fix: resolve blank node errors

__tests__/main-test.ts

@@ -2,6 +2,7 @@ import { DataFactory, Parser } from 'n3';
 import fs from 'fs';
 import path from 'path';
 import { write } from '../lib';
+import 'jest-rdf';
 
 async function getQuads(file: string) {
   const parser = new Parser({ rdfStar: true } as any);
@@ -16,10 +17,21 @@ async function getQuads(file: string) {
   };
 }
 
+const loose: Record<string, boolean | undefined> = {
+  'bnodes5.ttl': true,
+};
+
 it('It should correctly write turtle files', async () => {
   for (const file of fs.readdirSync(path.join(__dirname, '..', 'data'))) {
-    const { string } = await getQuads(file);
-    expect(string.replace(/b0_/g, '')).toEqual(fs.readFileSync(path.join(__dirname, '..', 'data', file)).toString());
+    const { string, quads } = await getQuads(file);
+
+    if (loose[file]) {
+      // If loose we only need the quads to match when we re-parse the string
+      expect((new Parser()).parse(string)).toBeRdfIsomorphic(quads);
+    } else {
+      // If not loose we expect an exact string match
+      expect(string.replace(/b\d+_/g, '')).toEqual(fs.readFileSync(path.join(__dirname, '..', 'data', file)).toString());
+    }
   }
 });
 

data/bnodes.ttl

@@ -6,4 +6,4 @@ ex:t a _:b1 .
 
 ex:k a _:b1 .
 
-_:b2 a ex:Thing .
+[] a ex:Thing .

data/bnodes1.ttl

@@ -0,0 +1,9 @@
+@prefix ex: <http://example.com/ns#> .
+
+ex:j a _:b1 .
+
+ex:t a _:b1, [] .
+
+ex:k a _:b1, [] .
+
+[] a ex:Thing .

data/bnodes2.ttl

@@ -0,0 +1,5 @@
+@prefix ex: <http://example.com/ns#> .
+
+[] ex:p [
+    ex:p2 ex:o
+  ] .

data/bnodes3.ttl

@@ -0,0 +1,3 @@
+@prefix ex: <http://example.com/ns#> .
+
+[] ex:p [], [], ([] []) .

data/bnodes4.ttl

@@ -0,0 +1,3 @@
+@prefix ex: <http://example.com/ns#> .
+
+_:s3 a _:s3 .

data/bnodes5.ttl

@@ -0,0 +1,15 @@
+@prefix ex: <http://example.com/ns#> .
+
+[] ex:p [
+    ex:p []
+  ] .
+
+_:s1 ex:t ex:k .
+
+ex:s ex:p _:s1 .
+
+_:s2 ex:t ex:k .
+
+_:s3 ex:p _:s2 .
+
+_:s3 a _:s3 .

data/bnodes6.ttl

@@ -0,0 +1,7 @@
+@prefix : <http://example.com/ns#> .
+
+:o :p _:s1 .
+
+:o1 :p _:s1 .
+
+_:s1 :p :o .

lib/ttlwriter.ts

@@ -51,21 +51,63 @@ export class TTLWriter {
 
     this.writer.newLine(1);
 
+    // First write Named Node subjects
     for (const subject of this.store.getSubjects(null, null, null)) {
       if (subject.termType === 'NamedNode') {
         await this.writeTurtleSubject(subject);
       }
     }
 
+    // Then write blank node subjects that can be anonymized at the top level
     for (const subject of this.store.getSubjects(null, null, null)) {
-      await this.writeTurtleSubject(subject);
+      if (
+        subject.termType === 'BlankNode'
+        && !this.explicitBnodes.has(subject.value)
+        // Ensure still in store as subject
+        && this.store.getQuads(subject, null, null, null).length > 0
+        && this.store.getQuads(null, subject, null, null).length === 0
+        && this.store.getQuads(null, null, subject, null).length === 0
+      ) {
+        await this.writeTurtleSubject(subject, true);
+      }
+    }
+
+    // Next write blank nodes that cannot be anonymized within another set of statements
+    // (it is not an explicit bnode,
+    // occurs as the object of one quad,
+    // and only as the subject in other quads)
+    for (const subject of this.store.getSubjects(null, null, null)) {
+      // Ensure still in store as subject
+      if (
+        subject.termType === 'BlankNode' && !(
+          this.store.getQuads(null, null, subject, null).length !== 1
+          || !this.store.getQuads(null, null, subject, null)[0].subject.equals(subject)
+        )
+      ) {
+        this.explicitBnodes.add(subject.value);
+        await this.writeTurtleSubject(subject);
+      }
+    }
+
+    for (const subject of this.store.getSubjects(null, null, null)) {
+      // Ensure still in store as subject
+      if (this.store.getQuads(subject, null, null, null).length > 0) {
+        if (subject.termType === 'BlankNode') {
+          this.explicitBnodes.add(subject.value);
+        }
+        await this.writeTurtleSubject(subject);
+      }
     }
 
     this.writer.end();
   }
 
-  private async writeTurtleSubject(term: Term) {
-    this.writer.add(await this.termToString(term));
+  private async writeTurtleSubject(term: Term, anonymizeSubject = false) {
+    if (anonymizeSubject) {
+      this.writer.add('[]');
+    } else {
+      this.writer.add(await this.termToString(term));
+    }
     this.writer.add(' ');
     this.writer.indent();
     await this.writeTurtlePredicates(term);
@@ -83,18 +125,17 @@ export class TTLWriter {
         }
       }
     } if (term.termType === 'Literal' && (term.datatypeString === 'http://www.w3.org/2001/XMLSchema#integer'
-    || term.datatypeString === 'http://www.w3.org/2001/XMLSchema#boolean')) {
+      || term.datatypeString === 'http://www.w3.org/2001/XMLSchema#boolean')) {
       return term.value;
     }
     if (term.termType === 'Quad') {
       if (!term.graph.equals(DataFactory.defaultGraph())) {
         throw new Error('Default graph expected on nested quads');
       }
-      return `<<${await this.termToString(term.subject as any)} ${
-        term.predicate.termType === 'NamedNode'
-        && term.predicate.value === 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type'
-          ? 'a'
-          : await this.termToString(term.predicate as any)} ${await this.termToString(term.object as any)}>>`;
+      return `<<${await this.termToString(term.subject as any)} ${term.predicate.termType === 'NamedNode'
+          && term.predicate.value === 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type'
+        ? 'a'
+        : await this.termToString(term.predicate as any)} ${await this.termToString(term.object as any)}>>`;
     }
     return termToString(term);
   }
@@ -146,11 +187,11 @@ export class TTLWriter {
     const nonBlankObjects: Term[] = [];
     for (const object of objects) {
       if (object.termType === 'BlankNode'
-      && [
-        ...this.store.match(null, null, object),
-        ...this.store.match(null, object, null),
-      ].length === 0
-      && !this.explicitBnodes.has(object.value)
+        && [
+          ...this.store.match(null, null, object),
+          ...this.store.match(null, object, null),
+        ].length === 0
+        && !this.explicitBnodes.has(object.value)
       ) {
         blankObjects.push(object);
       } else {
@@ -176,11 +217,13 @@ export class TTLWriter {
         }
         if (!(await this.writeList(blank))) {
           this.writer.add('[');
-          this.writer.indent();
-          this.writer.newLine(1);
-          await this.writeTurtlePredicates(blank);
-          this.writer.deindent();
-          this.writer.newLine(1);
+          if (this.store.getQuads(blank, null, null, null).length > 0) {
+            this.writer.indent();
+            this.writer.newLine(1);
+            await this.writeTurtlePredicates(blank);
+            this.writer.deindent();
+            this.writer.newLine(1);
+          }
           this.writer.add(']');
         }
       }

package.json

@@ -48,6 +48,7 @@
     "eslint-config-airbnb-base": "^15.0.0",
     "eslint-plugin-import": "^2.27.5",
     "jest": "^29.3.1",
+    "jest-rdf": "^1.8.0",
     "pre-commit": "^1.2.2",
     "semantic-release": "^20.0.2",
     "ts-jest": "^29.0.5",