improve slope calculation by delaying f64 cast

fastfield_codecs/src/linearinterpol.rs

@@ -97,7 +97,16 @@ pub(crate) fn get_slope(first_val: u64, last_val: u64, num_vals: u64) -> f32 {
     }
     //  We calculate the slope with f64 high precision and use the result in lower precision f32
     //  This is done in order to handle estimations for very large values like i64::MAX
-    ((last_val as f64 - first_val as f64) / (num_vals as u64 - 1) as f64) as f32
+    let diff = diff(last_val, first_val);
+    (diff / (num_vals as u64 - 1) as f64) as f32
+}
+
+fn diff(val1: u64, val2: u64) -> f64 {
+    if val1 >= val2 {
+        (val1 - val2) as f64
+    } else {
+        (val2 - val1) as f64 * -1.0
+    }
 }
 
 #[inline]

fastfield_codecs/src/multilinearinterpol.rs

@@ -16,7 +16,7 @@ use std::ops::Sub;
 use common::{BinarySerializable, CountingWriter, DeserializeFrom};
 use tantivy_bitpacker::{compute_num_bits, BitPacker, BitUnpacker};
 
-use crate::linearinterpol::{get_calculated_value, get_slope};
+use crate::linearinterpol::get_slope;
 use crate::{FastFieldCodecReader, FastFieldCodecSerializer, FastFieldDataAccess, FastFieldStats};
 
 const CHUNK_SIZE: u64 = 512;
@@ -356,6 +356,11 @@ fn distance<T: Sub<Output = T> + Ord>(x: T, y: T) -> T {
     }
 }
 
+#[inline]
+fn get_calculated_value(first_val: u64, pos: u64, slope: f32) -> u64 {
+    (first_val as i64 + (pos as f32 * slope) as i64) as u64
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -375,11 +380,11 @@ mod tests {
 
     #[test]
     fn test_compression_i64() {
-        let data = (-10..=6_000_i64)
-            .map(|el| i64_to_u64(el))
+        let data = (i64::MAX - 600_000..=i64::MAX - 550_000)
+            .map(i64_to_u64)
             .collect::<Vec<_>>();
         let (estimate, actual_compression) =
-            create_and_validate(&data, "simple monotonically large");
+            create_and_validate(&data, "simple monotonically large i64");
         assert!(actual_compression < 0.2);
         assert!(estimate < 0.20);
         assert!(estimate > 0.15);

src/fastfield/mod.rs

@@ -469,7 +469,8 @@ mod tests {
         }
         let file = directory.open_read(path).unwrap();
         // assert_eq!(file.len(), 17710 as usize); //bitpacked size
-        assert_eq!(file.len(), 10175_usize); // linear interpol size
+        // assert_eq!(file.len(), 10175_usize); // linear interpol size
+        assert_eq!(file.len(), 75_usize); // linear interpol size after calc improvement
         {
             let fast_fields_composite = CompositeFile::open(&file)?;
             let data = fast_fields_composite.open_read(i64_field).unwrap();