[write] Add common util for computing searchRange

This is shared in a few places, so it makes sense to have a single
implementation. (It is used in the kern table, which is coming up)

write-fonts/src/font_builder.rs

@@ -6,6 +6,8 @@ use std::{borrow::Cow, fmt::Display};
 use read_fonts::{FontRef, TableProvider};
 use types::{Tag, TT_SFNT_VERSION};
 
+use crate::util::SearchRange;
+
 include!("../generated/generated_font.rs");
 
 const TABLE_RECORD_LEN: usize = 16;
@@ -32,19 +34,14 @@ pub struct BuilderError {
 impl TableDirectory {
     pub fn from_table_records(table_records: Vec<TableRecord>) -> TableDirectory {
         assert!(table_records.len() <= u16::MAX as usize);
-
         // See https://learn.microsoft.com/en-us/typography/opentype/spec/otff#table-directory
-        // Computation works at the largest allowable num tables so don't stress the as u16's
-        let entry_selector = (table_records.len() as f64).log2().floor() as u16;
-        let search_range = (2.0_f64.powi(entry_selector as i32) * 16.0) as u16;
-        // The result doesn't really make sense with 0 tables but ... let's at least not fail
-        let range_shift = (table_records.len() * 16).saturating_sub(search_range as usize) as u16;
+        let computed = SearchRange::compute(table_records.len(), TABLE_RECORD_LEN);
 
         TableDirectory::new(
             TT_SFNT_VERSION,
-            search_range,
-            entry_selector,
-            range_shift,
+            computed.search_range,
+            computed.entry_selector,
+            computed.range_shift,
             table_records,
         )
     }

write-fonts/src/tables/cmap.rs

@@ -6,6 +6,8 @@ include!("../../generated/generated_cmap.rs");
 
 use std::collections::HashMap;
 
+use crate::util::SearchRange;
+
 // https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#windows-platform-platform-id--3
 const WINDOWS_BMP_ENCODING: u16 = 1;
 const WINDOWS_FULL_REPERTOIRE_ENCODING: u16 = 10;
@@ -96,26 +98,16 @@ impl CmapSubtable {
         );
 
         let seg_count: u16 = start_code.len().try_into().unwrap();
-        // Spec: Log2 of the maximum power of 2 less than or equal to segCount (log2(searchRange/2),
-        // which is equal to floor(log2(segCount)))
-        let entry_selector = (seg_count as f32).log2().floor();
-
-        // Spec: Maximum power of 2 less than or equal to segCount, times 2
-        // ((2**floor(log2(segCount))) * 2, where “**” is an exponentiation operator)
-        let search_range = 2u16.pow(entry_selector as u32).checked_mul(2).unwrap();
-
-        // if 2^entry_selector*2 is a u16 then so is entry_selector
-        let entry_selector = entry_selector as u16;
-        let range_shift = seg_count * 2 - search_range;
 
+        let computed = SearchRange::compute(seg_count as _, u16::RAW_BYTE_LEN);
         let id_range_offsets = vec![0; id_deltas.len()];
         Some(CmapSubtable::format_4(
             size_of_cmap4(seg_count, 0),
             0, // 'lang' set to zero for all 'cmap' subtables whose platform IDs are other than Macintosh
             seg_count * 2,
-            search_range,
-            entry_selector,
-            range_shift,
+            computed.search_range,
+            computed.entry_selector,
+            computed.range_shift,
             end_code,
             start_code,
             id_deltas,

write-fonts/src/util.rs

@@ -100,3 +100,59 @@ impl Default for FloatComparator {
 pub fn isclose(a: f64, b: f64) -> bool {
     FloatComparator::default().isclose(a, b)
 }
+
+/// Search range values used in various tables
+#[derive(Clone, Copy, Debug)]
+pub struct SearchRange {
+    pub search_range: u16,
+    pub entry_selector: u16,
+    pub range_shift: u16,
+}
+
+impl SearchRange {
+    //https://github.com/fonttools/fonttools/blob/729b3d2960ef/Lib/fontTools/ttLib/ttFont.py#L1147
+    /// calculate searchRange, entrySelector, and rangeShift
+    ///
+    /// these values are used in various places, such as [the base table directory]
+    /// and [cmap format 4].
+    ///
+    /// [the base table directory]: https://learn.microsoft.com/en-us/typography/opentype/spec/otff#table-directory
+    /// [cmap format 4]: https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values
+    pub fn compute(n_items: usize, item_size: usize) -> Self {
+        let entry_selector = (n_items as f64).log2().floor() as usize;
+        let search_range = (2.0_f64.powi(entry_selector as i32) * item_size as f64) as usize;
+        // The result doesn't really make sense with 0 tables but ... let's at least not fail
+        let range_shift = (n_items * item_size).saturating_sub(search_range);
+        SearchRange {
+            search_range: search_range.try_into().unwrap(),
+            entry_selector: entry_selector.try_into().unwrap(),
+            range_shift: range_shift.try_into().unwrap(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// based on example at
+    /// <https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#format-4-segment-mapping-to-delta-values>
+    #[test]
+    fn simple_search_range() {
+        let SearchRange {
+            search_range,
+            entry_selector,
+            range_shift,
+        } = SearchRange::compute(39, 2);
+        assert_eq!((search_range, entry_selector, range_shift), (64, 5, 14));
+    }
+
+    #[test]
+    fn search_range_no_crashy() {
+        let foo = SearchRange::compute(0, 0);
+        assert_eq!(
+            (foo.search_range, foo.entry_selector, foo.range_shift),
+            (0, 0, 0)
+        )
+    }
+}