remove user.zig dependency on shell.zig

src/user.zig

@@ -1,5 +1,4 @@
 const std = @import("std");
-const shell = @import("shell.zig");
 
 const Allocator = std.mem.Allocator;
 
@@ -27,18 +26,26 @@ pub const User = struct {
 
 // UserWriter accepts a naive User struct and returns a PackedUser
 pub const UserWriter = struct {
-    allocator: Allocator,
 
-    pub fn init(allocator: Allocator) UserWriter {
+    // shellIndexFnType is a signature for a function that accepts a shell
+    // string and returns it's index in the global shell section. Passing a
+    // function makes tests easier, and removes the Shell dependency of this
+    // module.
+    const shellIndexFnType = fn ([]const u8) ?u10;
+    allocator: Allocator,
+    shellIndexFn: shellIndexFnType,
+
+    pub fn init(allocator: Allocator, shellIndexFn: shellIndexFnType) UserWriter {
         return UserWriter{
             .allocator = allocator,
+            .shellIndexFn = shellIndexFn,
         };
     }
 
-    pub fn fromUser(user: User, shellw: shell.ShellWriter) !PackedUser {
+    pub fn fromUser(self: *UserWriter, user: User) !PackedUser {
         var shell_here: u1 = undefined;
         var shell_len_or_place: u6 = undefined;
-        if (shellw.getIndex(user.shell)) |idx| {
+        if (self.shellIndexFn(user.shell)) |idx| {
             shell_here = false;
             shell_len_or_place = idx;
         } else {
@@ -58,7 +65,6 @@ pub const UserWriter = struct {
             .gecos_len = undefined,
         };
 
-        _ = shellw;
         return puser;
     }
 };