1 files changed, 63 insertions, 0 deletions
diff --git a/kernel/pid.cc b/kernel/pid.cc
new file mode 100644
index 0000000..04b846b
--- /dev/null
+++ b/kernel/pid.cc
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Copyright (C) 2026 Richard Qin */
+
+/*
+ * PID Allocator — Bitmap-based O(1) block search
+ * Reference: docs/specs/task.md §3.2
+ */
+
+#include <lume/pid.h>
+
+void PidAllocator::init()
+{
+    for (int i = 0; i < kBitmapWords; i++)
+        bitmap_[i] = 0;
+    next_hint_ = 1;
+    bitmap_[0] |= 0x1ULL; /* Reserve PID 0 (idle/swapper) only.
+                           * PID 1 (init) is allocated normally — the first
+                           * kthread_create() call gets it, matching Linux. */
+}
+
+int32 PidAllocator::alloc()
+{
+    LockGuard guard(lock_);
+
+    int32 word_start = next_hint_ / 64;
+
+    for (int32 passes = 0; passes < kBitmapWords; passes++) {
+        int32 w = (word_start + passes) % kBitmapWords;
+        uint64 free_bits = ~bitmap_[w];
+        if (free_bits == 0)
+            continue;
+
+        int bit = 0;
+        uint64 v = free_bits;
+        while ((v & 1ULL) == 0) { v >>= 1; bit++; }
+        int32 pid = w * 64 + bit;
+
+        if (pid >= kMaxPid)
+            continue;
+
+        bitmap_[w] |= (1ULL << bit);
+        next_hint_ = pid + 1;
+        if (next_hint_ >= kMaxPid)
+            next_hint_ = 2;
+
+        return pid;
+    }
+
+    return -1;
+}
+
+void PidAllocator::free(int32 pid)
+{
+    if (pid < 0 || pid >= kMaxPid)
+        return;
+
+    LockGuard guard(lock_);
+    int32 w = pid / 64;
+    int32 bit = pid % 64;
+    bitmap_[w] &= ~(1ULL << bit);
+}
+
+PidAllocator g_pid_alloc;