begin move to u3r_unpack() (new function) for consistency in crypto jets

combining the padding and length checking into one function helps make
the crypto jets really boring and easy to scan, improving auditability.

pkg/urbit/include/noun/retrieve.h

@@ -346,6 +346,15 @@
                   c3_y*   c_y,
                   u3_atom d);
 
+      /* u3r_unpack():
+      **
+      **   Copy (len_w) bytes of (a) into (buf_y) if it fits, returning overage
+      */
+        c3_w
+        u3r_unpack(c3_w len_w,
+                   c3_y *buf_y,
+                   u3_atom a);
+
       /* u3r_chop():
       **
       **   Into the bloq space of `met`, from position `fum` for a

pkg/urbit/jets/e/ed_add_double_scalarmult.c

@@ -12,29 +12,18 @@
                              u3_atom c,
                              u3_atom d)
   {
-    c3_w ate_w, bet_w, get_w, det_w;
+    c3_y a_y[32], b_y[32], c_y[32], d_y[32], out_y[32];
 
-    if ( ((ate_w = u3r_met(3, a)) > 32) ||
+    if ( (0 == u3r_unpack(32, a_y, a)) &&
-         ((bet_w = u3r_met(3, b)) > 32) ||
+         (0 == u3r_unpack(32, b_y, b)) &&
-         ((get_w = u3r_met(3, c)) > 32) ||
+         (0 == u3r_unpack(32, c_y, c)) &&
-         ((det_w = u3r_met(3, d)) > 32) ) {
+         (0 == u3r_unpack(32, d_y, d)) &&
-      return u3_none;
+         (0 == urcrypt_ed_add_double_scalarmult(a_y, b_y, c_y, d_y, out_y)) )
+    {
+      return u3i_bytes(32, out_y);
     }
     else {
-      c3_y a_y[32], b_y[32], c_y[32], d_y[32], out_y[32];
+      return u3_none;
-
-      memset(a_y, 0, 32);
-      memset(b_y, 0, 32);
-      memset(c_y, 0, 32);
-      memset(d_y, 0, 32);
-      u3r_bytes(0, ate_w, a_y, a);
-      u3r_bytes(0, bet_w, b_y, b);
-      u3r_bytes(0, get_w, c_y, c);
-      u3r_bytes(0, det_w, d_y, d);
-
-      return ( 0 == urcrypt_ed_add_double_scalarmult(a_y, b_y, c_y, d_y, out_y) )
-        ? u3i_bytes(32, out_y)
-        : u3_none;
     }
   }
 

pkg/urbit/noun/retrieve.c

@@ -1076,6 +1076,23 @@ u3r_bytes(c3_w    a_w,
   }
 }
 
+/* u3r_unpack():
+**
+**  Copy (len_w) bytes of (a) into (buf_y) if it fits, returning overage
+*/
+c3_w
+u3r_unpack(c3_w len_w, c3_y *buf_y, u3_atom a)
+{
+  c3_w met_w = u3r_met(3, a);
+  if ( met_w <= len_w ) {
+    u3r_bytes(0, len_w, buf_y, a);
+    return 0;
+  }
+  else {
+    return len_w - met_w;
+  }
+}
+
 /* u3r_mp():
 **
 **   Copy (b) into (a_mp).