specify binary format for vectors in comment

Author: sivukhin

libsql-sqlite3/src/vectorInt.h

@@ -19,6 +19,26 @@ typedef u32 VectorDims;
 */
 #define MAX_VECTOR_SZ 65536
 
+/*
+ * on-disk binary format for vector of different types:
+ * 1. float32
+ *  [data[0] as f32] [data[1] as f32] ... [data[dims - 1] as f32] [1 as u8]?
+ *  - last 'type'-byte is optional for float32 vectors
+ *
+ * 2. float64
+ *  [data[0] as f64] [data[1] as f64] ... [data[dims - 1] as f64] [2 as u8]
+ *  - last 'type'-byte is mandatory for float64 vectors
+ *
+ * 3. float1bit
+ *  [data[0] as u8] [data[1] as u8] ... [data[(dims + 7) / 8] as u8] [_ as u8; padding]? [leftover as u8] [3 as u8]
+ *  - every data byte (except for the last) represents exactly 8 components of the vector
+ *  - last data byte represents [1..8] components of the vector
+ *  - optional padding byte ensures that leftover byte will be written at the odd blob position (0-based)
+ *  - leftover byte specify amount of trailing *bits* in the blob without last 'type'-byte which must be omitted
+ *    (so, vector dimensions are equal to 8 * (blob_size - 1) - leftover)
+ *  - last 'type'-byte is mandatory for float1bit vectors
+*/
+
 /*
  * Enumerate of supported vector types (0 omitted intentionally as we can use zero as "undefined" value)
 */