Fixes space leak by switching to objects

The integer-arena approach (parallel global arrays) never freed nodes,
causing 6GB+ memory usage when running the self-hosted kernel. PHP
arrays don't shrink and we have no GC for them.

- Replace int-arena with plain Node objects so PHP's refcounting GC
  reclaims unreachable subtrees automatically.
- Hash-cons Stem/Fork nodes to collapse duplicate immutable subtrees.
- Cache ofNumber(0..255) to avoid re-creating byte trees.
- Eliminate indirection (tag 4) nodes entirely; projection rules now
  eagerly reduce and copy tag/a/b into the original App node.
- Remove all deref() loops — every node is exactly what it says.

Memory: 6GB+ → 24MB for `id "hello"` with runArboricxToString kernel

ext/php/run.php

@@ -26,6 +26,7 @@ require __DIR__ . '/src/functions.php';
 require __DIR__ . '/src/codecs.php';
 require __DIR__ . '/src/kernel.php';
 
+use Arboricx\Node;
 use function Arboricx\{app, reduce, ofString, ofNumber, ofBytes, ofList,
     formatTree, unwrapResult, unwrapHostValue, decodeHostPayload,
     getRunArboricxToString};
@@ -67,7 +68,7 @@ function cmdRun(string $bundlePath, array $args): void
     // Kernel application: runArboricxToString bundle args
     $expr = app(app($kernel, $bundleTree), $argsTree);
     fwrite(STDERR, "Reducing kernel application...\n");
-    $result = reduce($expr, 1_000_000_000);
+    $result = reduce($expr, 1_000_000_000_000);
 
     // The kernel returns an ok/err pair.  On ok, the value is a
     // Host ABI envelope: Fork(tag_number, payload_tree).
@@ -96,7 +97,7 @@ function cmdRun(string $bundlePath, array $args): void
     }
 }
 
-function encodeArg(string $arg): int
+function encodeArg(string $arg): Node
 {
     return ctype_digit($arg) ? ofNumber((int)$arg) : ofString($arg);
 }

ext/php/src/codecs.php

@@ -6,15 +6,24 @@ namespace Arboricx;
 
 use function Arboricx\{leaf, stem, fork, isLeaf, isStem, isFork, reduce, stemChild, forkLeft, forkRight, same_tree, formatTree};
 
-function ofNumber(int $n): int
+$GLOBALS['ARB_NUM_CACHE'] = [];
+
+function ofNumber(int $n): Node
 {
     if ($n < 0) throw new \InvalidArgumentException('ofNumber: negative values not supported');
     if ($n === 0) return leaf();
+    if ($n < 256 && isset($GLOBALS['ARB_NUM_CACHE'][$n])) {
+        return $GLOBALS['ARB_NUM_CACHE'][$n];
+    }
     $bitTree = ($n % 2) === 1 ? stem(leaf()) : leaf();
-    return fork($bitTree, ofNumber(intdiv($n, 2)));
+    $result = fork($bitTree, ofNumber(intdiv($n, 2)));
+    if ($n < 256) {
+        $GLOBALS['ARB_NUM_CACHE'][$n] = $result;
+    }
+    return $result;
 }
 
-function toNumber(int $tree): array
+function toNumber(Node $tree): array
 {
     $tree = reduce($tree);
     if (isLeaf($tree)) return [true, 0];
@@ -33,7 +42,7 @@ function toNumber(int $tree): array
     return $ok ? [true, $bit + 2 * $rest] : [false, $rest];
 }
 
-function ofList(array $elements): int
+function ofList(array $elements): Node
 {
     $result = leaf();
     for ($i = count($elements) - 1; $i >= 0; $i--) {
@@ -42,7 +51,7 @@ function ofList(array $elements): int
     return $result;
 }
 
-function toList(int $tree): array
+function toList(Node $tree): array
 {
     $tree = reduce($tree);
     if (isLeaf($tree)) return [true, []];
@@ -57,7 +66,7 @@ function toList(int $tree): array
 /**
  * Strings are lists of byte values (each character encoded as a number tree).
  */
-function ofString(string $s): int
+function ofString(string $s): Node
 {
     $bytes = [];
     for ($i = 0, $len = strlen($s); $i < $len; $i++) {
@@ -66,7 +75,7 @@ function ofString(string $s): int
     return ofList($bytes);
 }
 
-function toString(int $tree): array
+function toString(Node $tree): array
 {
     [$ok, $elements] = toList($tree);
     if (!$ok) return [false, $elements];
@@ -82,17 +91,17 @@ function toString(int $tree): array
     return [true, $result];
 }
 
-function ofBytes(string $s): int
+function ofBytes(string $s): Node
 {
     return ofString($s);
 }
 
-function toBytes(int $tree): array
+function toBytes(Node $tree): array
 {
     return toString($tree);
 }
 
-function unwrapResult(int $tree): array
+function unwrapResult(Node $tree): array
 {
     $tree = reduce($tree);
     if (!isFork($tree)) return ['error', null, 'Result is not a valid ok/err pair'];
@@ -120,7 +129,7 @@ const HOST_BOOL_TAG   = 3;
 const HOST_LIST_TAG   = 4;
 const HOST_BYTES_TAG  = 5;
 
-function unwrapHostValue(int $tree): array
+function unwrapHostValue(Node $tree): array
 {
     $tree = reduce($tree);
     if (!isFork($tree)) throw new \InvalidArgumentException('Host ABI value must be a pair');
@@ -132,13 +141,13 @@ function unwrapHostValue(int $tree): array
     return [(int)$tagNum, $payload];
 }
 
-function isBool(int $tree): bool
+function isBool(Node $tree): bool
 {
     $tree = reduce($tree);
     return isLeaf($tree) || (isStem($tree) && isLeaf(stemChild($tree)));
 }
 
-function decodeHostPayload(int $tag, int $payload): array
+function decodeHostPayload(int $tag, Node $payload): array
 {
     switch ($tag) {
         case HOST_TREE_TAG:

ext/php/src/functions.php

@@ -5,185 +5,251 @@ declare(strict_types=1);
 namespace Arboricx;
 
 /**
- * Arena-backed Tree Calculus graph.
+ * Node-based Tree Calculus graph.
+ *
+ * Nodes are plain PHP objects so the runtime's refcounting GC
+ * can reclaim unreachable subtrees automatically.
  *
- * Nodes are integers (refs) into four parallel arrays:
  *   tag 0 = Leaf
  *   tag 1 = Stem(child)
  *   tag 2 = Fork(left, right)
  *   tag 3 = App(function, argument)  -- evaluator thunk
- *   tag 4 = Ind(target)              -- update-in-place indirection for sharing
- *
- * Using ints instead of objects keeps the hot path allocation-free
- * and lets us do cheap pointer-equality (same ref == same subtree).
  */
-$GLOBALS['ARB_TAG'] = [0 => 0];
-$GLOBALS['ARB_A']   = [0 => 0];
-$GLOBALS['ARB_B']   = [0 => 0];
-$GLOBALS['ARB_NEXT'] = 1;
-
-function spendFuel(int &$fuel): void
+final class Node
 {
-    if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
-    $fuel--;
-}
+    public int $tag;
+    public ?Node $a;
+    public ?Node $b;
 
-function newNode(int $tag, int $a, int $b = 0): int
-{
-    $id = $GLOBALS['ARB_NEXT']++;
-    $GLOBALS['ARB_TAG'][$id] = $tag;
-    $GLOBALS['ARB_A'][$id] = $a;
-    $GLOBALS['ARB_B'][$id] = $b;
-    return $id;
-}
-
-/**
- * Chase indirection (tag 4) chains to the real node.
- * Self-loops are treated as terminal to avoid infinite loops.
- */
-function deref(int $t): int
-{
-    while ($t !== 0 && ($GLOBALS['ARB_TAG'][$t] ?? -1) === 4) {
-        $next = $GLOBALS['ARB_A'][$t];
-        if ($next === $t) break;
-        $t = $next;
-    }
-    return $t;
-}
-
-/**
- * Overwrite a node with an indirection to its reduced form.
- * This short-circuits future reads so shared sub-terms are only
- * reduced once.
- */
-function setInd(int $o, int $t): void
-{
-    if ($o !== $t) {
-        $GLOBALS['ARB_TAG'][$o] = 4;
-        $GLOBALS['ARB_A'][$o] = $t;
+    public function __construct(int $tag = 0, ?Node $a = null, ?Node $b = null)
+    {
+        $this->tag = $tag;
+        $this->a = $a;
+        $this->b = $b;
     }
 }
 
-function leaf(): int { return 0; }
-function stem(int $c): int { return newNode(1, $c); }
-function fork(int $l, int $r): int { return newNode(2, $l, $r); }
-function app(int $f, int $x): int { return newNode(3, $f, $x); }
+$GLOBALS['ARB_LEAF'] = new Node();
+$GLOBALS['ARB_CONS'] = [];
 
-function tag(int $t): int { $t = deref($t); return $GLOBALS['ARB_TAG'][$t] ?? -1; }
-function isLeaf(int $t): bool { return deref($t) === 0; }
-function isStem(int $t): bool { return tag($t) === 1; }
-function isFork(int $t): bool { return tag($t) === 2; }
-function isApp(int $t): bool { return tag($t) === 3; }
+function newNode(int $tag, ?Node $a = null, ?Node $b = null): Node
+{
+    if ($tag !== 3) {
+        $key = $tag === 1
+            ? '1:' . spl_object_id($a)
+            : '2:' . spl_object_id($a) . ':' . spl_object_id($b);
+        if (isset($GLOBALS['ARB_CONS'][$key])) {
+            return $GLOBALS['ARB_CONS'][$key];
+        }
+        $n = new Node($tag, $a, $b);
+        $GLOBALS['ARB_CONS'][$key] = $n;
+        return $n;
+    }
 
-function stemChild(int $t): int { $t = deref($t); return $GLOBALS['ARB_A'][$t]; }
-function forkLeft(int $t): int { $t = deref($t); return $GLOBALS['ARB_A'][$t]; }
-function forkRight(int $t): int { $t = deref($t); return $GLOBALS['ARB_B'][$t]; }
-function appFunc(int $t): int { $t = deref($t); return $GLOBALS['ARB_A'][$t]; }
-function appArg(int $t): int { $t = deref($t); return $GLOBALS['ARB_B'][$t]; }
+    return new Node($tag, $a, $b);
+}
 
-/**
- * Reduce a term to weak-head normal form (WHNF).
- *
- * The evaluator is iterative (no PHP stack growth) and reduces
- * left-to-right: function first, then argument when needed.
- * Fuel prevents infinite loops on divergent terms.
- */
-function reduce(int $term, int $fuel = 100_000_000_000_000_000): int
+function leaf(): Node
+{
+    return $GLOBALS['ARB_LEAF'];
+}
+
+function stem(Node $c): Node
+{
+    return newNode(1, $c);
+}
+
+function fork(Node $l, Node $r): Node
+{
+    return newNode(2, $l, $r);
+}
+
+function app(Node $f, Node $x): Node
+{
+    return newNode(3, $f, $x);
+}
+
+function tag(Node $t): int
+{
+    return $t->tag;
+}
+
+function isLeaf(Node $t): bool
+{
+    return $t->tag === 0;
+}
+
+function isStem(Node $t): bool
+{
+    return $t->tag === 1;
+}
+
+function isFork(Node $t): bool
+{
+    return $t->tag === 2;
+}
+
+function isApp(Node $t): bool
+{
+    return $t->tag === 3;
+}
+
+function stemChild(Node $t): Node
+{
+    return $t->a;
+}
+
+function forkLeft(Node $t): Node
+{
+    return $t->a;
+}
+
+function forkRight(Node $t): Node
+{
+    return $t->b;
+}
+
+function appFunc(Node $t): Node
+{
+    return $t->a;
+}
+
+function appArg(Node $t): Node
+{
+    return $t->b;
+}
+
+function reduce(Node $term, int $fuel = 100_000_000_000_000_000): Node
 {
     return whnf($term, $fuel);
 }
 
-function whnf(int $term, int &$fuel): int
+function whnf(Node $term, int &$fuel): Node
 {
-    $TAG =& $GLOBALS['ARB_TAG'];
-    $A   =& $GLOBALS['ARB_A'];
-    $B   =& $GLOBALS['ARB_B'];
-
     while (true) {
-        $term = deref($term);
-        if ($TAG[$term] !== 3) return $term;
+        if ($term->tag !== 3) {
+            return $term;
+        }
 
         $orig = $term;
-        $f = whnf($A[$term], $fuel);
-        $x = $B[$term];
-
-        $f = deref($f);
-        $ftag = $TAG[$f];
+        $f = whnf($term->a, $fuel);
+        $x = $term->b;
+        $ftag = $f->tag;
 
         // apply Leaf b = Stem b
-        if ($f === 0) {
-            $r = newNode(1, $x); setInd($orig, $r); return $r;
+        if ($ftag === 0) {
+            $orig->tag = 1;
+            $orig->a = $x;
+            $orig->b = null;
+            return $orig;
         }
 
         // apply (Stem a) b = Fork a b
         if ($ftag === 1) {
-            $r = newNode(2, $A[$f], $x); setInd($orig, $r); return $r;
+            $orig->tag = 2;
+            $orig->a = $f->a;
+            $orig->b = $x;
+            return $orig;
         }
 
-        if ($ftag !== 2) throw new \RuntimeException('apply: function did not reduce to tree');
+        if ($ftag !== 2) {
+            throw new \RuntimeException('apply: function did not reduce to tree');
+        }
 
-        $left = whnf($A[$f], $fuel);
-        $right = $B[$f];
-        $left = deref($left);
-        $ltag = $TAG[$left];
+        $left = whnf($f->a, $fuel);
+        $right = $f->b;
+        $ltag = $left->tag;
 
         // apply (Fork Leaf a) _ = a
-        if ($left === 0) {
-            $term = $right; setInd($orig, $term); spendFuel($fuel); continue;
+        if ($ltag === 0) {
+            $result = whnf($right, $fuel);
+            if ($orig !== $result) {
+                $orig->tag = $result->tag;
+                $orig->a = $result->a;
+                $orig->b = $result->b;
+            }
+            if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
+            $fuel--;
+            return $orig;
         }
 
         // apply (Fork (Stem a) b) c = (a c) (b c)
         if ($ltag === 1) {
-            $term = newNode(3, newNode(3, $A[$left], $x), newNode(3, $right, $x));
-            setInd($orig, $term); spendFuel($fuel); continue;
+            $inner1 = newNode(3, $left->a, $x);
+            $inner2 = newNode(3, $right, $x);
+            $orig->tag = 3;
+            $orig->a = $inner1;
+            $orig->b = $inner2;
+            $term = $orig;
+            if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
+            $fuel--;
+            continue;
         }
 
-        if ($ltag !== 2) throw new \RuntimeException('apply: invalid Fork left child');
+        if ($ltag !== 2) {
+            throw new \RuntimeException('apply: invalid Fork left child');
+        }
 
         $arg = whnf($x, $fuel);
-        $arg = deref($arg);
-        $atag = $TAG[$arg];
+        $atag = $arg->tag;
 
         // apply (Fork (Fork a b) c) Leaf = a
-        if ($arg === 0) {
-            $term = $A[$left]; setInd($orig, $term); spendFuel($fuel); continue;
+        if ($atag === 0) {
+            $result = whnf($left->a, $fuel);
+            if ($orig !== $result) {
+                $orig->tag = $result->tag;
+                $orig->a = $result->a;
+                $orig->b = $result->b;
+            }
+            if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
+            $fuel--;
+            return $orig;
         }
 
         // apply (Fork (Fork a b) c) (Stem u) = b u
         if ($atag === 1) {
-            $term = newNode(3, $B[$left], $A[$arg]);
-            setInd($orig, $term); spendFuel($fuel); continue;
+            $orig->tag = 3;
+            $orig->a = $left->b;
+            $orig->b = $arg->a;
+            $term = $orig;
+            if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
+            $fuel--;
+            continue;
         }
 
         // apply (Fork (Fork a b) c) (Fork u v) = (c u) v
         if ($atag === 2) {
-            $term = newNode(3, newNode(3, $right, $A[$arg]), $B[$arg]);
-            setInd($orig, $term); spendFuel($fuel); continue;
+            $inner = newNode(3, $right, $arg->a);
+            $orig->tag = 3;
+            $orig->a = $inner;
+            $orig->b = $arg->b;
+            $term = $orig;
+            if ($fuel <= 0) throw new \RuntimeException('fuel exhausted');
+            $fuel--;
+            continue;
         }
 
         throw new \RuntimeException('apply: argument did not reduce to tree');
     }
 }
 
-function same_tree(int $a, int $b): bool
+function same_tree(Node $a, Node $b): bool
 {
-    $a = deref($a); $b = deref($b);
     if ($a === $b) return true;
-    if (tag($a) !== tag($b)) return false;
-    if (isLeaf($a)) return true;
-    if (isStem($a)) return same_tree(stemChild($a), stemChild($b));
-    if (isFork($a)) return same_tree(forkLeft($a), forkLeft($b)) && same_tree(forkRight($a), forkRight($b));
-    if (isApp($a)) return same_tree(appFunc($a), appFunc($b)) && same_tree(appArg($a), appArg($b));
+    if ($a->tag !== $b->tag) return false;
+    if ($a->tag === 0) return true;
+    if ($a->tag === 1) return same_tree($a->a, $b->a);
+    if ($a->tag === 2) return same_tree($a->a, $b->a) && same_tree($a->b, $b->b);
+    if ($a->tag === 3) return same_tree($a->a, $b->a) && same_tree($a->b, $b->b);
     return false;
 }
 
-function formatTree(int $t, int $depth = 0): string
+function formatTree(Node $t, int $depth = 0): string
 {
-    $t = deref($t);
     if ($depth > 200) return '...';
-    if (isLeaf($t)) return 'Leaf';
-    if (isStem($t)) return 'Stem(' . formatTree(stemChild($t), $depth + 1) . ')';
-    if (isFork($t)) return 'Fork(' . formatTree(forkLeft($t), $depth + 1) . ', ' . formatTree(forkRight($t), $depth + 1) . ')';
-    if (isApp($t)) return 'App(' . formatTree(appFunc($t), $depth + 1) . ', ' . formatTree(appArg($t), $depth + 1) . ')';
+    if ($t->tag === 0) return 'Leaf';
+    if ($t->tag === 1) return 'Stem(' . formatTree($t->a, $depth + 1) . ')';
+    if ($t->tag === 2) return 'Fork(' . formatTree($t->a, $depth + 1) . ', ' . formatTree($t->b, $depth + 1) . ')';
+    if ($t->tag === 3) return 'App(' . formatTree($t->a, $depth + 1) . ', ' . formatTree($t->b, $depth + 1) . ')';
     return 'Unknown';
 }

ext/php/src/kernel.php

@@ -26,7 +26,7 @@ function loadKernelTernary(string $name): string
     return $content !== false ? trim($content) : '';
 }
 
-function parseTernary(string $s, int &$pos = 0): int
+function parseTernary(string $s, int &$pos = 0): Node
 {
     if ($pos >= strlen($s)) {
         throw new \RuntimeException('parseTernary: unexpected end of string');
@@ -40,7 +40,7 @@ function parseTernary(string $s, int &$pos = 0): int
     };
 }
 
-function getRunArboricxToString(): ?int
+function getRunArboricxToString(): ?Node
 {
     $term = loadKernelTernary('kernel_run_arboricx_to_string');
     if ($term === '') return null;