Reimplement ResolveMemos with a ridiculous hand-rolled hash table

This is intended to band-aid #330 with a hash table that sucks,
but ought to be much faster to copy than the stock Java one.
The stock Java hash table rehashes everything every time, while
this hash table can often just copy an array. That said, I didn't
benchmark it or anything, it may well also be super slow.

This is more or less a troll commit to get someone to do better.

config/src/main/java/com/typesafe/config/impl/BadMap.java

@@ -0,0 +1,134 @@
+package com.typesafe.config.impl;
+
+/**
+ * A terrible Map that isn't as expensive as HashMap to copy and
+ * add one item to. Please write something real if you see this
+ * and get cranky.
+ */
+final class BadMap<K,V> {
+    final static class Entry {
+        final int hash;
+        final Object key;
+        final Object value;
+        final Entry next;
+
+        Entry(int hash, Object k, Object v, Entry next) {
+            this.hash = hash;
+            this.key = k;
+            this.value = v;
+            this.next = next;
+        }
+
+        Object find(Object k) {
+            if (key.equals(k))
+                return value;
+            else if (next != null)
+                return next.find(k);
+            else
+                return null;
+        }
+    }
+
+    final int size;
+    final Entry[] entries;
+
+    private final static Entry[] emptyEntries = {};
+
+    BadMap() {
+        this(0, (Entry[]) emptyEntries);
+    }
+
+    private BadMap(int size, Entry[] entries) {
+        this.size = size;
+        this.entries = entries;
+    }
+
+    BadMap<K,V> copyingPut(K k, V v) {
+        int newSize = size + 1;
+        Entry[] newEntries;
+        // The "- 1" is so we pick size 2 when newSize is 1.
+        int threshold = (newSize * 2) - 1;
+        if (threshold > (entries.length * 2)) {
+            // nextPrime doesn't always return a prime larger than
+            // we passed in, so this block may not actually change
+            // the entries size.
+            newEntries = new Entry[nextPrime(threshold)];
+        } else {
+            newEntries = new Entry[entries.length];
+        }
+
+        if (newEntries.length == entries.length)
+            System.arraycopy(entries, 0, newEntries, 0, entries.length);
+        else
+            rehash(entries, newEntries);
+
+        int hash = Math.abs(k.hashCode());
+        store(newEntries, hash, k, v);
+        return new BadMap<K,V>(newSize, newEntries);
+    }
+
+    private static <K,V> void store(Entry[] entries, int hash, K k, V v) {
+        int i = hash % entries.length;
+        Entry old = entries[i];  // old may be null
+        entries[i] = new Entry(hash, k, v, old);
+    }
+
+    private static <K,V> void store(Entry[] entries, Entry e) {
+        int i = e.hash % entries.length;
+        Entry old = entries[i]; // old may be null
+        if (old == null && e.next == null) {
+            // share the entry since it has no "next"
+            entries[i] = e;
+        } else {
+            // bah, have to copy it
+            entries[i] = new Entry(e.hash, e.key, e.value, old);
+        }
+    }
+
+    private static <K,V> void rehash(Entry[] src, Entry[] dest) {
+        for (Entry old : src) {
+            if (old != null)
+                store(dest, old);
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    V get(K k) {
+        if (entries.length == 0) {
+            return null;
+        } else {
+            int hash = Math.abs(k.hashCode());
+            int i = hash % entries.length;
+            Entry e = entries[i];
+            if (e == null)
+                return null;
+            else
+                return (V) e.find(k);
+        }
+    }
+
+
+    private final static int[] primes = {
+        /* Skip some early ones that are close together */
+        2, /* 3, */ 5, /* 7, */ 11, /* 13, */ 17, /* 19, */ 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71,
+        73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173,
+        179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281,
+        283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409,
+        419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541,
+        547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659,
+        661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809,
+        811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941,
+        947, 953, 967, 971, 977, 983, 991, 997, 1009,
+        /* now we start skipping some, this is arbitrary */
+        2053, 3079, 4057, 7103
+    };
+
+    private final static int nextPrime(int i) {
+        for (int p : primes) {
+            if (p > i)
+                return p;
+        }
+        /* oh well */
+        return primes[primes.length - 1];
+    }
+}

config/src/main/java/com/typesafe/config/impl/ResolveMemos.java

@@ -11,25 +11,21 @@
 final class ResolveMemos {
     // note that we can resolve things to undefined (represented as Java null,
     // rather than ConfigNull) so this map can have null values.
-    final private Map<MemoKey, AbstractConfigValue> memos;
+    final private BadMap<MemoKey, AbstractConfigValue> memos;
 
-    private ResolveMemos(Map<MemoKey, AbstractConfigValue> memos) {
+    private ResolveMemos(BadMap<MemoKey, AbstractConfigValue> memos) {
         this.memos = memos;
     }
 
     ResolveMemos() {
-        this(new HashMap<MemoKey, AbstractConfigValue>());
+        this(new BadMap<MemoKey, AbstractConfigValue>());
     }
 
     AbstractConfigValue get(MemoKey key) {
         return memos.get(key);
     }
 
     ResolveMemos put(MemoKey key, AbstractConfigValue value) {
-        // completely inefficient, but so far nobody cares about resolve()
-        // performance, we can clean it up someday...
-        Map<MemoKey, AbstractConfigValue> copy = new HashMap<MemoKey, AbstractConfigValue>(memos);
-        copy.put(key, value);
-        return new ResolveMemos(copy);
+        return new ResolveMemos(memos.copyingPut(key, value));
     }
 }