1 files changed, 60 insertions, 0 deletions
diff --git a/template/src/additional_cid.rs b/template/src/additional_cid.rs
new file mode 100644
index 0000000..eecf2e7
--- /dev/null
+++ b/template/src/additional_cid.rs
@@ -0,0 +1,60 @@
+use std::collections::{
+    HashMap, HashSet
+};
+
+use serde::{Serialize, Serializer};
+
+use mail_core::Resource;
+use mail_headers::header_components::ContentId;
+
+pub struct AdditionalCIds<'a> {
+    additional_resources: &'a [&'a HashMap<String, Resource>]
+}
+
+impl<'a> AdditionalCIds<'a> {
+
+    /// Creates a new `AdditionalCIds` instance.
+    ///
+    /// All resources in the all hash maps have to be loaded to the
+    /// `Data` or `EncData` variants or using `get` can panic.
+    pub(crate) fn new(additional_resources: &'a [&'a HashMap<String, Resource>]) -> Self {
+        AdditionalCIds { additional_resources }
+    }
+
+
+    /// Returns the content id associated with the given name.
+    ///
+    /// If multiple of the maps used to create this type contain the
+    /// key the first match is returned and all later ones are ignored.
+    ///
+    /// # Panic
+    ///
+    /// If the resource exists but is not loaded (i.e. has no content id)
+    /// this will panic as this can only happen if there is a bug in the
+    /// mail code, or this type was used externally.
+    pub fn get(&self, name: &str) -> Option<&ContentId> {
+        for possible_source in self.additional_resources {
+            if let Some(res) = possible_source.get(name) {
+                return Some(res.content_id().expect("all resources should be loaded/have a content id"));
+            }
+        }
+        return None;
+    }
+}
+
+impl<'a> Serialize for AdditionalCIds<'a> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+        where S: Serializer
+    {
+        let mut existing_keys = HashSet::new();
+        serializer.collect_map(
+            self.additional_resources
+            .iter()
+            .flat_map(|m| m.iter().map(|(k, resc)| {
+                (k, resc.content_id().expect("all resources should be loaded/have a content id"))
+            }))
+            .filter(|key| existing_keys.insert(key.to_owned()))
+        )
+    }
+}
+