Allow override of a protected term that uses the same definition.

- The only caveat to the "same definition" is that if the new
  definition does not include `@protected`, this flag is carried
  forward to maintain the protection status.

Author: dlongley

lib/context.js

@@ -301,33 +301,10 @@ api.createTermDefinition = (
       {code: 'invalid term definition', context: localCtx});
   }
 
-  // FIXME if(1.1) ... ?
-  if(activeCtx.protected.hasOwnProperty(term) &&
-    !isPropertyTermScopedContext) {
-    const protectedMode = (options && options.protectedMode) || 'error';
-    if(protectedMode === 'error') {
-      throw new JsonLdError(
-        'Invalid JSON-LD syntax; tried to redefine a protected term.',
-        'jsonld.SyntaxError',
-        {code: 'protected term redefinition', context: localCtx, term});
-    } else if(protectedMode === 'warn') {
-      // FIXME: remove logging and use a handler
-      console.warn('WARNING: protected term redefinition', {term});
-      return;
-    }
-    throw new JsonLdError(
-      'Invalid protectedMode.',
-      'jsonld.SyntaxError',
-      {code: 'invalid protected mode', context: localCtx, term,
-        protectedMode});
-  }
-
   // remove old mapping
   let previousMapping = null;
   if(activeCtx.mappings.has(term)) {
-    if(isTypeScopedContext) {
-      previousMapping = activeCtx.mappings.get(term);
-    }
+    previousMapping = activeCtx.mappings.get(term);
     activeCtx.mappings.delete(term);
   }
 
@@ -652,6 +629,33 @@ api.createTermDefinition = (
       'Invalid JSON-LD syntax; @context and @preserve cannot be aliased.',
       'jsonld.SyntaxError', {code: 'invalid keyword alias', context: localCtx});
   }
+
+  // FIXME if(1.1) ... ?
+  if(previousMapping && previousMapping.protected &&
+    !isPropertyTermScopedContext) {
+    // force new term to continue to be protected and see if the mappings would
+    // be equal
+    activeCtx.protected[term] = true;
+    mapping.protected = true;
+    if(!_areTermDefinitionsEqual(previousMapping, mapping)) {
+      const protectedMode = (options && options.protectedMode) || 'error';
+      if(protectedMode === 'error') {
+        throw new JsonLdError(
+          'Invalid JSON-LD syntax; tried to redefine a protected term.',
+          'jsonld.SyntaxError',
+          {code: 'protected term redefinition', context: localCtx, term});
+      } else if(protectedMode === 'warn') {
+        // FIXME: remove logging and use a handler
+        console.warn('WARNING: protected term redefinition', {term});
+        return;
+      }
+      throw new JsonLdError(
+        'Invalid protectedMode.',
+        'jsonld.SyntaxError',
+        {code: 'invalid protected mode', context: localCtx, term,
+          protectedMode});
+    }
+  }
 };
 
 /**
@@ -1303,3 +1307,44 @@ function _findContextUrls(input, urls, replace, base) {
     }
   }
 }
+
+function _areTermDefinitionsEqual(d1, d2) {
+  const k1 = Object.keys(d1);
+  const k2 = Object.keys(d2);
+  if(k1.length !== k2.length) {
+    return false;
+  }
+
+  for(const k of k1) {
+    const v1 = d1[k];
+    const v2 = d2[k];
+    if(k === '@context') {
+      if(JSON.stringify(v1) !== JSON.stringify(v2)) {
+        return false;
+      }
+      continue;
+    }
+    const isArray = Array.isArray(v1);
+    if(isArray !== Array.isArray(v2)) {
+      return false;
+    }
+    // `@container`
+    if(isArray) {
+      if(v1.length !== v2.length) {
+        return false;
+      }
+      v1.sort();
+      v2.sort();
+      for(let i = 0; i < v1.length; ++i) {
+        if(v1[i] !== v2[i]) {
+          return false;
+        }
+      }
+    }
+    // strings
+    if(v1 !== v2) {
+      return false;
+    }
+  }
+  return true;
+}