Module: BinData::BitField

Defined in:
lib/bindata/bits.rb

Overview

Defines a number of classes that contain a bit based integer. The integer is defined by endian and number of bits.

Class Method Summary collapse

Class Method Details

.create_clamp_code(nbits, signed) ⇒ Object 



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# File 'lib/bindata/bits.rb', line 55

def create_clamp_code(nbits, signed)
  if nbits == 1 and signed == :signed
    raise "signed bitfield must have more than one bit" 
  end

  if signed == :signed
    max = (1 << (nbits - 1)) - 1
    min = -(max + 1)
  else
    min = 0
    max = (1 << nbits) - 1
  end

  clamp = "(val < #{min}) ? #{min} : (val > #{max}) ? #{max} : val"

  if nbits == 1
    # allow single bits to be used as booleans
    clamp = "(val == true) ? 1 : (not val) ? 0 : #{clamp}"
  end

  "val = #{clamp}"
end

.create_int2uint_code(nbits) ⇒ Object 



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# File 'lib/bindata/bits.rb', line 78

def create_int2uint_code(nbits)
  "val = val & #{(1 << nbits) - 1}"
end

.create_uint2int_code(nbits) ⇒ Object 



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# File 'lib/bindata/bits.rb', line 82

def create_uint2int_code(nbits)
  "val = val - #{1 << nbits} if (val >= #{1 << (nbits - 1)})"
end

.define_class(nbits, endian, signed = :unsigned) ⇒ Object 



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# File 'lib/bindata/bits.rb', line 9

def define_class(nbits, endian, signed = :unsigned)
  name = ((signed == :signed ) ? "Sbit" : "Bit") + nbits.to_s
  name << "le" if endian == :little
  unless BinData.const_defined?(name)
    BinData.module_eval <<-END
      class #{name} < BinData::BasePrimitive
        BitField.define_methods(self, #{nbits}, :#{endian}, :#{signed})
      end
    END
  end

  BinData.const_get(name)
end

.define_methods(bit_class, nbits, endian, signed) ⇒ Object 



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# File 'lib/bindata/bits.rb', line 23

def define_methods(bit_class, nbits, endian, signed)
  bit_class.module_eval <<-END
    def assign(val)
      #{create_clamp_code(nbits, signed)}
      super(val)
    end

    def do_write(io)
      val = _value
      #{create_int2uint_code(nbits) if signed == :signed}
      io.writebits(val, #{nbits}, :#{endian})
    end

    def do_num_bytes
      #{nbits / 8.0}
    end

    #---------------
    private

    def read_and_return_value(io)
      val = io.readbits(#{nbits}, :#{endian})
      #{create_uint2int_code(nbits) if signed == :signed}
      val
    end

    def sensible_default
      0
    end
  END
end