Take an explicit search parameter

benches/bench.rs

@@ -8,5 +8,6 @@ benchmark_main!(benches);
 fn short(bench: &mut Bencher) {
     let segmenter = instant_segment::test_data::segmenter();
     let mut out = Vec::new();
-    bench.iter(|| segmenter.segment("thisisatest", &mut out));
+    let mut search = instant_segment::Search::default();
+    bench.iter(|| segmenter.segment("thisisatest", &mut out, &mut search));
 }

src/lib.rs

@@ -54,9 +54,15 @@ impl Segmenter {
     /// Appends list of words that is the best segmentation of `text` to `out`
     ///
     /// Requires that the input `text` consists of lowercase ASCII characters only. Otherwise,
-    /// returns `Err(InvalidCharacter)`.
-    pub fn segment(&self, text: &str, out: &mut Vec<String>) -> Result<(), InvalidCharacter> {
-        Ok(SegmentState::new(Ascii::new(text)?, &self, out).run())
+    /// returns `Err(InvalidCharacter)`. The `search` parameter contains caches that are used
+    /// segmentation; passing it in allows the callers to reuse the cache allocations.
+    pub fn segment(
+        &self,
+        text: &str,
+        out: &mut Vec<String>,
+        search: &mut Search,
+    ) -> Result<(), InvalidCharacter> {
+        Ok(SegmentState::new(Ascii::new(text)?, &self, out, search).run())
     }
 
     fn score(&self, word: &str, previous: Option<&str>) -> f64 {
@@ -94,21 +100,23 @@ impl Segmenter {
 struct SegmentState<'a> {
     data: &'a Segmenter,
     text: Ascii<'a>,
-    memo: HashMap<MemoKey, (f64, Range<usize>)>,
-    split_cache: Vec<usize>,
     result: &'a mut Vec<String>,
-    best: Vec<Vec<usize>>,
+    search: &'a mut Search,
 }
 
 impl<'a> SegmentState<'a> {
-    fn new(text: Ascii<'a>, data: &'a Segmenter, result: &'a mut Vec<String>) -> Self {
+    fn new(
+        text: Ascii<'a>,
+        data: &'a Segmenter,
+        result: &'a mut Vec<String>,
+        search: &'a mut Search,
+    ) -> Self {
+        search.clear();
         Self {
             data,
             text,
-            memo: HashMap::default(),
-            split_cache: Vec::new(),
             result,
-            best: vec![vec![]; SEGMENT_SIZE],
+            search,
         }
     }
 
@@ -119,7 +127,7 @@ impl<'a> SegmentState<'a> {
             end = self.text.len().min(end + SEGMENT_SIZE);
             self.search(0, start..end, None);
 
-            let mut splits = &self.best[0][..];
+            let mut splits = &self.search.best[0][..];
             if end < self.text.len() {
                 splits = &splits[..splits.len().saturating_sub(5)];
             }
@@ -134,7 +142,7 @@ impl<'a> SegmentState<'a> {
     /// Score `word` in the context of `previous` word
     fn search(&mut self, level: usize, range: Range<usize>, previous: Option<Range<usize>>) -> f64 {
         if range.is_empty() {
-            self.best[level].clear();
+            self.search.best[level].clear();
             return 0.0;
         }
 
@@ -145,24 +153,28 @@ impl<'a> SegmentState<'a> {
             let prefix_score = self.data.score(&self.text[start..split], previous).log10();
 
             let pair = (split..end, start..split);
-            let (suffix_score, suffix_splits) = match self.memo.get(&pair) {
-                Some((score, splits)) => (*score, &self.split_cache[splits.start..splits.end]),
+            let (suffix_score, suffix_splits) = match self.search.memo.get(&pair) {
+                Some((score, splits)) => {
+                    (*score, &self.search.split_cache[splits.start..splits.end])
+                }
                 None => {
                     let suffix_score = self.search(level + 1, split..end, Some(start..split));
 
-                    let start = self.split_cache.len();
-                    self.split_cache.extend(&self.best[level + 1][..]);
-                    let end = self.split_cache.len();
-                    self.memo.insert(pair, (suffix_score, start..end));
+                    let start = self.search.split_cache.len();
+                    self.search
+                        .split_cache
+                        .extend(&self.search.best[level + 1][..]);
+                    let end = self.search.split_cache.len();
+                    self.search.memo.insert(pair, (suffix_score, start..end));
 
-                    (suffix_score, &self.split_cache[start..end])
+                    (suffix_score, &self.search.split_cache[start..end])
                 }
             };
 
             let score = prefix_score + suffix_score;
             if score > best {
                 best = score;
-                let splits = &mut self.best[level];
+                let splits = &mut self.search.best[level];
                 splits.clear();
                 splits.push(split);
                 splits.extend(suffix_splits);
@@ -173,6 +185,32 @@ impl<'a> SegmentState<'a> {
     }
 }
 
+pub struct Search {
+    memo: HashMap<MemoKey, (f64, Range<usize>)>,
+    split_cache: Vec<usize>,
+    best: Vec<Vec<usize>>,
+}
+
+impl Default for Search {
+    fn default() -> Self {
+        Self {
+            memo: HashMap::default(),
+            split_cache: Vec::with_capacity(32),
+            best: vec![vec![]; SEGMENT_SIZE],
+        }
+    }
+}
+
+impl Search {
+    fn clear(&mut self) {
+        self.memo.clear();
+        self.split_cache.clear();
+        for inner in self.best.iter_mut() {
+            inner.clear();
+        }
+    }
+}
+
 type MemoKey = (Range<usize>, Range<usize>);
 
 #[derive(Debug)]

src/test_cases.rs

@@ -1,23 +1,24 @@
-use crate::Segmenter;
+use crate::{Search, Segmenter};
 
 /// Run a segmenter against the built-in test cases
 pub fn run(segmenter: &Segmenter) {
+    let mut search = Search::default();
     for test in TEST_CASES.iter().copied() {
-        assert_segments(segmenter, test);
+        assert_segments(test, &mut search, segmenter);
     }
-    assert_segments(segmenter, FAIL);
+    assert_segments(FAIL, &mut search, segmenter);
 }
 
-pub fn assert_segments(segmenter: &Segmenter, s: &[&str]) {
+pub fn assert_segments(s: &[&str], search: &mut Search, segmenter: &Segmenter) {
     let mut out = Vec::new();
-    segmenter.segment(&s.join(""), &mut out).unwrap();
+    segmenter.segment(&s.join(""), &mut out, search).unwrap();
     let cmp = out.iter().map(|s| &*s).collect::<Vec<_>>();
     assert_eq!(cmp, s);
 }
 
-pub fn check_segments(segmenter: &Segmenter, s: &[&str]) -> bool {
+pub fn check_segments(s: &[&str], search: &mut Search, segmenter: &Segmenter) -> bool {
     let mut out = Vec::new();
-    match segmenter.segment(&s.join(""), &mut out) {
+    match segmenter.segment(&s.join(""), &mut out, search) {
         Ok(()) => s == out.iter().map(|s| &*s).collect::<Vec<_>>(),
         Err(_) => false,
     }