AntoinePrv commented on code in PR #47294:
URL: https://github.com/apache/arrow/pull/47294#discussion_r2349465815
##########
cpp/src/arrow/util/bit_util.h:
##########
@@ -365,5 +365,118 @@ void PackBits(const uint32_t* values, uint8_t* out) {
}
}
+constexpr int64_t MaxLEB128ByteLen(int64_t n_bits) { return CeilDiv(n_bits,
7); }
+
+template <typename Int>
+constexpr int64_t MaxLEB128ByteLenFor = MaxLEB128ByteLen(sizeof(Int) * 8);
+
+/// Write a integer as LEB128
+///
+/// Write the input value as LEB128 into the outptu buffer and return the
number of bytes
+/// written.
+/// If the output buffer size is insufficient, return 0 but the output may
have been
+/// written to.
+///
+/// \see https://en.wikipedia.org/wiki/LEB128
+/// \see MaxLEB128ByteLenFor
+template <typename Int>
+constexpr int32_t WriteLEB128(Int value, uint8_t* out, int32_t max_out_size) {
+ constexpr Int kLow7Mask = Int(0x7F);
+ constexpr Int kHigh7Mask = ~kLow7Mask;
+ constexpr uint8_t kContinuationBit = 0x80;
+
+ auto const out_first = out;
+
+ // Write as many bytes as the could be for the given input
+ while ((value & kHigh7Mask) != Int(0)) {
+ // We do not have enough room to write the LEB128
+ if (out - out_first >= max_out_size) {
Review Comment:
I had done a branch-less version of the LEB128 reading for in32 on the fast
path that we can read all needed bytes at once. Turned out to be slower.
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