From 39c09be237da10dcc31c60e545c52f6c89c1a6de Mon Sep 17 00:00:00 2001
From: Ethan Dalool <ethan@voussoir.net>
Date: Fri, 3 Apr 2020 17:07:51 -0700
Subject: [PATCH] Add configlayers.py.

---
 voussoirkit/configlayers.py | 95 +++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 voussoirkit/configlayers.py

diff --git a/voussoirkit/configlayers.py b/voussoirkit/configlayers.py
new file mode 100644
index 0000000..678f49a
--- /dev/null
+++ b/voussoirkit/configlayers.py
@@ -0,0 +1,95 @@
+'''
+The purpose of this file is to work with JSON-based config files, where you
+load a default configuration, then overlay a user-supplied configuration on
+top, overwriting the matching keys and keeping default values for the rest.
+The functions will then suggest that the user config needs to be re-saved if
+the default key set contains keys that the user key set does not.
+'''
+import copy
+import json
+
+from voussoirkit import pathclass
+
+def recursive_dict_keys(d):
+    '''
+    Given a dictionary, return a set containing all of its keys and the keys of
+    all other dictionaries that appear as values within. The subkeys will use \\
+    to indicate their lineage.
+
+    {'hi': {'ho': 'neighbor'}}
+
+    returns
+
+    {'hi', 'hi\\ho'}
+    '''
+    keys = set(d.keys())
+    for (key, value) in d.items():
+        if isinstance(value, dict):
+            subkeys = {f'{key}\\{subkey}' for subkey in recursive_dict_keys(value)}
+            keys.update(subkeys)
+    return keys
+
+def recursive_dict_update(target, supply):
+    '''
+    Update target using supply, but when the value is a dictionary update the
+    insides instead of replacing the dictionary itself. This prevents keys that
+    exist in the target but don't exist in the supply from being erased.
+    Note that we are modifying target in place.
+
+    eg:
+    target = {'hi': 'ho', 'neighbor': {'name': 'Wilson'}}
+    supply = {'neighbor': {'behind': 'fence'}}
+
+    result: {'hi': 'ho', 'neighbor': {'name': 'Wilson', 'behind': 'fence'}}
+    whereas a regular dict.update would have produced:
+    {'hi': 'ho', 'neighbor': {'behind': 'fence'}}
+    '''
+    for (key, value) in supply.items():
+        if isinstance(value, dict):
+            existing = target.get(key, None)
+            if existing is None:
+                target[key] = value
+            else:
+                recursive_dict_update(target=existing, supply=value)
+        else:
+            target[key] = value
+
+def layer_json(target, supply):
+    '''
+    target is the dictionary into which the final values will be placed
+    (presumably loaded from a default set), and supply is a layer being applied
+    on top of the target (presumably a user-supplied set). needs_rewrite is
+    True if the target contains keys that the supply did not, indicating that
+    the supply is incomplete.
+    '''
+    target_keys = recursive_dict_keys(target)
+    supply_keys = recursive_dict_keys(supply)
+    needs_rewrite = target_keys > supply_keys
+    recursive_dict_update(target=target, supply=supply)
+    return (target, needs_rewrite)
+
+def load_file(filepath, defaults):
+    '''
+    Given a filepath to a user-supplied config file, and a dict of default
+    values, return a new dict containing the user-supplied values overlaid onto
+    the defaults, and needs_rewrite indicating that the user config is missing
+    some keys from the default config.
+    '''
+    path = pathclass.Path(filepath)
+    user_config_exists = path.exists
+
+    # This config will hold the final values. We start by loading it with the
+    # defaults, so that as we go through the user's config we can overwrite the
+    # user-specified keys, and the keys which the user does not specify will
+    # remain default.
+    config = copy.deepcopy(defaults)
+    needs_rewrite = False
+
+    if user_config_exists:
+        with open(path.absolute_path, 'r', encoding='utf-8') as handle:
+            user_config = json.load(handle)
+        (config, needs_rewrite) = layer_json(config, user_config)
+    else:
+        needs_rewrite = True
+
+    return (config, needs_rewrite)