Implement initial config loading

Config goes in $XDG_CONFIG_HOME/beansprout/config.kdl or
$HOME/.config/beansprout/config.kdl

Config is in the kdl format. Right now, the supported options are

```zig
/// Width of window borders in pixels
border_width: u8 = 2,
/// Color of focused window's border in 0xRRGGBBAA or 0xRRGGBB form
border_color_focused: RiverColor = utils.parseRgbaComptime("0x89b4fa"),
/// Color of uffocused windows' borders in 0xRRGGBBAA or 0xRRGGBB form
border_color_unfocused: RiverColor = utils.parseRgbaComptime("0x1e1e2e"),

/// Where a new window should attach, top or bottom of the stack
attach_mode: AttachMode = .top,
/// Should focus change when the cursor moves onto a new window
focus_follows_pointer: bool = true,
/// Should the pointer warp to the center of newly-focused windows
pointer_warp_on_focus_change: bool = true,
```

I plan to add Keybinds shortly. If parsing the configuration fails,
the default config will be used and the WM will continue loading.

src/Config.zig

@@ -4,10 +4,238 @@
 
 const Config = @This();
 
-border_width: u3,
-border_color_focused: RiverColor,
-border_color_unfocused: RiverColor,
+const CONFIG_FILE = "beansprout/config.kdl";
+
+/// Width of window borders in pixels
+border_width: u8 = 2,
+/// Color of focused window's border in 0xRRGGBBAA or 0xRRGGBB form
+border_color_focused: RiverColor = utils.parseRgbaComptime("0x89b4fa"),
+/// Color of uffocused windows' borders in 0xRRGGBBAA or 0xRRGGBB form
+border_color_unfocused: RiverColor = utils.parseRgbaComptime("0x1e1e2e"),
+
+/// Where a new window should attach, top or bottom of the stack
+attach_mode: AttachMode = .top,
+/// Should focus change when the cursor moves onto a new window
+focus_follows_pointer: bool = true,
+/// Should the pointer warp to the center of newly-focused windows
+pointer_warp_on_focus_change: bool = true,
+
+pub const AttachMode = enum {
+    top,
+    bottom,
+};
+
+const NodeName = enum {
+    attach_mode,
+    focus_follows_pointer,
+    pointer_warp_on_focus_change,
+    borders,
+};
+
+const BorderNodeName = enum {
+    width,
+    color_focused,
+    color_unfocused,
+};
+
+pub fn create() !*Config {
+    var config: *Config = try utils.allocator.create(Config);
+    errdefer config.destroy();
+    config.* = .{}; // create() gives us undefined memory
+
+    if (try known_folders.getPath(utils.allocator, .local_configuration)) |config_dir| blk: {
+        defer utils.allocator.free(config_dir);
+
+        const config_path = try std.fmt.allocPrint(utils.allocator, "{s}/{s}", .{ config_dir, CONFIG_FILE });
+        defer utils.allocator.free(config_path);
+
+        const file = fs.openFileAbsolute(config_path, .{}) catch break :blk;
+
+        var read_buffer: [1024]u8 = undefined;
+        var file_reader = file.reader(&read_buffer);
+
+        config.load(&file_reader.interface) catch |err| {
+            log.err("Error while loading config: {s}. Continuing with default config", .{@errorName(err)});
+            // Overwrite any partially-loaded config
+            config.* = .{};
+        };
+    }
+
+    return config;
+}
+
+pub fn destroy(config: *Config) void {
+    utils.allocator.destroy(config);
+}
+
+fn load(config: *Config, reader: *Io.Reader) !void {
+    var parser = try kdl.Parser.init(utils.allocator, reader, .{});
+    defer parser.deinit(utils.allocator);
+
+    var next_child_block: ?NodeName = null;
+
+    // Parse the KDL config
+    while (try parser.next()) |event| {
+        // First, we switch on the type of event
+        switch (event) {
+            .node => |node| {
+                if (next_child_block) |child_block| {
+                    log.warn("Expected child block for {s}, but got another node instead", .{@tagName(child_block)});
+                }
+                // If it's a node, we check if it's a valid NodeName
+                const node_name = std.meta.stringToEnum(NodeName, node.name);
+                if (node_name) |name| {
+                    // Next, we have to check the specifics for the NodeName
+                    switch (name) {
+                        .attach_mode => {
+                            const attach_mode_str = node.arg(&parser, 0) orelse "";
+                            if (std.meta.stringToEnum(AttachMode, attach_mode_str)) |mode| {
+                                config.attach_mode = mode;
+                                log.debug("Setting attach_mode to {s}", .{@tagName(mode)});
+                            } else {
+                                log.warn("Invalid \"attach_mode\". Using default", .{});
+                                continue;
+                            }
+                        },
+                        .focus_follows_pointer => {
+                            const focus_follows_pointer_str = node.arg(&parser, 0) orelse "";
+                            if (config.boolFromKdlStr(focus_follows_pointer_str)) |focus_follows_pointer| {
+                                config.focus_follows_pointer = focus_follows_pointer;
+                                log.debug("Setting focus_follows_pointer to {}", .{focus_follows_pointer});
+                            } else {
+                                log.warn("Invalid \"focus_follows_pointer\". Using default", .{});
+                                continue;
+                            }
+                        },
+                        .pointer_warp_on_focus_change => {
+                            const pointer_warp_on_focus_change_str = node.arg(&parser, 0) orelse "";
+                            if (config.boolFromKdlStr(pointer_warp_on_focus_change_str)) |pointer_warp_on_focus_change| {
+                                config.pointer_warp_on_focus_change = pointer_warp_on_focus_change;
+                                log.debug("Setting pointer_warp_on_focus_change to {}", .{pointer_warp_on_focus_change});
+                            } else {
+                                log.warn("Invalid \"pointer_warp_on_focus_change\". Using default", .{});
+                                continue;
+                            }
+                        },
+                        .borders => {
+                            next_child_block = .borders;
+                        },
+                    }
+                } else {
+                    log.warn("Invalid KDL node {s}. Ignoring it and carrying on", .{node.name});
+                }
+            },
+            .child_block_begin => {
+                if (next_child_block) |child_block| {
+                    switch (child_block) {
+                        .borders => try config.loadBordersChildBlock(&parser),
+                        else => {
+                            // Nothing else should ever be marked as a next_child_block
+                            unreachable;
+                        },
+                    }
+                    next_child_block = null;
+                } else {
+                    try config.skipChildBlock(&parser);
+                }
+            },
+            .child_block_end => {
+                @panic("Reached end of non-existant child block. A bug in zig-kdl?");
+            },
+        }
+    }
+}
+
+fn loadBordersChildBlock(config: *Config, parser: *kdl.Parser) !void {
+    while (try parser.next()) |event| {
+        switch (event) {
+            .node => |node| {
+                // If it's a node, we check if it's a valid NodeName
+                const node_name = std.meta.stringToEnum(BorderNodeName, node.name);
+                if (node_name) |name| {
+                    switch (name) {
+                        .width => {
+                            const width_str = node.arg(parser, 0) orelse "";
+                            config.border_width = fmt.parseInt(u8, width_str, 10) catch {
+                                log.warn("Invalid border.width \"{s}\"", .{width_str});
+                                continue;
+                            };
+                            log.debug("Setting border.width to {d}", .{config.border_width});
+                        },
+                        .color_focused => {
+                            const color_str = node.arg(parser, 0) orelse "";
+                            config.border_color_focused = utils.parseRgba(color_str) catch {
+                                log.warn("Invalid border.color_focused \"{s}\"", .{color_str});
+                                continue;
+                            };
+                            log.debug("Setting border.color_focused to {s}", .{color_str});
+                        },
+                        .color_unfocused => {
+                            const color_str = node.arg(parser, 0) orelse "";
+                            config.border_color_unfocused = utils.parseRgba(color_str) catch {
+                                log.warn("Invalid border.color_unfocused \"{s}\"", .{color_str});
+                                continue;
+                            };
+                            log.debug("Setting border.color_unfocused to {s}", .{color_str});
+                        },
+                    }
+                } else {
+                    log.warn("Invalid KDL node {s}. Ignoring it and carrying on", .{node.name});
+                }
+            },
+            .child_block_begin => {
+                // borders should never have a nested child block
+                try config.skipChildBlock(parser);
+            },
+            .child_block_end => {
+                // Done parsing the borders block; return
+                return;
+            },
+        }
+    }
+}
+
+/// Skips an entire child block in KDL
+fn skipChildBlock(_: *Config, parser: *kdl.Parser) !void {
+    log.warn("Unexpected child block. Skipping it", .{});
+    while (try parser.next()) |event| {
+        switch (event) {
+            .child_block_end => return,
+            else => {
+                // We don't care about anything else in this child block
+            },
+        }
+    }
+}
+
+/// Convert a KDL argument into a bool or null if the string is not a bool
+/// "#true" and "true" => true
+/// "#false" and "false" => false
+/// else => null
+fn boolFromKdlStr(_: Config, arg_str: []const u8) ?bool {
+    if (mem.eql(u8, arg_str, "#true") or
+        mem.eql(u8, arg_str, "true"))
+    {
+        return true;
+    } else if (mem.eql(u8, arg_str, "#false") or
+        mem.eql(u8, arg_str, "false"))
+    {
+        return false;
+    }
+    return null;
+}
 
 const std = @import("std");
+const fmt = std.fmt;
+const fs = std.fs;
+const mem = std.mem;
+const Io = std.Io;
 
-const RiverColor = @import("utils.zig").RiverColor;
+const kdl = @import("kdl");
+const known_folders = @import("known_folders");
+const KdlNode = kdl.Parser.Node;
+
+const utils = @import("utils.zig");
+const RiverColor = utils.RiverColor;
+
+const log = std.log.scoped(.Config);