Buffer protocol proposal (#3261)

* Add buffer protocol

* Modify for some review comments

* Per-object buffer availability

* Rename to MemoryView from Buffer and make compilable

* Support integral repeat count in memory view format

* Support 'x' for padding bytes

* Add rb_memory_view_parse_item_format

* Check type in rb_memory_view_register

* Update dependencies in common.mk

* Add test of MemoryView

* Add test of rb_memory_view_init_as_byte_array

* Add native size format test

* Add MemoryView test utilities

* Add test of rb_memory_view_fill_contiguous_strides

* Skip spaces in format string

* Support endianness specifiers

* Update documentation

* Support alignment

* Use RUBY_ALIGNOF

* Fix format parser to follow the pack format

* Support the _ modifier

* Parse count specifiers in get_format_size function.

* Use STRUCT_ALIGNOF

* Fix test

* Fix test

* Fix total size for the case with tail padding

* Fix rb_memory_view_get_item_pointer

* Fix rb_memory_view_parse_item_format again

3 files changed, 567 insertions, 0 deletions

diff --git a/ext/-test-/memory_view/depend b/ext/-test-/memory_view/depend
new file mode 100644
index 0000000000..bcbd98d41f
--- /dev/null
+++ b/ext/-test-/memory_view/depend
@@ -0,0 +1,164 @@
+# AUTOGENERATED DEPENDENCIES START
+memory_view.o: $(RUBY_EXTCONF_H)
+memory_view.o: $(arch_hdrdir)/ruby/config.h
+memory_view.o: $(hdrdir)/ruby.h
+memory_view.o: $(hdrdir)/ruby/assert.h
+memory_view.o: $(hdrdir)/ruby/backward.h
+memory_view.o: $(hdrdir)/ruby/backward/2/assume.h
+memory_view.o: $(hdrdir)/ruby/backward/2/attributes.h
+memory_view.o: $(hdrdir)/ruby/backward/2/bool.h
+memory_view.o: $(hdrdir)/ruby/backward/2/gcc_version_since.h
+memory_view.o: $(hdrdir)/ruby/backward/2/inttypes.h
+memory_view.o: $(hdrdir)/ruby/backward/2/limits.h
+memory_view.o: $(hdrdir)/ruby/backward/2/long_long.h
+memory_view.o: $(hdrdir)/ruby/backward/2/stdalign.h
+memory_view.o: $(hdrdir)/ruby/backward/2/stdarg.h
+memory_view.o: $(hdrdir)/ruby/defines.h
+memory_view.o: $(hdrdir)/ruby/intern.h
+memory_view.o: $(hdrdir)/ruby/internal/anyargs.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/char.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/double.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/fixnum.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/gid_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/int.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/intptr_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/long.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/long_long.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/mode_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/off_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/pid_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/short.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/size_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/st_data_t.h
+memory_view.o: $(hdrdir)/ruby/internal/arithmetic/uid_t.h
+memory_view.o: $(hdrdir)/ruby/internal/assume.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/alloc_size.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/artificial.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/cold.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/const.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/constexpr.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/deprecated.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/diagnose_if.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/enum_extensibility.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/error.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/flag_enum.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/forceinline.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/format.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/maybe_unused.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/noalias.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/nodiscard.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/noexcept.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/noinline.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/nonnull.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/noreturn.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/pure.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/restrict.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/returns_nonnull.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/warning.h
+memory_view.o: $(hdrdir)/ruby/internal/attr/weakref.h
+memory_view.o: $(hdrdir)/ruby/internal/cast.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/apple.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/clang.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/gcc.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/intel.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/msvc.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_is/sunpro.h
+memory_view.o: $(hdrdir)/ruby/internal/compiler_since.h
+memory_view.o: $(hdrdir)/ruby/internal/config.h
+memory_view.o: $(hdrdir)/ruby/internal/constant_p.h
+memory_view.o: $(hdrdir)/ruby/internal/core.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rarray.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rbasic.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rbignum.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rclass.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rdata.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rfile.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rhash.h
+memory_view.o: $(hdrdir)/ruby/internal/core/robject.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rregexp.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rstring.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rstruct.h
+memory_view.o: $(hdrdir)/ruby/internal/core/rtypeddata.h
+memory_view.o: $(hdrdir)/ruby/internal/ctype.h
+memory_view.o: $(hdrdir)/ruby/internal/dllexport.h
+memory_view.o: $(hdrdir)/ruby/internal/dosish.h
+memory_view.o: $(hdrdir)/ruby/internal/error.h
+memory_view.o: $(hdrdir)/ruby/internal/eval.h
+memory_view.o: $(hdrdir)/ruby/internal/event.h
+memory_view.o: $(hdrdir)/ruby/internal/fl_type.h
+memory_view.o: $(hdrdir)/ruby/internal/gc.h
+memory_view.o: $(hdrdir)/ruby/internal/glob.h
+memory_view.o: $(hdrdir)/ruby/internal/globals.h
+memory_view.o: $(hdrdir)/ruby/internal/has/attribute.h
+memory_view.o: $(hdrdir)/ruby/internal/has/builtin.h
+memory_view.o: $(hdrdir)/ruby/internal/has/c_attribute.h
+memory_view.o: $(hdrdir)/ruby/internal/has/cpp_attribute.h
+memory_view.o: $(hdrdir)/ruby/internal/has/declspec_attribute.h
+memory_view.o: $(hdrdir)/ruby/internal/has/extension.h
+memory_view.o: $(hdrdir)/ruby/internal/has/feature.h
+memory_view.o: $(hdrdir)/ruby/internal/has/warning.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/array.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/bignum.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/class.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/compar.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/complex.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/cont.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/dir.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/enum.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/enumerator.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/error.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/eval.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/file.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/gc.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/hash.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/io.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/load.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/marshal.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/numeric.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/object.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/parse.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/proc.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/process.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/random.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/range.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/rational.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/re.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/ruby.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/select.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/select/largesize.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/signal.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/sprintf.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/string.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/struct.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/thread.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/time.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/variable.h
+memory_view.o: $(hdrdir)/ruby/internal/intern/vm.h
+memory_view.o: $(hdrdir)/ruby/internal/interpreter.h
+memory_view.o: $(hdrdir)/ruby/internal/iterator.h
+memory_view.o: $(hdrdir)/ruby/internal/memory.h
+memory_view.o: $(hdrdir)/ruby/internal/method.h
+memory_view.o: $(hdrdir)/ruby/internal/module.h
+memory_view.o: $(hdrdir)/ruby/internal/newobj.h
+memory_view.o: $(hdrdir)/ruby/internal/rgengc.h
+memory_view.o: $(hdrdir)/ruby/internal/scan_args.h
+memory_view.o: $(hdrdir)/ruby/internal/special_consts.h
+memory_view.o: $(hdrdir)/ruby/internal/static_assert.h
+memory_view.o: $(hdrdir)/ruby/internal/stdalign.h
+memory_view.o: $(hdrdir)/ruby/internal/stdbool.h
+memory_view.o: $(hdrdir)/ruby/internal/symbol.h
+memory_view.o: $(hdrdir)/ruby/internal/token_paste.h
+memory_view.o: $(hdrdir)/ruby/internal/value.h
+memory_view.o: $(hdrdir)/ruby/internal/value_type.h
+memory_view.o: $(hdrdir)/ruby/internal/variable.h
+memory_view.o: $(hdrdir)/ruby/internal/warning_push.h
+memory_view.o: $(hdrdir)/ruby/internal/xmalloc.h
+memory_view.o: $(hdrdir)/ruby/memory_view.h
+memory_view.o: $(hdrdir)/ruby/missing.h
+memory_view.o: $(hdrdir)/ruby/ruby.h
+memory_view.o: $(hdrdir)/ruby/st.h
+memory_view.o: $(hdrdir)/ruby/subst.h
+memory_view.o: memory_view.c
+# AUTOGENERATED DEPENDENCIES END
diff --git a/ext/-test-/memory_view/extconf.rb b/ext/-test-/memory_view/extconf.rb
new file mode 100644
index 0000000000..d786b15db9
--- /dev/null
+++ b/ext/-test-/memory_view/extconf.rb
@@ -0,0 +1,3 @@
+# frozen_string_literal: false
+require_relative "../auto_ext.rb"
+auto_ext(inc: true)
diff --git a/ext/-test-/memory_view/memory_view.c b/ext/-test-/memory_view/memory_view.c
new file mode 100644
index 0000000000..33124e2551
--- /dev/null
+++ b/ext/-test-/memory_view/memory_view.c
@@ -0,0 +1,400 @@
+#include "ruby.h"
+#include "ruby/memory_view.h"
+
+#define STRUCT_ALIGNOF(T, result) do { \
+    struct S { char _; T t; }; \
+    (result) = (int)offsetof(struct S, t); \
+} while(0)
+
+static ID id_str;
+static VALUE sym_format;
+static VALUE sym_native_size_p;
+static VALUE sym_offset;
+static VALUE sym_size;
+static VALUE sym_repeat;
+static VALUE sym_obj;
+static VALUE sym_len;
+static VALUE sym_readonly;
+static VALUE sym_format;
+static VALUE sym_item_size;
+static VALUE sym_ndim;
+static VALUE sym_shape;
+static VALUE sym_strides;
+static VALUE sym_sub_offsets;
+static VALUE sym_endianness;
+static VALUE sym_little_endian;
+static VALUE sym_big_endian;
+
+static VALUE exported_objects;
+
+static int
+exportable_string_get_memory_view(VALUE obj, rb_memory_view_t *view, int flags)
+{
+    VALUE str = rb_ivar_get(obj, id_str);
+    rb_memory_view_init_as_byte_array(view, obj, RSTRING_PTR(str), RSTRING_LEN(str), true);
+
+    VALUE count = rb_hash_lookup2(exported_objects, obj, INT2FIX(0));
+    count = rb_funcall(count, '+', 1, INT2FIX(1));
+    rb_hash_aset(exported_objects, obj, count);
+
+    return 1;
+}
+
+static int
+exportable_string_release_memory_view(VALUE obj, rb_memory_view_t *view)
+{
+    VALUE count = rb_hash_lookup2(exported_objects, obj, INT2FIX(0));
+    if (INT2FIX(1) == count) {
+        rb_hash_delete(exported_objects, obj);
+    }
+    else if (INT2FIX(0) == count) {
+        rb_raise(rb_eRuntimeError, "Duplicated releasing of a memory view has been occurred for %"PRIsVALUE, obj);
+    }
+    else {
+        count = rb_funcall(count, '-', 1, INT2FIX(1));
+        rb_hash_aset(exported_objects, obj, count);
+    }
+
+    return 1;
+}
+
+static int
+exportable_string_memory_view_available_p(VALUE obj)
+{
+    return Qtrue;
+}
+
+static const rb_memory_view_entry_t exportable_string_memory_view_entry = {
+    exportable_string_get_memory_view,
+    exportable_string_release_memory_view,
+    exportable_string_memory_view_available_p
+};
+
+static VALUE
+memory_view_available_p(VALUE mod, VALUE obj)
+{
+    return rb_memory_view_available_p(obj) ? Qtrue : Qfalse;
+}
+
+static VALUE
+memory_view_register(VALUE mod, VALUE obj)
+{
+    return rb_memory_view_register(obj, &exportable_string_memory_view_entry) ? Qtrue : Qfalse;
+}
+
+static VALUE
+memory_view_item_size_from_format(VALUE mod, VALUE format)
+{
+    const char *c_str = NULL;
+    if (!NIL_P(format))
+        c_str = StringValueCStr(format);
+    const char *err = NULL;
+    ssize_t item_size = rb_memory_view_item_size_from_format(c_str, &err);
+    if (!err)
+        return rb_assoc_new(SSIZET2NUM(item_size), Qnil);
+    else 
+        return rb_assoc_new(SSIZET2NUM(item_size), rb_str_new_cstr(err));
+}
+
+static VALUE
+memory_view_parse_item_format(VALUE mod, VALUE format)
+{
+    const char *c_str = NULL;
+    if (!NIL_P(format))
+        c_str = StringValueCStr(format);
+    const char *err = NULL;
+
+    rb_memory_view_item_component_t *members;
+    ssize_t n_members;
+    ssize_t item_size = rb_memory_view_parse_item_format(c_str, &members, &n_members, &err);
+
+    VALUE result = rb_ary_new_capa(3);
+    rb_ary_push(result, SSIZET2NUM(item_size));
+
+    if (!err) {
+        VALUE ary = rb_ary_new_capa(n_members);
+        ssize_t i;
+        for (i = 0; i < n_members; ++i) {
+            VALUE member = rb_hash_new();
+            rb_hash_aset(member, sym_format, rb_str_new(&members[i].format, 1));
+            rb_hash_aset(member, sym_native_size_p, members[i].native_size_p ? Qtrue : Qfalse);
+            rb_hash_aset(member, sym_endianness, members[i].little_endian_p ? sym_little_endian : sym_big_endian);
+            rb_hash_aset(member, sym_offset, SSIZET2NUM(members[i].offset));
+            rb_hash_aset(member, sym_size, SSIZET2NUM(members[i].size));
+            rb_hash_aset(member, sym_repeat, SSIZET2NUM(members[i].repeat));
+            rb_ary_push(ary, member);
+        }
+        xfree(members);
+        rb_ary_push(result, ary);
+        rb_ary_push(result, Qnil);
+    }
+    else {
+        rb_ary_push(result, Qnil); // members
+        rb_ary_push(result, rb_str_new_cstr(err));
+    }
+
+    return result;
+}
+
+static VALUE
+memory_view_get_memory_view_info(VALUE mod, VALUE obj)
+{
+    rb_memory_view_t view;
+
+    if (!rb_memory_view_get(obj, &view, 0)) {
+        return Qnil;
+    }
+
+    VALUE hash = rb_hash_new();
+    rb_hash_aset(hash, sym_obj, view.obj);
+    rb_hash_aset(hash, sym_len, SSIZET2NUM(view.len));
+    rb_hash_aset(hash, sym_readonly, view.readonly ? Qtrue : Qfalse);
+    rb_hash_aset(hash, sym_format, view.format ? rb_str_new_cstr(view.format) : Qnil);
+    rb_hash_aset(hash, sym_item_size, SSIZET2NUM(view.item_size));
+    rb_hash_aset(hash, sym_ndim, SSIZET2NUM(view.ndim));
+
+    if (view.shape) {
+        VALUE shape = rb_ary_new_capa(view.ndim);
+        rb_hash_aset(hash, sym_shape, shape);
+    }
+    else {
+        rb_hash_aset(hash, sym_shape, Qnil);
+    }
+
+    if (view.strides) {
+        VALUE strides = rb_ary_new_capa(view.ndim);
+        rb_hash_aset(hash, sym_strides, strides);
+    }
+    else {
+        rb_hash_aset(hash, sym_strides, Qnil);
+    }
+
+    if (view.sub_offsets) {
+        VALUE sub_offsets = rb_ary_new_capa(view.ndim);
+        rb_hash_aset(hash, sym_sub_offsets, sub_offsets);
+    }
+    else {
+        rb_hash_aset(hash, sym_sub_offsets, Qnil);
+    }
+
+    rb_memory_view_release(&view);
+
+    return hash;
+}
+
+static VALUE
+memory_view_fill_contiguous_strides(VALUE mod, VALUE ndim_v, VALUE item_size_v, VALUE shape_v, VALUE row_major_p)
+{
+    int i, ndim = FIX2INT(ndim_v);
+
+    Check_Type(shape_v, T_ARRAY);
+    ssize_t *shape = ALLOC_N(ssize_t, ndim);
+    for (i = 0; i < ndim; ++i) {
+        shape[i] = NUM2SSIZET(RARRAY_AREF(shape_v, i));
+    }
+
+    ssize_t *strides = ALLOC_N(ssize_t, ndim);
+    rb_memory_view_fill_contiguous_strides(ndim, NUM2SSIZET(item_size_v), shape, RTEST(row_major_p), strides);
+
+    VALUE result = rb_ary_new_capa(ndim);
+    for (i = 0; i < ndim; ++i) {
+        rb_ary_push(result, SSIZET2NUM(strides[i]));
+    }
+
+    xfree(strides);
+    xfree(shape);
+
+    return result;
+}
+
+static VALUE
+expstr_initialize(VALUE obj, VALUE s)
+{
+    rb_ivar_set(obj, id_str, s);
+    return Qnil;
+}
+
+static int
+mdview_get_memory_view(VALUE obj, rb_memory_view_t *view, int flags)
+{
+    VALUE buf_v = rb_ivar_get(obj, id_str);
+    VALUE shape_v = rb_ivar_get(obj, SYM2ID(sym_shape));
+    VALUE strides_v = rb_ivar_get(obj, SYM2ID(sym_strides));
+
+    ssize_t i, ndim = RARRAY_LEN(shape_v);
+    ssize_t *shape = ALLOC_N(ssize_t, ndim);
+    ssize_t *strides = NULL;
+    if (!NIL_P(strides_v)) {
+        if (RARRAY_LEN(strides_v) != ndim) {
+            rb_raise(rb_eArgError, "strides has an invalid dimension");
+        }
+
+        strides = ALLOC_N(ssize_t, ndim);
+        for (i = 0; i < ndim; ++i) {
+            shape[i] = NUM2SSIZET(RARRAY_AREF(shape_v, i));
+            strides[i] = NUM2SSIZET(RARRAY_AREF(strides_v, i));
+        }
+    }
+    else {
+        for (i = 0; i < ndim; ++i) {
+            shape[i] = NUM2SSIZET(RARRAY_AREF(shape_v, i));
+        }
+    }
+
+    rb_memory_view_init_as_byte_array(view, obj, RSTRING_PTR(buf_v), RSTRING_LEN(buf_v), true);
+    view->format = "l";
+    view->item_size = sizeof(long);
+    view->ndim = ndim;
+    view->shape = shape;
+    view->strides = strides;
+
+    VALUE count = rb_hash_lookup2(exported_objects, obj, INT2FIX(0));
+    count = rb_funcall(count, '+', 1, INT2FIX(1));
+    rb_hash_aset(exported_objects, obj, count);
+
+    return 1;
+}
+
+static int
+mdview_release_memory_view(VALUE obj, rb_memory_view_t *view)
+{
+    VALUE count = rb_hash_lookup2(exported_objects, obj, INT2FIX(0));
+    if (INT2FIX(1) == count) {
+        rb_hash_delete(exported_objects, obj);
+    }
+    else if (INT2FIX(0) == count) {
+        rb_raise(rb_eRuntimeError, "Duplicated releasing of a memory view has been occurred for %"PRIsVALUE, obj);
+    }
+    else {
+        count = rb_funcall(count, '-', 1, INT2FIX(1));
+        rb_hash_aset(exported_objects, obj, count);
+    }
+
+    return 1;
+}
+
+static int
+mdview_memory_view_available_p(VALUE obj)
+{
+    return true;
+}
+
+static const rb_memory_view_entry_t mdview_memory_view_entry = {
+    mdview_get_memory_view,
+    mdview_release_memory_view,
+    mdview_memory_view_available_p
+};
+
+static VALUE
+mdview_initialize(VALUE obj, VALUE buf, VALUE shape, VALUE strides)
+{
+    Check_Type(buf, T_STRING);
+    Check_Type(shape, T_ARRAY);
+    if (!NIL_P(strides)) Check_Type(strides, T_ARRAY);
+
+    rb_ivar_set(obj, id_str, buf);
+    rb_ivar_set(obj, SYM2ID(sym_shape), shape);
+    rb_ivar_set(obj, SYM2ID(sym_strides), strides);
+    return Qnil;
+}
+
+static VALUE
+mdview_aref(VALUE obj, VALUE indices_v)
+{
+    Check_Type(indices_v, T_ARRAY);
+
+    rb_memory_view_t view;
+    if (!rb_memory_view_get(obj, &view, 0)) {
+        rb_raise(rb_eRuntimeError, "rb_memory_view_get: failed");
+    }
+
+    if (RARRAY_LEN(indices_v) != view.ndim) {
+        rb_raise(rb_eKeyError, "Indices has an invalid dimension");
+    }
+
+    VALUE buf_indices;
+    ssize_t *indices = ALLOCV_N(ssize_t, buf_indices, view.ndim);
+
+    ssize_t i;
+    for (i = 0; i < view.ndim; ++i) {
+        indices[i] = NUM2SSIZET(RARRAY_AREF(indices_v, i));
+    }
+
+    char *ptr = rb_memory_view_get_item_pointer(&view, indices);
+    ALLOCV_END(buf_indices);
+
+    long x = *(long *)ptr;
+    VALUE result = LONG2FIX(x);
+    rb_memory_view_release(&view);
+
+    return result;
+}
+
+void
+Init_memory_view(void)
+{
+    VALUE mMemoryViewTestUtils = rb_define_module("MemoryViewTestUtils");
+
+    rb_define_module_function(mMemoryViewTestUtils, "available?", memory_view_available_p, 1);
+    rb_define_module_function(mMemoryViewTestUtils, "register", memory_view_register, 1);
+    rb_define_module_function(mMemoryViewTestUtils, "item_size_from_format", memory_view_item_size_from_format, 1);
+    rb_define_module_function(mMemoryViewTestUtils, "parse_item_format", memory_view_parse_item_format, 1);
+    rb_define_module_function(mMemoryViewTestUtils, "get_memory_view_info", memory_view_get_memory_view_info, 1);
+    rb_define_module_function(mMemoryViewTestUtils, "fill_contiguous_strides", memory_view_fill_contiguous_strides, 4);
+
+    VALUE cExportableString = rb_define_class_under(mMemoryViewTestUtils, "ExportableString", rb_cObject);
+    rb_define_method(cExportableString, "initialize", expstr_initialize, 1);
+    rb_memory_view_register(cExportableString, &exportable_string_memory_view_entry);
+
+    VALUE cMDView = rb_define_class_under(mMemoryViewTestUtils, "MultiDimensionalView", rb_cObject);
+    rb_define_method(cMDView, "initialize", mdview_initialize, 3);
+    rb_define_method(cMDView, "[]", mdview_aref, 1);
+    rb_memory_view_register(cMDView, &mdview_memory_view_entry);
+
+    id_str = rb_intern("__str__");
+    sym_format = ID2SYM(rb_intern("format"));
+    sym_native_size_p = ID2SYM(rb_intern("native_size_p"));
+    sym_offset = ID2SYM(rb_intern("offset"));
+    sym_size = ID2SYM(rb_intern("size"));
+    sym_repeat = ID2SYM(rb_intern("repeat"));
+    sym_obj = ID2SYM(rb_intern("obj"));
+    sym_len = ID2SYM(rb_intern("len"));
+    sym_readonly = ID2SYM(rb_intern("readonly"));
+    sym_format = ID2SYM(rb_intern("format"));
+    sym_item_size = ID2SYM(rb_intern("item_size"));
+    sym_ndim = ID2SYM(rb_intern("ndim"));
+    sym_shape = ID2SYM(rb_intern("shape"));
+    sym_strides = ID2SYM(rb_intern("strides"));
+    sym_sub_offsets = ID2SYM(rb_intern("sub_offsets"));
+    sym_endianness = ID2SYM(rb_intern("endianness"));
+    sym_little_endian = ID2SYM(rb_intern("little_endian"));
+    sym_big_endian = ID2SYM(rb_intern("big_endian"));
+
+#ifdef WORDS_BIGENDIAN
+    rb_const_set(mMemoryViewTestUtils, rb_intern("NATIVE_ENDIAN"), sym_big_endian);
+#else
+    rb_const_set(mMemoryViewTestUtils, rb_intern("NATIVE_ENDIAN"), sym_little_endian);
+#endif
+
+#define DEF_ALIGNMENT_CONST(type, TYPE) do { \
+    int alignment; \
+    STRUCT_ALIGNOF(type, alignment); \
+    rb_const_set(mMemoryViewTestUtils, rb_intern(#TYPE "_ALIGNMENT"), INT2FIX(alignment)); \
+} while(0)
+
+    DEF_ALIGNMENT_CONST(short, SHORT);
+    DEF_ALIGNMENT_CONST(int, INT);
+    DEF_ALIGNMENT_CONST(long, LONG);
+    DEF_ALIGNMENT_CONST(LONG_LONG, LONG_LONG);
+    DEF_ALIGNMENT_CONST(int16_t, INT16);
+    DEF_ALIGNMENT_CONST(int32_t, INT32);
+    DEF_ALIGNMENT_CONST(int64_t, INT64);
+    DEF_ALIGNMENT_CONST(intptr_t, INTPTR);
+    DEF_ALIGNMENT_CONST(float, FLOAT);
+    DEF_ALIGNMENT_CONST(double, DOUBLE);
+
+#undef DEF_ALIGNMENT_CONST
+
+    exported_objects = rb_hash_new();
+    rb_gc_register_mark_object(exported_objects);
+}