HP-GL: a .plt file needs at least one instruction the reader knows

A file read as HP-GL by its extension that holds only instructions of
another set (`GC5;`) is now refused as "not an HP-GL file" instead of
opening as an empty plot. At least one of the instructions the check
reads must be one the reader knows.

crates/cad/src/hpgl.rs

@@ -71,16 +71,18 @@ pub fn is_hpgl(bytes: &[u8]) -> bool {
 
 /// Does `bytes` hold HP-GL instructions as [`is_hpgl`] checks? When `strict` is false, any of them
 /// may come first: a file picked by its extension may start with one a plot file seldom starts
-/// with (`SI`, `DI`).
+/// with (`SI`, `DI`). Either way, at least one of the instructions checked must be one the reader
+/// knows.
 fn holds_instructions(bytes: &[u8], strict: bool) -> bool {
     let mut lex = Lexer::new(bytes);
     let mut terminator = lex::ETX;
     let mut first = 0;
+    let mut any_known = false;
     for n in 0..8 {
         if n > 0 && lex.pos() > first + 4096 {
-            return true;
+            return any_known;
         }
-        let Some(op) = lex.op(true) else { return n > 0 && lex.done() };
+        let Some(op) = lex.op(true) else { return n > 0 && any_known && lex.done() };
         if n == 0 {
             first = lex.pos();
         }
@@ -88,6 +90,7 @@ fn holds_instructions(bytes: &[u8], strict: bool) -> bool {
             return false;
         }
         let known = PLAYED.contains(&op) || IGNORED.contains(&op);
+        any_known |= known;
         match &op {
             b"CO" => {
                 if !lex.comment() {
@@ -114,11 +117,12 @@ fn holds_instructions(bytes: &[u8], strict: bool) -> bool {
             }
         }
     }
-    true
+    any_known
 }
 
 /// Read an HP-GL file. It may start with any instruction the reader knows, or one of another set
-/// that a semicolon ends; the rest of [`is_hpgl`]'s check applies.
+/// that a semicolon ends; the rest of [`is_hpgl`]'s check applies, and at least one instruction the
+/// reader knows must be among those it checks.
 pub fn import(bytes: &[u8]) -> Result<Imported, String> {
     if !holds_instructions(bytes, false) {
         return Err("not an HP-GL file".into());

crates/cad/src/hpgl/tests.rs

@@ -451,6 +451,11 @@ fn a_plot_file_may_start_with_any_instruction() {
     for f in [&b"GC5"[..], b"SIhello world", b"set terminal png\n"] {
         assert_eq!(import(f).err().as_deref(), Some("not an HP-GL file"), "{:?}", String::from_utf8_lossy(f));
     }
+    // Instructions of another set alone, however many or long, are not taken for HP-GL.
+    let long = [&b"GC"[..], "1".repeat(5000).as_bytes(), b";ZZ1;"].concat();
+    for f in [&b"GC5;"[..], b"GC5;ZZ1;", b"GC5;ZZ1;YY2;XX3;WW4;VV5;UU6;TT7;", &long] {
+        assert_eq!(import(f).err().as_deref(), Some("not an HP-GL file"), "{:?}", String::from_utf8_lossy(f));
+    }
 }
 
 #[test]