Add occs_range in README.md

Author: rossanoventurini

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "qwt"
-version = "0.3.2"
+version = "0.3.4"
 edition = "2021"
 authors = ["Rossano Venturini <rossano.venturini@unipi.it>"]
 readme = "README.md"

README.md

@@ -165,6 +165,29 @@ assert_eq!(qwt.select(4, 0), Some(5));
 assert_eq!(qwt.select(1, 3), None);
 ```
 
+The data structure also supports an efficient `occs_range` operation:
+- `occs_range(range)` returns an iterator over pairs `(symbol, count)` for all symbols occurring in the given range of positions.
+
+This operation efficiently computes the distribution of symbol occurrences within a range by traversing only the tree nodes corresponding to symbols actually present, achieving O(k log σ) time complexity where k is the number of distinct symbols in the range. This feature was contributed by Eric Izoita ([@nytopop](https://github.com/nytopop)).
+
+```rust
+use qwt::{QWT256, OccsRangeUnsigned};
+
+let data = vec![1u8, 0, 1, 0, 2, 4, 5, 3];
+let qwt = QWT256::from(data);
+
+// Count occurrences of each symbol in the range [2..6)
+let occs: Vec<_> = qwt.occs_range(2..6).unwrap().collect();
+// Returns: [(0, 1), (1, 1), (2, 1), (4, 1)] - symbols 0,1,2,4 each appear once
+
+// Full sequence distribution
+let occs: Vec<_> = qwt.occs_range(..).unwrap().collect();
+// Returns: [(0, 2), (1, 2), (2, 1), (3, 1), (4, 1), (5, 1)]
+
+// Out-of-bounds ranges return None
+assert!(qwt.occs_range(0..100).is_none());
+```
+
 In the following example, we use [`QWT256`] to index a sequence over a larger alphabet.
 
 ```rust

src/binwt/mod.rs

@@ -445,6 +445,7 @@ where
 {
     type Item = (T, usize);
 
+    #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         while let Some(cur) = self.stack.pop() {
             // have we reached the bottom of the tree? huffman tree needs slightly different logic to check

src/quadwt/huffqwt.rs

@@ -755,6 +755,7 @@ where
 {
     type Item = (T, usize);
 
+    #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         while let Some(cur) = self.stack.pop() {
             // SAFETY: assumes tree depth corresponds exactly to max code length in codes_decode

src/quadwt/mod.rs

@@ -590,6 +590,7 @@ where
 {
     type Item = (T, usize);
 
+    #[inline]
     fn next(&mut self) -> Option<Self::Item> {
         while let Some(cur) = self.stack.pop() {
             // have we reached the bottom of the tree?