transition to l2

src/l2/ast.rs

@@ -0,0 +1,200 @@
+use std::cell::Cell;
+use std::fmt;
+use std::ops::{Add, AddAssign};
+use std::vec::Vec;
+
+pub type Var = String;
+
+pub type Atom = String;
+
+pub enum TopLevel {
+    Fact(Term),
+    Rule(Rule),
+    Query(Term)
+}
+
+#[derive(Clone, Copy)]
+pub enum Level {
+    Shallow, Deep
+}
+
+impl fmt::Display for Level {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &Level::Shallow => write!(f, "A"),
+            &Level::Deep => write!(f, "X")
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub enum RegType {
+    Perm(usize),
+    Temp(usize)
+}
+
+impl RegType {
+    pub fn reg_num(self) -> usize {
+        match self {
+            RegType::Perm(reg_num) | RegType::Temp(reg_num) => reg_num
+        }
+    }
+
+    pub fn is_perm(self) -> bool {
+        match self {
+            RegType::Perm(_) => true,
+            _ => false
+        }
+    }
+}
+
+impl From<RegType> for Addr {
+    fn from(reg: RegType) -> Addr {
+        match reg {
+            RegType::Perm(reg) => Addr::StackCell(reg),
+            RegType::Temp(reg) => Addr::RegNum(reg)
+        }
+    }
+}
+
+impl fmt::Display for RegType {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &RegType::Perm(val) => write!(f, "Y{}", val),
+            &RegType::Temp(val) => write!(f, "X{}", val)
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub enum VarReg {
+    ArgAndNorm(RegType, usize),
+    Norm(RegType)
+}
+
+impl VarReg {
+    pub fn norm(self) -> RegType {
+        match self {
+            VarReg::ArgAndNorm(reg, _) | VarReg::Norm(reg) => reg
+        }
+    }
+}
+
+pub enum Term {
+    Atom(Cell<RegType>, Atom),
+    Clause(Cell<RegType>, Atom, Vec<Box<Term>>),
+    Var(Cell<VarReg>, Var)
+}
+
+pub struct Rule {
+    pub head: (Term, Term),
+    pub clauses: Vec<Term>
+}
+
+pub enum TermRef<'a> {
+    Atom(Level, &'a Cell<RegType>, &'a Atom),
+    Clause(Level, &'a Cell<RegType>, &'a Atom, &'a Vec<Box<Term>>),
+    Var(Level, &'a Cell<VarReg>, &'a Var)
+}
+
+pub enum FactInstruction {
+    GetStructure(Level, Atom, usize, RegType),
+    GetValue(RegType, usize),
+    GetVariable(RegType, usize),
+    UnifyVariable(RegType),
+    UnifyValue(RegType)
+}
+
+pub enum QueryInstruction {
+    PutStructure(Level, Atom, usize, RegType),
+    PutValue(RegType, usize),
+    PutVariable(RegType, usize),
+    SetVariable(RegType),
+    SetValue(RegType)
+}
+
+pub enum ControlInstruction {
+    Allocate(usize),
+    Call(Atom, usize),
+    Deallocate,
+    Proceed
+}
+
+pub type CompiledFact = Vec<FactInstruction>;
+
+pub type CompiledQuery = Vec<QueryInstruction>;
+
+pub enum Line {
+    Control(ControlInstruction),    
+    Fact(CompiledFact),
+    Query(CompiledQuery)
+}
+
+pub type Code = Vec<Line>;
+
+#[derive(Clone, Copy, PartialEq)]
+pub enum Addr {
+    HeapCell(usize),
+    RegNum(usize),
+    StackCell(usize),
+}
+
+#[derive(Clone)]
+pub enum HeapCellValue {
+    NamedStr(usize, Atom),
+    Ref(usize),
+    Str(usize)
+}
+
+#[derive(Clone, Copy)]
+pub enum CodePtr {
+    DirEntry(usize),
+    TopLevel
+}
+
+impl Add<usize> for CodePtr {
+    type Output = CodePtr;
+    fn add(self, rhs: usize) -> Self::Output {
+        match self {
+            CodePtr::DirEntry(p) => CodePtr::DirEntry(p + rhs),
+            CodePtr::TopLevel => CodePtr::TopLevel
+        }
+    }
+}
+
+impl AddAssign<usize> for CodePtr {
+    fn add_assign(&mut self, rhs: usize) {
+        match self {
+            &mut CodePtr::DirEntry(ref mut p) => *p += rhs,
+            _ => {}
+        }
+    }
+}
+
+pub type Heap = Vec<HeapCellValue>;
+
+pub type Registers = Vec<HeapCellValue>;
+
+impl Term {
+    pub fn subterms(&self) -> usize {
+        match self {
+            &Term::Clause(_, _, ref terms) => terms.len(),
+            _ => 1
+        }
+    }
+
+    pub fn name(&self) -> &Atom {
+        match self {
+            &Term::Atom(_, ref atom)
+            | &Term::Var(_, ref atom)
+            | &Term::Clause(_, ref atom, _) => atom
+        }
+    }
+
+    pub fn arity(&self) -> usize {
+        match self {
+            &Term::Atom(_, _) | &Term::Var(_, _) => 0,
+            &Term::Clause(_, _, ref child_terms) => child_terms.len()
+        }
+    }
+}

src/l2/codegen.rs

@@ -0,0 +1,447 @@
+use l2::ast::*;
+use l2::iterators::{FactIterator, QueryIterator};
+
+use std::cell::Cell;
+use std::collections::HashMap;
+use std::fmt;
+use std::vec::Vec;
+
+impl fmt::Display for FactInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &FactInstruction::GetStructure(Level::Deep, ref name, ref arity, ref r) =>
+                write!(f, "get_structure {}/{}, {}", name, arity, r),
+            &FactInstruction::GetStructure(Level::Shallow, ref name, ref arity, ref r) =>
+                write!(f, "get_structure {}/{}, A{}", name, arity, r.reg_num()),
+            &FactInstruction::GetValue(ref x, ref a) =>
+                write!(f, "get_value {}, A{}", x, a),
+            &FactInstruction::GetVariable(ref x, ref a) =>
+                write!(f, "get_variable {}, A{}", x, a),
+            &FactInstruction::UnifyVariable(ref r) =>
+                write!(f, "unify_variable {}", r),
+            &FactInstruction::UnifyValue(ref r) =>
+                write!(f, "unify_value {}", r)
+        }
+    }
+}
+
+impl fmt::Display for QueryInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &QueryInstruction::PutStructure(Level::Deep, ref name, ref arity, ref r) =>
+                write!(f, "put_structure {}/{}, A{}", name, arity, r.reg_num()),
+            &QueryInstruction::PutStructure(Level::Shallow, ref name, ref arity, ref r) =>
+                write!(f, "put_structure {}/{}, {}", name, arity, r),
+            &QueryInstruction::PutValue(ref x, ref a) =>
+                write!(f, "put_value {}, A{}", x, a),
+            &QueryInstruction::PutVariable(ref x, ref a) =>
+                write!(f, "put_variable {}, A{}", x, a),
+            &QueryInstruction::SetVariable(ref r) =>
+                write!(f, "set_variable {}", r),
+            &QueryInstruction::SetValue(ref r) =>
+                write!(f, "set_value {}", r),
+        }
+    }
+}
+
+impl fmt::Display for ControlInstruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &ControlInstruction::Allocate(num_cells) =>
+                write!(f, "allocate {}", num_cells),
+            &ControlInstruction::Call(ref name, ref arity) =>
+                write!(f, "call {}/{}", name, arity),
+            &ControlInstruction::Deallocate =>
+                write!(f, "deallocate"),
+            &ControlInstruction::Proceed =>
+                write!(f, "proceed")
+        }
+    }
+}
+
+trait CompilationTarget<'a> {
+    type Iterator : Iterator<Item=TermRef<'a>>;
+
+    fn iter(&'a Term) -> Self::Iterator;
+
+    fn to_structure(Level, Atom, usize, RegType) -> Self;
+
+    fn argument_to_variable(RegType, usize) -> Self;
+    fn argument_to_value(RegType, usize) -> Self;
+    fn subterm_to_variable(RegType) -> Self;
+    fn subterm_to_value(RegType) -> Self;
+
+    fn clause_arg_to_instr(RegType) -> Self;
+}
+
+impl<'a> CompilationTarget<'a> for FactInstruction {
+    type Iterator = FactIterator<'a>;
+
+    fn iter(term: &'a Term) -> Self::Iterator {
+        term.breadth_first_iter()
+    }
+
+    fn to_structure(lvl: Level, atom: Atom, arity: usize, reg: RegType) -> Self {
+        FactInstruction::GetStructure(lvl, atom, arity, reg)
+    }
+
+    fn argument_to_variable(arg: RegType, val: usize) -> Self {
+        FactInstruction::GetVariable(arg, val)
+    }
+
+    fn argument_to_value(arg: RegType, val: usize) -> Self {
+        FactInstruction::GetValue(arg, val)
+    }
+
+    fn subterm_to_variable(val: RegType) -> Self {
+        FactInstruction::UnifyVariable(val)
+    }
+
+    fn subterm_to_value(val: RegType) -> Self {
+        FactInstruction::UnifyValue(val)
+    }
+
+    fn clause_arg_to_instr(val: RegType) -> Self {
+        FactInstruction::UnifyVariable(val)
+    }
+}
+
+impl<'a> CompilationTarget<'a> for QueryInstruction {
+    type Iterator = QueryIterator<'a>;
+
+    fn iter(term: &'a Term) -> Self::Iterator {
+        term.post_order_iter()
+    }
+
+    fn to_structure(lvl: Level, atom: Atom, arity: usize, reg: RegType) -> Self {
+        QueryInstruction::PutStructure(lvl, atom, arity, reg)
+    }
+
+    fn argument_to_variable(arg: RegType, val: usize) -> Self {
+        QueryInstruction::PutVariable(arg, val)
+    }
+
+    fn argument_to_value(arg: RegType, val: usize) -> Self {
+        QueryInstruction::PutValue(arg, val)
+    }
+
+    fn subterm_to_variable(val: RegType) -> Self {
+        QueryInstruction::SetVariable(val)
+    }
+
+    fn subterm_to_value(val: RegType) -> Self {
+        QueryInstruction::SetValue(val)
+    }
+
+    fn clause_arg_to_instr(val: RegType) -> Self {
+        QueryInstruction::SetValue(val)
+    }
+}
+
+struct TermMarker<'a> {
+    bindings: HashMap<&'a Var, VarReg>,
+    arg_c:    usize,
+    perm_c:   usize,
+    temp_c:   usize
+}
+
+impl<'a> TermMarker<'a> {
+    fn new() -> TermMarker<'a> {
+        TermMarker { bindings: HashMap::new(),
+                     arg_c:    1,
+                     perm_c:   1,
+                     temp_c:   1 }
+    }
+
+    fn contains_var(&self, var: &'a Var) -> bool {
+        self.bindings.contains_key(var)
+    }
+
+    fn get(&self, var: &'a Var) -> VarReg {
+        *self.bindings.get(var).unwrap()
+    }
+
+    fn insert(&mut self, var: &'a Var, r: VarReg) {
+        self.bindings.insert(var, r);
+    }
+
+    fn mark_non_var(&mut self, lvl: Level, cell: &Cell<RegType>) {
+        let reg_type = cell.get();
+
+        if reg_type.reg_num() == 0 {
+            match lvl {
+                Level::Deep if reg_type.is_perm() => {
+                    let perm = self.perm_c;
+                    self.perm_c += 1;
+                    cell.set(RegType::Perm(perm));
+                },
+                Level::Deep => {
+                    let temp = self.temp_c;
+                    self.temp_c += 1;
+                    cell.set(RegType::Temp(temp));
+                },
+                Level::Shallow if reg_type.is_perm() => {
+                    let arg = self.arg_c;
+                    self.arg_c += 1;
+                    cell.set(RegType::Perm(arg));
+                },
+                Level::Shallow => {
+                    let arg = self.arg_c;
+                    self.arg_c += 1;
+                    cell.set(RegType::Temp(arg));
+                }
+            };
+        }
+    }
+
+    fn mark_old_var(&mut self, lvl: Level, var: &'a Var) -> VarReg
+    {
+        let reg = self.get(var);
+
+        match lvl {
+            Level::Deep => VarReg::Norm(reg.norm()),
+            Level::Shallow => {
+                let reg = VarReg::ArgAndNorm(reg.norm(), self.arg_c);
+
+                self.arg_c += 1;
+                self.insert(var, reg);
+
+                reg
+            }
+        }
+    }
+
+    fn mark_new_var(&mut self, lvl: Level, var: &'a Var, reg: RegType) -> VarReg
+    {
+        let inner_reg = if reg.is_perm() {
+            let perm = self.perm_c;
+            self.perm_c += 1;
+            RegType::Perm(perm)
+        } else {
+            let temp = self.temp_c;
+            self.temp_c += 1;
+            RegType::Temp(temp)
+        };
+
+        let reg = match lvl {
+            Level::Deep => VarReg::Norm(inner_reg),
+            Level::Shallow => {
+                let reg = VarReg::ArgAndNorm(inner_reg, self.arg_c);
+                self.arg_c += 1;
+                reg
+            }
+        };
+
+        self.insert(var, reg);
+        reg
+    }
+
+    fn advance(&mut self, term: &'a Term) {
+        self.arg_c  = 1;
+        self.temp_c = term.subterms() + 1;
+    }
+}
+
+#[derive(Copy, Clone)]
+enum TermStatus {
+    New, Old, Recurrent
+}
+
+pub struct CodeGenerator<'a> {
+    marker: TermMarker<'a>
+}
+
+type VariableFixture<'a>  = (TermStatus, Vec<&'a Cell<VarReg>>);
+type VariableFixtures<'a> = HashMap<&'a Var, VariableFixture<'a>>;
+
+impl<'a> CodeGenerator<'a> {
+    pub fn new() -> Self {
+        CodeGenerator { marker: TermMarker::new() }
+    }
+
+    fn to_structure<Target>(&mut self,
+                            lvl: Level,
+                            name: &'a Atom,
+                            cell: &'a Cell<RegType>,
+                            arity: usize)
+                            -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(lvl, cell);
+        Target::to_structure(lvl, name.clone(), arity, cell.get())
+    }
+
+    fn var_term<Target>(&mut self,
+                        lvl: Level,
+                        cell: &'a Cell<VarReg>,
+                        var: &'a Var)
+                        -> Target
+        where Target: CompilationTarget<'a>
+    {
+        if !self.marker.contains_var(var) {
+            let reg = self.marker.mark_new_var(lvl, var, cell.get().norm());
+            cell.set(reg);
+
+            match reg {
+                VarReg::ArgAndNorm(arg, norm) =>
+                    Target::argument_to_variable(arg, norm),
+                VarReg::Norm(norm) =>
+                    Target::subterm_to_variable(norm)
+            }
+        } else {
+            let reg = self.marker.mark_old_var(lvl, var);
+            cell.set(reg);
+
+            match reg {
+                VarReg::ArgAndNorm(arg, norm) =>
+                    Target::argument_to_value(arg, norm),
+                VarReg::Norm(norm) =>
+                    Target::subterm_to_value(norm)
+            }
+        }
+    }
+
+    fn non_var_subterm<Target>(&mut self, cell: &'a Cell<RegType>) -> Target
+        where Target: CompilationTarget<'a>
+    {
+        self.marker.mark_non_var(Level::Deep, cell);
+        Target::clause_arg_to_instr(cell.get())
+    }
+
+    fn subterm_to_instr<Target>(&mut self, subterm: &'a Term) -> Target
+        where Target: CompilationTarget<'a>
+    {
+        match subterm {
+            &Term::Atom(ref cell, _) | &Term::Clause(ref cell, _, _) =>
+                self.non_var_subterm(cell),
+            &Term::Var(ref cell, ref var) =>
+                self.var_term(Level::Deep, cell, var)
+        }
+    }
+
+    fn compile_target<Target>(&mut self, term: &'a Term) -> Vec<Target>
+        where Target: CompilationTarget<'a>
+    {
+        let iter       = Target::iter(term);
+        let mut target = Vec::<Target>::new();
+
+        self.marker.advance(term);
+
+        for term in iter {
+            match term {
+                TermRef::Atom(lvl, term, atom) =>
+                    target.push(self.to_structure(lvl, atom, term, 0)),
+                TermRef::Clause(lvl, term, atom, terms) => {
+                    target.push(self.to_structure(lvl, atom, term, terms.len()));
+
+                    for subterm in terms {
+                        target.push(self.subterm_to_instr(subterm.as_ref()));
+                    }
+                },
+                TermRef::Var(lvl @ Level::Shallow, ref cell, ref var) =>
+                    target.push(self.var_term(lvl, cell, var)),
+                _ => {}
+            };
+        }
+
+        target
+    }
+
+    fn mark_vars_in_term<Iter>(iter: Iter, vs: &mut VariableFixtures<'a>)
+        where Iter : Iterator<Item=TermRef<'a>>
+    {
+        for term in iter {
+            if let TermRef::Var(_, reg_cell, var) = term {
+                let mut status =
+                    vs.entry(var)
+                      .or_insert((TermStatus::New, Vec::new()));
+
+                status.1.push(reg_cell);
+
+                match status.0 {
+                    TermStatus::Old | TermStatus::Recurrent =>
+                        status.0 = TermStatus::Recurrent,
+                    _ => {}
+                };
+            }
+        }
+
+        for &mut (ref mut term_status, ref mut cb) in vs.values_mut() {
+            match *term_status {
+                TermStatus::New => *term_status = TermStatus::Old,
+                TermStatus::Recurrent => {
+                    for cell_reg in cb.drain(0..) {
+                        cell_reg.set(VarReg::Norm(RegType::Perm(0)));
+                    }
+                },
+                _ => {}
+            }
+        }
+    }
+
+    fn mark_perm_vars(rule: &'a Rule) -> VariableFixtures {
+        let &Rule { head: (ref p0, ref p1), ref clauses } = rule;
+        let mut vfs = HashMap::new();
+
+        let iter = p0.breadth_first_iter().chain(p1.breadth_first_iter());
+
+        Self::mark_vars_in_term(iter, &mut vfs);
+
+        for term in clauses {
+            Self::mark_vars_in_term(term.breadth_first_iter(), &mut vfs);
+        }
+
+        vfs
+    }
+
+    pub fn compile_rule(&mut self, rule: &'a Rule) -> Code {
+        let vfs = Self::mark_perm_vars(&rule);
+        let &Rule { head: (ref p0, ref p1), ref clauses } = rule;
+        let mut perm_vars = 0;
+
+        for &(term_status, _) in vfs.values() {
+            if let TermStatus::Recurrent = term_status {
+                perm_vars += 1;
+            }
+        }
+
+        let mut body = Vec::new();
+
+        body.push(Line::Control(ControlInstruction::Allocate(perm_vars)));
+        body.push(Line::Fact(self.compile_target(p0)));
+
+        body.append(&mut self.compile_query(p1));
+
+        let mut body = clauses.iter()
+            .map(|ref term| self.compile_query(term))
+            .fold(body, |mut body, ref mut cqs| {
+                body.append(cqs);
+                body
+            });
+
+        body.push(Line::Control(ControlInstruction::Deallocate));
+
+        body
+    }
+
+    pub fn compile_fact(&mut self, term: &'a Term) -> Code {
+        let mut compiled_fact = vec![Line::Fact(self.compile_target(term))];
+        let proceed = Line::Control(ControlInstruction::Proceed);
+
+        compiled_fact.push(proceed);
+        compiled_fact
+    }
+
+    pub fn compile_query(&mut self, term: &'a Term) -> Code {
+        let mut compiled_query =
+            vec![Line::Query(self.compile_target(term))];
+
+        if let &Term::Clause(_, ref atom, ref terms) = term {
+            let call = Line::Control(ControlInstruction::Call(atom.clone(),
+                                                              terms.len()));
+            compiled_query.push(call);
+        }
+
+        compiled_query
+    }
+}

src/l2/iterators.rs

@@ -0,0 +1,175 @@
+use l2::ast::*;
+
+use std::cell::Cell;
+use std::collections::VecDeque;
+use std::vec::Vec;
+
+enum IteratorState<'a> {
+    Atom(Level, &'a Cell<RegType>, &'a Atom),
+    Clause(Level, usize, &'a Cell<RegType>, &'a Atom, &'a Vec<Box<Term>>),
+    IsolatedAtom(&'a Cell<RegType>, &'a Atom),
+    IsolatedVar(&'a Cell<VarReg>, &'a Var),
+    RootClause(usize, &'a Vec<Box<Term>>),
+    Var(Level, &'a Cell<VarReg>, &'a Var)
+}
+
+impl<'a> IteratorState<'a> {
+    fn to_state(lvl: Level, term: &'a Term) -> IteratorState<'a>
+    {
+        match term {
+            &Term::Atom(ref cell, ref atom) =>
+                IteratorState::Atom(lvl, cell, atom),
+            &Term::Clause(ref cell, ref atom, ref child_terms) =>
+                IteratorState::Clause(lvl, 0, cell, atom, child_terms),
+            &Term::Var(ref cell, ref var) =>
+                IteratorState::Var(lvl, cell, var)
+        }
+    }
+}
+
+pub struct QueryIterator<'a> {
+    state_stack: Vec<IteratorState<'a>>
+}
+
+impl<'a> QueryIterator<'a> {
+    fn push_clause(&mut self,
+                   lvl: Level,
+                   child_num: usize,
+                   cell: &'a Cell<RegType>,
+                   name: &'a Atom,
+                   child_terms: &'a Vec<Box<Term>>)
+    {
+        self.state_stack.push(IteratorState::Clause(lvl,
+                                                    child_num,
+                                                    cell,
+                                                    name,
+                                                    child_terms));
+    }
+
+    fn push_root_clause(&mut self,
+                        child_num: usize,
+                        child_terms: &'a Vec<Box<Term>>)
+    {
+        self.state_stack.push(IteratorState::RootClause(child_num, child_terms));
+    }
+
+    fn push_subterm(&mut self, lvl: Level, term: &'a Term) {
+        self.state_stack.push(IteratorState::to_state(lvl, term));
+    }
+
+    fn new(term: &'a Term) -> QueryIterator<'a> {
+        let state = match term {
+            &Term::Atom(ref cell, ref atom) =>
+                IteratorState::IsolatedAtom(cell, atom),
+            &Term::Clause(_, _, ref terms) =>
+                IteratorState::RootClause(0, terms),
+            &Term::Var(ref cell, ref var) =>
+                IteratorState::IsolatedVar(cell, var)
+        };
+
+        QueryIterator { state_stack: vec![state] }
+    }
+}
+
+impl<'a> Iterator for QueryIterator<'a> {
+    type Item = TermRef<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(iter_state) = self.state_stack.pop() {
+            match iter_state {
+                IteratorState::Atom(lvl, cell, atom) =>
+                    return Some(TermRef::Atom(lvl, cell, atom)),
+                IteratorState::Clause(lvl, child_num, cell, atom, child_terms) => {
+                    if child_num == child_terms.len() {
+                        return Some(TermRef::Clause(lvl, cell, atom, child_terms));
+                    } else {
+                        self.push_clause(lvl, child_num + 1, cell, atom, child_terms);
+                        self.push_subterm(Level::Deep, child_terms[child_num].as_ref());
+                    }
+                },
+                IteratorState::IsolatedAtom(cell, atom) =>
+                    return Some(TermRef::Atom(Level::Shallow, cell, atom)),
+                IteratorState::IsolatedVar(cell, var) =>
+                    return Some(TermRef::Var(Level::Shallow, cell, var)),
+                IteratorState::RootClause(child_num, child_terms) => {
+                    if child_num == child_terms.len() {
+                        return None;
+                    } else {
+                        self.push_root_clause(child_num + 1, child_terms);
+                        self.push_subterm(Level::Shallow, child_terms[child_num].as_ref());
+                    }
+                },
+                IteratorState::Var(lvl, cell, var) =>
+                    return Some(TermRef::Var(lvl, cell, var))
+            };
+        }
+
+        None
+    }
+}
+
+pub struct FactIterator<'a> {
+    state_queue: VecDeque<IteratorState<'a>>,
+}
+
+impl<'a> FactIterator<'a> {
+    fn push_subterm(&mut self, lvl: Level, term: &'a Term) {
+        self.state_queue.push_back(IteratorState::to_state(lvl, term));
+    }
+
+    fn new(term: &'a Term) -> FactIterator<'a> {
+        let states = match term {
+            &Term::Atom(ref cell, ref atom) =>
+                vec![IteratorState::IsolatedAtom(cell, atom)],
+            &Term::Clause(_, _, ref terms) =>
+                vec![IteratorState::RootClause(0, terms)],
+            &Term::Var(ref cell, ref var) =>
+                vec![IteratorState::IsolatedVar(cell, var)]
+        };
+
+        FactIterator { state_queue:  VecDeque::from(states) }
+    }
+}
+
+impl<'a> Iterator for FactIterator<'a> {
+    type Item = TermRef<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(state) = self.state_queue.pop_front() {
+            match state {
+                IteratorState::Atom(lvl, cell, atom) =>
+                    return Some(TermRef::Atom(lvl, cell, atom)),
+                IteratorState::Clause(lvl, _, cell, atom, child_terms) => {
+                    for child_term in child_terms {
+                        self.push_subterm(Level::Deep, child_term);
+                    }
+
+                    return Some(TermRef::Clause(lvl, cell, atom, child_terms));
+                },
+                IteratorState::IsolatedAtom(cell, atom) =>
+                    return Some(TermRef::Atom(Level::Shallow, cell, atom)),
+                IteratorState::IsolatedVar(cell, var) =>
+                    return Some(TermRef::Var(Level::Shallow, cell, var)),
+                IteratorState::RootClause(_, child_terms) => {
+                    for child_term in child_terms {
+                        self.push_subterm(Level::Shallow, child_term);
+                    }
+                },
+                IteratorState::Var(lvl, cell, var) =>
+                    return Some(TermRef::Var(lvl, cell, var))
+            }
+        }
+
+        None
+    }
+}
+
+impl Term {
+    pub fn post_order_iter(&self) -> QueryIterator {
+        QueryIterator::new(self)
+    }
+
+    pub fn breadth_first_iter(&self) -> FactIterator {
+        FactIterator::new(self)
+    }
+}

src/l2/l2_parser.lalrpop

@@ -0,0 +1,42 @@
+use l2::ast::*;
+
+use std::cell::Cell;
+
+grammar;
+
+pub TopLevel: TopLevel = {
+    "?-" <t:Term> "." => TopLevel::Query(t),
+    <r:Rule> "."      => TopLevel::Rule(r),
+    <t:Term> "."      => TopLevel::Fact(t),
+};
+
+Atom : Atom = {
+    r"[a-z][a-z0-9_]*" => <>.trim().to_string(),
+};
+
+BoxedTerm : Box<Term> = {
+    <t:Term> => Box::new(t),
+};
+
+Clause : Term = {
+    <a:Atom> "(" <ts: (<BoxedTerm> ",")*> <t:BoxedTerm> ")" => {
+     	let mut ts = ts;
+     	ts.push(t);
+	Term::Clause(Cell::new(RegType::Temp(0)), a, ts)
+    },
+};
+
+Rule : Rule = {
+    <c:Clause> ":-" <h:Term> <cs: ("," <Term>)*> =>
+         Rule { head: (c, h), clauses: cs },
+};
+
+Term : Term = {
+    <Clause> => <>,
+    <Atom> => Term::Atom(Cell::new(RegType::Temp(0)), <>),
+    <Var>  => Term::Var(Cell::new(VarReg::Norm(RegType::Temp(0))), <>),
+};
+
+Var : Var = {
+    r"[A-Z][a-z0-9_]*" => <>.trim().to_string(),
+};

src/l2/machine.rs

@@ -0,0 +1,400 @@
+use l2::ast::*;
+use l2::stack::*;
+
+use std::collections::HashMap;
+use std::ops::{Index, IndexMut};
+use std::vec::Vec;
+
+#[derive(Clone, Copy)]
+enum MachineMode {
+    Read,
+    Write
+}
+
+struct MachineState {
+    h: usize,
+    s: usize,
+    p: CodePtr,
+    cp: CodePtr,
+    fail: bool,
+    heap: Heap,
+    mode: MachineMode,
+    stack: Stack,
+    registers: Registers
+}
+
+type CodeDir = HashMap<(Atom, usize), usize>;
+
+pub struct Machine {
+    ms: MachineState,
+    code: Code,
+    code_dir: CodeDir
+}
+
+impl Index<Addr> for MachineState {
+    type Output = HeapCellValue;
+
+    fn index(&self, index: Addr) -> &Self::Output {
+        match index {
+            Addr::HeapCell(hc)  => &self.heap[hc],
+            Addr::RegNum(reg)   => &self.registers[reg],
+            Addr::StackCell(sc) => &self.stack[sc]
+        }
+    }
+}
+
+impl IndexMut<Addr> for MachineState {
+    fn index_mut(&mut self, index: Addr) -> &mut Self::Output {
+        match index {
+            Addr::HeapCell(hc)  => &mut self.heap[hc],
+            Addr::RegNum(reg)   => &mut self.registers[reg],
+            Addr::StackCell(sc) => &mut self.stack[sc]
+        }
+    }
+}
+
+impl Machine {
+    pub fn new() -> Self {
+        Machine {
+            ms: MachineState::new(),
+            code: Vec::new(),
+            code_dir: HashMap::new()
+        }
+    }
+
+    fn failed(&self) -> bool {
+        self.ms.fail
+    }
+
+    pub fn add_fact(&mut self, fact: &Term, mut code: Code) {
+        let p = self.code.len();
+        let name = fact.name().clone();
+        let arity = fact.arity();
+
+        self.code.append(&mut code);
+        self.code_dir.insert((name, arity), p);
+    }
+
+    pub fn add_rule(&mut self, rule: &Rule, mut code: Code) {
+        let p = self.code.len();
+        let name = rule.head.0.name().clone();
+        let arity = rule.head.1.arity();
+
+        self.code.append(&mut code);
+        self.code_dir.insert((name, arity), p);
+    }
+
+    fn execute_instr(&mut self, instr: &Line) -> bool {
+        let mut instr = instr;
+
+        loop {
+            match instr {
+                &Line::Fact(ref fact) => {
+                    for fact_instr in fact {
+                        self.ms.execute_fact_instr(&fact_instr);
+                    }
+                    self.ms.p += 1;
+                },
+                &Line::Query(ref query) => {
+                    for query_instr in query {
+                        self.ms.execute_query_instr(&query_instr);
+                    }
+                    self.ms.p += 1;
+                },
+                &Line::Control(ref control_instr) =>
+                    self.ms.execute_ctrl_instr(&self.code_dir, control_instr),
+            }
+            
+            if self.failed() {
+                return false;
+            }
+            
+            match self.ms.p {
+                CodePtr::DirEntry(p) if p < self.code.len() =>
+                    instr = &self.code[p],
+                _ => break
+            }
+        }
+
+        true
+    }
+
+    pub fn execute_query(&mut self, query: Code) -> bool {
+        let mut succeeded = true;
+        
+        for instr in query {
+            succeeded = self.execute_instr(&instr);
+            if !succeeded {
+                break;
+            }
+        }
+
+        self.ms.reset();
+        succeeded
+    }
+}
+
+impl MachineState {
+    fn new() -> MachineState {
+        MachineState { h: 0,
+                       s: 0,
+                       p: CodePtr::TopLevel,
+                       cp: CodePtr::TopLevel,
+                       fail: false,
+                       heap: Vec::with_capacity(256),
+                       mode: MachineMode::Write,
+                       stack: Stack::new(),
+                       registers: vec![HeapCellValue::Ref(0); 32] }
+    }
+
+    fn lookup(&self, a: Addr) -> &HeapCellValue {
+        match a {
+            Addr::HeapCell(hc)  => &self.heap[hc],
+            Addr::RegNum(reg)   => &self.registers[reg],
+            Addr::StackCell(sc) => &self.stack[sc]
+        }
+    }
+
+    fn deref(&self, a: Addr) -> Addr {
+        let mut a = a;
+
+        loop {
+            if let &HeapCellValue::Ref(value) = self.lookup(a) {
+                if let Addr::HeapCell(av) = a {
+                    if value != av {
+                        a = Addr::HeapCell(value);
+                        continue;
+                    }
+                } else {
+                    a = Addr::HeapCell(value);
+                    continue;
+                }
+            }
+
+            return a;
+        };
+    }
+
+    fn is_unbound(hc: &HeapCellValue, index: usize) -> bool {
+        match hc {
+            &HeapCellValue::Ref(r) => r == index,
+            _ => false
+        }
+    }
+
+    fn bind(&mut self, a: Addr, val: usize) {
+        let mut a = a;
+
+        loop {
+            match a {
+                addr @ Addr::RegNum(_) | addr @ Addr::StackCell(_) => {
+                    if let HeapCellValue::Ref(hc) = self[addr] {
+                        a = Addr::HeapCell(hc);
+                    } else if Self::is_unbound(&self.heap[val], val) {
+                        self.heap[val] = self[addr].clone();
+                        break;
+                    } else {
+                        self.fail = true;
+                        break;
+                    }
+                },
+                Addr::HeapCell(hc) => {
+                    if Self::is_unbound(&self.heap[hc], hc) {
+                        self.heap[hc] = HeapCellValue::Ref(val);
+                        break;
+                    } else if Self::is_unbound(&self.heap[val], val) {
+                        self.heap[val] = HeapCellValue::Ref(hc);
+                        break;
+                    } else {
+                        self.fail = true;
+                        break;
+                    }
+                }
+            };
+        }
+    }
+
+    fn unify(&mut self, a1: Addr, a2: Addr) {
+        let mut pdl = vec![a1, a2];
+
+        self.fail = false;
+
+        while !(pdl.is_empty() || self.fail) {
+            let d1 = self.deref(pdl.pop().unwrap());
+            let d2 = self.deref(pdl.pop().unwrap());
+
+            if d1 != d2 {
+                match (self.lookup(d1), self.lookup(d2)) {
+                    (&HeapCellValue::Ref(hc), _) =>
+                        self.bind(d2, hc),
+                    (_, &HeapCellValue::Ref(hc)) =>
+                        self.bind(d1, hc),
+                    (&HeapCellValue::Str(a1), &HeapCellValue::Str(a2)) => {
+                        let r1 = &self.heap[a1];
+                        let r2 = &self.heap[a2];
+
+                        if let &HeapCellValue::NamedStr(n1, ref f1) = r1 {
+                            if let &HeapCellValue::NamedStr(n2, ref f2) = r2 {
+                                if n1 == n2 && *f1 == *f2 {
+                                    for i in 1 .. n1 {
+                                        pdl.push(Addr::HeapCell(a1 + i));
+                                        pdl.push(Addr::HeapCell(a2 + i));
+                                    }
+
+                                    continue;
+                                }
+                            }
+                        }
+
+                        self.fail = true;
+                    },
+                    _ => self.fail = true,
+                };
+            }
+        }
+    }
+
+    fn execute_query_instr(&mut self, instr: &QueryInstruction) {
+        match instr {
+            &QueryInstruction::PutStructure(_, ref name, arity, reg) => {
+                self.heap.push(HeapCellValue::Str(self.h + 1));
+                self.heap.push(HeapCellValue::NamedStr(arity, name.clone()));
+
+                self[Addr::from(reg)] = self.heap[self.h].clone();
+
+                self.h += 2;
+            },
+            &QueryInstruction::PutValue(norm, arg) =>
+                self.registers[arg] = self[Addr::from(norm)].clone(),
+            &QueryInstruction::PutVariable(norm, arg) => {
+                self.heap.push(HeapCellValue::Ref(self.h));
+
+                self[Addr::from(norm)] = self.heap[self.h].clone();
+                self.registers[arg] = self.heap[self.h].clone();
+
+                self.h += 1;
+            },
+            &QueryInstruction::SetVariable(reg) => {
+                self.heap.push(HeapCellValue::Ref(self.h));
+                self[Addr::from(reg)] = self.heap[self.h].clone();
+
+                self.h += 1;
+            },
+            &QueryInstruction::SetValue(reg) => {
+                let heap_val = self[Addr::from(reg)].clone();
+                self.heap.push(heap_val);
+                self.h += 1;
+            },
+        }
+    }
+
+    fn execute_fact_instr(&mut self, instr: &FactInstruction) {
+        match instr {
+            &FactInstruction::GetStructure(_, ref name, arity, reg) => {
+                let addr = self.deref(Addr::from(reg));
+
+                match self.lookup(addr) {
+                    &HeapCellValue::Str(a) => {
+                        let result = &self.heap[a];
+
+                        if let &HeapCellValue::NamedStr(narity, ref str) = result {
+                            if narity == arity && *name == *str {
+                                self.s = a + 1;
+                                self.mode = MachineMode::Read;
+                            } else {
+                                self.fail = true;
+                            }
+                        }
+                    },
+                    &HeapCellValue::Ref(r) => {
+                        self.heap.push(HeapCellValue::Str(self.h + 1));
+                        self.heap.push(HeapCellValue::NamedStr(arity, name.clone()));
+
+                        let h = self.h;
+
+                        self.bind(Addr::HeapCell(r), h);
+
+                        self.h += 2;
+                        self.mode = MachineMode::Write;
+                    },
+                    _ => self.fail = true,
+                };
+            },
+            &FactInstruction::GetVariable(norm, arg) =>
+                self[Addr::from(norm)] = self.registers[arg].clone(),
+            &FactInstruction::GetValue(norm, arg) =>
+                self.unify(Addr::from(norm), Addr::RegNum(arg)),
+            &FactInstruction::UnifyVariable(reg) => {
+                match self.mode {
+                    MachineMode::Read  =>
+                        self[Addr::from(reg)] = self.heap[self.s].clone(),
+                    MachineMode::Write => {
+                        self.heap.push(HeapCellValue::Ref(self.h));
+
+                        self[Addr::from(reg)] = self.heap[self.h].clone();
+                        self.h += 1;
+                    }
+                };
+
+                self.s += 1;
+            },
+            &FactInstruction::UnifyValue(reg) => {
+                let s = self.s;
+
+                match self.mode {
+                    MachineMode::Read  =>
+                        self.unify(Addr::from(reg), Addr::HeapCell(s)),
+                    MachineMode::Write => {
+                        let heap_val = self[Addr::from(reg)].clone();
+                        self.heap.push(heap_val);
+                        self.h += 1;
+                    }
+                };
+
+                self.s += 1;
+            }
+        }
+    }
+
+    fn execute_ctrl_instr(&mut self, code_dir: &CodeDir, instr: &ControlInstruction)
+    {
+        match instr {
+            &ControlInstruction::Allocate(num_cells) => {
+                self.stack.push(self.cp, num_cells);
+                self.p += 1;
+            },
+            &ControlInstruction::Call(ref name, arity) => {
+                let compiled_tl_index = code_dir.get(&(name.clone(), arity))
+                                                .map(|index| *index);
+
+                match compiled_tl_index {
+                    Some(compiled_tl_index) => {
+                        self.cp = self.p + 1;
+                        self.p  = CodePtr::DirEntry(compiled_tl_index);
+                    },
+                    None => self.fail = true
+                };
+            },
+            &ControlInstruction::Deallocate => {
+                self.p = self.stack.get_cp();
+                self.stack.pop();
+            },
+            &ControlInstruction::Proceed => {
+                self.p = self.cp;
+            }
+        };
+    }
+    
+    fn reset(&mut self) {
+        self.h = 0;
+        self.s = 0;
+        self.p = CodePtr::TopLevel;
+        self.cp = CodePtr::TopLevel;
+
+        self.fail = false;
+        self.heap.clear();
+        self.mode = MachineMode::Write;
+        self.stack = Stack::new();
+        self.registers = vec![HeapCellValue::Ref(0); 32];
+    }
+}

src/l2/mod.rs

@@ -0,0 +1,6 @@
+pub mod ast;
+pub mod iterators;
+pub mod l2_parser;
+pub mod codegen;
+pub mod machine;
+pub mod stack;

src/l2/stack.rs

@@ -0,0 +1,60 @@
+use l2::ast::*;
+
+use std::ops::{Index, IndexMut};
+use std::vec::Vec;
+
+struct Frame {
+    cp: CodePtr,
+    perms: Vec<HeapCellValue>
+}
+
+impl Frame {
+    fn new(cp: CodePtr, n: usize) -> Self {
+        Frame {
+            cp: cp,
+            perms: vec![HeapCellValue::Ref(0); n]
+        }
+    }
+
+    fn read_pv(&self, i: usize) -> &HeapCellValue {
+        self.perms.index(i)
+    }
+
+    fn read_pv_mut(&mut self, i: usize) -> &mut HeapCellValue {
+        self.perms.index_mut(i)
+    }
+}
+
+pub struct Stack(Vec<Frame>);
+
+impl Stack {
+    pub fn new() -> Self {
+        Stack(Vec::new())
+    }
+    
+    pub fn push(&mut self, cp: CodePtr, n: usize) {
+        self.0.push(Frame::new(cp, n));
+    }
+
+    pub fn get_cp(&self) -> CodePtr {
+        self.0.last().unwrap().cp
+    }
+    
+    pub fn pop(&mut self) {
+        self.0.pop();
+    }
+}
+
+impl Index<usize> for Stack {
+    type Output = HeapCellValue;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        self.0.last().unwrap().read_pv(index - 1)
+    }
+}
+
+impl IndexMut<usize> for Stack {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        self.0.last_mut().unwrap().read_pv_mut(index - 1)
+    }
+}