pitrou commented on code in PR #221:
URL: https://github.com/apache/parquet-format/pull/221#discussion_r2128468354
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src/main/thrift/parquet.thrift:
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@@ -1098,6 +1107,53 @@ union ColumnOrder {
* `-0.0` should be written into the min statistics field.
*/
1: TypeDefinedOrder TYPE_ORDER;
+
+ /*
+ * The floating point type is ordered according to the totalOrder predicate,
+ * as defined in section 5.10 of IEEE-754 (2008 revision). Only columns of
+ * physical type FLOAT or DOUBLE, or logical type FLOAT16 may use this
ordering.
+ *
+ * Intuitively, this orders floats mathematically, but defines -0 to be less
+ * than +0, -NaN to be less than anything else, and +NaN to be greater than
+ * anything else. It also defines an order between different bit
representations
+ * of the same value.
+ *
+ * The formal definition is as follows:
+ * a) If x<y, totalOrder(x, y) is true.
+ * b) If x>y, totalOrder(x, y) is false.
+ * c) If x=y:
+ * 1) totalOrder(−0, +0) is true.
+ * 2) totalOrder(+0, −0) is false.
+ * 3) If x and y represent the same floating-point datum:
+ * i) If x and y have negative sign, totalOrder(x, y) is true if and
+ * only if the exponent of x ≥ the exponent of y
+ * ii) otherwise totalOrder(x, y) is true if and only if the exponent
+ * of x ≤ the exponent of y.
+ * d) If x and y are unordered numerically because x or y is NaN:
+ * 1) totalOrder(−NaN, y) is true where −NaN represents a NaN with
+ * negative sign bit and y is a non-NaN floating-point number.
+ * 2) totalOrder(x, +NaN) is true where +NaN represents a NaN with
+ * positive sign bit and x is a non-NaN floating-point number.
+ * 3) If x and y are both NaNs, then totalOrder reflects a total ordering
+ * based on:
+ * i) negative sign orders below positive sign
+ * ii) signaling orders below quiet for +NaN, reverse for −NaN
+ * iii) lesser payload, when regarded as an integer, orders below
+ * greater payload for +NaN, reverse for −NaN.
+ *
+ * Note that this ordering can be implemented efficiently in software by
bit-wise
+ * operations on the integer representation of the floating point values.
+ * E.g., this is a possible implementation for DOUBLE in Rust:
+ *
+ * pub fn totalOrder(x: f64, y: f64) -> bool {
+ * let mut x_int = x.to_bits() as i64;
+ * let mut y_int = y.to_bits() as i64;
+ * x_int ^= (((x_int >> 63) as u64) >> 1) as i64;
+ * y_int ^= (((y_int >> 63) as u64) >> 1) as i64;
Review Comment:
Let's add a comment explaining those lines?
```suggestion
* // Turn sign+mantissa into 2's complement representation
* x_int ^= (((x_int >> 63) as u64) >> 1) as i64;
* y_int ^= (((y_int >> 63) as u64) >> 1) as i64;
```
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