add '', type checking for incomplete lists in sort, keysort (re: #17)

src/prolog/ast.rs

@@ -808,7 +808,7 @@ impl ClauseType {
             &ClauseType::Op(ref name, ..) => name.clone(),
             &ClauseType::Named(ref name, ..) => name.clone(),
             &ClauseType::SetupCallCleanup => clause_name!("setup_call_cleanup"),
-            &ClauseType::SkipMaxList => clause_name!("'$skip_max_list'"),
+            &ClauseType::SkipMaxList => clause_name!("$skip_max_list"),
             &ClauseType::Sort => clause_name!("sort"),
             &ClauseType::Throw => clause_name!("throw")
         }
@@ -838,7 +838,7 @@ impl ClauseType {
             ("keysort", 2) => ClauseType::KeySort,
             ("\\==", 2) => ClauseType::NotEq,
             ("setup_call_cleanup", 3) => ClauseType::SetupCallCleanup,
-            ("'$skip_max_list'", 4) => ClauseType::SkipMaxList,
+            ("$skip_max_list", 4) => ClauseType::SkipMaxList,
             ("sort", 2) => ClauseType::Sort,
             ("throw", 1) => ClauseType::Throw,
             _ => if let Some(fixity) = fixity {
@@ -1360,6 +1360,13 @@ impl Addr {
             _ => true
         }
     }
+
+    pub fn is_empty_list(&self) -> bool {
+        match self {
+            &Addr::Con(Constant::EmptyList) => true,
+            _ => false
+        }
+    }
 }
 
 impl From<Ref> for Addr {
@@ -1405,7 +1412,7 @@ pub struct ModuleCodeIndex(pub IndexPtr, pub ClauseName);
 
 impl From<ModuleCodeIndex> for CodeIndex {
     fn from(value: ModuleCodeIndex) -> Self {
-        CodeIndex(Rc::new(RefCell::new((value.0, value.1.clone()))))
+        CodeIndex(Rc::new(RefCell::new((value.0, value.1))))
     }
 }
 

src/prolog/machine/machine_state.rs

@@ -554,6 +554,12 @@ pub(crate) trait CallPolicy: Any {
             },
             &ClauseType::Inlined(ref inlined) => {
                 machine_st.execute_inlined(inlined, &vec![temp_v!(1), temp_v!(2)]);
+                Ok(())
+            },
+            &ClauseType::SkipMaxList => {
+                machine_st.skip_max_list();
+                machine_st.p += 1;
+                
                 Ok(())
             }
         }

src/prolog/machine/machine_state_impl.rs

@@ -4,7 +4,7 @@ use prolog::copier::*;
 use prolog::heap_iter::*;
 use prolog::heap_print::*;
 use prolog::machine::machine_state::*;
-use prolog::num::{Integer, ToPrimitive, Zero};
+use prolog::num::{Integer, Signed, ToPrimitive, Zero};
 use prolog::num::bigint::{BigInt, BigUint};
 use prolog::num::rational::Ratio;
 use prolog::or_stack::*;
@@ -26,6 +26,14 @@ macro_rules! try_or_fail {
     }}
 }
 
+// used by '$skip_max_list'.
+enum CycleSearchResult {
+    EmptyList,
+    NotList,
+    PartialOrProperList(usize, usize), // returns the list length (up to max), and an offset into the heap.
+    UntouchedList(usize) // return the offset of an uniterated Addr::Lis(offset).
+}
+
 impl MachineState {
     pub(super) fn new(atom_tbl: TabledData<Atom>) -> MachineState {
         MachineState {
@@ -1666,6 +1674,13 @@ impl MachineState {
     {
         let a1 = self.store(self.deref(self[r].clone()));
 
+        // detect cycles.
+        match self.detect_cycles(usize::max_value(), a1.clone()) {
+            CycleSearchResult::PartialOrProperList(_, h)
+                if self.store(self.deref(self.heap[h].as_addr(h))).is_empty_list() => {},
+            _ => return Err(functor!("type_error", 2, [heap_atom!("list"), HeapCellValue::Addr(a1)]))
+        };
+
         match a1.clone() {
             Addr::Lis(mut l) => {
                 let mut result = Vec::new();
@@ -1709,6 +1724,106 @@ impl MachineState {
         }
     }
 
+    fn detect_cycles(&self, max_steps: usize, addr: Addr) -> CycleSearchResult
+    {
+        let addr = self.store(self.deref(addr));
+
+        let mut hare = match addr {
+            Addr::Lis(offset) if max_steps > 0 => offset + 1,
+            Addr::Lis(offset) => return CycleSearchResult::UntouchedList(offset),
+            Addr::Con(Constant::EmptyList) => return CycleSearchResult::EmptyList,
+            _ => return CycleSearchResult::NotList
+        };
+
+        // use Brent's algorithm to detect cycles.
+        let mut tortoise = hare;
+        let mut power = 2;
+        let mut steps = 1;
+
+        loop {
+            if steps == max_steps {
+                return CycleSearchResult::PartialOrProperList(steps, hare);
+            }
+
+            match self.heap[hare].clone() {
+                HeapCellValue::Addr(Addr::Lis(l)) => {
+                    hare = l + 1;
+                    steps += 1;
+
+                    if tortoise == hare {
+                        return CycleSearchResult::NotList;
+                    } else if steps == power {
+                        tortoise = hare;
+                        power <<= 1;
+                    }
+                },
+                HeapCellValue::Addr(Addr::Con(Constant::EmptyList)) =>
+                    return CycleSearchResult::PartialOrProperList(steps, hare),
+                HeapCellValue::Addr(Addr::HeapCell(hc)) if hc == hare =>
+                    return CycleSearchResult::PartialOrProperList(steps, hare),
+                HeapCellValue::Addr(ref sc @ Addr::StackCell(..))
+                    if *sc == self.store(self.deref(sc.clone())) =>
+                      return CycleSearchResult::PartialOrProperList(steps, hare),
+                _ => return CycleSearchResult::NotList
+            }
+        }
+    }
+
+    fn finalize_skip_max_list(&mut self, n: usize, addr: Addr) {
+        let target_n = self[temp_v!(1)].clone();
+        self.unify(Addr::Con(integer!(n)), target_n);
+
+        if !self.fail {
+            let xs = self[temp_v!(4)].clone();
+            self.unify(addr, xs);
+        }
+    }
+
+/*
+  '$skip_max_list'(N, Max, Xs0, Xs):
+  valid modes: Max is always +Max (a non-negative integer), Xs, Xs0 and N are all ?_.
+
+  Modes        | Conditions for success
+  ======================================================================================
+      ?N, -Xs0 : N = 0, Xs = Xs0.
+      ?N, +Xs0 : Xs0 is a proper or partial list, Xs0 = [X1, X2, ..., XN | Xs], N = Max,
+                 if |Xs0| >= Max, or, Xs = [] and N = |Xs0|.
+*/
+    pub(super) fn skip_max_list(&mut self) {
+        let max = self.store(self.deref(self[temp_v!(2)].clone()));
+
+        match max {
+            Addr::Con(Constant::Number(Number::Integer(ref max)))
+                if !max.is_negative() => {
+                    let n = self.store(self.deref(self[temp_v!(1)].clone()));
+
+                    match n {
+                        Addr::Con(Constant::Number(Number::Integer(ref n))) if n.is_zero() => {
+                            let xs0 = self[temp_v!(3)].clone();
+                            let xs  = self[temp_v!(4)].clone();
+
+                            self.unify(xs0, xs);
+                        },
+                        _ => {
+                            let max = max.to_usize().unwrap_or(usize::max_value());
+
+                            match self.detect_cycles(max, self[temp_v!(3)].clone()) {
+                                CycleSearchResult::UntouchedList(l) =>
+                                    self.finalize_skip_max_list(0, Addr::Lis(l)),
+                                CycleSearchResult::EmptyList =>
+                                    self.finalize_skip_max_list(0, Addr::Con(Constant::EmptyList)),
+                                CycleSearchResult::PartialOrProperList(n, hc) =>
+                                    self.finalize_skip_max_list(n, Addr::HeapCell(hc)),
+                                CycleSearchResult::NotList =>
+                                    self.fail = true
+                            }
+                        }
+                    }
+                },
+            _ => self.fail = true
+        };
+    }
+
     pub(super) fn duplicate_term(&mut self) {
         let old_h = self.heap.h;
 

src/prolog/machine/mod.rs

@@ -15,9 +15,9 @@ use std::mem::swap;
 use std::ops::Index;
 use std::rc::Rc;
 
-struct MachineCodeIndex<'a> {
+pub(super) struct MachineCodeIndex<'a> {
-    code_dir: &'a mut CodeDir,
+    pub(super) code_dir: &'a mut CodeDir,
-    op_dir: &'a mut OpDir,
+    pub(super) op_dir: &'a mut OpDir,
 }
 
 pub struct Machine {
@@ -148,7 +148,7 @@ impl Machine {
                 let mut indices = MachineCodeIndex { code_dir: &mut self.code_dir,
                                                      op_dir: &mut self.op_dir };
 
-                indices.use_qualified_module(module, exports)
+                indices.use_qualified_module(module, &exports)
             },
             None => EvalSession::from(EvalError::ModuleNotFound)
         }

src/tests.rs

@@ -1593,3 +1593,55 @@ fn test_queries_on_call_with_inference_limit()
                            [["R = inference_limit_exceeded", "X = _1"]]);
 
 }
+
+#[test]
+fn test_queries_on_skip_max_list() {
+    let mut wam = Machine::new();
+
+    // test on proper and empty lists.
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 5, [], Xs).",
+                           [["Xs = []", "N = 0"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 5, [a,b,c], Xs).",
+                           [["Xs = []", "N = 3"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 2, [a,b,c], Xs).",
+                           [["Xs = [c]", "N = 2"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 3, [a,b,c], Xs).",
+                           [["Xs = []", "N = 3"]]);
+
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 0, [], Xs).",
+                           [["Xs = []", "N = 0"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 0, [a,b,c], Xs).",
+                           [["Xs = [a, b, c]", "N = 0"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 0, [a,b,c], Xs).",
+                           [["Xs = [a, b, c]", "N = 0"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(N, 0, [a,b,c], Xs).",
+                           [["Xs = [a, b, c]", "N = 0"]]);
+
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(4, 0, [], Xs).");
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(3, 0, [a,b,c], Xs).");
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(2, 0, [a,b,c], Xs).");
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(1, 0, [a,b,c], Xs).");
+
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(0, 5, [], Xs).",
+                           [["Xs = []"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(3, 5, [a,b,c], Xs).",
+                           [["Xs = []"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(2, 2, [a,b,c], Xs).",
+                           [["Xs = [c]"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(3, 3, [a,b,c], Xs).",
+                           [["Xs = []"]]);
+
+    // tests on partial lists.
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(3, 4, [a,b,c|X], Xs0).",
+                           [["X = _1", "Xs0 = _1"]]);
+    assert_prolog_success!(&mut wam, "?- '$skip_max_list'(3, 3, [a,b,c|X], Xs0).",
+                           [["X = _1", "Xs0 = _1"]]);
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(3, 2, [a,b,c|X], Xs0).");
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(3, 1, [a,b,c|X], Xs0).");
+    assert_prolog_failure!(&mut wam, "?- '$skip_max_list'(3, 0, [a,b,c|X], Xs0).");
+        
+    // tests on cyclic lists.
+    assert_prolog_failure!(&mut wam, "?- Xs = [a,b|Xs], '$skip_max_list'(3, 5, X, Xs0).");
+    assert_prolog_failure!(&mut wam, "?- X = [a,b|Y], Y = [c,d|X], '$skip_max_list'(4, 5, X, Xs0).");
+    assert_prolog_failure!(&mut wam, "?- X = [a,b|Y], Y = [c,d|X], '$skip_max_list'(4, 3, X, Xs0).");
+}