pitrou commented on a change in pull request #9582:
URL: https://github.com/apache/arrow/pull/9582#discussion_r585572255
##########
File path: cpp/src/parquet/statistics.cc
##########
@@ -133,56 +136,95 @@ struct CompareHelper<Int96Type, is_signed> {
static T Max(int type_length, const T& a, const T& b) {
return Compare(0, a, b) ? b : a;
}
-}; // namespace parquet
-
-template <typename DType, bool is_signed>
-struct ByteLikeCompareHelperBase {
- using T = ByteArrayType::c_type;
- using PtrType = typename std::conditional<is_signed, int8_t, uint8_t>::type;
+};
- static T DefaultMin() { return {}; }
- static T DefaultMax() { return {}; }
- static T Coalesce(T val, T fallback) { return val; }
+template <typename T, bool is_signed>
+struct BinaryLikeComparer {};
- static inline bool Compare(int type_length, const T& a, const T& b) {
- const auto* aptr = reinterpret_cast<const PtrType*>(a.ptr);
- const auto* bptr = reinterpret_cast<const PtrType*>(b.ptr);
- return std::lexicographical_compare(aptr, aptr + a.len, bptr, bptr +
b.len);
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/false> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+ // Unsigned comparison is used for non-numeric types so straight
+ // lexiographic comparison makes sense. (a.ptr is always unsigned)....
+ return std::lexicographical_compare(a.ptr, a.ptr + a_length, b.ptr, b.ptr
+ b_length);
}
+};
- static T Min(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? a : b;
- }
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/true> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ // Is signed is used for integers encoded as big-endian twos
+ // complement integers. (e.g. decimals).
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+
+ // At least of the lengths is zero.
+ if (a_length == 0 || b_length == 0) {
+ return a_length == 0 && b_length > 0;
+ }
- static T Max(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? b : a;
+ int8_t first_a = *a.ptr;
+ int8_t first_b = *b.ptr;
+ // We can short circuit for different signed numbers or
+ // for equal length bytes arrays that have different first bytes.
+ if ((0x80 & first_a) != (0x80 & first_b) ||
+ (a_length == b_length && first_a != first_b)) {
+ return first_a < first_b;
+ }
+ // When the lengths are unequal and the numbers are of the same
+ // sign we need to extend the digits.
+ const uint8_t* a_start = a.ptr;
+ const uint8_t* b_start = b.ptr;
+ if (a_length != b_length) {
+ const uint8_t* lead_start = nullptr;
+ const uint8_t* lead_end = nullptr;
+ if (a_length > b_length) {
+ int lead_length = a_length - b_length;
+ lead_start = a.ptr;
+ lead_end = a.ptr + lead_length;
+ a_start += lead_length;
+ } else {
+ DCHECK_LT(a_length, b_length);
+ int lead_length = b_length - a_length;
+ lead_start = b.ptr;
+ lead_end = b.ptr + lead_length;
+ b_start += lead_length;
+ }
+ // Compare extra bytes to the sign extension of the first
+ // byte of the other number.
+ uint8_t extension = first_a < 0 ? 0xFF : 0;
+ for (; lead_start != lead_end; lead_start++) {
+ if (*lead_start < extension) {
+ // The first bytes of the long value are less
+ // then the extended short one. So if a is the long value
+ // we can return true.
+ return a_length > b_length;
+ } else if (*lead_start > extension) {
Review comment:
Same here: if `extension` is 0xFF then this is never possible?
So perhaps it's possible to rewrite this:
```c++
if (first_a < 0) {
// Both numbers negative
for (; lead_start != lead_end; lead_start++) {
if (*lead_start != 0xff) {
return a_length > b_length;
}
}
} else {
// Both numbers positive
for (; lead_start != lead_end; lead_start++) {
if (*lead_start != 0) {
return a_length < b_length;
}
}
}
```
Note sure which one is more understandable.
##########
File path: cpp/src/parquet/statistics_test.cc
##########
@@ -71,19 +71,25 @@ static FLBA FLBAFromString(const std::string& s) {
TEST(Comparison, SignedByteArray) {
auto comparator = MakeComparator<ByteArrayType>(Type::BYTE_ARRAY,
SortOrder::SIGNED);
- std::string s1 = "12345";
- std::string s2 = "12345678";
- ByteArray s1ba = ByteArrayFromString(s1);
- ByteArray s2ba = ByteArrayFromString(s2);
- ASSERT_TRUE(comparator->Compare(s1ba, s2ba));
+ std::vector<uint8_t> byte_values[] = {
+ {0x80, 0x80, 0, 0}, {/*0xFF,*/ 0x80, 0, 0}, {/*0xFF,*/ 0xFF,
0x01, 0},
+ {/*0xFF,0xFF,*/ 0x80, 0}, {/*0xFF,0xFF,0xFF,*/ 0x80}, {/*0xFF, 0xFF,
0xFF,*/ 0xFF},
+ {/*0, 0,*/ 0x01, 0x01}, {/*0,*/ 0x01, 0x01, 0}, {0x01, 0x01, 0,
0}};
Review comment:
Can you also test pairs of equal values with differing lengths?
Perhaps even do exhaustive pairwise comparisons, e.g.:
```c++
struct Case {
std::vector<uint8_t> bytes;
int order;
};
std::vector<Case> cases[] = {
{{0x80, 0x80, 0, 0}, 1},
{{0xFF, 0x80, 0x80, 0, 0}, 1},
{{0x80, 0, 0}, 2},
{{0xFF, 0x80, 0, 0}, 2},
{{0xFF, 0xFF, 0x80, 0, 0}, 2},
{{0xFF, 0x01, 0}, 3},
{{0xFF, 0xFF, 0x01, 0}, 3},
// ...
};
for (const auto& a : cases) {
for (const auto& b : cases) {
ByteArray ba(static_cast<int>(a.bytes.size()), a.bytes.data()));
ByteArray bb(static_cast<int>(b.bytes.size()), b.bytes.data()));
if (a.order < b.order) {
EXPECT_TRUE(comparator->Compare(ba, bb)) << ba << " " << bb;
} else {
EXPECT_FALSE(comparator->Compare(ba, bb)) << ba << " " << bb;
}
}
}
```
##########
File path: cpp/src/parquet/statistics.cc
##########
@@ -133,56 +136,95 @@ struct CompareHelper<Int96Type, is_signed> {
static T Max(int type_length, const T& a, const T& b) {
return Compare(0, a, b) ? b : a;
}
-}; // namespace parquet
-
-template <typename DType, bool is_signed>
-struct ByteLikeCompareHelperBase {
- using T = ByteArrayType::c_type;
- using PtrType = typename std::conditional<is_signed, int8_t, uint8_t>::type;
+};
- static T DefaultMin() { return {}; }
- static T DefaultMax() { return {}; }
- static T Coalesce(T val, T fallback) { return val; }
+template <typename T, bool is_signed>
+struct BinaryLikeComparer {};
- static inline bool Compare(int type_length, const T& a, const T& b) {
- const auto* aptr = reinterpret_cast<const PtrType*>(a.ptr);
- const auto* bptr = reinterpret_cast<const PtrType*>(b.ptr);
- return std::lexicographical_compare(aptr, aptr + a.len, bptr, bptr +
b.len);
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/false> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+ // Unsigned comparison is used for non-numeric types so straight
+ // lexiographic comparison makes sense. (a.ptr is always unsigned)....
+ return std::lexicographical_compare(a.ptr, a.ptr + a_length, b.ptr, b.ptr
+ b_length);
}
+};
- static T Min(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? a : b;
- }
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/true> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ // Is signed is used for integers encoded as big-endian twos
+ // complement integers. (e.g. decimals).
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+
+ // At least of the lengths is zero.
+ if (a_length == 0 || b_length == 0) {
+ return a_length == 0 && b_length > 0;
+ }
- static T Max(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? b : a;
+ int8_t first_a = *a.ptr;
+ int8_t first_b = *b.ptr;
+ // We can short circuit for different signed numbers or
+ // for equal length bytes arrays that have different first bytes.
+ if ((0x80 & first_a) != (0x80 & first_b) ||
+ (a_length == b_length && first_a != first_b)) {
+ return first_a < first_b;
+ }
Review comment:
Ok, I understand and your assessment looks correct to me. Can you just
add a bit more explanations as comments, so this doesn't produce too much
head-scratching later on? :-)
##########
File path: cpp/src/parquet/statistics.cc
##########
@@ -133,56 +136,95 @@ struct CompareHelper<Int96Type, is_signed> {
static T Max(int type_length, const T& a, const T& b) {
return Compare(0, a, b) ? b : a;
}
-}; // namespace parquet
-
-template <typename DType, bool is_signed>
-struct ByteLikeCompareHelperBase {
- using T = ByteArrayType::c_type;
- using PtrType = typename std::conditional<is_signed, int8_t, uint8_t>::type;
+};
- static T DefaultMin() { return {}; }
- static T DefaultMax() { return {}; }
- static T Coalesce(T val, T fallback) { return val; }
+template <typename T, bool is_signed>
+struct BinaryLikeComparer {};
- static inline bool Compare(int type_length, const T& a, const T& b) {
- const auto* aptr = reinterpret_cast<const PtrType*>(a.ptr);
- const auto* bptr = reinterpret_cast<const PtrType*>(b.ptr);
- return std::lexicographical_compare(aptr, aptr + a.len, bptr, bptr +
b.len);
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/false> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+ // Unsigned comparison is used for non-numeric types so straight
+ // lexiographic comparison makes sense. (a.ptr is always unsigned)....
+ return std::lexicographical_compare(a.ptr, a.ptr + a_length, b.ptr, b.ptr
+ b_length);
}
+};
- static T Min(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? a : b;
- }
+template <typename T>
+struct BinaryLikeComparer<T, /*is_signed=*/true> {
+ static bool Compare(int type_length, const T& a, const T& b) {
+ // Is signed is used for integers encoded as big-endian twos
+ // complement integers. (e.g. decimals).
+ int a_length = value_length(type_length, a);
+ int b_length = value_length(type_length, b);
+
+ // At least of the lengths is zero.
+ if (a_length == 0 || b_length == 0) {
+ return a_length == 0 && b_length > 0;
+ }
- static T Max(int type_length, const T& a, const T& b) {
- if (a.ptr == nullptr) return b;
- if (b.ptr == nullptr) return a;
- return Compare(type_length, a, b) ? b : a;
+ int8_t first_a = *a.ptr;
+ int8_t first_b = *b.ptr;
+ // We can short circuit for different signed numbers or
+ // for equal length bytes arrays that have different first bytes.
+ if ((0x80 & first_a) != (0x80 & first_b) ||
+ (a_length == b_length && first_a != first_b)) {
+ return first_a < first_b;
+ }
+ // When the lengths are unequal and the numbers are of the same
+ // sign we need to extend the digits.
+ const uint8_t* a_start = a.ptr;
+ const uint8_t* b_start = b.ptr;
+ if (a_length != b_length) {
+ const uint8_t* lead_start = nullptr;
+ const uint8_t* lead_end = nullptr;
+ if (a_length > b_length) {
+ int lead_length = a_length - b_length;
+ lead_start = a.ptr;
+ lead_end = a.ptr + lead_length;
+ a_start += lead_length;
+ } else {
+ DCHECK_LT(a_length, b_length);
+ int lead_length = b_length - a_length;
+ lead_start = b.ptr;
+ lead_end = b.ptr + lead_length;
+ b_start += lead_length;
+ }
+ // Compare extra bytes to the sign extension of the first
+ // byte of the other number.
+ uint8_t extension = first_a < 0 ? 0xFF : 0;
+ for (; lead_start != lead_end; lead_start++) {
+ if (*lead_start < extension) {
Review comment:
If `extension` is 0 then this is never possible, right?
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