/*
* immutable.c
* An immutable table library by Bruce Hill. This library returns a single function
* that can be used to declare immutable classes, like so:
*
*     immutable = require 'immutable'
*     local Foo = immutable({"baz","qux"}, {class_var=5})
*     local foo = Foo("hello", 99)
*     assert(foo.baz == "hello")
*     assert(not pcall(function() foo.x = 'mutable' end))
*     local t = {[foo]="it works"}
*     assert(t[Foo("hello", 99)] == "it works")
*     assert(foo.class_var == 5)
* 
* Instances *are* garbage collected.
*
* Class layout:
*     __instances: weak-valued map from hash -> hash bucket
*         hash buckets: weak-keyed map from instance userdata -> table of values
*     __buckets: weak-keyed map from instance userdata -> hash bucket (used to manage hash bucket lifetimes)
*     __fields: list of named fields
*     __indices: map from field names to the index in the instance table where the value is stored
*     metamethods, methods, class variables, etc.
*/

#include "lua.h"
#include "lauxlib.h"

// The C API changed from 5.1 to 5.2, so these shims help the code compile on >=5.2
#if LUA_VERSION_NUM >= 502
#define lua_objlen(L, i) lua_rawlen(L, i)
#define luaL_register(L, _, R) luaL_setfuncs(L, R, 0)
#endif
// Lua 5.3 introduced lua_isinteger, fall back to lua_isnumber
#if LUA_VERSION_NUM < 503
#define lua_isinteger(L, i) lua_isnumber(L, i)
#endif

#if !defined(LUAI_HASHLIMIT)
#define LUAI_HASHLIMIT		5
#endif

// This is used to create a unique light userdata to store in the registry to allow all
// hash buckets to share the same {__mode='k'} metatable
static int WEAK_KEY_METATABLE;
// This is used to create a unique light userdata to store in the registry to allow all
// __instances tables to share the same {__mode='v'} metatable
static int WEAK_VALUE_METATABLE;
// This is used to create a unique light userdata to store in the registry to allow all
// constructed classes to have the same metatable:
//  {__new=Lcreate_instance, __tostring=function(cls) return cls.name or 'immutable(...)' end}
static int SHARED_CLASS_METATABLE;

typedef struct {
    int hash;
    size_t len;
} immutable_info_t;

static int Lcreate_instance(lua_State *L)
{
    int num_args = lua_gettop(L)-1;
    lua_getfield(L, 1, "__fields");
    int num_fields = lua_objlen(L, -1);
    lua_pop(L, 1);
    // arg 1: class table, ...
    lua_getfield(L, 1, "__new");
    if (! lua_isnil(L, -1)) {
        // stack: cls, ..., __new
        lua_pushvalue(L, 1);
        // stack: cls, ..., __new, cls
        lua_rotate(L, 2, 2);
        // stack: cls, __new, cls, ...
        lua_call(L, num_args+1, LUA_MULTRET);
        num_args = lua_gettop(L)-1;
        // stack: cls, ...
    } else {
        lua_pop(L, 1);
    }
    
    // Compute the hash:
    unsigned long long ull_hash = 0x9a937c4d; // Seed
    for (lua_Integer i=1; i <=(lua_Integer)num_fields || i <=(lua_Integer)num_args; i++) {
        unsigned long long item_hash;
        int type = i > num_args ? LUA_TNIL : lua_type(L, 1+i);
        switch (type) {
            case LUA_TNIL: case LUA_TNONE:
                // Arbitrarily chosen value
                item_hash = 0x97167da9;
                break;
            case LUA_TNUMBER:
                {
                    // Cast float bits to integer
                    lua_Number num = lua_tonumber(L, 1+i);
                    item_hash = *((lua_Integer*)&num);
                    if (item_hash == 0) {
                        item_hash = 0x2887c992;
                    }
                    break;
                }
            case LUA_TBOOLEAN:
                // Arbitrarily chosen values
                item_hash = lua_toboolean(L, 1+i)? 0x82684f71 : 0x88d66f2a;
                break;
            case LUA_TTABLE:
            case LUA_TFUNCTION:
            case LUA_TUSERDATA:
            case LUA_TTHREAD:
            case LUA_TLIGHTUSERDATA:
                item_hash = (1000003 * type) ^ (lua_Integer)lua_topointer(L, 1+i);
                break;
            case LUA_TSTRING:
                {
                    // Algorithm taken from Lua 5.3's implementation
                    size_t len;
                    const char *str = lua_tolstring(L, 1+i, &len);
                    item_hash = len ^ 0xd2e9e9ac; // Arbitrary seed
                    size_t step = (len >> LUAI_HASHLIMIT) + 1;
                    for (; len >= step; len -= step)
                        item_hash ^= ((item_hash<<5) + (item_hash>>2) + (unsigned char)(str[len - 1]));
                    break;
                }
            default:
                item_hash = 0;
        }
        ull_hash = (1000003 * ull_hash) ^ item_hash;
    }
    int hash = (int)ull_hash;

    lua_getfield(L, 1, "__instances");

    // Stack: [__instances]
    // Find bucket
    lua_rawgeti(L, -1, hash);
    // Stack: [__instances, bucket]
    if (lua_isnil(L, -1)) {
        // Make a new bucket
        // Stack: [__instances, nil]
        lua_pop(L, 1);
        // Stack: [__instances]
        lua_createtable(L, 0, 1);
        // Stack: [__instances, bucket]
        lua_pushlightuserdata(L, (void*)&WEAK_KEY_METATABLE);
        lua_gettable(L, LUA_REGISTRYINDEX);
        // Stack: [__instances, bucket, {__mode='k'}]
        lua_setmetatable(L, -2);
        // Stack: [__instances, bucket]
        lua_pushvalue(L, -1);
        // Stack: [__instances, bucket, bucket]
        lua_rawseti(L, -3, hash);
        // Stack: [__instances, bucket]
    } else {
        // Stack: [__instances, bucket]
        // scan bucket
        lua_pushnil(L);
        while (lua_next(L, -2) != 0) { // for hash_collider_inst, hash_collider in pairs(bucket) do
            // Stack: [__instances, bucket, hash_collider_inst, hash_collider]
            // Shallow equality check:
            lua_pushnil(L);
            while (lua_next(L, -2) != 0) { // for i, collider_value in pairs(hash_collider) do
                // Stack: [__instances, bucket, hash_collider_inst, hash_collider, i, value]
                if (!lua_rawequal(L, -1, 1+lua_tonumber(L, -2))) { // If the i'th entry doesn't match the i'th arg
                    // Stack: [__instances, bucket, hash_collider_inst, hash_collider, i, value]
                    lua_pop(L, 3);
                    // Stack: [__instances, bucket, hash_collider_inst]
                    goto next_bucket_item;
                } else {
                    // Stack: [__instances, bucket, hash_collider_inst, hash_collider, i, value]
                    lua_pop(L, 1);
                    // Stack: [__instances, bucket, hash_collider_inst, hash_collider, i]
                }
            }
            // bucket item matches
            // Stack: [__instances, bucket, hash_collider_inst, hash_collider]
            lua_pop(L, 1);
            return 1;

            next_bucket_item: ;
        }
    }
    lua_insert(L, -2);
    // Stack: [bucket, __instances]
    lua_pop(L, 1);
    // Stack: [bucket]
    int bucket_index = lua_gettop(L);

    // Failed to find an existing instance, so create a new one
    // Stack: [bucket]
    immutable_info_t *userdata = (immutable_info_t*)lua_newuserdata(L, sizeof(immutable_info_t));
    // Stack [bucket, inst_userdata]
    int userdata_index = lua_gettop(L);
    userdata->hash = hash;
    userdata->len = num_fields > num_args ? num_fields : num_args;
    
    lua_pushvalue(L, 1);
    // Stack [bucket, inst_userdata, cls]
    lua_setmetatable(L, -2);
    // Stack [bucket, inst_userdata]
    lua_createtable(L, userdata->len, 0); // Create the table to store the instance's data
    // Stack [bucket, inst_userdata, inst_table]
    for (lua_Integer i=1; i <= (lua_Integer)num_args; i++) {
        lua_pushvalue(L, i+1);
        // Stack [bucket, inst_userdata, inst_table, arg #1+i]
        lua_rawseti(L, -2, i);
    }

    // Set up a ref to the bucket so its lifetime is tied to inst_userdata
    lua_getfield(L, 1, "__buckets");
    // Stack [bucket, inst_userdata, inst_table, __buckets]
    lua_pushvalue(L, userdata_index);
    // Stack [bucket, inst_userdata, inst_table, __buckets, inst_userdata]
    lua_pushvalue(L, bucket_index);
    // Stack [bucket, inst_userdata, inst_table, __buckets, inst_userdata, bucket]
    lua_settable(L, -3);
    lua_pop(L, 1);
    
    // Stack [bucket, inst_userdata, inst_table]
    lua_pushvalue(L, userdata_index);
    // Stack [bucket, inst_userdata, inst_table, inst_userdata]
    lua_rotate(L, bucket_index, 1);
    // Stack [inst_userdata, bucket, inst_userdata, inst_table]
    lua_settable(L, -3); // buckets[inst_userdata] = inst_table
    lua_pop(L, 1);
    // Stack [inst_userdata]
    lua_gc(L, LUA_GCSTEP, 3);
    return 1;
}

static int Lfrom_table(lua_State *L)
{
    lua_getfield(L, 1, "__fields");
    int fields_index = lua_gettop(L);
    int num_fields = lua_objlen(L, -1);
    int num_values = num_fields + lua_objlen(L, 2);
    
    // Compute the hash:
    unsigned long long ull_hash = 0x9a937c4d; // Seed
    for (lua_Integer i=1; i <=(lua_Integer)num_values; i++) {
        unsigned long long item_hash;
        // Stack: [fields]
        if (i <= num_fields) {
            lua_rawgeti(L, fields_index, i);
            // Stack: [fields, fields[i]]
            lua_gettable(L, 2);
            // Stack: [fields, table[fields[i]]]
        } else {
            lua_rawgeti(L, 2, i - num_fields);
            // Stack: [fields, table[i-num_fields]]
        }
        // Stack: [fields, value[i]]
        int type = lua_type(L, -1);
        switch (type) {
            case LUA_TNIL: case LUA_TNONE:
                // Arbitrarily chosen value
                item_hash = 0x97167da9;
                break;
            case LUA_TNUMBER:
                {
                    // Cast float bits to integer
                    lua_Number num = lua_tonumber(L, -1);
                    item_hash = *((lua_Integer*)&num);
                    if (item_hash == 0) {
                        item_hash = 0x2887c992;
                    }
                    break;
                }
            case LUA_TBOOLEAN:
                // Arbitrarily chosen values
                item_hash = lua_toboolean(L, -1)? 0x82684f71 : 0x88d66f2a;
                break;
            case LUA_TTABLE:
            case LUA_TFUNCTION:
            case LUA_TUSERDATA:
            case LUA_TTHREAD:
            case LUA_TLIGHTUSERDATA:
                item_hash = (1000003 * type) ^ (lua_Integer)lua_topointer(L, -1);
                break;
            case LUA_TSTRING:
                {
                    // Algorithm taken from Lua 5.3's implementation
                    size_t len;
                    const char *str = lua_tolstring(L, -1, &len);
                    item_hash = len ^ 0xd2e9e9ac; // Arbitrary seed
                    size_t step = (len >> LUAI_HASHLIMIT) + 1;
                    for (; len >= step; len -= step)
                        item_hash ^= ((item_hash<<5) + (item_hash>>2) + (unsigned char)(str[len - 1]));
                    break;
                }
            default:
                item_hash = 0;
        }
        ull_hash = (1000003 * ull_hash) ^ item_hash;
        lua_pop(L, 1);
        // Stack: [fields]
    }
    int hash = (int)ull_hash;

    lua_getfield(L, 1, "__instances");

    // Stack: [fields, __instances]
    // Find bucket
    lua_rawgeti(L, -1, hash);
    // Stack: [fields, __instances, bucket]
    if (lua_isnil(L, -1)) {
        // Make a new bucket
        // Stack: [fields, __instances, nil]
        lua_pop(L, 1);
        // Stack: [fields, __instances]
        lua_createtable(L, 0, 1);
        // Stack: [fields, __instances, bucket]
        lua_pushlightuserdata(L, (void*)&WEAK_KEY_METATABLE);
        lua_gettable(L, LUA_REGISTRYINDEX);
        // Stack: [fields, __instances, bucket, {__mode='k'}]
        lua_setmetatable(L, -2);
        // Stack: [fields, __instances, bucket]
        lua_pushvalue(L, -1);
        // Stack: [fields, __instances, bucket, bucket]
        lua_rawseti(L, -3, hash);
        // Stack: [fields, __instances, bucket]
    } else {
        // Stack: [fields, __instances, bucket]
        // scan bucket
        lua_pushnil(L);
        while (lua_next(L, -2) != 0) { // for hash_collider_inst, hash_collider in pairs(bucket) do
            // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider]
            // Shallow equality check:
            lua_pushnil(L);
            while (lua_next(L, -2) != 0) { // for i, collider_value in pairs(hash_collider) do
                // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider, i, value]
                int i = lua_tonumber(L, -2);
                if (i <= num_fields) {
                    lua_rawgeti(L, fields_index, i);
                    lua_gettable(L, 2);
                } else {
                    lua_rawgeti(L, 2, i-num_fields);
                }
                // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider, i, value, table_value]
                if (!lua_rawequal(L, -1, -2)) { // If the i'th entry doesn't match the i'th arg
                    // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider, i, value, table_value]
                    lua_pop(L, 4);
                    // Stack: [fields, __instances, bucket, hash_collider_inst]
                    goto next_bucket_item;
                } else {
                    // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider, i, value, table_value]
                    lua_pop(L, 2);
                    // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider, i]
                }
            }
            // bucket item matches
            // Stack: [fields, __instances, bucket, hash_collider_inst, hash_collider]
            lua_pop(L, 1);
            return 1;

            next_bucket_item: ;
        }
    }
    lua_insert(L, -2);
    // Stack: [fields, bucket, __instances]
    lua_pop(L, 1);
    // Stack: [fields, bucket]
    int bucket_index = lua_gettop(L);

    // Failed to find an existing instance, so create a new one
    // Stack: [fields, bucket]
    immutable_info_t *userdata = (immutable_info_t*)lua_newuserdata(L, sizeof(immutable_info_t));
    // Stack [fields, bucket, inst_userdata]
    int userdata_index = lua_gettop(L);
    userdata->hash = hash;
    userdata->len = num_values;
    
    lua_pushvalue(L, 1);
    // Stack [fields, bucket, inst_userdata, cls]
    lua_setmetatable(L, -2);
    // Stack [fields, bucket, inst_userdata]
    lua_createtable(L, userdata->len, 0); // Create the table to store the instance's data
    // Stack [fields, bucket, inst_userdata, inst_table]
    for (lua_Integer i=1; i <= (lua_Integer)num_fields; i++) {
        lua_rawgeti(L, fields_index, i);
        lua_gettable(L, 2);
        lua_rawseti(L, -2, i);
    }
    for (lua_Integer i=num_fields+1; i <= num_values; i++) {
        lua_rawgeti(L, 2, i-num_fields);
        lua_rawseti(L, -2, i);
    }

    // Set up a ref to the bucket so its lifetime is tied to inst_userdata
    lua_getfield(L, 1, "__buckets");
    // Stack [fields, bucket, inst_userdata, inst_table, __buckets]
    lua_pushvalue(L, userdata_index);
    // Stack [fields, bucket, inst_userdata, inst_table, __buckets, inst_userdata]
    lua_pushvalue(L, bucket_index);
    // Stack [fields, bucket, inst_userdata, inst_table, __buckets, inst_userdata, bucket]
    lua_settable(L, -3);
    lua_pop(L, 1);
    
    // Stack [fields, bucket, inst_userdata, inst_table]
    lua_pushvalue(L, userdata_index);
    // Stack [fields, bucket, inst_userdata, inst_table, inst_userdata]
    lua_rotate(L, bucket_index, 1);
    // Stack [fields, inst_userdata, bucket, inst_userdata, inst_table]
    lua_settable(L, -3); // buckets[inst_userdata] = inst_table
    lua_pop(L, 1);
    // Stack [fields, inst_userdata]
    lua_gc(L, LUA_GCSTEP, 3);
    return 1;
}

static int Lis_instance(lua_State *L)
{
    if (lua_type(L, 2) != LUA_TUSERDATA) {
        lua_pushboolean(L, 0);
        return 1;
    }
    lua_getmetatable(L, 2);
    lua_pushboolean(L, lua_rawequal(L, -1, 1));
    return 1;
}

static int Llen(lua_State *L)
{
    luaL_checktype(L, 1, LUA_TUSERDATA);
    if (! lua_getmetatable(L, 1)) {
        luaL_error(L, "invalid type");
    }
    immutable_info_t *info = (immutable_info_t *)lua_touserdata(L, 1);
    lua_getfield(L, -1, "__fields");
    size_t len = info->len - lua_objlen(L, -1);
    lua_pushinteger(L, len);
    return 1;
}

static int Linst_index(lua_State *L)
{
    // Return inst[key], <whether inst has key>
    luaL_checktype(L, 1, LUA_TUSERDATA);
    if (! lua_getmetatable(L, 1)) {
        luaL_error(L, "invalid type");
    }
    // Stack: [mt]
    immutable_info_t *info = (immutable_info_t*)lua_touserdata(L, 1);
    if (! info) {
        luaL_error(L, "invalid type");
    }

    lua_getfield(L, -1, "__fields");
    size_t num_fields = lua_objlen(L, -1);
    lua_pop(L, 1);

    // Stack: [mt]
    lua_getfield(L, -1, "__indices");
    // Stack: [mt, indices]
    lua_pushvalue(L, 2);
    lua_gettable(L, -2);
    // Stack: [mt, indices, index]
    int index;
    if (lua_isnil(L, -1)) {
        if (! lua_isinteger(L, 2)) {
            return 0;
        }
        index = lua_tointeger(L, 2) + num_fields;
    } else {
        index = lua_tointeger(L, -1);
    }
    lua_pop(L, 2);
    // Stack: [mt]
    if (! (0 < index && index <= (int)info->len)) {
        return 0;
    }
    // Stack: [mt]
    lua_getfield(L, -1, "__instances");
    // Stack: [mt, buckets]
    lua_rawgeti(L, -1, info->hash);
    // Stack: [mt, buckets, bucket]
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find hash bucket");
    }
    lua_pushvalue(L, 1);
    // Stack: [mt, buckets, bucket, inst_udata]
    lua_rawget(L, -2);
    // Stack: [mt, buckets, bucket, inst_table]
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find instance");
    }
    lua_rawgeti(L, -1, index);
    lua_pushboolean(L, 1);
    return 2;
}

static int Lindex(lua_State *L)
{
    int ret = Linst_index(L);
    if (ret > 0) {
        if (ret > 1) {
            lua_pop(L, ret-1);
        }
        return 1;
    }
    // Fall back to class:
    lua_getmetatable(L, 1);
    // Stack: [..., mt]
    lua_pushvalue(L, 2);
    // Stack: [..., mt, key]
    lua_gettable(L, -2);
    // Stack: [..., mt, value]
    if (! lua_isnil(L, -1)) {
        return 1;
    }
    return 0;
}

static int Ltostring(lua_State *L)
{
    luaL_Buffer b;
    luaL_buffinit(L, &b);

    if (! lua_getmetatable(L, 1)) {
        luaL_error(L, "invalid type");
    }
    int cls_index = lua_gettop(L);
    // Stack: [mt]
    
    lua_getfield(L, cls_index, "name");
    if (!lua_isnil(L, -1)) {
        luaL_addvalue(&b);
    } else {
        lua_pop(L, 1);
    }
    luaL_addstring(&b, "(");
    
    lua_getfield(L, cls_index, "__instances");
    // Stack: [mt, buckets]
    immutable_info_t *info = (immutable_info_t*)lua_touserdata(L, 1);
    if (! info) {
        luaL_error(L, "invalid type");
    }
    lua_rawgeti(L, -1, info->hash);
    // Stack: [mt, buckets, bucket]
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find hash bucket");
    }
    lua_pushvalue(L, 1);
    // Stack: [mt, buckets, bucket, inst_udata]
    lua_rawget(L, -2);
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find instance table");
    }
    int inst_table_index = lua_gettop(L);
    // Stack: [mt, buckets, bucket, inst_table]
    lua_getglobal(L, "tostring");
    int tostring_index = lua_gettop(L);
    // Stack: [mt, buckets, bucket, inst_table, tostring]
    int n = info->len, i = 1;
    if (i <= n) {
        goto first_list_item;
    }
    while (++i <= n) {
        // Stack: [mt, buckets, bucket, inst_table, tostring, ???]
        luaL_addstring(&b, ", ");
      first_list_item:
        lua_pushvalue(L, tostring_index);
        // Stack: [mt, buckets, bucket, inst_table, tostring, ???, tostring]
        lua_rawgeti(L, inst_table_index, i);
        // Stack: [mt, buckets, bucket, inst_table, tostring, ???, tostring, value]
        int quotes = lua_type(L, -1) == LUA_TSTRING;
        lua_call(L, 1, 1);
        if (quotes) {
            luaL_addchar(&b, '"');
        }
        // Stack: [mt, buckets, bucket, inst_table, tostring, ???, value string]
        // TODO: properly escape strings? str:gsub('["\\]',"\\%1"):gsub("\n","\\n")
        luaL_addvalue(&b);
        // Stack: [mt, buckets, bucket, inst_table, tostring, ???]
        if (quotes) {
            luaL_addchar(&b, '"');
        }
    }
    luaL_addstring(&b, ")");
    luaL_pushresult(&b);
    return 1;
}

static int Lnexti(lua_State *L)
{
    if (! lua_getmetatable(L, 1)) {
        luaL_error(L, "invalid type");
    }
    // Stack: [mt]
    lua_getfield(L, -1, "__instances");
    // Stack: [mt, buckets]
    immutable_info_t *info = (immutable_info_t*)lua_touserdata(L, 1);
    if (! info) {
        luaL_error(L, "invalid type");
    }
    int i = lua_isnil(L, 2) ? 1 : lua_tointeger(L, 2)+1;
    if (i > (int)info->len) {
        return 0;
    }
    lua_rawgeti(L, -1, info->hash);
    // Stack: [mt, buckets, bucket]
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find hash bucket");
    }
    lua_pushvalue(L, 1);
    // Stack: [mt, buckets, bucket, inst_udata]
    lua_rawget(L, -2);
    // Stack: [mt, buckets, bucket, inst_table]
    lua_pushinteger(L, i);
    // Stack: [mt, buckets, bucket, inst_table, i]
    lua_getfield(L, -5, "__fields");
    // Stack: [mt, buckets, bucket, inst_table, i, __fields]
    i += lua_objlen(L, -1);
    lua_pop(L, 1);
    lua_rawgeti(L, -2, i);
    // Stack: [mt, buckets, bucket, inst_table, i, table[i]]
    return 2;
}

static int Lipairs(lua_State *L)
{
    lua_pushcfunction(L, Lnexti);
    // Stack: [Lnexti]
    lua_pushvalue(L, 1);
    // Stack: [Lnexti, inst_udata]
    lua_pushnil(L);
    // Stack: [Lnexti, inst_udata, nil]
    return 3;
}

static int Lnext(lua_State *L)
{
    if (! lua_getmetatable(L, 1)) {
        luaL_error(L, "invalid type");
    }
    int cls_index = lua_gettop(L);
    // Stack: [mt]
    lua_getfield(L, cls_index, "__fields");
    int fields_index = lua_gettop(L);
    lua_getfield(L, cls_index, "__indices");
    int indices_index = lua_gettop(L);
    lua_getfield(L, cls_index, "__instances");
    // Stack: [..., buckets]
    immutable_info_t *info = (immutable_info_t*)lua_touserdata(L, 1);
    if (! info) {
        luaL_error(L, "invalid type");
    }
    lua_rawgeti(L, -1, info->hash);
    // Stack: [..., buckets, bucket]
    if (lua_isnil(L, -1)) {
        luaL_error(L, "Failed to find hash bucket");
    }
    lua_pushvalue(L, 1);
    // Stack: [..., buckets, bucket, inst_udata]
    lua_rawget(L, -2);
    // Stack: [..., buckets, bucket, inst_table]
    size_t num_fields = lua_objlen(L, fields_index);
    int index;
    if (lua_isnil(L, 2)) {
        index = 1;
    } else {
        lua_pushvalue(L, 2);
        // Stack: [..., buckets, bucket, inst_table, k]
        lua_gettable(L, indices_index);
        // Stack: [..., buckets, bucket, inst_table, i]
        if (! lua_isnil(L, -1)) {
            index = lua_tointeger(L, -1) + 1;
        } else {
            index = lua_tointeger(L, 2) + num_fields + 1;
        }
        lua_pop(L, 1);
    }
    // Stack: [..., buckets, bucket, inst_table]
    if (! (0 < index && index <= (int)info->len)) {
        return 0;
    }

    // Now find field name
    if (index <= (int)num_fields) {
        lua_rawgeti(L, fields_index, index);
    } else {
        lua_pushinteger(L, index - num_fields);
    }
    // Stack: [..., buckets, bucket, inst_table, k2]
    lua_rawgeti(L, -2, index);
    // Stack: [..., buckets, bucket, inst_table, k2, value2]
    return 2;
}

static int Lpairs(lua_State *L)
{
    lua_pushcfunction(L, Lnext);
    // Stack: [Lnext]
    lua_pushvalue(L, 1);
    // Stack: [Lnext, inst_udata]
    lua_pushnil(L);
    // Stack: [Lnext, inst_udata, nil]
    return 3;
}

static int Lhash(lua_State *L)
{
    luaL_checktype(L, 1, LUA_TUSERDATA);
    immutable_info_t *info = (immutable_info_t *)lua_touserdata(L, 1);
    lua_pushinteger(L, info->hash);
    return 1;
}

static const luaL_Reg Rinstance_metamethods[] =
{
    {"__len", Llen},
    {"__rawlen", Llen},
    {"__index", Lindex},
    {"__rawindex", Linst_index},
    {"__tostring", Ltostring},
    {"__rawtostring", Ltostring},
    {"__ipairs", Lipairs},
    {"__rawipairs", Lipairs},
    {"__pairs", Lpairs},
    {"__rawpairs", Lpairs},
    {"__hash", Lhash},
    {"from_table", Lfrom_table},
    {"is_instance", Lis_instance},
    {NULL, NULL}
};

static int Lmake_class(lua_State *L)
{
    size_t num_args = lua_gettop(L);
    // immutable([fields], [methods/metamethods])
    lua_createtable(L, 0, 24); // Rough guess of number of fields from Rinstance_metamethods + __fields, etc.
    // Stack: [CLS]
    // Populate CLS.__len, CLS.__index, CLS.__pairs, etc.
    luaL_register(L,NULL,Rinstance_metamethods);

    // If methods were passed in, copy them over, overwriting defaults if desired
    if (num_args >= 2 && lua_type(L, 2) == LUA_TTABLE) {
        // Stack: [CLS]
        lua_pushnil(L);
        // Stack: [CLS, nil]
        while (lua_next(L, 2) != 0) {
            // Stack: [CLS, method_name, method_value]
            lua_pushvalue(L, -2);
            // Stack: [CLS, method_name, method_value, method_name]
            lua_insert(L, -2);
            // Stack: [CLS, method_name, method_name, method_value]
            lua_settable(L, -4);
            // Stack: [CLS, method_name]
        }
        // Stack: [CLS]
    }

    // Stack: [CLS]
    lua_createtable(L, 0, 32); // Rough guess: at least 32 instances concurrently
    // Stack: [CLS, __instances]
    lua_pushlightuserdata(L, (void*)&WEAK_VALUE_METATABLE);
    lua_gettable(L, LUA_REGISTRYINDEX);
    // Stack: [CLS, __instances, {__mode='v'}]
    lua_setmetatable(L, -2);
    // Stack: [CLS, __instances]
    lua_setfield(L, -2, "__instances");

    // Stack: [CLS]
    lua_createtable(L, 0, 32); // Rough guess: at least 32 instances concurrently
    // Stack: [CLS, __buckets]
    lua_pushlightuserdata(L, (void*)&WEAK_KEY_METATABLE);
    lua_gettable(L, LUA_REGISTRYINDEX);
    // Stack: [CLS, __buckets, {__mode='k'}]
    lua_setmetatable(L, -2);
    // Stack: [CLS, __buckets]
    lua_setfield(L, -2, "__buckets");

    // Stack: [CLS]
    switch (num_args == 0 ? LUA_TNIL : lua_type(L, 1)) {
        case LUA_TTABLE: {
            // CLS.__fields = arg1
            lua_pushvalue(L, 1);
            // Stack: [CLS, __fields]
            lua_setfield(L, -2, "__fields");
            // Stack: [CLS]

            size_t n = lua_objlen(L, 1);
            lua_createtable(L, 0, n);
            // Stack: [CLS, __indices]
            lua_pushnil(L);
            while (lua_next(L, 1) != 0) {
                // Stack: [CLS, __indices, i, fieldname]
                lua_pushvalue(L, -2);
                // Stack: [CLS, __indices, i, fieldname, i]
                lua_settable(L, -4);
                // Stack: [CLS, __indices, i]
            }
            lua_setfield(L, -2, "__indices");
            break;
        }
        case LUA_TNIL: case LUA_TNONE: {
            // If no fields were passed in, set __fields and __indices to empty tables
            lua_createtable(L, 0, 0);
            lua_setfield(L, -2, "__fields");
            lua_createtable(L, 0, 0);
            lua_setfield(L, -2, "__indices");
            break;
        }
        default: {
            luaL_error(L, "immutable expected the fields to be either table or nil");
        }
    }
    // Stack: [CLS]
    lua_pushlightuserdata(L, (void*)&SHARED_CLASS_METATABLE);
    lua_gettable(L, LUA_REGISTRYINDEX);
    lua_setmetatable(L, -2);
    // Stack [CLS]

    return 1;
}

static int Lclass_tostring(lua_State *L)
{
    lua_getfield(L, 1, "name");
    if (lua_isnil(L, -1)) {
        lua_pushfstring(L, "immutable type: %p", lua_topointer(L, 1));
    }
    return 1;
}

static const luaL_Reg Rclass_metamethods[] =
{
    { "__call", Lcreate_instance},
    { "__tostring", Lclass_tostring},
    { NULL, NULL}
};

LUALIB_API int luaopen_immutable(lua_State *L)
{
    lua_pushlightuserdata(L, (void*)&WEAK_VALUE_METATABLE);
    lua_createtable(L, 0, 1);
    lua_pushstring(L, "v");
    lua_setfield(L, -2, "__mode");
    lua_settable(L, LUA_REGISTRYINDEX);

    lua_pushlightuserdata(L, (void*)&WEAK_KEY_METATABLE);
    lua_createtable(L, 0, 1);
    lua_pushstring(L, "k");
    lua_setfield(L, -2, "__mode");
    lua_settable(L, LUA_REGISTRYINDEX);

    lua_pushlightuserdata(L, (void*)&SHARED_CLASS_METATABLE);
    lua_createtable(L, 0, 2);
    luaL_register(L,NULL,Rclass_metamethods);
    lua_settable(L, LUA_REGISTRYINDEX);

    lua_pushcfunction(L, Lmake_class);
    return 1;
}