diff --git a/src/core/array.c b/src/core/array.c
index faddab86..cbfb6bd7 100644
--- a/src/core/array.c
+++ b/src/core/array.c
@@ -63,10 +63,11 @@ JanetArray *janet_array_n(const Janet *elements, int32_t n) {
 }
 
 /* Ensure the array has enough capacity for elements */
-void janet_array_ensure(JanetArray *array, int32_t capacity) {
+void janet_array_ensure(JanetArray *array, int32_t capacity, int32_t growth) {
     Janet *newData;
     Janet *old = array->data;
     if (capacity <= array->capacity) return;
+    capacity *= growth;
     newData = realloc(old, capacity * sizeof(Janet));
     if (NULL == newData) {
         JANET_OUT_OF_MEMORY;
@@ -81,7 +82,7 @@ void janet_array_setcount(JanetArray *array, int32_t count) {
         return;
     if (count > array->count) {
         int32_t i;
-        janet_array_ensure(array, count);
+        janet_array_ensure(array, count, 1);
         for (i = array->count; i < count; i++) {
             array->data[i] = janet_wrap_nil();
         }
@@ -92,9 +93,7 @@ void janet_array_setcount(JanetArray *array, int32_t count) {
 /* Push a value to the top of the array */
 void janet_array_push(JanetArray *array, Janet x) {
     int32_t newcount = array->count + 1;
-    if (newcount >= array->capacity) {
-        janet_array_ensure(array, newcount * 2);
-    }
+    janet_array_ensure(array, newcount, 2);
     array->data[array->count] = x;
     array->count = newcount;
 }
@@ -148,7 +147,7 @@ static int cfun_push(JanetArgs args) {
     JANET_MINARITY(args, 1);
     JANET_ARG_ARRAY(array, args, 0);
     newcount = array->count - 1 + args.n;
-    janet_array_ensure(array, newcount);
+    janet_array_ensure(array, newcount, 2);
     if (args.n > 1) memcpy(array->data + array->count, args.v + 1, (args.n - 1) * sizeof(Janet));
     array->count = newcount;
     JANET_RETURN(args, args.v[0]);
@@ -168,11 +167,13 @@ static int cfun_setcount(JanetArgs args) {
 static int cfun_ensure(JanetArgs args) {
     JanetArray *array;
     int32_t newcount;
-    JANET_FIXARITY(args, 2);
+    int32_t growth;
+    JANET_FIXARITY(args, 3);
     JANET_ARG_ARRAY(array, args, 0);
     JANET_ARG_INTEGER(newcount, args, 1);
+    JANET_ARG_INTEGER(growth, args, 2);
     if (newcount < 0) JANET_THROW(args, "expected positive integer");
-    janet_array_ensure(array, newcount);
+    janet_array_ensure(array, newcount, growth);
     JANET_RETURN(args, args.v[0]);
 }
 
diff --git a/src/core/buffer.c b/src/core/buffer.c
index 909d7d7d..5556cc36 100644
--- a/src/core/buffer.c
+++ b/src/core/buffer.c
@@ -50,10 +50,11 @@ JanetBuffer *janet_buffer(int32_t capacity) {
 }
 
 /* Ensure that the buffer has enough internal capacity */
-void janet_buffer_ensure(JanetBuffer *buffer, int32_t capacity) {
+void janet_buffer_ensure(JanetBuffer *buffer, int32_t capacity, int32_t growth) {
     uint8_t *new_data;
     uint8_t *old = buffer->data;
     if (capacity <= buffer->capacity) return;
+    capacity *= growth;
     new_data = realloc(old, capacity * sizeof(uint8_t));
     if (NULL == new_data) {
         JANET_OUT_OF_MEMORY;
@@ -68,7 +69,7 @@ void janet_buffer_setcount(JanetBuffer *buffer, int32_t count) {
         return;
     if (count > buffer->count) {
         int32_t oldcount = buffer->count;
-        janet_buffer_ensure(buffer, count);
+        janet_buffer_ensure(buffer, count, 1);
         memset(buffer->data + oldcount, 0, count - oldcount);
     }
     buffer->count = count;
diff --git a/src/core/core.janet b/src/core/core.janet
index 5b3410d7..d7a50bb8 100644
--- a/src/core/core.janet
+++ b/src/core/core.janet
@@ -864,7 +864,7 @@ value, one key will be ignored."
     (def complex? (> (length y) 4))
     ((if complex? pp-dict-nested pp-dict-simple) y))
 
-  (def printers 
+  (def printers
     {:array  (fn [y] (do-ds y "@[" "]" true pp-seq))
      :tuple  (fn [y] (do-ds y "(" ")" false pp-seq))
      :table  (fn [y] (do-ds y "@{" "}" true pp-dict))
@@ -902,8 +902,8 @@ value, one key will be ignored."
 
   (defn expand-bindings [x]
     (case (type x)
-      :array (map expand-bindings x)
-      :tuple (tuple.slice (map expand-bindings x) 0)
+      :array (mapa expand-bindings x)
+      :tuple (map expand-bindings x)
       :table (dotable x expand-bindings)
       :struct (table.to-struct (dotable x expand-bindings))
       (macroexpand-1 x)))
@@ -916,11 +916,11 @@ value, one key will be ignored."
                                 @[(expand-bindings last2) (macroexpand-1 last)]) 0))
 
   (defn expandall [t]
-    (def args (map macroexpand-1 (tuple.slice t 1)))
+    (def args (mapa macroexpand-1 (tuple.slice t 1)))
     (apply tuple (get t 0) args))
 
   (defn expandfn [t]
-    (def args (map macroexpand-1 (tuple.slice t 2)))
+    (def args (mapa macroexpand-1 (tuple.slice t 2)))
     (apply tuple 'fn (get t 1) args))
 
   (def specs
@@ -942,12 +942,12 @@ value, one key will be ignored."
     (cond
       s (s t)
       m? (apply m (tuple.slice t 1))
-      (tuple.slice (map macroexpand-1 t) 0)))
+      (map macroexpand-1 t) 0))
 
   (def ret
     (case (type x)
       :tuple (dotup x)
-      :array (map macroexpand-1 x)
+      :array (mapa macroexpand-1 x)
       :struct (table.to-struct (dotable x macroexpand-1))
       :table (dotable x macroexpand-1)
       x))
@@ -1001,7 +1001,7 @@ value, one key will be ignored."
 ###
 ###
 
-(defn make-env 
+(defn make-env
   @[parent]
   (def parent (if parent parent _env))
   (def newenv (table.setproto @{} parent))
@@ -1166,7 +1166,7 @@ value, one key will be ignored."
       (string syspath janet.version "/?/??.so")
       (string syspath "/?.so")
       (string syspath "/?/??.so")]))
-      
+
 (if (= :windows (os.which))
    (loop [i :range [0 (length module.native-paths)]]
      (def x (get module.native-paths i))
diff --git a/src/core/string.c b/src/core/string.c
index b89da70e..485e8ca1 100644
--- a/src/core/string.c
+++ b/src/core/string.c
@@ -112,7 +112,7 @@ static int32_t real_to_string_impl(uint8_t *buf, double x) {
 }
 
 static void real_to_string_b(JanetBuffer *buffer, double x) {
-    janet_buffer_ensure(buffer, buffer->count + BUFSIZE);
+    janet_buffer_ensure(buffer, buffer->count + BUFSIZE, 2);
     buffer->count += real_to_string_impl(buffer->data + buffer->count, x);
 }
 
@@ -202,7 +202,7 @@ static int32_t string_description_impl(uint8_t *buf, const char *title, void *po
 }
 
 static void string_description_b(JanetBuffer *buffer, const char *title, void *pointer) {
-    janet_buffer_ensure(buffer, buffer->count + BUFSIZE);
+    janet_buffer_ensure(buffer, buffer->count + BUFSIZE, 2);
     buffer->count += string_description_impl(buffer->data + buffer->count, title, pointer);
 }
 
diff --git a/src/core/vm.c b/src/core/vm.c
index 11f95c30..3ac99e2a 100644
--- a/src/core/vm.c
+++ b/src/core/vm.c
@@ -906,20 +906,26 @@ static void *op_lookup[255] = {
                 retreg = ds;
                 goto vm_type_error;
             case JANET_ARRAY:
-                if (index >= janet_unwrap_array(ds)->count) {
-                    janet_array_ensure(janet_unwrap_array(ds), 2 * index);
-                    janet_unwrap_array(ds)->count = index + 1;
+                {
+                    JanetArray *array = janet_unwrap_array(ds);
+                    if (index >= array->count) {
+                        janet_array_ensure(array, index + 1, 2);
+                        array->count = index + 1;
+                    }
+                    array->data[index] = value;
+                    break;
                 }
-                janet_unwrap_array(ds)->data[index] = value;
-                break;
             case JANET_BUFFER:
-                vm_assert_type(value, JANET_INTEGER);
-                if (index >= janet_unwrap_buffer(ds)->count) {
-                    janet_buffer_ensure(janet_unwrap_buffer(ds), 2 * index);
-                    janet_unwrap_buffer(ds)->count = index + 1;
+                {
+                    JanetBuffer *buffer = janet_unwrap_buffer(ds);
+                    vm_assert_type(value, JANET_INTEGER);
+                    if (index >= buffer->count) {
+                        janet_buffer_ensure(buffer, index + 1, 2);
+                        buffer->count = index + 1;
+                    }
+                    buffer->data[index] = janet_unwrap_integer(value);
+                    break;
                 }
-                janet_unwrap_buffer(ds)->data[index] = janet_unwrap_integer(value);
-                break;
         }
         ++pc;
         vm_checkgc_next();
diff --git a/src/include/janet/janet.h b/src/include/janet/janet.h
index 4c360370..1a1899ff 100644
--- a/src/include/janet/janet.h
+++ b/src/include/janet/janet.h
@@ -916,7 +916,7 @@ JANET_API JanetArray *janet_array(int32_t capacity);
 JANET_API JanetArray *janet_array_n(const Janet *elements, int32_t n);
 JANET_API JanetArray *janet_array_init(JanetArray *array, int32_t capacity);
 JANET_API void janet_array_deinit(JanetArray *array);
-JANET_API void janet_array_ensure(JanetArray *array, int32_t capacity);
+JANET_API void janet_array_ensure(JanetArray *array, int32_t capacity, int32_t growth);
 JANET_API void janet_array_setcount(JanetArray *array, int32_t count);
 JANET_API void janet_array_push(JanetArray *array, Janet x);
 JANET_API Janet janet_array_pop(JanetArray *array);
@@ -926,7 +926,7 @@ JANET_API Janet janet_array_peek(JanetArray *array);
 JANET_API JanetBuffer *janet_buffer(int32_t capacity);
 JANET_API JanetBuffer *janet_buffer_init(JanetBuffer *buffer, int32_t capacity);
 JANET_API void janet_buffer_deinit(JanetBuffer *buffer);
-JANET_API void janet_buffer_ensure(JanetBuffer *buffer, int32_t capacity);
+JANET_API void janet_buffer_ensure(JanetBuffer *buffer, int32_t capacity, int32_t growth);
 JANET_API void janet_buffer_setcount(JanetBuffer *buffer, int32_t count);
 JANET_API int janet_buffer_extra(JanetBuffer *buffer, int32_t n);
 JANET_API int janet_buffer_push_bytes(JanetBuffer *buffer, const uint8_t *string, int32_t len);
@@ -1185,7 +1185,7 @@ JANET_API int janet_typeabstract_err(JanetArgs args, int32_t n, const JanetAbstr
 #define JANET_ARG_ARRAY(DEST, A, N) _JANET_ARG(JANET_ARRAY, array, DEST, A, N)
 #define JANET_ARG_TUPLE(DEST, A, N) _JANET_ARG(JANET_TUPLE, tuple, DEST, A, N)
 #define JANET_ARG_TABLE(DEST, A, N) _JANET_ARG(JANET_TABLE, table, DEST, A, N)
-#define JANET_ARG_STRUCT(DEST, A, N) _JANET_ARG(JANET_STRUCT, st, DEST, A, N)
+#define JANET_ARG_STRUCT(DEST, A, N) _JANET_ARG(JANET_STRUCT, struct, DEST, A, N)
 #define JANET_ARG_BUFFER(DEST, A, N) _JANET_ARG(JANET_BUFFER, buffer, DEST, A, N)
 #define JANET_ARG_FUNCTION(DEST, A, N) _JANET_ARG(JANET_FUNCTION, function, DEST, A, N)
 #define JANET_ARG_CFUNCTION(DEST, A, N) _JANET_ARG(JANET_CFUNCTION, cfunction, DEST, A, N)
diff --git a/src/mainclient/line.c b/src/mainclient/line.c
index b04d17f7..23463f21 100644
--- a/src/mainclient/line.c
+++ b/src/mainclient/line.c
@@ -465,7 +465,7 @@ void janet_line_get(const uint8_t *p, JanetBuffer *buffer) {
     }
     norawmode();
     fputc('\n', stdout);
-    janet_buffer_ensure(buffer, len + 1);
+    janet_buffer_ensure(buffer, len + 1, 2);
     memcpy(buffer->data, buf, len);
     buffer->data[len] = '\n';
     buffer->count = len + 1;