AntoinePrv commented on code in PR #47294:
URL: https://github.com/apache/arrow/pull/47294#discussion_r2336690910
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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) {
+ return 0;
+ }
+
+ // Write the encoded byte with continuation bit
+ *out = static_cast<uint8_t>(value & kLow7Mask) | kContinuationBit;
+ ++out;
+ // Shift remaining data
+ value >>= 7;
+ }
+
+ // We do not have enough room to write the LEB128
+ if (out - out_first >= max_out_size) {
+ return 0;
+ }
+
+ // Write last non-continuing byte
+ *out = static_cast<uint8_t>(value & kLow7Mask);
+ ++out;
+
+ return static_cast<int32_t>(out - out_first);
+}
+
+/// Parse a leading LEB128
+///
+/// Take as input a data pointer and the maximum number of bytes that can be
read from it
+/// (typically the array size).
+/// When a valid LEB128 is found at the start of the data, the function writes
it to the
+/// out pointer and return the number of bytes read.
+/// Otherwise, the out pointer is unmodified and zero is returned.
+///
+/// \see https://en.wikipedia.org/wiki/LEB128
+/// \see MaxLEB128ByteLenFor
+template <typename Int>
+constexpr int32_t ParseLeadingLEB128(uint8_t const* data, int32_t
max_data_size,
+ Int* out) {
+ constexpr auto kMaxBytes = static_cast<int32_t>(MaxLEB128ByteLenFor<Int>);
+ static_assert(kMaxBytes >= 1);
+ constexpr uint8_t kLow7Mask = 0x7F;
+ constexpr uint8_t kContinuationBit = 0x80;
+ constexpr int32_t kSignBitCount = std::is_signed_v<Int> ? 1 : 0;
Review Comment:
This decoding function allows reading into a signed integer type, but the
encoded value can only represent non-negatives anyways (the encoding function
refuses negative inputs).
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