Optimize String length computation. #1685
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JAVA-5842
| /** | ||
| * Detects the position of the first NULL (0x00) byte in a 64-bit word using SWAR technique. | ||
| * <a href="https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord"> | ||
| */ | ||
| private int computeCStringLength(final int prevPos) { | ||
| ensureOpen(); |
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Can we remove this check, as it's a private method and duplicates a check from the public method that calls it?
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| result: | ||
| 00000000 00000000 10000000 00000000 00000000 00000000 00000000 00000000 | ||
| ^^^^^^^^ |
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| ^^^^^^^^ | |
| ^^^^^^^^ |
| result: | ||
| 00000000 00000000 10000000 00000000 00000000 00000000 00000000 00000000 | ||
| ^^^^^^^^ | ||
| The high bit is set only at the 0x00 byte position. |
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| The high bit is set only at the 0x00 byte position. | |
| The high bit is set only at the 0x00 byte position. |
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Not sure if you meant to center this...
| @@ -182,11 +182,58 @@ public void skipCString() { | |||
| buffer.position(pos + length); | |||
| } | |||
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| /** | |||
| * Detects the position of the first NULL (0x00) byte in a 64-bit word using SWAR technique. | |||
| * <a href="https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord"> | |||
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Do you have reason to believe that this link will still be around in, say, 5 years? If not, maybe just go with Wikipedia.
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Yeah, seems Wiki URL like https://en.wikipedia.org/wiki/SWAR is better
| pos += 8; | ||
| } | ||
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| // Process remaining bytes one-by-one. |
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seems one-by-one is more commonly used as one by one
| long word = buffer.getLong(pos); | ||
| /* | ||
| Subtract 0x0101010101010101L to cause a borrow on 0x00 bytes. | ||
| if original byte is 00000000 -> 00000000 - 00000001 = 11111111 (borrow causes high bit set to 1). |
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Optional. A little bit less clear than
if original byte is 00000000, then 00000000 - 00000001 = 11111111 (borrow causes high bit set to 1).
| result: | ||
| 00000000 00000000 10000000 00000000 00000000 00000000 00000000 00000000 | ||
| ^^^^^^^^ | ||
| The high bit is set only at the 0x00 byte position. |
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Optional. the above comment seems duplicated and could be deleted
| 00000000 00000000 11111111 00000000 00000001 00000001 00000000 00000111 | ||
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| ANDing mask with 0x8080808080808080 isolates the high bit (0x80) in positions where | ||
| the original byte was 0x00, setting the high bit to 1 only at the 0x00 byte position. |
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Optional. maybe the following verbiage is little bit clearer:
, by setting the high bit to 1 only at the 0x00 byte position.
JAVA-5842