From 70f97b8291454efbf9606051352c15ea6ebca319 Mon Sep 17 00:00:00 2001
From: Richard Thier <magosit@outlook.hu>
Date: Sun, 29 Sep 2024 18:00:32 +0200
Subject: [PATCH] Initial version

---
 arena.h | 103 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)
 create mode 100644 arena.h

diff --git a/arena.h b/arena.h
new file mode 100644
index 0000000..f71396a
--- /dev/null
+++ b/arena.h
@@ -0,0 +1,103 @@
+// Gradual-commit arena demonstration
+// This is free and unencumbered software released into the public domain.
+/* Usage:
+ * 
+ *    arena a = newarena((ptrdiff_t)1 << 33);
+ *    if (!alloc(a, size, align, count)) {
+ *      break;
+ *    }
+ *    total += size * count;
+ */
+#ifndef ARENA_H
+#define ARENA_H
+
+#include <stddef.h>
+#include <string.h>
+
+static void *os_reserve(ptrdiff_t);
+static char os_commit(void *, ptrdiff_t);
+
+#define ARENA_PAGESIZE ((ptrdiff_t)1<<26)
+
+typedef struct {
+    char *begin;
+    char *commit;
+    char *end;
+} arena;
+
+static arena newarena(ptrdiff_t cap)
+{
+    arena a = {0};
+    cap += -cap & (ARENA_PAGESIZE - 1);
+    a.begin = a.commit = a.end = (char*) os_reserve(cap);
+    if (a.begin) {
+        a.end += cap;
+    }
+    return a;
+}
+
+static void *alloc(arena *a, ptrdiff_t size, ptrdiff_t align, ptrdiff_t count)
+{
+    ptrdiff_t padding   = -(size_t)a->begin & (align - 1);
+    ptrdiff_t committed = a->commit - a->begin;
+    if (count > (committed-padding)/size) {
+        ptrdiff_t reserved = a->end - a->begin;
+        if (count > (reserved-padding)/size) {
+            return 0;
+        }
+
+        ptrdiff_t needed = size*count + padding - committed;
+        needed += -needed & (ARENA_PAGESIZE - 1);
+        if (!os_commit(a->commit, needed)) {
+            return 0;
+        }
+        a->commit += needed;
+    }
+
+    void *ptr = a->begin + padding;
+    a->begin += padding + size*count;
+
+	// Change to this instead if you want zero-inited (but this gets slow with many arenas)
+    //return memset(ptr, 0, size*count);
+	return ptr;
+}
+
+#ifdef _WIN32
+// $ cc -g3 -nostartfiles -o arena.exe arena.c
+// $ cl /Z7 arena.c /link /subsystem:console kernel32.lib libvcruntime.lib
+#define W32(r) __declspec(dllimport) r __stdcall
+W32(void)   ExitProcess(int);
+W32(void *) VirtualAlloc(void *, ptrdiff_t, int, int);
+
+#define MEM_COMMIT     0x1000
+#define MEM_RESERVE    0x2000
+#define PAGE_NOACCESS  0x0001
+#define PAGE_READWRITE 0x0004
+
+static void *os_reserve(ptrdiff_t cap)
+{
+    return VirtualAlloc(0, cap, MEM_RESERVE, PAGE_NOACCESS);
+}
+
+static char os_commit(void *ptr, ptrdiff_t len)
+{
+    return VirtualAlloc(ptr, len, MEM_COMMIT, PAGE_READWRITE);
+}
+
+#else  // POSIX
+// $ cc -g3 -o arena arena.c
+#include <sys/mman.h>
+
+static void *os_reserve(ptrdiff_t cap)
+{
+    void *r = mmap(0, cap, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
+    return r==MAP_FAILED ? 0 : r;
+}
+
+static char os_commit(void *ptr, ptrdiff_t len)
+{
+    return !mprotect(ptr, len, PROT_READ|PROT_WRITE);
+}
+#endif // POSIX
+
+#endif /* ARENA_H */