add set_stream_position/2

src/prolog/machine/machine_errors.rs

@@ -468,9 +468,11 @@ pub enum Permission {
     Modify,
     Open,
     OutputStream,
+    Reposition,
 }
 
 impl Permission {
+    #[inline]
     pub fn as_str(self) -> &'static str {
         match self {
             Permission::Access => "access",
@@ -479,6 +481,7 @@ impl Permission {
             Permission::Modify => "modify",
             Permission::Open => "open",
             Permission::OutputStream => "output",
+            Permission::Reposition => "reposition",
         }
     }
 }

src/prolog/machine/streams.rs

@@ -27,6 +27,15 @@ impl StreamType {
     #[inline]
     pub(crate)
     fn as_str(&self) -> &'static str {
+        match self {
+            StreamType::Binary => "binary_stream",
+            StreamType::Text => "text_stream",
+        }
+    }
+
+    #[inline]
+    pub(crate)
+    fn as_property_str(&self) -> &'static str {
         match self {
             StreamType::Binary => "binary",
             StreamType::Text => "text",
@@ -296,6 +305,22 @@ impl Stream {
         }
     }
 
+    #[inline]
+    pub(crate)
+    fn set_position(&mut self, position: u64) {
+        match self.stream_inst.0.borrow_mut().deref_mut() {
+            (past_end_of_stream, StreamInstance::InputFile(_, ref mut file)) => {
+                file.seek(SeekFrom::Start(position)).unwrap();
+
+                if let Ok(metadata) = file.metadata() {
+                    *past_end_of_stream = position > metadata.len();
+                }
+            }
+            _ => {
+            }
+        }
+    }
+
     #[inline]
     pub(crate)
     fn past_end_of_stream(&self) -> bool {

src/prolog/machine/system_calls.rs

@@ -4556,6 +4556,49 @@ impl MachineState {
                     }
                 }
             }
+            &SystemClauseType::SetStreamPosition => {
+                let mut stream = self.get_stream_or_alias(
+                    self[temp_v!(1)],
+                    indices,
+                    "set_stream_position",
+                    2,
+                )?;
+
+                if !stream.options.reposition {
+                    let stub = MachineError::functor_stub(clause_name!("set_stream_position"), 2);
+
+                    let err = MachineError::permission_error(
+                        self.heap.h(),
+                        Permission::Reposition,
+                        "stream",
+                        vec![HeapCellValue::Stream(stream)],
+                    );
+
+                    return Err(self.error_form(err, stub));
+                }
+
+                let position = self.store(self.deref(self[temp_v!(2)]));
+
+                let position =
+                    match Number::try_from((position, &self.heap)) {
+                        Ok(Number::Fixnum(n)) => {
+                            n as u64
+                        }
+                        Ok(Number::Integer(n)) => {
+                            if let Some(n) = n.to_u64() {
+                                n
+                            } else {
+                                self.fail = true;
+                                return Ok(());
+                            }
+                        }
+                        _ => {
+                            unreachable!()
+                        }
+                    };
+
+                stream.set_position(position);
+            }
             &SystemClauseType::StreamProperty => {
                 let mut stream = self.get_stream_or_alias(
                     self[temp_v!(1)],
@@ -4648,7 +4691,7 @@ impl MachineState {
                                         }
                                         "type" => {
                                             HeapCellValue::Atom(
-                                                clause_name!(stream.options.stream_type.as_str()),
+                                                clause_name!(stream.options.stream_type.as_property_str()),
                                                 None,
                                             )
                                         }