There are several performance optimizations:
* ByteBuffer/CharBuffer/StringBuilder objects pre-allocated and are
reused on each call to readLine.
* The state for readLine is lazily allocated via JVM classloading
(using a singleton object).
* There is an auto-detection heuristic for "directEOL" encodings which
represent LF ('\n') directly as the corresponding byte
(UTF-8 and many single-byte encodings are like that).
When "directEOL" encoding is used, then bytes are batched into
ByteBuffer for a single call to CharsetDecoder.decode which
results in higher throughput. Otherwise (UTF-16, etc), slower
byte-by-byte approach is used.
* Bytes and chars are directly moved in/out of byte/char arrays and
ByteBuffer/CharBuffer wrappers are used only to interface with
JVM CharsetDecoder class (which is the slowest piece).
* StringBuilder is not used at all for short lines (<=32 chars).
There are also some function improvements to readLine functionality:
* Restriction on "max chars per byte" is lifted, so readLine works with
all encodings that JVM supports.
* It support on-the-fly changes to system default charset, because
it rechecks current charset on each call and updates it decoder
when needed.
All the other features of readLine function are retained:
* It does not read more bytes from System.in than needed, so it
is compatible with other ways to read System.in. On-the-fly
changes to System.in are supported.
* It is thread-safe. Its internal mutable state is protected by
synchronization.
* There is an internal method for tests that supports explicit
charset specification, but the name of this method has changed.
There are additional tests:
* Check all supported encodings on JVM to make sure that readLine
works correctly with them all.
* Check unicode code points of different bits length with all standard
unicode encodings (UTF-8, UTF-16, and UTF-32 in LE/HE byte orders).
Benchmarks that compare different implementations of readLine,
including this one (readLine6NoLV in the set) can be found here:
https://github.com/elizarov/ReadLineBenchmark
Taking BufferedReader as 100% baseline we see that:
* Current readLine is 7.5 times slower than BufferedReader baseline.
* New implementation in this commit is 2.5 timer slower than baseline.
It is ~3 times faster than existing implementation of readLine.
Altogether these optimizations are enough to enable reading of
~500K lines in sports programming setting under 2s time-limit with
plenty of headroom in time. Example that is using this version of
readLine can be found here:
https://codeforces.com/contest/1322/submission/73005366
#KT-37416 Fixed
Roman Elizarov
e26a3ad033