add header to witness bin

c/main.cpp

@@ -118,6 +118,18 @@ void writeOutBin(Circom_CalcWit *ctx, std::string filename) {
 
     write_ptr = fopen(filename.c_str(),"wb");
 
+    // Add header
+    // 4 bytes --> total number of constraints
+    uint32_t witnessLength = _circuit.NVars;
+    // 4 bytes --> witness element size (measured in number of 32bit words)
+    uint32_t witnessSize = 8; // witness size = 8*32 = 256 bits
+    // 8bytes --> empty
+    uint64_t otherField = 0;
+
+    fwrite(&witnessLength, 4, 1, write_ptr);
+    fwrite(&witnessSize, 4, 1, write_ptr);
+    fwrite(&otherField, 8, 1, write_ptr);
+
     FrElement v;
 
     u8 buffOut[256];

doc/r1cs_bin_format.md

@@ -152,9 +152,9 @@ Size in bytes of the section
 
 Section Type: 0x01
 ````
-     ┏━━━━┳━━━━━━━━━━━━━━━━━┳━━━┳━━━━━━━━━┳━━━━━━━━━━━━━━━━┓
-     ┃ 4  │  FieldDefSize   ┃ 4 ┃FieldDef ┃    field Id    ┃
-     ┗━━━━┻━━━━━━━━━━━━━━━━━┻━━━┻━━━━━━━━━┻━━━━━━━━━━━━━━━━┛
+     ┏━━━━┳━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
+     ┃ 4  │  FieldDefSize   ┃  FieldDef                    ┃      field Id
+     ┗━━━━┻━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
      ┏━━━━┳━━━━━━━━━━━━━━━━━┓
      ┃ 4  │   00 00 00 00   ┃        bigInt Format
      ┗━━━━┻━━━━━━━━━━━━━━━━━┛
@@ -501,7 +501,7 @@ The format will be:
 
 ````
      ┏━━━━━━━━━━━━━━┓
-     ┃ 72 31 63 73  ┃        Magic
+     ┃ 72 31 63 77  ┃        Magic
      ┣━━━━━━━━━━━━━━┫
      ┃ 01 00 00 00  ┃       Version
      ┣━━━━━━━━━━━━━━┫