Use nested `HashMap` for storing both unigram and bigram scores

README.md

@@ -70,7 +70,7 @@ print([word for word in search])
 use instant_segment::{Search, Segmenter};
 use std::collections::HashMap;
 
-let segmenter = Segmenter::from_maps(unigrams, bigrams);
+let segmenter = Segmenter::new(unigrams, bigrams);
 let mut search = Search::default();
 let words = segmenter
     .segment("instantdomainsearch", &mut search)

instant-segment-py/Cargo.toml

@@ -16,7 +16,6 @@ name = "instant_segment"
 crate-type = ["cdylib"]
 
 [dependencies]
-ahash = "0.8"
 bincode = "1.3.2"
 instant-segment = { version = "0.10", path = "../instant-segment", features = ["with-serde"] }
 pyo3 = { version = "0.20", features = ["extension-module"] }

instant-segment-py/src/lib.rs

@@ -39,7 +39,7 @@ impl Segmenter {
                 let val = item.get_item(1)?.extract::<f64>()?;
                 Ok((SmartString::from(key), val))
             })
-            .collect::<Result<HashMap<_, _>, PyErr>>()?;
+            .collect::<Result<Vec<_>, PyErr>>()?;
 
         let bigrams = bigrams
             .map(|item| {
@@ -52,10 +52,10 @@ impl Segmenter {
                 let val = item.get_item(1)?.extract::<f64>()?;
                 Ok(((SmartString::from(first), SmartString::from(second)), val))
             })
-            .collect::<Result<HashMap<_, _>, PyErr>>()?;
+            .collect::<Result<Vec<_>, PyErr>>()?;
 
         Ok(Self {
-            inner: instant_segment::Segmenter::from_maps(unigrams, bigrams),
+            inner: instant_segment::Segmenter::new(unigrams, bigrams),
         })
     }
 
@@ -148,5 +148,3 @@ impl Search {
         Some(word)
     }
 }
-
-type HashMap<K, V> = std::collections::HashMap<K, V, ahash::RandomState>;

instant-segment/examples/contrived.rs

@@ -2,7 +2,7 @@ use instant_segment::{Search, Segmenter};
 use std::collections::HashMap;
 
 fn main() {
-    let mut unigrams = HashMap::default();
+    let mut unigrams = HashMap::new();
 
     unigrams.insert("choose".into(), 80_000.0);
     unigrams.insert("chooses".into(), 7_000.0);
@@ -10,12 +10,12 @@ fn main() {
     unigrams.insert("spain".into(), 20_000.0);
     unigrams.insert("pain".into(), 90_000.0);
 
-    let mut bigrams = HashMap::default();
+    let mut bigrams = HashMap::new();
 
     bigrams.insert(("choose".into(), "spain".into()), 7.0);
     bigrams.insert(("chooses".into(), "pain".into()), 0.0);
 
-    let segmenter = Segmenter::from_maps(unigrams, bigrams);
+    let segmenter = Segmenter::new(unigrams, bigrams);
     let mut search = Search::default();
 
     let words = segmenter.segment("choosespain", &mut search).unwrap();

instant-segment/src/lib.rs

@@ -13,47 +13,57 @@ pub mod test_data;
 /// Central data structure used to calculate word probabilities
 #[cfg_attr(feature = "with-serde", derive(Deserialize, Serialize))]
 pub struct Segmenter {
-    unigrams: HashMap<String, f64>,
+    // Maps a word to both its unigram score, as well has a nested HashMap in
-    bigrams: HashMap<(String, String), f64>,
+    // which the bigram score can be looked up using the previous word. Scores
+    // are base-10 logarithms of relative word frequencies
+    scores: HashMap<String, (f64, HashMap<String, f64>)>,
+    // Base-10 logarithm of the total count of unigrams
     uni_total_log10: f64,
     limit: usize,
 }
 
 impl Segmenter {
-    /// Create `Segmenter` from the given iterators
+    /// Create `Segmenter` from the given unigram and bigram counts.
     ///
     /// Note: the `String` types used in this API are defined in the `smartstring` crate. Any
     /// `&str` or `String` can be converted into the `String` used here by calling `into()` on it.
-    pub fn from_iters<U, B>(unigrams: U, bigrams: B) -> Self
+    pub fn new<U, B>(unigrams: U, bigrams: B) -> Self
     where
-        U: Iterator<Item = (String, f64)>,
+        U: IntoIterator<Item = (String, f64)>,
-        B: Iterator<Item = ((String, String), f64)>,
+        B: IntoIterator<Item = ((String, String), f64)>,
     {
-        Self::from_maps(unigrams.collect(), bigrams.collect())
+        // Initially, `scores` contains the original unigram and bigram counts
+        let mut scores = HashMap::default();
+        let mut uni_total = 0.0;
+        for (word, uni) in unigrams {
+            scores.insert(word, (uni, HashMap::default()));
+            uni_total += uni;
+        }
+        let mut bi_total = 0.0;
+        for ((word1, word2), bi) in bigrams {
+            let Some((_, bi_scores)) = scores.get_mut(&word2) else {
+                // We throw away bigrams for which we do not have a unigram for
+                // the second word. This case shouldn't ever happen on
+                // real-world data, and in fact, it never happens on the word
+                // count lists shipped with this crate.
+                continue;
+            };
+            bi_scores.insert(word1, bi);
+            bi_total += bi;
         }
 
-    /// Create `Segmenter` from the given hashmaps (using ahash)
+        // Now convert the counts in `scores` to the values we actually want,
-    ///
+        // namely logarithms of relative frequencies
-    /// Note: the `String` types used in this API are defined in the `smartstring` crate. Any
+        for (uni, bi_scores) in scores.values_mut() {
-    /// `&str` or `String` can be converted into the `String` used here by calling `into()` on it.
-    /// The `HashMap` type here refers to `std::collections::HashMap` parametrized with the
-    /// `ahash::RandomState`.
-    pub fn from_maps(
-        mut unigrams: HashMap<String, f64>,
-        mut bigrams: HashMap<(String, String), f64>,
-    ) -> Self {
-        let uni_total = unigrams.values().sum::<f64>();
-        let bi_total = bigrams.values().sum::<f64>();
-        for uni in unigrams.values_mut() {
             *uni = (*uni / uni_total).log10();
-        }
+            for bi in bi_scores.values_mut() {
-        for bi in bigrams.values_mut() {
                 *bi = (*bi / bi_total).log10();
             }
+        }
+
         Self {
             uni_total_log10: uni_total.log10(),
-            unigrams,
+            scores,
-            bigrams,
             limit: DEFAULT_LIMIT,
         }
     }
@@ -91,24 +101,25 @@ impl Segmenter {
     }
 
     fn score(&self, word: &str, previous: Option<&str>) -> f64 {
+        let (uni, bi_scores) = match self.scores.get(word) {
+            Some((uni, bi_scores)) => (uni, bi_scores),
+            // Penalize words not found in the unigrams according
+            // to their length, a crucial heuristic.
+            None => return 1.0 - self.uni_total_log10 - word.len() as f64,
+        };
+
         if let Some(prev) = previous {
-            if let Some(bi) = self.bigrams.get(&(prev.into(), word.into())) {
+            if let Some(bi) = bi_scores.get(prev) {
-                if let Some(uni) = self.unigrams.get(prev) {
+                if let Some((uni_prev, _)) = self.scores.get(prev) {
                     // Conditional probability of the word given the previous
                     // word. The technical name is "stupid backoff" and it's
                     // not a probability distribution but it works well in practice.
-                    return bi - uni;
+                    return bi - uni_prev;
                 }
             }
         }
 
-        match self.unigrams.get(word) {
+        *uni
-            // Probability of the given word
-            Some(uni) => *uni,
-            // Penalize words not found in the unigrams according
-            // to their length, a crucial heuristic.
-            None => 1.0 - self.uni_total_log10 - word.len() as f64,
-        }
     }
 
     /// Customize the word length `limit`

instant-segment/src/test_data.rs

@@ -56,7 +56,7 @@ pub fn segmenter(dir: PathBuf) -> Segmenter {
         ln.clear();
     }
 
-    Segmenter::from_maps(unigrams, bigrams)
+    Segmenter::new(unigrams, bigrams)
 }
 
 pub fn crate_data_dir() -> PathBuf {