+ * Stream flags may be used to describe characteristics of several different entities associated with streams: + * stream sources, intermediate operations, and terminal operations. Not all stream flags are meaningful for all + * entities; the following table summarizes which flags are meaningful in what contexts: + *
+ * DISTINCT SORTED ORDERED SIZED SHORT_CIRCUIT PARALLEL + * Stream source Y Y Y Y N Y + * Intermediate operation PCI PCI PCI PC PI PC + * Terminal operation N N PC N PI N + *+ * + *
In the above table, "PCI" means "may preserve, clear, or inject"; "PC" means "may preserve or clear", + * "PI" means "may preserve or inject", and "N" means "not valid". + * + *
Stream flags are represented by unioned bit sets, so that a single word may describe all the + * characteristics of a given stream entity, and that, for example, the flags for a stream source can be + * efficiently combined with the flags for later operations on that stream. + * + *
The bit masks {@link #STREAM_MASK}, {@link #OP_MASK}, and {@link #TERMINAL_OP_MASK} can be ANDed with + * a bit set of stream flags to produce a mask containing only the valid flags for that entity type. + * + *
When describing a stream source, one only need describe what characteristics that stream has; when describing + * a stream operation, one need describe whether the operation preserves, injects, or clears that characteristic. + * Accordingly, two bits are used for each flag, so as to allow representing not only the presence of of a + * characteristic, but how an operation modifies that characteristic. There are two common forms in which + * flag bits are combined into an {@code int} bit set. Stream flags are a unioned bit set constructed by + * ORing the enum characteristic values of {@link #set()} (or, more commonly, ORing the corresponding static named + * constants prefixed with {@code IS_}). Operation flags are a unioned bit set constructed by + * ORing the enum characteristic values of {@link #set()} or {@link #clear()} (to inject, or clear, respectively, + * the corresponding flag), or more commonly ORing the corresponding named constants prefixed with {@code IS_} + * or {@code NOT_}. Flags that are not marked with {@code IS_} or {@code NOT_} are implicitly treated as preserved. + * Care must be taken when combining bitsets that the correct combining operations are applied in the correct order. + * + *
+ * With the exception of {@link #PARALLEL}, stream characteristics can be derived from the equivalent + * {@link java.util.Spliterator} characteristics: {@link java.util.Spliterator#DISTINCT}, + * {@link java.util.Spliterator#SORTED}, {@link java.util.Spliterator#ORDERED}, and + * {@link java.util.Spliterator#SIZED}. A spliterator characteristics bit set can be converted to stream flags + * using the method {@link #fromCharacteristics(int)} and converted back using {@link #toCharacteristics(int)}. + * (The bit set {@link #SPLITERATOR_CHARACTERISTICS_MASK} is used to AND with a bit set to produce a valid + * spliterator characteristics bit set that can be converted to stream flags.) + * + *
+ * The source of a stream encapsulates a spliterator. The characteristics of that source + * spliterator when transformed to stream flags will be a proper subset of stream flags of that stream. + * For example: + *
{@code
+ * Spliterator s = ...;
+ * Stream stream = Streams.stream(s);
+ * flagsFromSpliterator = fromCharacteristics(s.characteristics());
+ * assert(flagsFromSpliterator & stream.getStreamFlags() == flagsFromSpliterator);
+ * }
+ *
+ * + * An intermediate operation, performed on an input stream to create a new output stream, + * may preserve, clear or inject stream or operation characteristics. + * Similarly, a terminal operation, performed on an input stream to produce an output result + * may preserve, clear or inject stream or operation characteristics. + * Preservation means that if that characteristic is present on the input, then it is + * also present on the output. Clearing means that the characteristic is not present on the output regardless + * of the input. Injection means that the characteristic is present on the output regardless of the input. + * If a characteristic is not cleared or injected then it is implicitly preserved. + * + *
+ * A pipeline consists of a stream source encapsulating a spliterator, one or more intermediate operations, + * and finally a terminal operation that produces a result. At each stage of the pipeline, a combined + * stream and operation flags can be calculated, using {@link #combineOpFlags(int, int)}. + * Such flags ensure that preservation, clearing and injecting information is retained at each stage. + * + * The combined stream and operation flags for the source stage of the pipeline is calculated as + * follows: + *
{@code
+ * int flagsForSourceStage = combineOpFlags(sourceFlags, INITIAL_OPS_VALUE);
+ * }
+ *
+ * The combined stream and operation flags of each subsequent intermediate operation stage in the pipeline is
+ * calculated as follows:
+ * {@code
+ * int flagsForThisStage = combineOpFlags(flagsForPreviousStage, thisOpFlags);
+ * }
+ *
+ * Finally the flags output from the last intermediate operation of the pipeline are combined with the operation flags
+ * of the terminal operation to produce the flags output from the pipeline.
+ *
+ * Those flags can then be used to apply optimizations. For example, if {@code SIZED.isKnown(flags)} returns true + * then the stream size remains constant throughout the pipeline, this information can be utilized to pre-allocate data + * structures and combined with {@link java.util.Spliterator#SUBSIZED} that information can be utilized to perform + * concurrent in-place updates into a shared array. + * + * For specific details see the {@link AbstractPipeline} constructors. + * + * @since 1.8 + */ +// @@@ When a new flag is added what should happen for existing operations? +// Need to move to a builder approach used by ops where the masks for the new flag are +// taken into account for default behaviour. +enum StreamOpFlag { + + /* + * Each characteristic takes up 2 bits in a bit set to accommodate preserving, clearing and + * setting/injecting information. + * + * This applies to stream flags, intermediate/terminal operation flags, and combined stream and + * operation flags. Even though the former only requires 1 bit of information per characteristic, + * is it more efficient when combining flags to align set and inject bits. + * + * Characteristics belong to certain types, see the Type enum. Bit masks for the types are + * constructed as per the following table: + * + * DISTINCT SORTED ORDERED SIZED SHORT_CIRCUIT PARALLEL + * SPLITERATOR 01 01 01 01 00 00 + * STREAM 01 01 01 01 00 01 + * OP 11 11 11 10 01 10 + * TERMINAL_OP 00 00 10 00 01 00 + * UPSTREAM_TERMINAL_OP 00 00 10 00 00 00 + * + * 01 = set/inject + * 10 = clear + * 11 = preserve + * + * Construction of the columns is performed using a simple builder for non-zero values. + */ + + + // The following flags correspond to characteristics on Spliterator + // and the values MUST be equal. + // + + /** + * Characteristic value signifying that, for each pair of + * encountered elements in a stream {@code x, y}, {@code !x.equals(y)}. + *
+ * A stream may have this value or an intermediate operation can preserve, clear or inject + * this value. + */ + // 0, 0x00000001 + // Matches Spliterator.DISTINCT + DISTINCT(0, + set(Type.SPLITERATOR).set(Type.STREAM).setAndClear(Type.OP)), + + /** + * Characteristic value signifying that encounter order follows a + * defined sort order. + *
+ * A stream can have this value or an intermediate operation can preserve, clear or inject + * this value. + */ + // 1, 0x00000004 + // Matches Spliterator.SORTED + SORTED(1, + set(Type.SPLITERATOR).set(Type.STREAM).setAndClear(Type.OP)), + + /** + * Characteristic value signifying that an encounter order is + * defined for stream elements. + *
+ * A stream can have this value, an intermediate operation can preserve, clear or inject + * this value, or a terminal operation can preserve or clear this value. + */ + // 2, 0x00000010 + // Matches Spliterator.ORDERED + ORDERED(2, + set(Type.SPLITERATOR).set(Type.STREAM).setAndClear(Type.OP).clear(Type.TERMINAL_OP) + .clear(Type.UPSTREAM_TERMINAL_OP)), + + /** + * Characteristic value signifying that size of the stream + * is of a known finite size that is equal to the known finite + * size of the source spliterator input to the first stream + * in the pipeline. + *
+ * A stream can have this value or an intermediate operation can preserve or clear + * this value. + */ + // 3, 0x00000040 + // Matches Spliterator.SIZED + SIZED(3, + set(Type.SPLITERATOR).set(Type.STREAM).clear(Type.OP)), + + // The following Spliterator characteristics are not currently used but a + // gap in the bit set is deliberately retained to enable corresponding stream + // flags if//when required without modification to other flag values. + // + // 4, 0x00000100 INFINITE(4, ... + // 5, 0x00000400 NONNULL(5, ... + // 6, 0x00001000 IMMUTABLE(6, ... + // 7, 0x00004000 CONCURRENT(7, ... + // 8, 0x00010000 SUBSIZED(8, ...) + + // The following 3 flags are currently undefined and a free for any further + // spliterator characteristics. + // + // 9, 0x00040000 + // 10, 0x00100000 + // 11, 0x00400000 + + // The following flags are specific to streams and operations + // + + /** + * Characteristic value signifying that an operation may short-circuit the stream. + *
+ * An intermediate operation can preserve or inject this value, + * or a terminal operation can preserve or inject this value. + */ + // 12, 0x01000000 + SHORT_CIRCUIT(12, + set(Type.OP).set(Type.TERMINAL_OP)), + + + /** + * Characteristic value signifying that the stream is to be evaluated in parallel rather than + * sequentially. + *
+ * A stream can have this value or an intermediate operation can preserve or clear
+ * this value.
+ */
+ // 13, 0x04000000
+ PARALLEL(13,
+ set(Type.STREAM).clear(Type.OP));
+
+ // The following 2 flags are currently undefined and a free for any further
+ // stream flags if/when required
+ //
+ // 14, 0x10000000
+ // 15, 0x40000000
+
+ /**
+ * Type of a flag
+ */
+ enum Type {
+ /** The flag is associated with spliterator characteristics. */
+ SPLITERATOR,
+
+ /** The flag is associated with stream flags. */
+ STREAM,
+
+ /** The flag is associated with intermediate operation flags. */
+ OP,
+
+ /** The flag is associated with terminal operation flags. */
+ TERMINAL_OP,
+
+ /**
+ * The flag is associated with terminal operation flags that are propagated upstream
+ * across the last stateful operation boundary
+ */
+ UPSTREAM_TERMINAL_OP
+ }
+
+ /**
+ * The bit pattern for setting/injecting a flag.
+ */
+ private static final int SET_BITS = 0b01;
+
+ /**
+ * The bit pattern for clearing a flag.
+ */
+ private static final int CLEAR_BITS = 0b10;
+
+ /**
+ * The bit pattern for preserving a flag.
+ */
+ private static final int PRESERVE_BITS = 0b11;
+
+ private static MaskBuilder set(Type t) {
+ return new MaskBuilder(new EnumMap<>(Type.class)).set(t);
+ }
+
+ private static class MaskBuilder {
+ final Map
+ * A flag set on stream flags or injected on operation flags,
+ * and injected combined stream and operation flags,
+ * will be injected on the updated combined stream and operation flags.
+ *
+ * A flag set on stream flags or injected on operation flags,
+ * and cleared on the combined stream and operation flags,
+ * will be cleared on the updated combined stream and operation flags.
+ *
+ * A flag set on the stream flags or injected on operation flags,
+ * and preserved on the combined stream and operation flags,
+ * will be injected on the updated combined stream and operation flags.
+ *
+ * A flag not set on the stream flags or cleared/preserved on operation flags,
+ * and injected on the combined stream and operation flags,
+ * will be injected on the updated combined stream and operation flags.
+ *
+ * A flag not set on the stream flags or cleared/preserved on operation flags,
+ * and cleared on the combined stream and operation flags,
+ * will be cleared on the updated combined stream and operation flags.
+ *
+ * A flag not set on the stream flags,
+ * and preserved on the combined stream and operation flags
+ * will be preserved on the updated combined stream and operation flags.
+ *
+ * A flag cleared on operation flags,
+ * and preserved on the combined stream and operation flags
+ * will be cleared on the updated combined stream and operation flags.
+ *
+ * A flag preserved on operation flags,
+ * and preserved on the combined stream and operation flags
+ * will be preserved on the updated combined stream and operation flags.
+ * Each flag injected on the combined stream and operation flags will be set on the stream flags.
+ *
+ * @param combOpFlags the combined stream and operation flags.
+ * @return the stream flags.
+ */
+ static int toStreamFlags(int combOpFlags) {
+ // By flipping the nibbles 0x11 become 0x00 and 0x01 become 0x10
+ // Shift left 1 to restore set flags and mask off anything other than the set flags
+ return ((~combOpFlags) >> 1) & FLAG_MASK_IS & combOpFlags;
+ }
+
+ /**
+ * Convert stream flags to a spliterator characteristic bit set.
+ *
+ * @param streamFlags the stream flags.
+ * @return the spliterator characteristic bit set.
+ */
+ static int toCharacteristics(int streamFlags) {
+ return streamFlags & SPLITERATOR_CHARACTERISTICS_MASK;
+ }
+
+ /**
+ * Convert a spliterator characteristic bit set to stream flags.
+ *
+ * @param characteristics the spliterator characteristic bit set.
+ * @return the stream flags.
+ */
+ static int fromCharacteristics(int characteristics) {
+ return characteristics & SPLITERATOR_CHARACTERISTICS_MASK;
+ }
+}