From 7bae84d126c775ae2b5fd0c81da665f73b30863e Mon Sep 17 00:00:00 2001
From: Shanti Chellaram <shanti@shanti.im>
Date: Mon, 12 Jul 2021 22:28:33 -0400
Subject: [PATCH] Add arena headers

An arena is a data structure that makes memory management a fair bit simpler.
This is an arena type that doesn't do anything fancy; it simply maintains a
linked list of blocks from which it doles out memory. It's probably not even
that efficient!

Future directions for this work include adding more options and implementation details:

* custom allocators
* object destructors
* strcpy instead of just strncpy support
* large-block allocation support
* block data structure that is more concurrency safe
---
 src/arena.c | 121 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/arena.h |  14 ++++++
 2 files changed, 135 insertions(+)
 create mode 100644 src/arena.c
 create mode 100644 src/arena.h

diff --git a/src/arena.c b/src/arena.c
new file mode 100644
index 0000000..2522d66
--- /dev/null
+++ b/src/arena.c
@@ -0,0 +1,121 @@
+#include <stdlib.h>
+#include <strings.h>
+
+#include "arena.h"
+
+struct arena_block {
+    struct arena_block *next;
+    size_t used;
+    char data[];
+};
+
+struct arena {
+    size_t block_size; /* size of blocks to allocate */
+    struct arena_block *head, *last;
+};
+
+struct arena *arena_create(size_t block_size) {
+    // allocate an initial block
+    struct arena * result = (struct arena *) malloc(sizeof(struct arena) + sizeof(struct arena_block) + block_size);
+    if (result == NULL) {
+        return NULL;
+    }
+    result->block_size = block_size;
+    result->head = (struct arena_block *) (result + 1);
+    result->head->next = NULL;
+    result->head->used = 0;
+    result->last = result->head;
+
+    return result;
+}
+
+void arena_destroy(struct arena *arena) {
+    if (arena == NULL) {
+        return; // error?
+    }
+    struct arena_block *blk = arena->head->next;
+    while (blk != NULL) {
+        struct arena_block *tmp = blk->next;
+        free(blk);
+        blk = tmp;
+    };
+    free(arena);
+}
+
+void *arena_malloc(struct arena *arena, size_t size) {
+    struct arena_block *block;
+    if (size > arena->block_size) {
+        return NULL;
+    } else if (size < arena->block_size - arena->last->used) {
+        block = arena->last;
+    } else {
+        block = (struct arena_block *) malloc(sizeof(struct arena_block) + arena->block_size);
+        if (block == NULL) {
+            return NULL;
+        }
+        block->next = NULL;
+        block->used = 0;
+        arena->last->next = block;
+        arena->last = block;
+    }
+    void *ret = block->data + block->used;
+    block->used += size;
+    return ret;
+}
+
+void *arena_calloc(struct arena *arena, size_t count, size_t size) {
+    return arena_malloc(arena, count * size);
+}
+
+void *arena_copy_alloc(struct arena *arena, const void *src, size_t len) {
+    void *dst = arena_malloc(arena, len);
+    if (dst == NULL) {
+        return NULL;
+    }
+    return memcpy(dst, src, len);
+}
+
+void *arena_strcpy_alloc(struct arena *arena, const void *src, size_t len) {
+    if (arena == NULL) {
+        return NULL;
+    }
+    size_t true_len = strnlen(src, len) + 1;
+    void *dst = arena_malloc(arena, true_len);
+    if (dst == NULL) {
+        return NULL;
+    }
+    memcpy(dst, src, true_len - 1);
+    ((char *)dst)[true_len - 1] = '\0';
+    return dst;
+}
+
+size_t arena_used(struct arena *arena) {
+    size_t total = 0;
+    if (arena != NULL) {
+        for (struct arena_block *block = arena->head; block != NULL; block = block->next) {
+            total += block->used;
+        }
+    }
+    return total;
+}
+
+size_t arena_overhead(struct arena *arena) {
+    size_t total = 0;
+    if (arena != NULL) {
+        total += sizeof(struct arena);
+        for (struct arena_block *block = arena->head; block != NULL; block = block->next) {
+            total += sizeof(struct arena_block);
+        }
+    }
+    return total;
+}
+
+size_t arena_total_capacity(struct arena *arena) {
+    size_t total = 0;
+    if (arena != NULL) {
+        for (struct arena_block *block = arena->head; block != NULL; block = block->next) {
+            total += arena->block_size;
+        }
+    }
+    return total;
+}
diff --git a/src/arena.h b/src/arena.h
new file mode 100644
index 0000000..6456107
--- /dev/null
+++ b/src/arena.h
@@ -0,0 +1,14 @@
+
+struct arena;
+
+struct arena *arena_create(size_t block_size);
+void arena_destroy(struct arena *);
+
+void *arena_malloc(struct arena *, size_t size);
+void *arena_calloc(struct arena *, size_t count, size_t size);
+void *arena_copy_alloc(struct arena *, const void *src, size_t len);
+void *arena_strcpy_alloc(struct arena *, const void *src, size_t len);
+
+size_t arena_used(struct arena *);
+size_t arena_overhead(struct arena *);
+size_t arena_total_capacity(struct arena *);