commit f146f95efcc01b7aa6c8082f1b898b6f6a1890db
Author: Miroslav Misek <miroslav.misek@netgarden.cz>
Date:   Thu Mar 27 08:58:59 2025 +0100

    Initial commit

diff --git a/.idea/mergefs.iml b/.idea/mergefs.iml
new file mode 100644
index 0000000..c956989
--- /dev/null
+++ b/.idea/mergefs.iml
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="WEB_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$" />
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>
\ No newline at end of file
diff --git a/.idea/modules.xml b/.idea/modules.xml
new file mode 100644
index 0000000..b334d7e
--- /dev/null
+++ b/.idea/modules.xml
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/mergefs.iml" filepath="$PROJECT_DIR$/.idea/mergefs.iml" />
+    </modules>
+  </component>
+</project>
\ No newline at end of file
diff --git a/.idea/workspace.xml b/.idea/workspace.xml
new file mode 100644
index 0000000..0e0384f
--- /dev/null
+++ b/.idea/workspace.xml
@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectViewState">
+    <option name="autoscrollFromSource" value="true" />
+    <option name="autoscrollToSource" value="true" />
+    <option name="hideEmptyMiddlePackages" value="true" />
+    <option name="showLibraryContents" value="true" />
+  </component>
+</project>
\ No newline at end of file
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..1ca2857
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module netgarden.dev/netgarden/mergefs
+
+go 1.24.1
diff --git a/mergefs.go b/mergefs.go
new file mode 100644
index 0000000..2197792
--- /dev/null
+++ b/mergefs.go
@@ -0,0 +1,66 @@
+package mergefs
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+)
+
+// Merge merges the given filesystems together,
+func Merge(filesystems ...fs.FS) fs.FS {
+	return MergedFS{filesystems: filesystems}
+}
+
+// MergedFS combines filesystems. Each filesystem can serve different paths.
+// The first FS takes precedence
+type MergedFS struct {
+	filesystems []fs.FS
+}
+
+// Open opens the named file.
+func (mfs MergedFS) Open(name string) (fs.File, error) {
+	for _, fs := range mfs.filesystems {
+		file, err := fs.Open(name)
+		if err == nil { // TODO should we return early when it's not an os.ErrNotExist? Should we offer options to decide this behaviour?
+			return file, nil
+		}
+	}
+	return nil, os.ErrNotExist
+}
+
+// ReadDir reads from the directory, and produces a DirEntry array of different
+// directories.
+//
+// It iterates through all different filesystems that exist in the mfs MergeFS
+// filesystem slice and it identifies overlapping directories that exist in different
+// filesystems
+func (mfs MergedFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	dirsMap := make(map[string]fs.DirEntry)
+	notExistCount := 0
+	for _, filesystem := range mfs.filesystems {
+		dir, err := fs.ReadDir(filesystem, name)
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				notExistCount++
+				continue
+			}
+			return nil, err
+		}
+		for _, v := range dir {
+			if _, ok := dirsMap[v.Name()]; !ok {
+				dirsMap[v.Name()] = v
+			}
+		}
+		continue
+	}
+	if len(mfs.filesystems) == notExistCount {
+		return nil, fs.ErrNotExist
+	}
+	dirs := make([]fs.DirEntry, 0, len(dirsMap))
+
+	for _, value := range dirsMap {
+		dirs = append(dirs, value)
+	}
+
+	return dirs, nil
+}