Document the LEB128 patch

Add some comments and harmonize style for the LEB128 patch.

closes https://github.com/official-stockfish/Stockfish/pull/4642

No functional change

src/nnue/nnue_common.h

@@ -86,6 +86,7 @@ namespace Stockfish::Eval::NNUE {
       return (n + base - 1) / base * base;
   }
 
+
   // read_little_endian() is our utility to read an integer (signed or unsigned, any size)
   // from a stream in little-endian order. We swap the byte order after the read if
   // necessary to return a result with the byte ordering of the compiling machine.
@@ -110,6 +111,7 @@ namespace Stockfish::Eval::NNUE {
       return result;
   }
 
+
   // write_little_endian() is our utility to write an integer (signed or unsigned, any size)
   // to a stream in little-endian order. We swap the byte order before the write if
   // necessary to always write in little endian order, independently of the byte
@@ -140,6 +142,7 @@ namespace Stockfish::Eval::NNUE {
       }
   }
 
+
   // read_little_endian(s, out, N) : read integers in bulk from a little indian stream.
   // This reads N integers from stream s and put them in array out.
   template <typename IntType>
@@ -151,6 +154,7 @@ namespace Stockfish::Eval::NNUE {
               out[i] = read_little_endian<IntType>(stream);
   }
 
+
   // write_little_endian(s, values, N) : write integers in bulk to a little indian stream.
   // This takes N integers from array values and writes them on stream s.
   template <typename IntType>
@@ -162,77 +166,119 @@ namespace Stockfish::Eval::NNUE {
               write_little_endian<IntType>(stream, values[i]);
   }
 
+
+  // read_leb_128(s, out, N) : read N signed integers from the stream s, putting them in
+  // the array out. The stream is assumed to be compressed using the signed LEB128 format.
+  // See https://en.wikipedia.org/wiki/LEB128 for a description of the compression scheme.
   template <typename IntType>
   inline void read_leb_128(std::istream& stream, IntType* out, std::size_t count) {
-      static_assert(std::is_signed_v<IntType>, "Not implemented for unsigned types");
+
+      // Check the presence of our LEB128 magic string
       char leb128MagicString[Leb128MagicStringSize];
       stream.read(leb128MagicString, Leb128MagicStringSize);
       assert(strncmp(Leb128MagicString, leb128MagicString, Leb128MagicStringSize) == 0);
+
+      static_assert(std::is_signed_v<IntType>, "Not implemented for unsigned types");
+
       const std::uint32_t BUF_SIZE = 4096;
       std::uint8_t buf[BUF_SIZE];
+
       auto bytes_left = read_little_endian<std::uint32_t>(stream);
+
       std::uint32_t buf_pos = BUF_SIZE;
-      for (std::size_t i = 0; i < count; ++i) {
+      for (std::size_t i = 0; i < count; ++i)
+      {
           IntType result = 0;
           size_t shift = 0;
-          do {
+          do
-              if (buf_pos == BUF_SIZE) {
+          {
+              if (buf_pos == BUF_SIZE)
+              {
                   stream.read(reinterpret_cast<char*>(buf), std::min(bytes_left, BUF_SIZE));
                   buf_pos = 0;
               }
+
               std::uint8_t byte = buf[buf_pos++];
               --bytes_left;
               result |= (byte & 0x7f) << shift;
               shift += 7;
-              if ((byte & 0x80) == 0) {
+
-                  out[i] = sizeof(IntType) * 8 <= shift || (byte & 0x40) == 0 ? result : result | ~((1 << shift) - 1);
+              if ((byte & 0x80) == 0)
+              {
+                  out[i] = (sizeof(IntType) * 8 <= shift || (byte & 0x40) == 0) ? result
+                                                                                : result | ~((1 << shift) - 1);
                   break;
               }
-          } while (shift < sizeof(IntType) * 8);
           }
+          while (shift < sizeof(IntType) * 8);
+      }
+
       assert(bytes_left == 0);
   }
 
+
+  // write_leb_128(s, values, N) : write signed integers to a stream with LEB128 compression.
+  // This takes N integers from array values, compress them with the LEB128 algorithm and
+  // writes the result on the stream s.
+  // See https://en.wikipedia.org/wiki/LEB128 for a description of the compression scheme.
   template <typename IntType>
   inline void write_leb_128(std::ostream& stream, const IntType* values, std::size_t count) {
-      static_assert(std::is_signed_v<IntType>, "Not implemented for unsigned types");
+
+      // Write our LEB128 magic string
       stream.write(Leb128MagicString, Leb128MagicStringSize);
+
+      static_assert(std::is_signed_v<IntType>, "Not implemented for unsigned types");
+
       std::uint32_t byte_count = 0;
-      for (std::size_t i = 0; i < count; ++i) {
+      for (std::size_t i = 0; i < count; ++i)
+      {
           IntType value = values[i];
           std::uint8_t byte;
-          do {
+          do
+          {
               byte = value & 0x7f;
               value >>= 7;
               ++byte_count;
-          } while ((byte & 0x40) == 0 ? value != 0 : value != -1);
           }
+          while ((byte & 0x40) == 0 ? value != 0 : value != -1);
+      }
+
       write_little_endian(stream, byte_count);
+
       const std::uint32_t BUF_SIZE = 4096;
       std::uint8_t buf[BUF_SIZE];
       std::uint32_t buf_pos = 0;
+
       auto flush = [&]() {
-          if (buf_pos > 0) {
+          if (buf_pos > 0)
+          {
               stream.write(reinterpret_cast<char*>(buf), buf_pos);
               buf_pos = 0;
           }
       };
+
       auto write = [&](std::uint8_t byte) {
           buf[buf_pos++] = byte;
-          if (buf_pos == BUF_SIZE) flush();
+          if (buf_pos == BUF_SIZE)
+              flush();
       };
-      for (std::size_t i = 0; i < count; ++i) {
+
+      for (std::size_t i = 0; i < count; ++i)
+      {
           IntType value = values[i];
-          while (true) {
+          while (true)
+          {
               std::uint8_t byte = value & 0x7f;
               value >>= 7;
-              if ((byte & 0x40) == 0 ? value == 0 : value == -1) {
+              if ((byte & 0x40) == 0 ? value == 0 : value == -1)
+              {
                   write(byte);
                   break;
               }
               write(byte | 0x80);
           }
       }
+
       flush();
   }