From e503e312a187d816b40e306b90343f497ae58aec Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Sat, 11 Nov 2017 16:01:29 -0700
Subject: [PATCH] reuse defunct AND stack frames.

---
 src/prolog/and_stack.rs | 14 +++++++++++---
 src/prolog/machine.rs   | 33 ++++++++++++++++++++++++++++-----
 2 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/src/prolog/and_stack.rs b/src/prolog/and_stack.rs
index 77a729f9..f4d043da 100644
--- a/src/prolog/and_stack.rs
+++ b/src/prolog/and_stack.rs
@@ -49,9 +49,17 @@ impl AndStack {
         self.0.clear()
     }
 
-    pub fn pop(&mut self) {
-        self.0.pop();
-    }
+    pub fn resize(&mut self, fr: usize, n: usize) {                        
+        let len = self[fr].perms.len();               
+        
+        if len < n {
+            self[fr].perms.reserve(n - len);
+
+            for i in len .. n {                
+                self[fr].perms.push(Addr::StackCell(fr, i));
+            }
+        }
+    }    
 }
 
 impl Index<usize> for AndStack {
diff --git a/src/prolog/machine.rs b/src/prolog/machine.rs
index 0d924618..f22f3ba8 100644
--- a/src/prolog/machine.rs
+++ b/src/prolog/machine.rs
@@ -1510,11 +1510,34 @@ impl MachineState {
     {
         match instr {
             &ControlInstruction::Allocate(num_cells) => {
-                let num_frames = self.num_frames();
+                if let Some(ref or_fr) = self.or_stack.top() {
+                    let and_gi = if self.and_stack.len() > self.e {
+                        self.and_stack[self.e].global_index
+                    } else {
+                        0
+                    };
+                                       
+                    if and_gi <= or_fr.global_index {
+                        self.e = or_fr.e;
+                    }                    
+                }
+
+                if self.e + 1 < self.and_stack.len() {
+                    let index = self.e + 1;
+                    
+                    self.and_stack[index].e  = self.e;
+                    self.and_stack[index].cp = self.cp;
+
+                    self.and_stack.resize(index, num_cells);
+
+                    self.e = index;
+                } else {
+                    let num_frames = self.num_frames();
+
+                    self.and_stack.push(num_frames + 1, self.e, self.cp, num_cells);
+                    self.e = self.and_stack.len() - 1;
+                };
 
-                self.and_stack.push(num_frames + 1, self.e, self.cp, num_cells);
-                self.e = self.and_stack.len() - 1;
-                
                 self.p += 1;
             },
             &ControlInstruction::Call(ref name, arity, _) =>
@@ -1539,7 +1562,7 @@ impl MachineState {
 
                 self.cp = self.and_stack[e].cp;
                 self.e  = self.and_stack[e].e;
-                
+
                 self.p += 1;
             },
             &ControlInstruction::Execute(ref name, arity) =>
-- 
2.54.0