Implements a decoder using a table-driven alphabet to produce output represented in the US-ASCII character set.

TIdDecoder4to3 = class(TIdDecoder)

TIdDecoder4to3 is a TIdDecoder descendant that implements decoding capabilities for data encoded using TIdEncoder3to4. TIdDecoder4to3 provides a decoding table to represent the valid characters in the encoder alphabet. Note: TIdDecoder4to3 does not specify the valid characters in the decoding table. Application will generally use a descendant class, like TIdDeCoderMIME or TIdDecoderUUE, where the alphabet is specified.

TIdDecoder4to3 provides methods to decode string values to as stream destination, and method to handle the low-level conversion performed when processing string input values.

Indicates a character value that can be omitted from decoded output.

property FillChar: Char;

FillChar is a Char property that Indicates a character value that can be omitted from decoded output. FillChar should correspond to the value that may have been added by TIdEncoder3to4 the encoded output to force to the length required by TIdEncoder3to4.Encode.

Initializes the coding table used by the decoder.

class procedure ConstructDecodeTable(const ACodingTable: string; var ADecodeArray: TIdDecodeTable);

String containing the initial coder alphabet.

Table used by the decoder to detect valid characters and ordinal position.

ConstructDecodeTable is a Class procedure that initializes the CodingTable used by TIdDecoder4to3. All elements in ADecoderArray are initialize to decimal 255, and the valid characters in ACodingTable are overlaid to indicate their ordinal position in the coder alphabet.

ADecodeArray is used by descendant classes, like TIdDecoderMIME and TIdDecoderUUE, to initialize their DecodeTable properties based on the alphabet for each specific coder.

Implements the TIdDecoder4to3 decoding algorithm for decoding String values to a Stream.

procedure DecodeToStream(AIn: string; ADest: TStream); override;

Value to be decoded.

Stream that is the destination for the decoded value.

DecodeToStream is an overridden procedure in TIdDecoder4to3 that implements decoding the String value in AIn to the TStream destination in ADest. DecodeToStream implements a decoding algorithm that converts the 4-byte representation of encoded data to it's 3-byte un-encoded representation.

DecodeToStream can raise an EIdException with the constant message RSUnevenSizeInDecodeStream when the length of the encoded data in AIn is not a multiple of 4.

DecodeToStream reads 4 bytes of data from AIn and calls DecodeUnit until all contents in the encoded data have been handled. An individual byte in the 3-byte un-encoded value and it's preceding byte will be omitted when the byte value is equal to FillChar.

Performs the specifics of the TIdDecoder4to3 decoding algorithm.

procedure DecodeUnit(AIn: Cardinal; var VOut1: Byte; var VOut2: Byte; var VOut3: Byte);

Cardinal value of the encoded data.

value of Byte 1 in the un-encoded data.

value of Byte 2 in the un-encoded data.

value of Byte 3 in the un-encoded data.

DecodeUnit is a procedure that implements the specific processing required to perform the TIdDecoder4to3 decoding algorithm. AIn is the Cardinal value of the encoded data. DecodeUnit create an instance of TIdCardinalBytes used to concatenate the first 6-bits of each byte into their un-encoded representation as 3 Bytes values in VOut1, VOut2, and VOut3.