refactor occurrence utility for common use (to be used in ctx_simplifier) per Nuno's suggestion

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/tactic/core/collect_occs.cpp

@@ -0,0 +1,97 @@
+/*++
+Copyright (c) 2011 Microsoft Corporation
+
+Module Name:
+
+    collect_cccs.cpp
+
+Abstract:
+
+    Collect variables by occurrences.
+
+Author:
+
+    Leonardo (leonardo) 2011-10-22
+
+Notes:
+
+--*/
+
+#include "ast.h"
+#include "goal.h"
+#include "hashtable.h"
+#include "collect_occs.h"
+
+bool collect_occs::visit(expr * t) {
+    if (m_visited.is_marked(t)) {
+        if (is_uninterp_const(t))
+            m_more_than_once.mark(t);
+        return true;
+    }
+    m_visited.mark(t);
+    if (is_uninterp_const(t)) {
+        m_vars.push_back(to_app(t));
+        return true;
+    }
+    if (is_var(t))
+        return true;
+    if (is_app(t) && to_app(t)->get_num_args() == 0)
+        return true;
+    m_stack.push_back(frame(t, 0));
+    return false;
+}
+    
+void collect_occs::process(expr * t) {
+    SASSERT(m_stack.empty());
+    if (visit(t))
+        return;
+    SASSERT(!m_stack.empty());
+    unsigned num;
+    expr * child;
+    while (!m_stack.empty()) {
+    start:
+        frame & fr = m_stack.back();
+        expr *  t  = fr.first;
+        switch (t->get_kind()) {
+        case AST_APP:
+            num = to_app(t)->get_num_args();
+            while (fr.second < num) {
+                child = to_app(t)->get_arg(fr.second);
+                fr.second++;
+                if (!visit(child))
+                    goto start;
+            }
+            m_stack.pop_back();
+            break;
+        case AST_QUANTIFIER:
+            // don't need to visit patterns
+            child = to_quantifier(t)->get_expr();
+            fr.second++;
+            if (!visit(child))
+                goto start;
+            m_stack.pop_back();
+            break;
+        default:
+            UNREACHABLE();
+        }
+    }
+}
+    
+    
+void collect_occs::operator()(goal const & g, obj_hashtable<expr> & r) {
+    unsigned sz = g.size();
+    for (unsigned i = 0; i < sz; i++) {
+        expr * t = g.form(i);
+        process(t);
+    }
+    
+    ptr_vector<app>::const_iterator it  = m_vars.begin();
+    ptr_vector<app>::const_iterator end = m_vars.end();
+    for (; it != end; ++it) {
+        if (m_more_than_once.is_marked(*it))
+            continue;
+        r.insert(*it);
+    }
+    m_visited.reset();
+    m_more_than_once.reset();
+}