pcb-rnd - config for users

# The pcb-rnd config system

## User documentation

As of 1.1.0, pcb-rnd switched to a lihata based configuration system. The purpose of this document is to describes the basic system design going into enough details to provide the user with full control over the configuration. The other side, how the system is implemented is described in the programmer's manual and there is also a checklist to assist plugin programmers.

## Architecture: data flows, merging, dispatching

The final configuration is a collection of values for all known settings, arranged in a tree. The config tree is a theoretical concept; different representations of the tree are actually built runtime, in-memory. Pcb-rnd code, plugins and utilities are constantly reading these in-memory representations to decide how to carry out their tasks.

Config settings are imported from multiple sources: from different files, from environment variables, from command line arguments, from the opened (.pcb or .lht) board files on load. Any source can define any part of the config tree. When the configuration is processed, each source is read into a temporary tree and then all the temporary trees are merged into the final config tree. The following diagram demonstrates all configuration related data flows.

The leftmost column of nodes are the sources. (Note: paths mentioned there are the default paths, for reference, it is possible to change them compile-time.) Along the black arrows, from left to right, each source is imported into a tree representing a role: the role or purpose of the source. The next step is following the red arrows in two steps:

- first merge all the role trees into a flat list; this determines the value of each setting;
- then dispatch the values to the right component of the code.

Some components may change some of the settings run-time. The trivial example is the GUI (preferences dialog on this diagram) that provides menus and dialog boxes for the user to change settings. Such a change is always fed back (blue arrow) to the design role tree directly, from where the new value is again merged and dispatched along the red arrows. Changes in the design role are saved with the board file (thus the bidirectional black arrow between the source and the in-memory tree for the design role). Occasionally the user wants to save parts of the setting as a project setting or as an user setting - in this case, along the dashed blue lines, the corresponding project or user roles are modified. This again results in updating the hash and the binary representation; these roles also have bidirectional black arrows and their changes are also saved in the original source.

## Merge details

In the new system it is up to the user to decide what settings are system-level and what settings are user- or project-level. This is possible because any source can define any setting. In the merging step (red arrows between roles and the hash) it may turn out that there are overlaps (multiple sources defining value for the same setting) or blind spots (no source sets a given item).

### overlaps

Each setting in each source has a priority. The priority can be defined in the source, or if it is not defined, each source inherits a fallback default priority. The fallback is designed to provide the intuitive order: cli > design > project > user > system.

When multiple sources are describing a value for the same setting, priority decides the final value. Most settings are scalar: a single integer, string or a single "yes/no" or "on/off" value (e.g. the background color or whether polygons are thin-drawn). For scalars the rule is simple: the higher priority value wins and all lower priority values are discarded when putting the values into the hash. More details: how different roles and priorities can be used with scalars.

There are some settings that are represented as an array or list of values. They are described in a lihata list item ("li:") in the config files and are generally called lists in this document. How lists are merged is controlled by the merging policy, which can be in each source, just like the priority is set. Check out the list merging section for more details.

### blind spots

At the end the code does need a value for each setting, so in the final render (after the hash) every setting must have a value. To avoid blind spots, values not initialized, there is a built-in configuration file, compiled into the executable. This file is loaded into role CFR_INTERNAL, and has the lowest priority. This configuration file contains the default value for all settings.