pack.c: add `X` directive (back up byte).

mrbgems/mruby-pack/src/pack.c

@@ -39,6 +39,7 @@ enum pack_dir {
   PACK_DIR_BASE64,    /* m */
   PACK_DIR_QENC,      /* M */
   PACK_DIR_NUL,       /* x */
+  PACK_DIR_BACK,      /* X */
   PACK_DIR_INVALID
 };
 
@@ -1024,6 +1025,24 @@ unpack_x(mrb_state *mrb, int slen, int count)
   return count;
 }
 
+static int
+pack_X(mrb_state *mrb, mrb_value dst, mrb_int didx, int count)
+{
+  if (count > didx) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "X outside of string");
+  }
+  return count;
+}
+
+static int
+unpack_X(mrb_state *mrb, int sidx, int count)
+{
+  if (sidx < count) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "X outside of string");
+  }
+  return count;
+}
+
 static void
 prepare_tmpl(mrb_state *mrb, struct tmpl *tmpl)
 {
@@ -1214,13 +1233,16 @@ alias:
     dir = PACK_DIR_NUL;
     type = PACK_TYPE_NONE;
     break;
+  case 'X':
+    dir = PACK_DIR_BACK;
+    type = PACK_TYPE_NONE;
+    break;
   case 'Z':
     dir = PACK_DIR_STR;
     type = PACK_TYPE_STRING;
     flags |= PACK_FLAG_WIDTH | PACK_FLAG_COUNT2 | PACK_FLAG_Z;
     break;
   case 'p': case 'P':
-  case 'X':
   case '%': case '@':
     mrb_raisef(mrb, E_ARGUMENT_ERROR, "%c is not supported", (char)t);
     break;
@@ -1300,6 +1322,11 @@ mrb_pack_pack(mrb_state *mrb, mrb_value ary)
       ridx += pack_x(mrb, result, ridx, count);
       continue;
     }
+    else if (dir == PACK_DIR_BACK) {
+      if (count > 0 && ridx > INT_MAX - count) goto overflow;
+      ridx -= pack_X(mrb, result, ridx, count);
+      continue;
+    }
 
     if ((flags & PACK_FLAG_WIDTH) && aidx >= RARRAY_LEN(ary)) {
       mrb_raise(mrb, E_ARGUMENT_ERROR, "too few arguments");
@@ -1412,6 +1439,10 @@ pack_unpack(mrb_state *mrb, mrb_value str, int single)
       srcidx += unpack_x(mrb, srclen-srcidx, count);
       continue;
     }
+    else if (dir == PACK_DIR_BACK) {
+      srcidx -= unpack_X(mrb, srcidx, count);
+      continue;
+    }
 
     if (flags & PACK_FLAG_COUNT2) {
       sptr = (const unsigned char *)RSTRING_PTR(str) + srcidx;