Lua: bit.extract

manual.lyx

@@ -2141,6 +2141,28 @@ Shift bits-bit (max 48, default 48) number arithmetically right by amount
  The new bits are shifted in with copy of the high bit.
 \end_layout
 
+\begin_layout Subsubsection
+bit.extract(number base[, number bit0[, number bit1,...]])
+\end_layout
+
+\begin_layout Standard
+Returns number that has bit0-th bit as bit 0, bit1-th bit as 1 and so on.
+\end_layout
+
+\begin_layout Standard
+Notes: 
+\end_layout
+
+\begin_layout Itemize
+Bit numbers up to 51 should work reliably (then things start falling apart
+ due to double precision issues).
+\end_layout
+
+\begin_layout Itemize
+There are two special bit positions, true and false, standing for always
+ set bit and always clear bit.
+\end_layout
+
 \begin_layout Subsection
 Table gui:
 \end_layout

manual.txt

@@ -1041,6 +1041,19 @@ Shift bits-bit (max 48, default 48) number arithmetically right
 by amount (default 1) places. The new bits are shifted in with 
 copy of the high bit.
 
+8.2.10 bit.extract(number base[, number bit0[, number bit1,...]])
+
+Returns number that has bit0-th bit as bit 0, bit1-th bit as 1 
+and so on.
+
+Notes: 
+
+• Bit numbers up to 51 should work reliably (then things start 
+  falling apart due to double precision issues).
+
+• There are two special bit positions, true and false, standing 
+  for always set bit and always clear bit.
+
 8.3 Table gui:
 
 Most of these functions can only be called in on_paint and 

src/lua/bit.cpp

@@ -102,6 +102,23 @@ namespace
 		}
 	};
 
+	function_ptr_luafun lua_print("bit.extract", [](lua_State* LS, const std::string& fname) -> int {
+		uint64_t num = get_numeric_argument<uint64_t>(LS, 1, fname.c_str());
+		uint64_t ret = 0;
+		for(size_t i = 0;; i++) {
+			if(lua_isnumber(LS, i + 2)) {
+				uint8_t bit = get_numeric_argument<uint8_t>(LS, i + 2, fname.c_str());
+				ret |= (((num >> bit) & 1) << i);
+			} else if(lua_isboolean(LS, i + 2)) {
+				if(lua_toboolean(LS, i + 2))
+					ret |= (1ULL << i);
+			} else
+				break;
+		}
+		lua_pushnumber(LS, ret);
+		return 1;
+	});
+
 	lua_symmetric_bitwise<combine_none, BITWISE_MASK> bit_none("bit.none");
 	lua_symmetric_bitwise<combine_none, BITWISE_MASK> bit_bnot("bit.bnot");
 	lua_symmetric_bitwise<combine_any, 0> bit_any("bit.any");