feat: tantivy lexical index over Alpha-ID texts

src/lexical.rs

@@ -1 +1,74 @@
-// implemented in a later task
+use crate::model::{AlphaIdEntry, AppError};
+use crate::normalize::normalize;
+use tantivy::collector::TopDocs;
+use tantivy::query::QueryParser;
+use tantivy::schema::{Schema, Value, STORED, TEXT};
+use tantivy::{doc, Index, TantivyDocument};
+
+pub struct LexicalIndex {
+    index: Index,
+    text_field: tantivy::schema::Field,
+    idx_field: tantivy::schema::Field,
+}
+
+impl LexicalIndex {
+    fn schema() -> (Schema, tantivy::schema::Field, tantivy::schema::Field) {
+        let mut b = Schema::builder();
+        let text_field = b.add_text_field("text", TEXT);
+        let idx_field = b.add_u64_field("idx", STORED);
+        (b.build(), text_field, idx_field)
+    }
+
+    pub fn build_in_ram(entries: &[AlphaIdEntry]) -> Result<Self, AppError> {
+        let (schema, text_field, idx_field) = Self::schema();
+        let index = Index::create_in_ram(schema);
+        let mut w = index
+            .writer(50_000_000)
+            .map_err(|e| AppError::Io(format!("tantivy writer: {e}")))?;
+        for (i, e) in entries.iter().enumerate() {
+            w.add_document(doc!(text_field => normalize(&e.text), idx_field => i as u64))
+                .map_err(|e| AppError::Io(format!("tantivy add: {e}")))?;
+        }
+        w.commit()
+            .map_err(|e| AppError::Io(format!("tantivy commit: {e}")))?;
+        Ok(Self {
+            index,
+            text_field,
+            idx_field,
+        })
+    }
+
+    /// Returns (entry_index, bm25_score) descending.
+    pub fn search(&self, query: &str, k: usize) -> Result<Vec<(usize, f32)>, AppError> {
+        let reader = self
+            .index
+            .reader()
+            .map_err(|e| AppError::Io(format!("tantivy reader: {e}")))?;
+        let searcher = reader.searcher();
+        let norm = normalize(query);
+        let sanitized = norm.replace(['(', ')', '"', ':'], " ");
+        // Use OR semantics (default) for recall-favoring behaviour; fall back to
+        // the raw normalised query on parse error.
+        let parsed = QueryParser::for_index(&self.index, vec![self.text_field])
+            .parse_query(&sanitized)
+            .or_else(|_| {
+                let mut qp = QueryParser::for_index(&self.index, vec![self.text_field]);
+                qp.set_conjunction_by_default();
+                qp.parse_query(&norm)
+            })
+            .map_err(|e| AppError::Parse(format!("query: {e}")))?;
+        let top = searcher
+            .search(&parsed, &TopDocs::with_limit(k.max(1)))
+            .map_err(|e| AppError::Io(format!("tantivy search: {e}")))?;
+        let mut out = Vec::new();
+        for (score, addr) in top {
+            let d: TantivyDocument = searcher
+                .doc(addr)
+                .map_err(|e| AppError::Io(format!("tantivy doc: {e}")))?;
+            if let Some(v) = d.get_first(self.idx_field).and_then(|v| v.as_u64()) {
+                out.push((v as usize, score));
+            }
+        }
+        Ok(out)
+    }
+}