From 21d8c27e73a723e55b5659530d22cde1ddc804f1 Mon Sep 17 00:00:00 2001
From: Bruce Hill <bitbucket@bruce-hill.com>
Date: Sat, 10 Feb 2018 16:37:26 -0800
Subject: [PATCH] Improving hash.

---
 limmutable.c | 45 +++++++++++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 14 deletions(-)

diff --git a/limmutable.c b/limmutable.c
index 02210bd..1bb7f42 100644
--- a/limmutable.c
+++ b/limmutable.c
@@ -24,6 +24,10 @@
 #define luaL_register(L, _, R) luaL_setfuncs(L, R, 0)
 #endif
 
+#if !defined(LUAI_HASHLIMIT)
+#define LUAI_HASHLIMIT		5
+#endif
+
 static int Lcreate_instance(lua_State *L)
 {
     int n_args = lua_gettop(L);
@@ -47,20 +51,22 @@ static int Lcreate_instance(lua_State *L)
     // Stack [inst]
     
     // Copy in all the values, and simultaneously compute the hash:
-    lua_Integer hash = 0;
+    lua_Integer hash = (lua_Integer)lua_topointer(L, 1); // Hash depends on the metatable used in creation
     for (lua_Integer i=1; i <=(lua_Integer)n; i++) {
         lua_pushvalue(L, i+1);
         // Stack [inst, args[i+1]]
         lua_Integer item_hash;
         switch (lua_type(L, -1)) {
             case LUA_TNIL:
-                item_hash = 0;
+                // Arbitrarily chosen value
+                item_hash = 0x97167da9;
                 break;
             case LUA_TNUMBER:
                 item_hash = (lua_Integer)lua_tonumber(L, -1);
                 break;
             case LUA_TBOOLEAN:
-                item_hash = (lua_Integer)lua_toboolean(L, -1);
+                // Arbitrarily chosen values
+                item_hash = lua_toboolean(L, -1)? 0x82684f71 : 0x88d66f2a;
                 break;
             case LUA_TTABLE:
             case LUA_TFUNCTION:
@@ -71,12 +77,13 @@ static int Lcreate_instance(lua_State *L)
                 break;
             case LUA_TSTRING:
                 {
-                    size_t strlen;
-                    const char *str = lua_tolstring(L, -1, &strlen);
-                    item_hash = *str << 7;
-                    for (const char *end = &str[strlen]; str < end; str++)
-                        item_hash = (1000003*item_hash) ^ *str;
-                    item_hash ^= strlen;
+                    // 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:
@@ -196,7 +203,9 @@ static int Lfrom_table(lua_State *L)
 
 static int Llen(lua_State *L)
 {
-    lua_getmetatable(L, 1);
+    if (! lua_getmetatable(L, 1)) {
+        luaL_error(L, "invalid type");
+    }
     // Stack: [mt]
     lua_getfield(L, -1, "__fields");
     // Stack: [mt, fields]
@@ -206,7 +215,9 @@ static int Llen(lua_State *L)
 
 static int Lindex(lua_State *L)
 {
-    lua_getmetatable(L, 1);
+    if (! lua_getmetatable(L, 1)) {
+        luaL_error(L, "invalid type");
+    }
     // Stack: [mt]
     lua_getfield(L, -1, "__indices");
     // Stack: [mt, indices]
@@ -242,7 +253,9 @@ static int Ltostring(lua_State *L)
     luaL_Buffer b;
     luaL_buffinit(L, &b);
 
-    lua_getmetatable(L, 1);
+    if (! lua_getmetatable(L, 1)) {
+        luaL_error(L, "invalid type");
+    }
     // Stack: [mt]
     
     lua_getfield(L, -1, "name");
@@ -298,7 +311,9 @@ static int Ltostring(lua_State *L)
 
 static int Lnexti(lua_State *L)
 {
-    lua_getmetatable(L, 1);
+    if (! lua_getmetatable(L, 1)) {
+        luaL_error(L, "invalid type");
+    }
     // Stack: [mt]
     lua_getfield(L, -1, "__instances");
     // Stack: [mt, buckets]
@@ -336,7 +351,9 @@ static int Lipairs(lua_State *L)
 
 static int Lnext(lua_State *L)
 {
-    lua_getmetatable(L, 1);
+    if (! lua_getmetatable(L, 1)) {
+        luaL_error(L, "invalid type");
+    }
     // Stack: [mt]
     lua_getfield(L, -1, "__instances");
     // Stack: [mt, buckets]