ICharStream Interface

The index of the character relative to the beginning of the line 0..n-1

Only makes sense for streams that buffer everything up probably, but might be useful to display the entire stream or for testing. This value includes a single EOF.

Return the current input symbol index 0..n where n indicates the last symbol has been read. The index is the symbol about to be read not the most recently read symbol.

ANTLR tracks the line information automatically

Where are you getting symbols from? Normally, implementations will pass the buck all the way to the lexer who can ask its input stream for the file name or whatever.

Get int at current input pointer + i ahead where i=1 is next int. Negative indexes are allowed. LA(-1) is previous token (token just matched). LA(-i) where i is before first token should yield -1, invalid char / EOF.

Get the ith character of lookahead. This is the same usually as LA(i). This will be used for labels in the generated lexer code. I'd prefer to return a char here type-wise, but it's probably better to be 32-bit clean and be consistent with LA.

Tell the stream to start buffering if it hasn't already. Return current input position, Index, or some other marker so that when passed to rewind() you get back to the same spot. rewind(mark()) should not affect the input cursor. The Lexer track line/col info as well as input index so its markers are not pure input indexes. Same for tree node streams.

You may want to commit to a backtrack but don't want to force the stream to keep bookkeeping objects around for a marker that is no longer necessary. This will have the same behavior as rewind() except it releases resources without the backward seek. This must throw away resources for all markers back to the marker argument. So if you're nested 5 levels of mark(), and then release(2) you have to release resources for depths 2..5.

Rewind to the input position of the last marker. Used currently only after a cyclic DFA and just before starting a sem/syn predicate to get the input position back to the start of the decision. Do not "pop" the marker off the state. mark(i) and rewind(i) should balance still. It is like invoking rewind(last marker) but it should not "pop" the marker off. It's like seek(last marker's input position).

Reset the stream so that next call to index would return marker. The marker will usually be Index but it doesn't have to be. It's just a marker to indicate what state the stream was in. This is essentially calling release() and seek(). If there are markers created after this marker argument, this routine must unroll them like a stack. Assume the state the stream was in when this marker was created.

Set the input cursor to the position indicated by index. This is normally used to seek ahead in the input stream. No buffering is required to do this unless you know your stream will use seek to move backwards such as when backtracking.

For infinite streams, you don't need this; primarily I'm providing a useful interface for action code. Just make sure actions don't use this on streams that don't support it.