fix decode test after always using DEFLATE canonicalization

Update decode test to construct a table in canonical form and fix the
associated test. This commit also adds additional decode test cases.

Change-Id: Ic46cbcee812f5812ab55f446c7d05c0e9e9d3e8a

huffman/test/decode_test.cpp

@@ -2,70 +2,291 @@
 
 #include <boost/ut.hpp>
 
+#include <algorithm>
 #include <array>
+#include <climits>
 #include <cstddef>
 #include <stdexcept>
 #include <utility>
 
-constexpr auto reverse_bits(int b) -> std::byte
+// TODO(move a test utility?)
+struct byte_printer
 {
-  std::byte result{};
-  for (auto i = 0; i < CHAR_BIT; ++i) {
-    result <<= 1;
-    result |= std::byte{(b & 1) == 1};
-    b >>= 1;
+  std::uint8_t value;
+
+  byte_printer(std::byte b) : value{static_cast<std::uint8_t>(b)} {}
+};
+
+auto operator<<(std::ostream& os, byte_printer bp) -> std::ostream&
+{
+  for (auto mask = std::uint8_t{0x80U}; mask != std::uint8_t{}; mask >>= 1U) {
+    const auto on = (bp.value & mask) == std::uint8_t{1};
+    os << (on ? '1' : '0');
+  }
+
+  return os;
+}
+
+static constexpr auto eot = '\4';
+
+static constexpr auto code_table = [] {
+  using namespace ::starflate::huffman::literals;
+
+  // clang-format off
+  return ::starflate::huffman::table{
+      ::starflate::huffman::table_contents,
+      {
+          std::pair{0_c, 'e'},
+                   {10_c, 'i'},
+                   {110_c, 'n'},
+                   {1110_c, 'q'},
+                   {11110_c, eot},
+                   {11111_c, 'x'},
+      }};
+  // clang-format on
+}();
+
+namespace test {
+
+constexpr auto verify(bool cond)
+{
+  if (not cond) {
+    throw std::runtime_error{""};
   }
-  return result;
 }
 
+}  // namespace test
+
 auto main() -> int
 {
+  using ::boost::ut::eq;
   using ::boost::ut::expect;
   using ::boost::ut::test;
 
   namespace huffman = ::starflate::huffman;
-  using namespace huffman::literals;
-
-  // FIXME table contents are not in canonical form
-  test("basic") = [] {
-    // encoded data from soxofaan/dahuffman readme.rst.
-    // We reverse the bits in each byte to match the encoding used in DEFLATE.
-    constexpr std::array encoded_bytes = {
-        reverse_bits(134),
-        reverse_bits(124),
-        reverse_bits(37),
-        reverse_bits(19),
-        reverse_bits(105),
-        reverse_bits(64)};
-
-    constexpr char eot = {'\4'};
-    static constexpr auto code_table =  // clang-format off
-      huffman::table{
-        huffman::table_contents,
-        {std::pair{1_c,     'e'},
-                  {01_c,    'i'},
-                  {001_c,   'n'},
-                  {0001_c,  'q'},
-                  {00000_c, eot},
-                  {00001_c, 'x'}
-        }
-      };  // clang-format on
-
-    constexpr std::array expected = {
+
+  test("empty") = [] {
+    constexpr auto result = [] {
+      auto buf = std::array<char, 1>{};
+
+      auto it = huffman::decode(code_table, huffman::bit_span{}, buf.begin());
+
+      return it == buf.begin();
+    }();
+
+    static_assert(result);
+  };
+
+  test("just `e` - beginning not byte aligned") = [] {
+    static constexpr auto expected = std::array{'e'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array<std::byte, 1>{};
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(
+          code_table,
+          huffman::bit_span{encoded}.consume(CHAR_BIT - 1),
+          buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("just `e` - beginning not byte aligned, different padding") = [] {
+    static constexpr auto expected = std::array{'e'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{
+          //
+          std::byte{0b0111'1111}
+          //           ^^^ ^^^^
+          //           padding
+      };
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(
+          code_table,
+          huffman::bit_span{encoded}.consume(CHAR_BIT - 1),
+          buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`e` x8") = [] {
+    static constexpr auto expected = [] {
+      auto arr = std::array<char, 8>{};
+      std::ranges::fill(arr, 'e');
+      return arr;
+    }();
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array<std::byte, 1>{};
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(code_table, encoded, buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("invalid code") = [] {
+    static constexpr auto different_table = [] {
+      using namespace ::starflate::huffman::literals;
+
+      // clang-format off
+      return ::starflate::huffman::table{
+        ::starflate::huffman::table_contents,
+        {
+          std::pair{0_c, 'e'},
+        }};
+      // clang-format on
+    }();
+
+    static constexpr auto expected = [] {
+      auto arr = std::array<char, 8>{};
+      std::ranges::fill(arr, 'e');
+      return arr;
+    }();
+
+    static constexpr auto encoded =
+        std::array{std::byte{0x00U}, std::byte{0xFFU}};
+
+    constexpr auto decoded = [] {
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(different_table, encoded, buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`nx`") = [] {
+    static constexpr auto expected = std::array{'n', 'x'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{std::byte{0b1111'1011}};
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(code_table, encoded, buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`nxqiee`") = [] {
+    static constexpr auto expected = std::array{'n', 'x', 'q', 'i', 'e', 'e'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{
+          std::byte{0b1111'1011},  //
+          std::byte{0b0001'0111}};
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(code_table, encoded, buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`nxqi` - end is not byte aligned") = [] {
+    static constexpr auto expected = std::array{'n', 'x', 'q', 'i'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{
+          std::byte{0b1111'1011},  //
+          std::byte{0b0001'0111}
+          //          ^^
+          //          padding
+      };
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(
+          code_table,
+          huffman::bit_span{encoded.data(), encoded.size() * CHAR_BIT - 2},
+          buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`exeneeeexni`") = [] {
+    static constexpr auto expected =
+        std::array{'e', 'x', 'e', 'n', 'e', 'e', 'e', 'e', 'x', 'n', 'i'};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{
+          std::byte{0b1011'1110},
+          std::byte{0b1100'0001},
+          std::byte{0b0101'1111}};
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(code_table, encoded, buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
+    }();
+
+    expect(eq(expected, decoded));
+  };
+
+  test("`exeneeeexniqneiein`") = [] {
+    static constexpr auto expected = std::array{
         'e', 'x', 'e', 'n', 'e', 'e', 'e', 'e', 'x', 'n',
-        'i', 'q', 'n', 'e', 'i', 'e', 'i', 'n', 'i', eot,
-    };
-
-    const auto output_buf = [&] {
-      std::array<char, expected.size()> output_buf{};
-      auto result = decode(code_table, encoded_bytes, output_buf.begin());
-      // result should point to the back of output_buf.
-      if (output_buf.end() != result) {
-        throw std::runtime_error("assertion failed");
-      }
-      return output_buf;
+        'i', 'q', 'n', 'e', 'i', 'e', 'i', 'n', 'i', eot};
+
+    constexpr auto decoded = [] {
+      constexpr auto encoded = std::array{
+          std::byte{0b1011'1110},
+          std::byte{0b1100'0001},
+          std::byte{0b0101'1111},
+          std::byte{0b0011'0111},
+          std::byte{0b0110'1001},
+          std::byte{0b0011'1101}
+          //          ^
+          //          padding
+      };
+
+      auto buf = std::array<char, expected.size()>{};
+
+      auto it = huffman::decode(
+          code_table,
+          huffman::bit_span{encoded.data(), encoded.size() * CHAR_BIT - 1},
+          buf.begin());
+
+      ::test::verify(it == buf.end());
+      return buf;
     }();
 
-    expect(output_buf == expected);
+    expect(eq(expected, decoded));
   };
 }