Add float and double support for struct

@edubart suggested it would be still better to have it done within C as
extra module (just like lbitlib).

"f" a float (4 bytes).
"d" a double (8 bytes).

http://en.wikipedia.org/wiki/IEEE_floating_point

modules/corelib/struct.lua

@@ -1,7 +1,7 @@
 Struct = {}
 
 function Struct.pack(format, ...)
-  local stream = ''
+  local stream = {}
   local vars = {...}
   local endianness = true
 
@@ -11,35 +11,78 @@ function Struct.pack(format, ...)
     if opt == '<' or opt == '>' then
       endianness = opt == '<' and true or false
     elseif opt == 'b' or opt == 'B' or opt == 'h' or opt == 'H' or opt == 'i' or opt == 'I' or opt == 'l' or opt == 'L' then
-      local val = tonumber(table.remove(vars, 1))
       local n = ((opt == 'h' or opt == 'H') and 2) or ((opt == 'i' or opt == 'I') and 4) or ((opt == 'l' or opt == 'L') and 8) or 1
+      local val = tonumber(table.remove(vars, 1))
 
       if val < 0 then
         val = val + 2 ^ (n * 8 - 1)
       end
 
-      local binary = ''
+      local bytes = {}
       for j = 1, n do
-        binary = binary .. string.char(val % (2 ^ 8))
+        table.insert(bytes, string.char(val % (2 ^ 8)))
         val = math.floor(val / (2 ^ 8))
       end
 
       if not endianness then
-        binary = string.reverse(binary)
+        table.insert(stream, string.reverse(table.concat(bytes)))
+      else
+        table.insert(stream, table.concat(bytes))
+      end
+    elseif opt == 'f' or opt == 'd' then
+      local val = tonumber(table.remove(vars, 1))
+      local sign = 0
+
+      if val < 0 then
+        sign = 1 
+        val = -val 
       end
 
-      stream = stream .. binary
+      local mantissa, exponent = math.frexp(val)
+      if val == 0 then
+        mantissa = 0
+        exponent = 0
+      else
+        mantissa = (mantissa * 2 - 1) * math.ldexp(0.5, (opt == 'd') and 53 or 24)
+        exponent = exponent + ((opt == 'd') and 1022 or 126)
+      end
+
+      local bytes = {}
+      if opt == 'd' then
+        val = mantissa
+        for i = 1, 6 do
+          table.insert(bytes, string.char(math.floor(val) % (2 ^ 8)))
+          val = math.floor(val / (2 ^ 8))
+        end
+      else
+        table.insert(bytes, string.char(math.floor(mantissa) % (2 ^ 8)))
+        val = math.floor(mantissa / (2 ^ 8))
+        table.insert(bytes, string.char(math.floor(val) % (2 ^ 8)))
+        val = math.floor(val / (2 ^ 8))
+      end
+
+      table.insert(bytes, string.char(math.floor(exponent * ((opt == 'd') and 16 or 128) + val) % (2 ^ 8)))
+      val = math.floor((exponent * ((opt == 'd') and 16 or 128) + val) / (2 ^ 8))
+      table.insert(bytes, string.char(math.floor(sign * 128 + val) % (2 ^ 8)))
+      val = math.floor((sign * 128 + val) / (2 ^ 8))
+
+      if not endianness then
+        table.insert(stream, string.reverse(table.concat(bytes)))
+      else
+        table.insert(stream, table.concat(bytes))
+      end
     elseif opt == 's' then
-      stream = stream .. tostring(table.remove(vars, 1))
-      stream = stream .. string.char(0)
+      table.insert(stream, tostring(table.remove(vars, 1)))
+      table.insert(stream, string.char(0))
     end
   end
 
-  return stream
+  return table.concat(stream)
 end
 
 function Struct.unpack(format, stream)
   local vars = {}
+  local iterator = 1
   local endianness = true
 
   for i = 1, string.len(format) do
@@ -53,13 +96,13 @@ function Struct.unpack(format, stream)
 
       local val = 0
       for j = 1, n do
-        local byte = string.byte(string.sub(stream, 1, 1))
+        local byte = string.byte(string.sub(stream, iterator, iterator))
         if endianness then
           val = val + byte * (2 ^ ((j - 1) * 8))
         else
           val = val + byte * (2 ^ ((n - j) * 8))
         end
-        stream = string.sub(stream, 2)
+        iterator = iterator + 1
       end
 
       if signed then
@@ -67,17 +110,44 @@ function Struct.unpack(format, stream)
       end
 
       table.insert(vars, val)
+    elseif opt == 'f' or opt == 'd' then
+      local n = (opt == 'd') and 8 or 4
+      local x = string.sub(stream, iterator, iterator + n - 1)
+      iterator = iterator + n
+
+      if not endianness then
+        x = string.reverse(x)
+      end
+
+      local sign = 1
+      local mantissa = string.byte(x, (opt == 'd') and 7 or 3) % ((opt == 'd') and 16 or 128)
+      for i = n - 2, 1, -1 do
+        mantissa = mantissa * (2 ^ 8) + string.byte(x, i)
+      end
+
+      if string.byte(x, n) > 127 then
+        sign = -1
+      end
+
+      local exponent = (string.byte(x, n) % 128) * ((opt == 'd') and 16 or 2) + math.floor(string.byte(x, n - 1) / ((opt == 'd') and 16 or 128))
+      if exponent == 0 then
+        table.insert(vars, 0.0)
+      else
+        mantissa = (math.ldexp(mantissa, (opt == 'd') and -52 or -23) + 1) * sign
+        table.insert(vars, math.ldexp(mantissa, exponent - ((opt == 'd') and 1023 or 127)))
+      end
     elseif opt == 's' then
-      local str = ''
-      for j = 1, string.len(stream) do
+      local bytes = {}
+      for j = 1 + iterator, string.len(stream) do
         if string.sub(stream, j, j) == string.char(0) then
           break
         end
 
-        str = str .. string.sub(stream, j, j)
+        table.insert(bytes, string.sub(stream, j, j))
       end
 
-      stream = string.sub(stream, string.len(str) + 2)
+      local str = table.concat(bytes)
+      iterator = iterator + string.len(str) + 1
       table.insert(vars, str)
     end
   end