better error messages for checkcolor (#1211)

src/api/renderer.c

@@ -2,6 +2,7 @@
 #include "api.h"
 #include "../renderer.h"
 #include "../rencache.h"
+#include "lua.h"
 
 // a reference index to a table that stores the fonts
 static int RENDERER_FONT_REF = LUA_NOREF;
@@ -215,20 +216,47 @@ static int f_font_set_size(lua_State *L) {
   return 0;
 }
 
+static int color_value_error(lua_State *L, int idx, int table_idx) {
+  const char *type, *msg;
+  // generate an appropriate error message
+  if (luaL_getmetafield(L, -1, "__name") == LUA_TSTRING) {
+    type = lua_tostring(L, -1); // metatable name
+  } else if (lua_type(L, -1) == LUA_TLIGHTUSERDATA) {
+    type = "light userdata"; // special name for light userdata
+  } else {
+    type = lua_typename(L, lua_type(L, -1)); // default name
+  }
+  // the reason it went through so much hoops is to generate the correct error
+  // message (with function name and proper index).
+  msg = lua_pushfstring(L, "table[%d]: %s expected, got %s", table_idx, lua_typename(L, LUA_TNUMBER), type);
+  return luaL_argerror(L, idx, msg);
+}
+
+static int get_color_value(lua_State *L, int idx, int table_idx) {
+  lua_rawgeti(L, idx, table_idx);
+  return lua_isnumber(L, -1) ? lua_tonumber(L, -1) : color_value_error(L, idx, table_idx);
+}
+
+static int get_color_value_opt(lua_State *L, int idx, int table_idx, int default_value) {
+  lua_rawgeti(L, idx, table_idx);
+  if (lua_isnoneornil(L, -1))
+    return default_value;
+  else if (lua_isnumber(L, -1))
+    return lua_tonumber(L, -1);
+  else
+    return color_value_error(L, idx, table_idx);
+}
+
 static RenColor checkcolor(lua_State *L, int idx, int def) {
   RenColor color;
   if (lua_isnoneornil(L, idx)) {
     return (RenColor) { def, def, def, 255 };
   }
   luaL_checktype(L, idx, LUA_TTABLE);
-  lua_rawgeti(L, idx, 1);
+  color.r = get_color_value(L, idx, 1);
-  lua_rawgeti(L, idx, 2);
+  color.g = get_color_value(L, idx, 2);
-  lua_rawgeti(L, idx, 3);
+  color.b = get_color_value(L, idx, 3);
-  lua_rawgeti(L, idx, 4);
+  color.a = get_color_value_opt(L, idx, 4, 255);
-  color.r = luaL_checknumber(L, -4);
-  color.g = luaL_checknumber(L, -3);
-  color.b = luaL_checknumber(L, -2);
-  color.a = luaL_optnumber(L, -1, 255);
   lua_pop(L, 4);
   return color;
 }