fallintoplace commented on code in PR #872:
URL: https://github.com/apache/arrow-go/pull/872#discussion_r3502180803
##########
arrow/array/data.go:
##########
@@ -270,12 +268,59 @@ func NewSliceData(data arrow.ArrayData, i, j int64)
arrow.ArrayData {
func Hash(h *maphash.MapHash, data arrow.ArrayData) {
a := data.(*Data)
- h.Write((*[bits.UintSize / 8]byte)(unsafe.Pointer(&a.length))[:])
- h.Write((*[bits.UintSize / 8]byte)(unsafe.Pointer(&a.length))[:])
- if len(a.buffers) > 0 && a.buffers[0] != nil {
- h.Write(a.buffers[0].Bytes())
+ hashString(h, a.dtype.Fingerprint())
+ hashInt(h, a.length)
+ hashInt(h, a.offset)
+
+ hashInt(h, len(a.buffers))
+ for _, b := range a.buffers {
+ if b == nil {
+ hashByte(h, 0)
+ continue
+ }
+
+ hashByte(h, 1)
+ hashBytes(h, b.Bytes())
}
+
+ hashInt(h, len(a.childData))
for _, c := range a.childData {
+ if c == nil {
+ hashByte(h, 0)
+ continue
+ }
+
+ hashByte(h, 1)
Hash(h, c)
}
+
+ if a.dictionary == nil {
+ hashByte(h, 0)
+ return
+ }
+
+ hashByte(h, 1)
+ Hash(h, a.dictionary)
+}
+
+func hashByte(h *maphash.MapHash, v byte) {
+ var b [1]byte
+ b[0] = v
+ h.Write(b[:])
+}
+
+func hashInt(h *maphash.MapHash, v int) {
+ var b [8]byte
+ binary.LittleEndian.PutUint64(b[:], uint64(int64(v)))
+ h.Write(b[:])
+}
+
+func hashBytes(h *maphash.MapHash, b []byte) {
+ hashInt(h, len(b))
Review Comment:
h.Write(b) is sufficient for hashing one isolated byte slice, but here the
hash input is a structured stream made of multiple variable-width fields.
maphash does not preserve Write call boundaries; it only sees the final
sequence of bytes. So without length-prefixing, different structured inputs can
serialize to the same byte stream, e.g. splitting bytes differently across
adjacent buffers/strings
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