issues/65 Added i64 fast fields

1 files changed, 15 insertions, 0 deletions

diff --git a/src/common/mod.rs b/src/common/mod.rs
index 86a56a7..bc31b18 100644
--- a/src/common/mod.rs
+++ b/src/common/mod.rs
@@ -43,11 +43,26 @@ pub fn allocate_vec<T>(capacity: usize) -> Vec<T> {
 
 const HIGHEST_BIT: u64 = 1 << 63;
 
+
+/// Maps `i64` to `u64` so that
+/// `-2^63 .. 2^63-1` is mapped
+///     to 
+/// `0 .. 2^64`
+/// in that order.
+///
+/// This is more suited than simply casting (`val as u64`)
+/// because of bitpacking.
+/// 
+/// Imagine a list of `i64` ranging from -10 to 10.
+/// When casting negative values, the negative values are projected
+/// to values over 2^63, and all values end up requiring 64 bits.
 #[inline(always)]
 pub fn i64_to_u64(val: i64) -> u64 {
     (val as u64) ^ HIGHEST_BIT
 }
 
+/// Reverse the mapping given by 
+/// `i64_to_u64`.
 #[inline(always)]
 pub fn u64_to_i64(val: u64) -> i64 {
     (val ^ HIGHEST_BIT) as i64