-s

Added comments for clarity

Author: medhatiwari

src/MongoDB.Bson/Serialization/BinaryVectorReader.cs

@@ -51,14 +51,15 @@ public static (TItem[] Items, byte Padding, BinaryVectorDataType VectorDataType)
                         throw new NotSupportedException($"Expected float for Float32 vector type, but found {typeof(TItem)}.");
                     }
 
-                    int count = vectorDataBytes.Length / 4;
+                    int count = vectorDataBytes.Length / 4; // 4 bytes per float
                     float[] floatArray = new float[count];
 
                     for (int i = 0; i < count; i++)
                     {
+                        // Each float32 is 4 bytes. So to extract the i-th float, we slice 4 bytes from offset i * 4. Use little-endian or big-endian decoding based on platform.
                         floatArray[i] = BitConverter.IsLittleEndian
-                            ? MemoryMarshal.Read<float>(vectorDataBytes.Span.Slice(i * 4, 4))
-                            : BinaryPrimitives.ReadSingleBigEndian(vectorDataBytes.Span.Slice(i * 4, 4));
+                            ? MemoryMarshal.Read<float>(vectorDataBytes.Span.Slice(i * 4, 4))   // fast, unaligned read on little endian
+                            : BinaryPrimitives.ReadSingleBigEndian(vectorDataBytes.Span.Slice(i * 4, 4));   // correctly reassemble 4 bytes as big-endian float
                     }
 
                     items = (TItem[])(object)floatArray;
@@ -158,3 +159,4 @@ private static void ValidateItemTypeForBinaryVector<TItem, TItemExpectedType, TB
         }
     }
 }
+

src/MongoDB.Bson/Serialization/BinaryVectorWriter.cs

@@ -48,15 +48,16 @@ public static byte[] WriteToBytes<TItem>(ReadOnlySpan<TItem> vectorData, BinaryV
             {
                 case BinaryVectorDataType.Float32:
                     int length = vectorData.Length * sizeof(float);
-                    resultBytes = new byte[2 + length];
-                    resultBytes[0] = (byte)binaryVectorDataType;
-                    resultBytes[1] = padding;
-                    var floatSpan = MemoryMarshal.Cast<TItem, float>(vectorData);
-                    Span<byte> floatOutput = resultBytes.AsSpan(2);
+                    resultBytes = new byte[2 + length]; 				          // Allocate output buffer:
+                    resultBytes[0] = (byte)binaryVectorDataType; 			      // - [0]: vector type
+                    resultBytes[1] = padding;						              // - [1]: padding
+                    var floatSpan = MemoryMarshal.Cast<TItem, float>(vectorData);	
+                    Span<byte> floatOutput = resultBytes.AsSpan(2);			      // - [2...]: actual float data , skipping header
                     foreach (var value in floatSpan)
                     {
+			            // Each float is 4 bytes - write in Big Endian format
                         BinaryPrimitives.WriteSingleBigEndian(floatOutput, value);
-                        floatOutput = floatOutput.Slice(4);
+                        floatOutput = floatOutput.Slice(4); // advance to next 4-byte block
                     }
                     return resultBytes;
 
@@ -75,3 +76,4 @@ public static byte[] WriteToBytes<TItem>(ReadOnlySpan<TItem> vectorData, BinaryV
         }
     }
 }
+