原创 log2(x) number of bits needed to encode x choices

function log2(x : natural) return integer;

-------------------------------------------------------------------------------
-- Function log2 -- returns number of bits needed to encode x choices
--   x = 0  returns 0
--   x = 1  returns 0
--   x = 2  returns 1
--   x = 4  returns 2, etc.
-------------------------------------------------------------------------------
--
function log2(x : natural) return integer is
  variable i  : integer := 0;
  variable val: integer := 1;
begin
  if x = 0 then return 0;
  else
    for j in 0 to 29 loop -- for loop for XST
      if val >= x then null;
      else
        i := i+1;
        val := val*2;
      end if;
    end loop;
    assert val >= x
      report "Function log2 received argument larger than its capability of 2^30. "
      severity failure;
    return i;
  end if; 
end function log2;

--- find minimum number of bits required to
--- represent N as an unsigned binary number
---
function log2_ceil(N: natural) return positive is
begin
 if N < 2 then
  return 1;
 else
  return 1 + log2_ceil(N/2);
 end if;
end;

function log2_ceil_x(N : integer) return integer is
begin
 if (N <= 2) then
  return 1;
 else
  if (N mod 2 = 0) then
   return 1 + log2_ceil(N/2);
  else
   return 1 + log2_ceil((N+1)/2);
  end if;
 end if;
end function log2_ceil_x;

  function log2_floor (x : positive) return natural is
  begin
    if x <= 1 then
      return 0;
    else
      return log2_floor (x / 2) + 1;
    end if;
  end log2_floor;

  function log2_ceil (x : positive) return natural is
  begin
    if x <= 1 then
      return 0;
    else
      return log2_floor (x - 1) + 1;
    end if;
  end log2_ceil;