From d288eeb79abc08d09f535bfd5d64b623d26bd129 Mon Sep 17 00:00:00 2001
From: Jacques Lucke <jacques@blender.org>
Date: Thu, 13 May 2021 13:39:23 +0200
Subject: BLI: support looking up a key stored in Map or VectorSet

Sometimes it is useful to find the key that compares equal
to a known key. Typically that happens when the key itself
has additional data attached that is not part of its hash.

Note that the returned key reference/pointer is const, because
the caller must not change the key in a way that changes its
hash or how it compares to other keys.
---
 source/blender/blenlib/BLI_map.hh | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

(limited to 'source/blender/blenlib/BLI_map.hh')

diff --git a/source/blender/blenlib/BLI_map.hh b/source/blender/blenlib/BLI_map.hh
index 95afbfc2ec6..4d254960f34 100644
--- a/source/blender/blenlib/BLI_map.hh
+++ b/source/blender/blenlib/BLI_map.hh
@@ -604,6 +604,37 @@ class Map {
     return this->lookup_or_add_cb_as(std::forward<ForwardKey>(key), []() { return Value(); });
   }
 
+  /**
+   * Returns the key that is stored in the set that compares equal to the given key. This invokes
+   * undefined behavior when the key is not in the map.
+   */
+  const Key &lookup_key(const Key &key) const
+  {
+    return this->lookup_key_as(key);
+  }
+  template<typename ForwardKey> const Key &lookup_key_as(const ForwardKey &key) const
+  {
+    const Slot &slot = this->lookup_slot(key, hash_(key));
+    return *slot.key();
+  }
+
+  /**
+   * Returns a pointer to the key that is stored in the map that compares equal to the given key.
+   * If the key is not in the map, null is returned.
+   */
+  const Key *lookup_key_ptr(const Key &key) const
+  {
+    return this->lookup_key_ptr_as(key);
+  }
+  template<typename ForwardKey> const Key *lookup_key_ptr_as(const ForwardKey &key) const
+  {
+    const Slot *slot = this->lookup_slot_ptr(key, hash_(key));
+    if (slot == nullptr) {
+      return nullptr;
+    }
+    return slot->key();
+  }
+
   /**
    * Calls the provided callback for every key-value-pair in the map. The callback is expected
    * to take a `const Key &` as first and a `const Value &` as second parameter.
-- 
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