From d4f4fe970da59d734089c3d60acbd2288d5f61f6 Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Mon, 27 Jan 2020 11:33:10 -0800
Subject: [PATCH] add Value::nameOf()

---
 src/data.cpp      | 19 ++++++++++++++++++-
 src/pvxs/data.h   | 10 ++++++++++
 test/testdata.cpp | 18 +++++++++++++++++-
 3 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/src/data.cpp b/src/data.cpp
index 0c5aaf6..1b3d20b 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -288,6 +288,24 @@ bool Value::idStartsWith(const std::string& prefix) const
     return ID.size()>=prefix.size() && prefix==ID.substr(0u, prefix.size());
 }
 
+const std::string &Value::nameOf(const Value& decendent) const
+{
+    if(!store || !decendent.store)
+        throw NoField();
+    auto pidx = store->index();
+    auto didx = decendent.store->index();
+    if(pidx >= didx || didx >= store->top->members.size())
+        throw std::logic_error("not a decendent");
+
+    // inefficient, but we don't keep a reverse mapping
+    for(auto& it : desc->mlookup) {
+        if(it.second == didx-pidx)
+            return it.first;
+    }
+
+    throw std::logic_error("missing decendent");
+}
+
 namespace {
 // C-style cast between scalar storage types, and print to string (base 10)
 template<typename Src>
@@ -796,7 +814,6 @@ FieldStorage::~FieldStorage()
 size_t FieldStorage::index() const
 {
     const size_t ret = this-top->members.data();
-    assert(this==&top->members[ret]);
     return ret;
 }
 
diff --git a/src/pvxs/data.h b/src/pvxs/data.h
index a106f2d..9763901 100644
--- a/src/pvxs/data.h
+++ b/src/pvxs/data.h
@@ -385,6 +385,16 @@ public:
 //    int compareValue(const Value&);
     inline int compareType(const Value& o) { return desc==o.desc; }
 
+    /** Return our name for a decendent field.
+     * @code
+     *   Value v = ...;
+     *   assert(v.nameOf(v["some.field"])=="some.field");
+     * @endcode
+     * @throws NoField unless both this and decendent are valid()
+     * @throws std::logic_error if decendent is not actually a decendent
+     */
+    const std::string& nameOf(const Value& decendent) const;
+
     // access to Value's ... value
     // not for Struct
 
diff --git a/test/testdata.cpp b/test/testdata.cpp
index 0b0741c..20e2c22 100644
--- a/test/testdata.cpp
+++ b/test/testdata.cpp
@@ -445,11 +445,26 @@ void testAssign()
     testOk1(!val["alarm"].isMarked(true, false));
 }
 
+void testName()
+{
+    testDiag("%s", __func__);
+
+    auto def = nt::NTScalar{TypeCode::String}.build();
+    auto val = def.create();
+
+    testEq(val.nameOf(val["value"]), "value");
+    testEq(val.nameOf(val["alarm.status"]), "alarm.status");
+
+    testThrows<std::logic_error>([&val]() {
+        val.nameOf(val);
+    });
+}
+
 } // namespace
 
 MAIN(testdata)
 {
-    testPlan(67);
+    testPlan(70);
     testSerialize1();
     testDeserialize1();
     testSimpleDef();
@@ -457,6 +472,7 @@ MAIN(testdata)
     testDeserialize2();
     testTraverse();
     testAssign();
+    testName();
     cleanup_for_valgrind();
     return testDone();
 }