diff --git a/git.zig b/git.zig
index 3af34f7224719f60ce1d569bfc8e945e9a37987f..ba168662533a983a84549e3ecc771cf40746c0a1 100644
--- a/git.zig
+++ b/git.zig
@@ -1,24 +1,29 @@
 const std = @import("std");
 const string = []const u8;
 
+// 40 is length of sha1 hash
+pub const Id = *const [40]u8;
+
 /// Returns the result of running `git rev-parse HEAD`
 /// dir must already be pointing at the .git folder
-pub fn getHEAD(alloc: std.mem.Allocator, dir: std.fs.Dir) !string {
+pub fn getHEAD(alloc: std.mem.Allocator, dir: std.fs.Dir) !Id {
     const h = std.mem.trimRight(u8, try dir.readFileAlloc(alloc, "HEAD", 1024), "\n");
 
     if (std.mem.startsWith(u8, h, "ref:")) {
-        return std.mem.trimRight(u8, try dir.readFileAlloc(alloc, h[5..], 1024), "\n");
+        const r = std.mem.trimRight(u8, try dir.readFileAlloc(alloc, h[5..], 1024), "\n");
+        std.debug.assert(r.len == 40);
+        return r[0..40];
     }
 
     // content should be 40-char sha1 hash
     std.debug.assert(h.len == 40);
-    return h;
+    return h[0..40];
 }
 
 // TODO make this inspect .git/objects
 // https://git-scm.com/book/en/v2/Git-Internals-Git-Objects
 // https://git-scm.com/book/en/v2/Git-Internals-Packfiles
-pub fn getObject(alloc: std.mem.Allocator, dir: std.fs.Dir, obj: string) !string {
+pub fn getObject(alloc: std.mem.Allocator, dir: std.fs.Dir, obj: Id) !string {
     const result = try std.ChildProcess.exec(.{
         .allocator = alloc,
         .cwd_dir = dir,
@@ -30,7 +35,7 @@ pub fn getObject(alloc: std.mem.Allocator, dir: std.fs.Dir, obj: string) !string
 pub fn parseCommit(alloc: std.mem.Allocator, commitfile: string) !Commit {
     var iter = std.mem.split(u8, commitfile, "\n");
     var result: Commit = undefined;
-    var parents = std.ArrayList(string).init(alloc);
+    var parents = std.ArrayList(Id).init(alloc);
     errdefer parents.deinit();
     while (true) {
         const line = iter.next().?;
@@ -38,10 +43,10 @@ pub fn parseCommit(alloc: std.mem.Allocator, commitfile: string) !Commit {
         const space = std.mem.indexOfScalar(u8, line, ' ').?;
         const k = line[0..space];
 
-        if (std.mem.eql(u8, k, "tree")) result.tree = line[space + 1 ..];
+        if (std.mem.eql(u8, k, "tree")) result.tree = line[space + 1 ..][0..40];
         if (std.mem.eql(u8, k, "author")) result.author = line[space + 1 ..];
         if (std.mem.eql(u8, k, "committer")) result.committer = line[space + 1 ..];
-        if (std.mem.eql(u8, k, "parent")) try parents.append(line[space + 1 ..]);
+        if (std.mem.eql(u8, k, "parent")) try parents.append(line[space + 1 ..][0..40]);
     }
     result.parents = try parents.toOwnedSlice();
     result.message = iter.rest();
@@ -49,8 +54,8 @@ pub fn parseCommit(alloc: std.mem.Allocator, commitfile: string) !Commit {
 }
 
 pub const Commit = struct {
-    tree: string,
-    parents: []const string,
+    tree: Id,
+    parents: []const Id,
     author: string,
     committer: string,
     gpgsig: string,