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Improve readString performance #792

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35 changes: 29 additions & 6 deletions hollow/src/main/java/com/netflix/hollow/core/memory/encoding/VarInt.java
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Original file line number Diff line number Diff line change
Expand Up @@ -243,7 +243,7 @@ public static int readVInt(byte[] arr, int position) {
* @param arr the byte data to read from
* @param position the position in the byte data to read from
* @param length the number of bytes that should be read
* @param output an array where outputs will be placed, which should be at least {@code length} long and zero.
* @param output an array where outputs will be placed, which should be at least {@code length} long.
* @return the number of values written
*/
public static int readVIntsInto(ByteData arr, long position, int length, int[] output) {
Expand All @@ -256,12 +256,24 @@ public static int readVIntsInto(ByteData arr, long position, int length, int[] o
output[i] = b;
}

// Second loop handles multi-byte encoded values using branchless logic
int count = i;
int accumulator = 0;
for(; i < length; i++) {
int b = arr.get(position + i);

output[count] = (output[count] << 7) | (b & 0x7f);
count += (~b >> 7) & 0x1;
// Accumulate the next 7 bits into the value
accumulator = (accumulator << 7) | (b & 0x7f);

// Check if this byte completes a value (continuation bit not set)
int isComplete = (~b >> 7) & 0x1; // 1 if complete, 0 if continuing
output[count] = accumulator; // Write current accumulator (may overwrite same position)
count += isComplete; // Advance output position only when value is complete

// Clear accumulator for next value using sign extension trick:
// - If b has continuation bit (b negative): b >> 7 = 0xFFFFFFFF, keeps accumulator
// - If b completes value (b positive): b >> 7 = 0, clears accumulator
accumulator &= (b >> 7);
}

return count;
Expand All @@ -272,7 +284,7 @@ public static int readVIntsInto(ByteData arr, long position, int length, int[] o
* @param arr the byte data to read from
* @param position the position in the byte data to read from
* @param length the number of bytes that should be read
* @param output an array where outputs will be placed, which should be at least {@code length} long and zero.
* @param output an array where outputs will be placed, which should be at least {@code length} long.
* @return the number of values written
*/
public static int readVIntsInto(ByteData arr, long position, int length, char[] output) {
Expand All @@ -286,11 +298,22 @@ public static int readVIntsInto(ByteData arr, long position, int length, char[]
}

int count = i;
int accumulator = 0;
for(; i < length; i++) {
int b = arr.get(position + i);

output[count] = (char) ((output[count] << 7) | (b & 0x7f));
count += (~b >> 7) & 0x1;
// Accumulate the next 7 bits into the value
accumulator = (accumulator << 7) | (b & 0x7f);

// Check if this byte completes a value (continuation bit not set)
int isComplete = (~b >> 7) & 0x1; // 1 if complete, 0 if continuing
output[count] = (char) accumulator; // Write current accumulator (may overwrite same position)
count += isComplete; // Advance output position only when value is complete

// Clear accumulator for next value using sign extension trick:
// - If b has continuation bit (b negative): b >> 7 = 0xFFFFFFFF, keeps accumulator
// - If b completes value (b positive): b >> 7 = 0, clears accumulator
accumulator &= (b >> 7);
}

return count;
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2 changes: 0 additions & 2 deletions .../main/java/com/netflix/hollow/core/read/engine/object/HollowObjectTypeReadStateShard.java
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Original file line number Diff line number Diff line change
Expand Up @@ -170,8 +170,6 @@ private String readString(ByteData data, long position, int length) {
char[] chararr = HollowObjectTypeReadStateShard.chararr.get();
if (length > chararr.length) {
chararr = new char[length];
} else {
Arrays.fill(chararr, 0, length, '\0');
}

int count = VarInt.readVIntsInto(data, position, length, chararr);
Expand Down