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Loyc.Collections.ICharSource Interface Reference

A read-only list of characters plus a Slice(int,int) method. More...


Source file:
Inheritance diagram for Loyc.Collections.ICharSource:
Loyc.Collections.IListSource< char > Loyc.Collections.StringSlice Loyc.Syntax.StreamCharSource Loyc.UString

Remarks

A read-only list of characters plus a Slice(int,int) method.

To get an instance of this interface from a string, write new StringSlice("string") or (UString)"string".

This is the standard interface for lexers to use as a source of characters; is it defined in Loyc.Essentials rather than Loyc.Syntax so that UString and StringSlice can implement it.

This interface was created to read characters more efficiently. Although a lexer could read characters one-at-a-time from IReadOnlyList{char} or IListSource{char}, it requires dynamic interface dispatch for every character. On the other hand, if lexers avoid this overhead by requiring the entire file in the form of a string, it becomes necessary to hold the entire file in memory at once, in a very specific format (a string).

Slice() allows the lexer to request small pieces of the file that it can read without dynamic dispatch. Typically a lexer will be derived from Loyc.Syntax.Lexing.BaseLexer, which requests rather small chunks; the ICharSource implementation is is free to read larger blocks at once, since the return type, StringSlice, can be a slice of a larger string.

This interface provides good efficiency when reading from strings, or from data structures composed of large strings (most notably, this interface could efficiently return sections of a gap buffer), but unlike String itself, it is flexible, and does not require the entire file to be held in memory as a single contiguous block.

It's unfortunate that .NET treats strings as something completely different than arrays. Otherwise, this interface could support not just substrings, but subarrays of any element type, which would be useful any time you want to optimize your code by reducing dynamic dispatch.

Note about Count: if ICharSource represents to a file or other Stream, reading Count forces the entire stream to be scanned in order to determine the number of characters in the file (which may be different from the number of bytes). Rather than do a test like if (index >= charSource.Count) it is better to use if (Slice(index, 1).Count == 0), or better yet, TryGet(index, out fail).

Public Member Functions

new StringSlice Slice (int startIndex, int length)
 Returns a substring from the character source. If some of the requested characters are past the end of the stream, the string is truncated to the available number of characters. More...
 
- Public Member Functions inherited from Loyc.Collections.IListSource< char >
TryGet (int index, out bool fail)
 Gets the item at the specified index, and does not throw an exception on failure. More...
 
IRange< T > Slice (int start, int count=int.MaxValue)
 Returns a sub-range of this list. More...
 

Member Function Documentation

new StringSlice Loyc.Collections.ICharSource.Slice ( int  startIndex,
int  length 
)

Returns a substring from the character source. If some of the requested characters are past the end of the stream, the string is truncated to the available number of characters.

Parameters
startIndexIndex of first character to return. If startIndex >= Count, an empty string is returned.
lengthNumber of characters desired.
Exceptions
ArgumentExceptionThrown if startIndex or length are negative.

Implemented in Loyc.UString, Loyc.UString, Loyc.Collections.StringSlice, Loyc.Collections.StringSlice, and Loyc.Syntax.StreamCharSource.

Referenced by Loyc.Syntax.Lexing.Token.SourceText().