feat: supports CLIENTTAGDENY parsing and "is blocked" function

This introduces a `MessageTags.parseDenylist` which parses
the value of `CLIENTTAGDENY=<value>` string into a structure containing
everything you need to perform O(#allowed tags) or O(#denied tags)
check for whether a tag is allowed or not.

Then, a `MessageTags.isBlocked` function can be used to leverage that
structure to perform this check.

src/messagetags.js

@@ -6,6 +6,8 @@ module.exports.decodeValue = decodeValue;
 module.exports.encodeValue = encodeValue;
 module.exports.decode = decode;
 module.exports.encode = encode;
+module.exports.parseDenylist = parseDenylist;
+module.exports.isBlocked = isBlocked;
 
 const tokens_map = {
     '\\\\': '\\',
@@ -68,3 +70,45 @@ function encode(tags, separator = ';') {
 
     return parts.join(separator);
 }
+
+// Parses a raw CLIENTTAGDENY= denylist
+// into a { allBlockedByDefault: boolean, explicitlyAccepted: string[], explicitlyDenied: string[] }
+// structure.
+function parseDenylist(raw) {
+    const denylist = {
+        allBlockedByDefault: false,
+        explicitlyAccepted: [],
+        explicitlyDenied: []
+    };
+    const parts = raw.split(',');
+
+    for (let idx = 0; idx < parts.length; idx++) {
+        const tag = parts[idx];
+        if (tag === '') {
+            continue;
+        }
+
+        if (tag === '*') {
+            denylist.allBlockedByDefault = true;
+            continue;
+        }
+
+        if (tag[0] === '-') {
+            denylist.explicitlyAccepted.push(tag.slice(1));
+        } else {
+            denylist.explicitlyDenied.push(tag);
+        }
+    }
+
+    return denylist;
+}
+
+// Takes a parsed denylist and returns whether tag is allowed
+// according to current denial policies.
+function isBlocked(denylist, tag) {
+    if (denylist.allBlockedByDefault) {
+        return !denylist.explicitlyAccepted.includes(tag);
+    } else {
+        return denylist.explicitlyDenied.includes(tag);
+    }
+}

test/messagetags.js

@@ -8,6 +8,49 @@ const assert = chai.assert;
 chai.use(require('chai-subset'));
 
 describe('src/messagetags.js', function() {
+    describe('CLIENTTAGDENY= parsing', function() {
+        it('should parse CLIENTTAGDENY=', function() {
+            assert.deepEqual(MessageTags.parseDenylist(''), {
+                allBlockedByDefault: false,
+                explicitlyDenied: [],
+                explicitlyAccepted: []
+            });
+        });
+
+        it('should parse CLIENTTAGDENY=*,-a', function() {
+            assert.deepEqual(MessageTags.parseDenylist('*,-a'), {
+                allBlockedByDefault: true,
+                explicitlyAccepted: ['a'],
+                explicitlyDenied: []
+            });
+        });
+
+        it('should parse CLIENTTAGDENY=a,b', function() {
+            assert.deepEqual(MessageTags.parseDenylist('a,b'), {
+                allBlockedByDefault: false,
+                explicitlyAccepted: [],
+                explicitlyDenied: ['a', 'b']
+            });
+        });
+    });
+
+    describe('CLIENTTAGDENY= logic', function() {
+        it('should block all tags (`b`) with * and no exception', function() {
+            assert.isTrue(MessageTags.isBlocked(MessageTags.parseDenylist('*'), 'b'));
+        });
+
+        it('should not block all tags with * and exceptions (`c`, `a`)', function() {
+            assert.isFalse(MessageTags.isBlocked(MessageTags.parseDenylist('*,-c,-a'), 'a'));
+            assert.isFalse(MessageTags.isBlocked(MessageTags.parseDenylist('*,-c,-a'), 'c'));
+            assert.isTrue(MessageTags.isBlocked(MessageTags.parseDenylist('*,-c,-a'), 'b'));
+        });
+
+        it('should block a specific tag if no * is present', function() {
+            assert.isTrue(MessageTags.isBlocked(MessageTags.parseDenylist('a'), 'a'));
+            assert.isFalse(MessageTags.isBlocked(MessageTags.parseDenylist('a'), 'b'));
+        });
+    });
+
     describe('value encoding', function() {
         it('should decode characters to correct strings', function() {
             const plain = "Some people use IRC; others don't \\o/ Note: Use IRC\r\n";