From 56037c4b0574c39305347c57f9b74f4296ca61ba Mon Sep 17 00:00:00 2001
From: Ryan McGuire <ryand_mcguire@sweetwater.com>
Date: Wed, 8 Jan 2025 16:48:34 -0500
Subject: [PATCH] Support custom types

---
 pkg/app/setup.go        |   7 +++
 pkg/app/setup_custom.go | 107 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+)
 create mode 100644 pkg/app/setup_custom.go

diff --git a/pkg/app/setup.go b/pkg/app/setup.go
index 8797312..0383aea 100644
--- a/pkg/app/setup.go
+++ b/pkg/app/setup.go
@@ -23,3 +23,10 @@ func MustSetupConfigAndLogging(ctx context.Context) context.Context {
 	zerolog.Ctx(ctx).Trace().Any("config", *cfg).Send()
 	return ctx
 }
+
+// Unmarshal config into a custom type
+// Type MUST include *config.AppConfig
+// Stored in context as *config.AppConfig but can be asserted back
+func MustSetupConfigAndLoggingInto[T any](ctx context.Context, into T) (context.Context, T) {
+	return ctx, into
+}
diff --git a/pkg/app/setup_custom.go b/pkg/app/setup_custom.go
new file mode 100644
index 0000000..5f8e258
--- /dev/null
+++ b/pkg/app/setup_custom.go
@@ -0,0 +1,107 @@
+package app
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"reflect"
+
+	"gopkg.in/yaml.v3"
+
+	"gitea.libretechconsulting.com/rmcguire/go-app/pkg/config"
+)
+
+// Used to unmarshal config and environment into a custom type
+// that overloads *config.AppConfig. Will perform normal env
+// substitutions for AppConfig, but env overrides for custom type
+// are up to the caller.
+func MustLoadConfigInto[T any](ctx context.Context, into T) (context.Context, T) {
+	// Step 1: Check our custom type for required *config.AppConfig
+	if err := hasAppConfig(into); err != nil {
+		panic(err)
+	}
+
+	// Step 2: Do the normal thing
+	ctx = MustSetupConfigAndLogging(ctx)
+
+	// Step 3: Extract the config
+	cfg := config.MustFromCtx(ctx)
+
+	// Step 4: Unmarshal custom config
+	configPath := flag.Lookup("config")
+	if configPath != nil && configPath.Value.String() != "" {
+		file, err := os.Open(configPath.Value.String())
+		if err != nil {
+			panic(fmt.Errorf("could not open config file: %w", err))
+		}
+		defer file.Close()
+
+		decoder := yaml.NewDecoder(file)
+		if err := decoder.Decode(into); err != nil {
+			panic(fmt.Errorf("could not decode config file: %w", err))
+		}
+	}
+
+	// Step 5: Re-apply AppConfig to custom type
+	if err := setAppConfig(into, cfg); err != nil {
+		panic(err)
+	}
+
+	// Step 6: Update context, return custom type
+	return ctx, into
+}
+
+func setAppConfig[T any](target T, appConfig *config.AppConfig) error {
+	// Ensure target is a pointer to a struct
+	v := reflect.ValueOf(target)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return errors.New("target must be a non-nil pointer to a struct")
+	}
+
+	v = v.Elem() // Dereference the pointer
+	if v.Kind() != reflect.Struct {
+		return errors.New("target must be a pointer to a struct")
+	}
+
+	// Replace *config.AppConfig
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Field(i)
+		if field.Type() == reflect.TypeOf((*config.AppConfig)(nil)) {
+			if !field.CanSet() {
+				return fmt.Errorf("field %q cannot be set", v.Type().Field(i).Name)
+			}
+			field.Set(reflect.ValueOf(appConfig))
+			return nil
+		}
+	}
+
+	return errors.New("no *config.AppConfig field found in target struct")
+}
+
+func hasAppConfig[T any](target T) error {
+	v := reflect.ValueOf(target)
+	if v.Kind() != reflect.Ptr || v.IsNil() {
+		return errors.New("target must be a non-nil pointer to a struct")
+	}
+	v = v.Elem()
+
+	if v.Kind() != reflect.Struct {
+		return errors.New("target must be a pointer to a struct")
+	}
+
+	hasAppConfig := false
+	for i := 0; i < v.NumField(); i++ {
+		field := v.Type().Field(i)
+		if field.Type == reflect.TypeOf((*config.AppConfig)(nil)) {
+			hasAppConfig = true
+			break
+		}
+	}
+	if !hasAppConfig {
+		return errors.New("struct does not contain a *config.AppConfig field")
+	}
+
+	return nil
+}