diff --git a/internal/repomaint/build_map.go b/internal/repomaint/build_map.go
new file mode 100644
index 0000000..049a734
--- /dev/null
+++ b/internal/repomaint/build_map.go
@@ -0,0 +1,119 @@
+package repomaint
+
+import (
+	"archive/tar"
+	"bufio"
+	"compress/gzip"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+)
+
+func (r *RepoSync) buildMap(repo string) (map[string]string, error) {
+	// create slice all filenames in repo
+	pkgs, err := r.cachedPkgs(repo)
+	if err != nil {
+		return nil, err
+	}
+
+	// open db file
+	db, fr, gzr, err := r.openDb(repo)
+	if err != nil {
+		return nil, err
+	}
+	defer fr.Close()
+	defer gzr.Close()
+
+	// for entry in db
+	pkgMap := make(map[string]string)
+	for {
+		hdr, err := db.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return nil, err
+		}
+		if filepath.Base(hdr.Name) != "desc" {
+			continue
+		}
+		if !slices.Contains(pkgs, filepath.Dir(hdr.Name)) {
+			continue
+		}
+		pkgName, pkgFile, err := parseDesc(db)
+		if err != nil {
+			slog.Warn("failed to parse desc file", "pkg", hdr.Name, "err", err)
+			continue
+		}
+
+		pkgMap[pkgName] = pkgFile
+	}
+
+	return pkgMap, nil
+}
+
+func (r *RepoSync) cachedPkgs(repo string) ([]string, error) {
+	repoPath := filepath.Join(repo, repoArch)
+	f, err := r.root.Open(repoPath)
+	if err != nil {
+		return nil, err
+	}
+	files, err := f.ReadDir(-1)
+	if err != nil {
+		return nil, err
+	}
+	var pkgs []string
+	for _, f := range files {
+		pkg := strings.TrimSuffix(f.Name(), pkgSuffix)
+		pkgs = append(pkgs, pkg)
+	}
+
+	return pkgs, nil
+}
+
+func (r *RepoSync) openDb(repo string) (*tar.Reader, *os.File, *gzip.Reader, error) {
+
+	repoPath := filepath.Join(repo, repoArch)
+	path := filepath.Join(repoPath, repo+dbSuffix)
+	f, err := r.root.Open(path)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+	gr, err := gzip.NewReader(f)
+	if err != nil {
+		f.Close()
+		return nil, nil, nil, err
+	}
+	tr := tar.NewReader(gr)
+
+	return tr, f, gr, nil
+}
+
+func parseDesc(f io.Reader) (pkgname, filename string, err error) {
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		switch scanner.Text() {
+		case "%NAME%":
+			scanner.Scan()
+			pkgname = scanner.Text()
+		case "%FILENAME%":
+			scanner.Scan()
+			filename = scanner.Text()
+		}
+		if pkgname != "" && filename != "" {
+			break
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", "", err
+	}
+	if pkgname == "" || filename == "" {
+		return "", "", fmt.Errorf("incomplete or corrupt desc")
+	}
+
+	return pkgname, filename, nil
+}
diff --git a/internal/repomaint/create_map.go b/internal/repomaint/create_map.go
deleted file mode 100644
index 32d69ee..0000000
--- a/internal/repomaint/create_map.go
+++ /dev/null
@@ -1,11 +0,0 @@
-package repomaint
-
-func (r *RepoSync) createMap(repo string) (map[string]string, error) {
-	// create slice all filenames in repo
-	// open db file
-	// for file in slice
-	// find in db
-	// insert into map as map[packagename] = filename
-	// return map
-	return nil, nil
-}
diff --git a/internal/repomaint/repomaint.go b/internal/repomaint/repomaint.go
index b5a2a7a..90f3106 100644
--- a/internal/repomaint/repomaint.go
+++ b/internal/repomaint/repomaint.go
@@ -6,6 +6,12 @@ import (
 	"github.com/ewpt3ch/pkgstash/internal/cache"
 )
 
+const (
+	repoArch  = "os/x86_64"
+	dbSuffix  = ".db.tar.gz"
+	pkgSuffix = "-x86_64.pkg.tar.zst"
+)
+
 type CacheClient interface {
 	FetchDB() error
 	Fetch(relpath string) (*cache.CacheFile, error)
diff --git a/internal/repomaint/repomaint_test.go b/internal/repomaint/repomaint_test.go
index 49a1c02..584b9d4 100644
--- a/internal/repomaint/repomaint_test.go
+++ b/internal/repomaint/repomaint_test.go
@@ -4,10 +4,12 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/ewpt3ch/pkgstash/internal/cache"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 // create a CacheClient to mock cache.Fetch and cache.FetchDB to fulfill
@@ -22,6 +24,48 @@ func (m *mockCache) Fetch(relPath string) (*cache.CacheFile, error) {
 	return nil, nil
 }
 
+func TestParseDesc(t *testing.T) {
+	t.Run("parse good desc file", func(t *testing.T) {
+		r := strings.NewReader("%NAME%\nlinux\n\n\n%FAKEKEY%\nFAKEINFO\n\n%FILENAME%\nlinux-7.0.9\n")
+		pkgName, fileName, err := parseDesc(r)
+		require.NoError(t, err)
+		assert.Equal(t, "linux", pkgName)
+		assert.Equal(t, "linux-7.0.9", fileName)
+	})
+
+	t.Run("parse bad desc file", func(t *testing.T) {
+		r := strings.NewReader("%NAME\nlinux\n\n%FAKEKEY%\nwhat\n")
+		pkgName, fileName, err := parseDesc(r)
+		assert.Error(t, err)
+		assert.Equal(t, "", pkgName)
+		assert.Equal(t, "", fileName)
+	})
+}
+
+func TestCachedFiles(t *testing.T) {
+	testFiles := []string{
+		"linux-x86_64.pkg.tar.zst",
+		"gcc-rust-x86_64.pkg.tar.zst",
+		"linux-api-headers-x86_64.pkg.tar.zst",
+	}
+	c := &mockCache{
+		fetched: make([]string, 5),
+	}
+	rs, err := NewRepoSync(c, t.TempDir(), []string{"core"})
+	if err != nil {
+		t.Fatalf("failed to create RepoMaint: %v", err)
+	}
+	rs.root.MkdirAll("core/os/x86_64", 0750)
+	for _, f := range testFiles {
+		rs.root.WriteFile(filepath.Join("core/os/x86_64", f), []byte{}, 0644)
+	}
+
+	cachedFiles, err := rs.cachedPkgs("core")
+	assert.Contains(t, cachedFiles, "linux")
+	assert.Contains(t, cachedFiles, "gcc-rust")
+	assert.Contains(t, cachedFiles, "linux-api-headers")
+}
+
 func TestSyncMapBuild(t *testing.T) {
 	testFiles := make(map[string]string)
 	testFiles["linux"] = "linux-7.0.9.arch1-1-x86_64.pkg.tar.zst"