[PATCH 2/3] Add module to ease using Lua iterators from C

[Alexander Turenko <alexander.turenko@tarantool.org>]

Needed for #3276.
---
 src/lua/lua_iterator.h | 117 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 src/lua/lua_iterator.h

diff --git a/src/lua/lua_iterator.h b/src/lua/lua_iterator.h
new file mode 100644
index 000000000..3e1a88c93
--- /dev/null
+++ b/src/lua/lua_iterator.h
@@ -0,0 +1,117 @@
+#ifndef TARANTOOL_LUA_LUA_ITERATOR_H_INCLUDED
+#define TARANTOOL_LUA_LUA_ITERATOR_H_INCLUDED 1
+/*
+ * Copyright 2010-2018, Tarantool AUTHORS, please see AUTHORS file.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above
+ *    copyright notice, this list of conditions and the
+ *    following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above
+ *    copyright notice, this list of conditions and the following
+ *    disclaimer in the documentation and/or other materials
+ *    provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY AUTHORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+ * AUTHORS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
+ * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+ * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/**
+ * This module contains helper functions to interact with a Lua
+ * iterator from C.
+ */
+
+#include <lua.h>
+
+/**
+ * Holds iterator state (references to Lua objects).
+ */
+struct lua_iterator {
+	int gen;
+	int param;
+	int state;
+};
+
+/**
+ * Create a Lua iterator from {gen, param, state}.
+ */
+struct lua_iterator *
+lua_iterator_new_fromtable(lua_State *L, int idx)
+{
+	struct lua_iterator *it = (struct lua_iterator *) malloc(
+		sizeof(struct lua_iterator));
+
+	lua_rawgeti(L, idx, 1); /* Popped by luaL_ref(). */
+	it->gen = luaL_ref(L, LUA_REGISTRYINDEX);
+	lua_rawgeti(L, idx, 2); /* Popped by luaL_ref(). */
+	it->param = luaL_ref(L, LUA_REGISTRYINDEX);
+	lua_rawgeti(L, idx, 3); /* Popped by luaL_ref(). */
+	it->state = luaL_ref(L, LUA_REGISTRYINDEX);
+
+	return it;
+}
+
+/**
+ * Move iterator to the next value. Push values returned by
+ * gen(param, state) and return its count. Zero means no more
+ * results available.
+ */
+int
+lua_iterator_next(lua_State *L, struct lua_iterator *it)
+{
+	int frame_start = lua_gettop(L);
+
+	/* Call gen(param, state). */
+	lua_rawgeti(L, LUA_REGISTRYINDEX, it->gen);
+	lua_rawgeti(L, LUA_REGISTRYINDEX, it->param);
+	lua_rawgeti(L, LUA_REGISTRYINDEX, it->state);
+	lua_call(L, 2, LUA_MULTRET);
+	int nresults = lua_gettop(L) - frame_start;
+	if (nresults == 0) {
+		luaL_error(L, "lua_iterator_next: gen(param, state) must "
+			      "return at least one result");
+		unreachable();
+		return 0;
+	}
+
+	/* The call above returns nil as the first result. */
+	if (lua_isnil(L, frame_start + 1)) {
+		lua_settop(L, frame_start);
+		return 0;
+	}
+
+	/* Save the first result to it->state. */
+	luaL_unref(L, LUA_REGISTRYINDEX, it->state);
+	lua_pushvalue(L, frame_start + 1); /* Popped by luaL_ref(). */
+	it->state = luaL_ref(L, LUA_REGISTRYINDEX);
+
+	return nresults;
+}
+
+/**
+ * Free all resources hold by the iterator.
+ */
+void lua_iterator_free(lua_State *L, struct lua_iterator *it)
+{
+	luaL_unref(L, LUA_REGISTRYINDEX, it->gen);
+	luaL_unref(L, LUA_REGISTRYINDEX, it->param);
+	luaL_unref(L, LUA_REGISTRYINDEX, it->state);
+	free(it);
+}
+
+#endif /* TARANTOOL_LUA_LUA_ITERATOR_H_INCLUDED */
-- 
2.19.2