts;
+  using ::boost::ut::expect;
+  using ::boost::ut::test;
+
+  namespace huffman = ::gpu_deflate::huffman;
+  using namespace huffman::literals;
+
+  test("basic") = [] {
+    static constexpr std::array<std::byte, 2> data{
+        std::byte{0b10101010}, std::byte{0xff}};
+    // leave off the last bit of the last byte
+    constexpr huffman::bit_span span{data.data(), (data.size() * CHAR_BIT) - 1};
+    constexpr std::string_view expected = "101010101111111";
+    expect(std::ranges::equal(
+        span,
+        expected | std::views::transform([](char c) {
+          return huffman::bit{c};
+        })));
+  };
+}