Add configuration and CLI foundation

internal/config/load.go

@@ -0,0 +1,68 @@
+package config
+
+import (
+	"errors"
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+type LoadOptions struct {
+	Path     string
+	Units    string
+	Timezone string
+	Output   string
+}
+
+func Load(opts LoadOptions) (Config, error) {
+	cfg := Defaults()
+
+	path := opts.Path
+	if path == "" {
+		path = DefaultPath
+	}
+
+	if err := mergeFile(&cfg, path); err != nil {
+		if opts.Path != "" || !errors.Is(err, os.ErrNotExist) {
+			return Config{}, err
+		}
+	}
+
+	if opts.Units != "" {
+		cfg.WeatherAPI.Units = opts.Units
+	}
+	if opts.Timezone != "" {
+		cfg.WeatherAPI.Timezone = opts.Timezone
+	}
+	if opts.Output != "" {
+		cfg.Reports.OutputDir = opts.Output
+	}
+
+	if err := Validate(cfg); err != nil {
+		return Config{}, err
+	}
+
+	return cfg, nil
+}
+
+func LoadFile(path string) (Config, error) {
+	return Load(LoadOptions{Path: path})
+}
+
+func mergeFile(cfg *Config, path string) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return fmt.Errorf("read config %q: %w", path, err)
+	}
+	if err := yaml.Unmarshal(data, cfg); err != nil {
+		return fmt.Errorf("parse config %q: %w", path, err)
+	}
+	if cfg.MissingSource.Sources == nil {
+		cfg.MissingSource.Sources = map[string]MissingSourcePolicy{}
+	}
+	if cfg.Reports.Paths == nil {
+		cfg.Reports.Paths = map[string]string{}
+	}
+	return nil
+}