add resizeable scratch space API

Alignment support by Pieter Wuille.

src/scratch_impl.h

@@ -0,0 +1,77 @@
+/**********************************************************************
+ * Copyright (c) 2017 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_SCRATCH_IMPL_H_
+#define _SECP256K1_SCRATCH_IMPL_H_
+
+#include "scratch.h"
+
+/* Using 16 bytes alignment because common architectures never have alignment
+ * requirements above 8 for any of the types we care about. In addition we
+ * leave some room because currently we don't care about a few bytes.
+ * TODO: Determine this at configure time. */
+#define ALIGNMENT 16
+
+static secp256k1_scratch* secp256k1_scratch_create(const secp256k1_callback* error_callback, size_t init_size, size_t max_size) {
+    secp256k1_scratch* ret = (secp256k1_scratch*)checked_malloc(error_callback, sizeof(*ret));
+    if (ret != NULL) {
+        ret->data = checked_malloc(error_callback, init_size);
+        if (ret->data == NULL) {
+            free (ret);
+            return NULL;
+        }
+        ret->offset = 0;
+        ret->init_size = init_size;
+        ret->max_size = max_size;
+        ret->error_callback = error_callback;
+    }
+    return ret;
+}
+
+static void secp256k1_scratch_destroy(secp256k1_scratch* scratch) {
+    if (scratch != NULL) {
+        free(scratch->data);
+        free(scratch);
+    }
+}
+
+static size_t secp256k1_scratch_max_allocation(const secp256k1_scratch* scratch, size_t objects) {
+    if (scratch->max_size <= objects * ALIGNMENT) {
+        return 0;
+    }
+    return scratch->max_size - objects * ALIGNMENT;
+}
+
+static int secp256k1_scratch_resize(secp256k1_scratch* scratch, size_t n, size_t objects) {
+    n += objects * ALIGNMENT;
+    if (n > scratch->init_size && n <= scratch->max_size) {
+        void *tmp = checked_realloc(scratch->error_callback, scratch->data, n);
+        if (tmp == NULL) {
+            return 0;
+        }
+        scratch->init_size = n;
+        scratch->data = tmp;
+    }
+    return n <= scratch->max_size;
+}
+
+static void *secp256k1_scratch_alloc(secp256k1_scratch* scratch, size_t size) {
+    void *ret;
+    size = ((size + ALIGNMENT - 1) / ALIGNMENT) * ALIGNMENT;
+    if (size + scratch->offset > scratch->init_size) {
+        return NULL;
+    }
+    ret = (void *) ((unsigned char *) scratch->data + scratch->offset);
+    memset(ret, 0, size);
+    scratch->offset += size;
+    return ret;
+}
+
+static void secp256k1_scratch_reset(secp256k1_scratch* scratch) {
+    scratch->offset = 0;
+}
+
+#endif