Rafferty97 commented on code in PR #9453:
URL: https://github.com/apache/arrow-rs/pull/9453#discussion_r2927926500
##########
arrow-buffer/src/bigint/mod.rs:
##########
@@ -614,6 +615,62 @@ impl i256 {
let n = (n.high >> 64) as i64; // throw away the lower 192 bits
(n as f64) * f64::powi(2.0, 192 - (k as i32)) // convert to f64 and
scale it, as we left-shift k bit previous, so we need to scale it by 2^(192-k)
}
+
+ /// Computes the `base` logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero, or if `base`
is less than 2.
+ #[inline]
+ pub fn checked_ilog(self, base: i256) -> Option<u32> {
+ if self <= Self::ZERO || base < i256::from(2) {
+ None
+ } else {
+ // Invariants are enforced, so this call cannot fail
+ self.to_ext_i256().checked_ilog(base.to_ext_i256())
+ }
+ }
+
+ /// Computes the `base` logarithm of the number `self`
+ /// Panic if `self` is less than or equal to zero, or if `base` is less
than 2.
+ #[inline]
+ pub fn ilog(self, base: i256) -> u32 {
+ self.checked_ilog(base)
+ .unwrap_or_else(|| panic!("ilog overflow with {self} and base
{base}"))
+ }
+
+ /// Computes the decimal logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero.
+ #[inline]
+ pub fn checked_ilog10(self) -> Option<u32> {
+ // Switch to I256::checked_ilog10 when I256 switches MSRV
Review Comment:
Why can't it be used now?
##########
arrow-buffer/src/bigint/mod.rs:
##########
@@ -614,6 +615,62 @@ impl i256 {
let n = (n.high >> 64) as i64; // throw away the lower 192 bits
(n as f64) * f64::powi(2.0, 192 - (k as i32)) // convert to f64 and
scale it, as we left-shift k bit previous, so we need to scale it by 2^(192-k)
}
+
+ /// Computes the `base` logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero, or if `base`
is less than 2.
+ #[inline]
+ pub fn checked_ilog(self, base: i256) -> Option<u32> {
+ if self <= Self::ZERO || base < i256::from(2) {
Review Comment:
Since these invariants are checked by the call to `ExtI256::checked_ilog`
anyway, what's the point of restating the logic here? The `to_ext_i256` calls
should be no-ops anyway.
##########
arrow-buffer/src/bigint/mod.rs:
##########
@@ -1580,4 +1637,69 @@ mod tests {
assert_eq!(i256::MAX.trailing_zeros(), 0);
assert_eq!(i256::from(-1).trailing_zeros(), 0);
}
+
+ #[test]
+ fn test_ilog() {
+ let value = i256::from(128);
+
+ // log2
+ assert_eq!(value.ilog(i256::from(2)), 7);
+ assert_eq!(value.ilog2(), 7);
+
+ // log10
+ assert_eq!(value.ilog(i256::from(10)), 2);
+ assert_eq!(value.ilog10(), 2);
+
+ // negative base
+ assert_eq!(value.checked_ilog(i256::from(-2)), None);
+ assert_eq!(value.checked_ilog(i256::from(-10)), None);
+ assert_eq!(value.checked_ilog(i256::from(0)), None);
+ assert_eq!(value.checked_ilog(i256::from(1)), None);
+
+ // negative self
+ let neg_value = i256::from(-128);
+ assert_eq!(neg_value.checked_ilog(i256::from(2)), None);
+ assert_eq!(neg_value.checked_ilog(i256::from(10)), None);
+ assert_eq!(neg_value.checked_ilog10(), None);
+ assert_eq!(neg_value.checked_ilog2(), None);
+
+ // zero self
+ assert_eq!(i256::ZERO.checked_ilog(i256::from(2)), None);
+ assert_eq!(i256::ZERO.checked_ilog(i256::from(10)), None);
+ assert_eq!(i256::ZERO.checked_ilog10(), None);
+ assert_eq!(i256::ZERO.checked_ilog2(), None);
+
+ // Large i256 values
+ let large = i256::from_parts(100000000, i128::MAX);
+ // log2 of 2 powered to approximately 255 should be 254
+ assert_eq!(large.checked_ilog(i256::from(2)), Some(254));
+
+ // log10 should be 76 or 77
+ let result = large.checked_ilog(i256::from(10));
+ assert!(result.is_some());
+ assert!(result.unwrap() >= 76 && result.unwrap() <= 77);
Review Comment:
Why can't an `assert_eq!` be used here? Surely the result is deterministic?
##########
arrow-buffer/src/bigint/mod.rs:
##########
@@ -614,6 +615,62 @@ impl i256 {
let n = (n.high >> 64) as i64; // throw away the lower 192 bits
(n as f64) * f64::powi(2.0, 192 - (k as i32)) // convert to f64 and
scale it, as we left-shift k bit previous, so we need to scale it by 2^(192-k)
}
+
+ /// Computes the `base` logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero, or if `base`
is less than 2.
+ #[inline]
+ pub fn checked_ilog(self, base: i256) -> Option<u32> {
+ if self <= Self::ZERO || base < i256::from(2) {
+ None
+ } else {
+ // Invariants are enforced, so this call cannot fail
+ self.to_ext_i256().checked_ilog(base.to_ext_i256())
+ }
+ }
+
+ /// Computes the `base` logarithm of the number `self`
+ /// Panic if `self` is less than or equal to zero, or if `base` is less
than 2.
+ #[inline]
+ pub fn ilog(self, base: i256) -> u32 {
+ self.checked_ilog(base)
+ .unwrap_or_else(|| panic!("ilog overflow with {self} and base
{base}"))
+ }
+
+ /// Computes the decimal logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero.
+ #[inline]
+ pub fn checked_ilog10(self) -> Option<u32> {
+ // Switch to I256::checked_ilog10 when I256 switches MSRV
+ self.checked_ilog(i256::from(10))
+ }
+
+ /// Computes the decimal logarithm of the number `self`
+ /// Panics if `self` is less than or equal to zero.
+ #[inline]
+ pub fn ilog10(self) -> u32 {
+ self.checked_ilog10()
+ .unwrap_or_else(|| panic!("ilog10 overflow with {self}"))
+ }
+
+ /// Computes the binary logarithm of the number `self`
+ /// Returns `None` if `self` is less than or equal to zero.
+ #[inline]
+ pub fn checked_ilog2(self) -> Option<u32> {
+ // Switch to I256::checked_ilog2 when I256 switches MSRV
Review Comment:
Same as above: why not now?
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