* Started hacking on a ATermTable replacement, since ATermTable uses

gigantic amounts of memory --- 65536 bytes per table at least ---
  which makes it unsuitable for representing short-lived substitution
  tables and attribute sets.

table/aterm-map.cc

@@ -0,0 +1,231 @@
+#include <string>
+#include <iostream>
+#include <assert.h>
+#include <math.h>
+#include <aterm2.h>
+
+using namespace std;
+
+
+class ATermMap
+{
+private:
+
+    struct KeyValue
+    {
+        ATerm key;
+        ATerm value;
+    };
+
+    /* Hash table for the map.  We use open addressing, i.e., all
+       key/value pairs are stored directly in the table, and there are
+       no pointers.  Collisions are resolved through probing. */
+    KeyValue * hashTable;
+
+    /* Current size of the hash table. */
+    unsigned int size;
+
+    /* Number of elements in the hash table. */
+    unsigned int count;
+
+    /* Maximum number of elements in the hash table.  If `count'
+       exceeds this number, the hash table is expanded. */
+    unsigned int maxCount;
+    
+public:
+    /* Create a map.  `expectedCount' is the number of elements the
+       map is expected to hold. */
+    ATermMap(unsigned int expectedCount);
+    
+    ATermMap(const ATermMap & map);
+    
+    ~ATermMap();
+
+    ATermMap & operator = (const ATermMap & map);
+        
+    void set(ATerm key, ATerm value);
+
+    ATerm get(ATerm key) const;
+
+    void remove(ATerm key);
+    void remove(const string & key);
+
+private:
+    void init(unsigned int expectedCount);
+
+    void resizeTable(unsigned int expectedCount);
+
+    unsigned int hash1(ATerm key) const;
+    unsigned int hash2(ATerm key) const;
+};
+
+
+ATermMap::ATermMap(unsigned int expectedCount)
+{
+    init(expectedCount);
+}
+
+
+ATermMap::~ATermMap()
+{
+    if (hashTable) free(hashTable); 
+}
+
+
+void ATermMap::init(unsigned int expectedCount)
+{
+    size = 0;
+    count = 0;
+    maxCount = 0;
+    hashTable = 0;
+    resizeTable(expectedCount);
+}
+
+
+void ATermMap::resizeTable(unsigned int expectedCount)
+{
+    assert(size == 0);
+
+    this->maxCount = expectedCount;
+
+    unsigned int newSize = 128;
+
+    hashTable = (KeyValue *) calloc(sizeof(KeyValue), newSize);
+
+    size = newSize;
+}
+
+
+unsigned int ATermMap::hash1(ATerm key) const
+{
+    /* Don't care about the least significant bits of the ATerm
+       pointer since they're always 0. */
+    unsigned int key2 = ((unsigned int) key) >> 2;
+
+#if 0
+    double d1 = key2 * 0.6180339887;
+    unsigned int h1 = (int) (size * (d1 - floor(d1)));
+#endif
+
+#if 0
+    unsigned int h1 = size * (key2 * 61803 % 100000);
+#endif
+
+    unsigned int h1 = (size * ((key2 * 40503) & 0xffff)) >> 16;
+
+//     cout << key2 << " " << h1 << endl;
+    
+//     unsigned int h1 = (key2 * 134217689) & (size - 1);
+
+    return h1 % size;
+}
+
+
+unsigned int ATermMap::hash2(ATerm key) const
+{
+    unsigned int key2 = ((unsigned int) key) >> 2;
+
+#if 0    
+    double d2 = key2 * 0.6180339887;
+    unsigned int h2 = 1 | (int) (size * (d2 - floor(d2)));
+#endif
+
+    unsigned int h3 = ((key2 * 134217689) & (size - 1)) | 1;
+    return h3;
+}
+
+
+unsigned int nrItemsSet = 0;
+unsigned int nrSetProbes = 0;
+unsigned int nrMaxProbes = 0;
+
+
+void ATermMap::set(ATerm key, ATerm value)
+{
+    unsigned int probes = 0;
+    nrItemsSet++;
+    for (unsigned int i = 0, h = hash1(key); i < size;
+         ++i, h = (h + hash2(key)) & (size - 1))
+    {
+        assert(h < size);
+        probes++;
+        nrSetProbes++;
+        if (hashTable[h].key == 0) {
+            if (probes > nrMaxProbes) nrMaxProbes = probes;
+            hashTable[h].key = key;
+            hashTable[h].value = value;
+            count++;
+            return;
+        }
+    }
+    abort();
+}
+
+
+unsigned int nrItemsGet = 0;
+unsigned int nrGetProbes = 0;
+
+
+ATerm ATermMap::get(ATerm key) const
+{
+    nrItemsGet++;
+    for (unsigned int i = 0, h = hash1(key); i < size;
+         ++i, h = (h + hash2(key)) & (size - 1))
+    {
+        nrGetProbes++;
+        if (hashTable[h].key == 0) return 0;
+        if (hashTable[h].key == key) return hashTable[h].value;
+    }
+    return 0;
+}
+
+
+int main(int argc, char * * argv)
+{
+    ATerm bottomOfStack;
+    ATinit(argc, argv, &bottomOfStack);
+
+    /* Make test terms. */
+    int nrTestTerms = 100000;
+    ATerm testTerms[nrTestTerms];
+
+    for (int i = 0; i < nrTestTerms; ++i) {
+        char name[10];
+        sprintf(name, "%d", (int) random() % 37);
+
+        int arity = i == 0 ? 0 : (random() % 37);
+        ATerm kids[arity];
+        for (int j = 0; j < arity; ++j)
+            kids[j] = testTerms[random() % i];
+        
+        testTerms[i] = (ATerm) ATmakeApplArray(ATmakeAFun(name, arity, ATfalse), kids);
+//         ATwriteToSharedTextFile(testTerms[i], stdout);
+//         printf("\n");
+    }
+
+
+    cout << "testing...\n";
+
+    
+    #define someTerm() (testTerms[(int) random() % nrTestTerms])
+
+
+    for (int test = 0; test < 100000; ++test) {
+        ATermMap map(100);
+        for (int i = 0; i < 30; ++i) 
+            map.set(someTerm(), someTerm());
+        for (int i = 0; i < 100; ++i)
+            map.get(someTerm());
+    }
+
+    cout << "SET: "
+         << nrItemsSet << " "
+         << nrSetProbes << " "
+         << (double) nrSetProbes / nrItemsSet << " "
+         << nrMaxProbes << endl;
+
+    cout << "GET: "
+         << nrItemsGet << " "
+         << nrGetProbes << " "
+         << (double) nrGetProbes / nrItemsGet << endl;
+}

table/timings.txt

@@ -0,0 +1,19 @@
+[eelco@tyros:~/Dev/nix/table]$ time ./a.out
+testing...
+SET: 3000000 3400626 1.13354 10
+GET: 10000000 13034450 1.30344
+
+real    0m1.981s
+user    0m1.972s
+sys     0m0.008s
+
+
+# Floating point -> fixed point
+[eelco@tyros:~/Dev/nix/table]$ time ./a.out
+testing...
+SET: 3000000 3400506 1.1335 9
+GET: 10000000 13032311 1.30323
+
+real    0m0.956s
+user    0m0.928s
+sys     0m0.024s