diff --git a/src/test_util/mock_client.rs b/src/test_util/mock_client.rs
index 4cbefc0..626833f 100644
--- a/src/test_util/mock_client.rs
+++ b/src/test_util/mock_client.rs
@@ -1,11 +1,105 @@
+use std::{
+    cell::{RefCell, RefMut},
+    collections::{HashMap, hash_map::Entry},
+};
+
 use crate::{
-    Result, file,
+    Error, Result,
+    file::{self, FileTrait},
     sharry::{AliasID, Client, FileID, ShareID, Uri, json},
 };
 
 use super::mock_ids::CheckID;
 
-pub struct MockClient;
+#[derive(Debug)]
+pub struct MockClient {
+    shares: RefCell<HashMap<String, MockShare>>,
+}
+
+#[derive(Debug, Default)]
+struct MockShare {
+    files: HashMap<String, MockFile>,
+}
+
+#[derive(Debug)]
+struct MockFile {
+    name: String,
+    size: u64,
+    offset: u64,
+}
+
+impl MockClient {
+    fn insert_share(&self, share_id: &ShareID) -> Result<()> {
+        let mut shares = self.shares.borrow_mut();
+
+        let Entry::Vacant(entry) = shares.entry(share_id.to_string()) else {
+            return Err(Error::response(format_args!(
+                "Can't create share {share_id:?}!"
+            )));
+        };
+
+        entry.insert(MockShare::default());
+
+        Ok(())
+    }
+
+    fn insert_file(
+        &self,
+        share_id: &ShareID,
+        file_id: &FileID,
+        name: String,
+        size: u64,
+    ) -> Result<()> {
+        let mut share = self.get_share_mut(share_id)?;
+
+        let Entry::Vacant(entry) = share.files.entry(file_id.to_string()) else {
+            return Err(Error::response(format_args!(
+                "Can't create file {file_id:?}!"
+            )));
+        };
+
+        entry.insert(MockFile {
+            name,
+            size,
+            offset: 0,
+        });
+
+        Ok(())
+    }
+
+    fn get_share_mut<'t>(&'t self, share_id: &ShareID) -> Result<RefMut<'t, MockShare>> {
+        let share_id = &share_id.to_string();
+        let shares = self.shares.borrow_mut();
+
+        shares
+            .get(share_id)
+            .ok_or_else(|| Error::response(format_args!("Can't find share {share_id:?}!")))?;
+
+        // share exists
+        Ok(RefMut::map(shares, |shares| {
+            shares.get_mut(share_id).expect("checked but None!")
+        }))
+    }
+
+    fn get_file_mut<'t>(
+        &'t self,
+        share_id: &ShareID,
+        file_id: &FileID,
+    ) -> Result<RefMut<'t, MockFile>> {
+        let file_id = &file_id.to_string();
+        let share = self.get_share_mut(share_id)?;
+
+        share
+            .files
+            .get(file_id)
+            .ok_or_else(|| Error::response(format_args!("Can't find file {file_id:?}!")))?;
+
+        // file exists
+        Ok(RefMut::map(share, move |share| {
+            share.files.get_mut(file_id).expect("checked but None!")
+        }))
+    }
+}
 
 impl Client for MockClient {
     fn share_create(
@@ -16,7 +110,10 @@ impl Client for MockClient {
     ) -> Result<ShareID> {
         (uri, alias_id).check()?;
 
-        Ok(true.into())
+        let share_id = true.into();
+        self.insert_share(&share_id)?;
+
+        Ok(share_id)
     }
 
     fn share_notify(&self, uri: &Uri, alias_id: &AliasID, share_id: &ShareID) -> crate::Result<()> {
@@ -31,12 +128,20 @@ impl Client for MockClient {
         uri: &Uri,
         alias_id: &AliasID,
         share_id: &ShareID,
-        _: &file::Checked,
+        file: &file::Checked,
     ) -> Result<FileID> {
         (uri, alias_id).check()?;
         share_id.check()?;
 
-        Ok(true.into())
+        let file_id = true.into();
+        self.insert_file(
+            share_id,
+            &file_id,
+            file.get_name().to_string(),
+            file.get_size(),
+        )?;
+
+        Ok(file_id)
     }
 
     fn file_patch(
@@ -50,9 +155,10 @@ impl Client for MockClient {
         (share_id, chunk.get_file_id()).check()?;
 
         // TODO: `chunk` must align to a full MiB
-
-        // Ok(())
+        let file = self.get_file_mut(share_id, chunk.get_file_id())?;
 
         todo!()
+
+        // Ok(())
     }
 }
diff --git a/src/test_util/mock_ids.rs b/src/test_util/mock_ids.rs
index 9660c4e..5618fdd 100644
--- a/src/test_util/mock_ids.rs
+++ b/src/test_util/mock_ids.rs
@@ -8,15 +8,22 @@ const VALID_ALIAS: &str = "valid-alias";
 const VALID_SHARE: &str = "valid-share";
 const VALID_FILE: &str = "valid-file";
 
+fn make_invalid(valid: &str) -> String {
+    let invalid = valid.replace("valid", "invalid");
+    assert_ne!(valid, invalid);
+
+    invalid
+}
+
 pub trait CheckID {
     fn check(self) -> Result<()>;
 }
 
 impl CheckID for (&Uri, &AliasID) {
     fn check(self) -> Result<()> {
-        if self.0.to_string() != VALID_URI {
+        if self.0.to_string() == make_invalid(VALID_URI) {
             Err(self.0.into())
-        } else if self.1.as_ref() != VALID_ALIAS {
+        } else if self.1.as_ref() == make_invalid(VALID_ALIAS) {
             Err(self.1.into())
         } else {
             Ok(())
@@ -26,10 +33,10 @@ impl CheckID for (&Uri, &AliasID) {
 
 impl CheckID for &ShareID {
     fn check(self) -> Result<()> {
-        if self.to_string() == VALID_SHARE {
-            Ok(())
-        } else {
+        if self.to_string() == make_invalid(VALID_SHARE) {
             Err(self.into())
+        } else {
+            Ok(())
         }
     }
 }
@@ -38,21 +45,14 @@ impl CheckID for (&ShareID, &FileID) {
     fn check(self) -> Result<()> {
         self.0.check()?;
 
-        if self.1.to_string() == VALID_FILE {
-            Ok(())
-        } else {
+        if self.1.to_string() == make_invalid(VALID_FILE) {
             Err(self.1.into())
+        } else {
+            Ok(())
         }
     }
 }
 
-fn make_invalid(valid: &str) -> String {
-    let invalid = valid.replace("valid", "invalid");
-    assert_ne!(valid, invalid);
-
-    invalid
-}
-
 impl From<bool> for Uri {
     fn from(value: bool) -> Self {
         if value {
@@ -111,6 +111,17 @@ mod tests {
         assert!(matches!((&share_id, &file_id).check(), Ok(())));
     }
 
+    #[test]
+    fn default_is_valid() {
+        let uri = Uri::default();
+        let share_id = ShareID::default();
+        let file_id = FileID::default();
+
+        assert!(matches!((&uri, &AliasID::from(true)).check(), Ok(())));
+        assert!(matches!(share_id.check(), Ok(())));
+        assert!(matches!((&share_id, &file_id).check(), Ok(())));
+    }
+
     #[test]
     fn false_makes_invalids() {
         let uri = Uri::from(true);