package config

import (
	"bytes"
	"encoding"
	"encoding/json"
	"errors"
	"fmt"
	apierrors "git.yuntuo.hk/k2safe/go-omni/errors"
	"math"
	"os"
	"reflect"
	"strconv"
	"strings"
	"time"
	"unicode"
)

// EnvOptions controls typed project configuration overlays. Explicit env tags
// are absolute names; otherwise JSON names become PREFIX_NESTED_FIELD names.
// Empty is allow (default), ignore or reject. No input values appear in errors.
type EnvOptions struct {
	Prefix string
	Empty  string
	Lookup func(string) (string, bool)
	// Aliases maps canonical absolute environment names to ordered fallback names.
	// A present canonical value wins, including an explicit empty value.
	Aliases map[string][]string
}

// ApplyDefaults applies envDefault tags to zero fields before file overlays.
func ApplyDefaults[T any](base T) (T, error) { return projectOverlay(base, EnvOptions{}, true) }

// LoadProject applies code/tag defaults, a strict optional file, environment,
// then the project's Validate method, before any logger or dependency is opened.
// envDefault is only applied to zero-valued defaults before reading the file.
func LoadProject[T any](path string, defaults T, options EnvOptions) (T, error) {
	var zero T
	cfg, err := projectOverlay(defaults, options, true)
	if err != nil {
		return zero, err
	}
	if path != "" {
		cfg, err = DecodeFile(path, cfg)
		if err != nil {
			return zero, errors.New("config: cannot decode file; check its path, format, field names and value types")
		}
	}
	cfg, err = OverlayEnv(cfg, options)
	if err != nil {
		return zero, err
	}
	if valid, ok := any(&cfg).(interface{ Validate() error }); ok {
		if err = valid.Validate(); err != nil {
			return zero, err
		}
	}
	return cfg, nil
}

// OverlayEnv overlays the entire project struct without mutating the input.
// Tags: env:"CUSTOM_NAME,required,secret,empty=reject"; env:"-" excludes a
// subtree. required means nonempty string/non-nil pointer or collection; false
// and zero are valid scalars. Use project Validate for semantic constraints.
// Slices/maps use JSON syntax, avoiding ambiguous comma and empty semantics.
func OverlayEnv[T any](base T, options EnvOptions) (T, error) {
	return projectOverlay(base, options, false)
}

var textType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
var durationType = reflect.TypeOf(time.Duration(0))

func projectOverlay[T any](base T, options EnvOptions, defaults bool) (T, error) {
	var zero T
	if options.Lookup == nil {
		options.Lookup = os.LookupEnv
	}
	if options.Empty == "" {
		options.Empty = "allow"
	}
	if options.Empty != "allow" && options.Empty != "ignore" && options.Empty != "reject" {
		return zero, errors.New("config: empty policy must be allow, ignore or reject")
	}
	v := reflect.ValueOf(&base).Elem()
	if v.Kind() != reflect.Struct {
		return zero, errors.New("config: project configuration must be a struct")
	}
	result := reflect.New(v.Type()).Elem()
	result.Set(v)
	seen := map[string]bool{}
	var walk func(reflect.Value, string, int) error
	walk = func(value reflect.Value, prefix string, depth int) error {
		if depth > 32 {
			return errors.New("config: nesting exceeds 32")
		}
		for i := 0; i < value.NumField(); i++ {
			field := value.Type().Field(i)
			dst := value.Field(i)
			if !dst.CanSet() || field.PkgPath != "" {
				continue
			}
			tag := strings.Split(field.Tag.Get("env"), ",")
			if tag[0] == "-" {
				continue
			}
			jsonName := strings.Split(field.Tag.Get("json"), ",")[0]
			if jsonName == "-" && tag[0] == "" {
				continue
			}
			if jsonName == "" {
				jsonName = field.Name
			}
			key := prefix + envName(jsonName)
			if tag[0] != "" {
				key = tag[0]
			}
			empty, required := options.Empty, false
			for _, opt := range tag[1:] {
				switch opt {
				case "required":
					required = true
				case "secret", "":
				case "empty=allow":
					empty = "allow"
				case "empty=ignore":
					empty = "ignore"
				case "empty=reject":
					empty = "reject"
				default:
					return fmt.Errorf("config: invalid env tag for %s", key)
				}
			}
			if dst.Kind() == reflect.Struct && !dst.Addr().Type().Implements(textType) {
				if err := walk(dst, key+"_", depth+1); err != nil {
					return err
				}
				continue
			}
			if dst.Kind() == reflect.Pointer && dst.Type().Elem().Kind() == reflect.Struct && !dst.Type().Implements(textType) {
				// Clone before traversing so a failed overlay never edits defaults.
				child := reflect.New(dst.Type().Elem())
				if !dst.IsNil() {
					child.Elem().Set(dst.Elem())
				}
				if err := walk(child.Elem(), key+"_", depth+1); err != nil {
					return err
				}
				if !dst.IsNil() || !child.Elem().IsZero() {
					dst.Set(child)
				}
				if required && !defaults && dst.IsNil() {
					return fmt.Errorf("config: %s is required", key)
				}
				continue
			}
			// Unexported runtime capabilities are skipped unless explicitly tagged.
			if dst.Kind() == reflect.Func || dst.Kind() == reflect.Chan || dst.Kind() == reflect.Interface {
				if tag[0] != "" {
					return fmt.Errorf("config: %s has unsupported environment type", key)
				}
				continue
			}
			if seen[key] {
				return fmt.Errorf("config: duplicate environment key %s", key)
			}
			seen[key] = true
			raw, present := options.Lookup(key)
			selectedKey := key
			if !present && !defaults {
				for _, alias := range options.Aliases[key] {
					if raw, present = options.Lookup(alias); present {
						selectedKey = alias
						break
					}
				}
			}
			if defaults {
				raw, present = field.Tag.Lookup("envDefault")
				present = present && dst.IsZero()
			}
			if present && raw == "" && !defaults {
				if empty == "reject" {
					return fmt.Errorf("config: %s must not be empty", selectedKey)
				}
				if empty == "ignore" {
					present = false
				}
			}
			if present {
				parsed := reflect.New(dst.Type()).Elem()
				if err := parseEnvValue(parsed, raw); err != nil {
					return fmt.Errorf("config: %s must be a valid %s", selectedKey, dst.Type())
				}
				dst.Set(parsed)
			}
			if required && !defaults && emptyRequired(dst) {
				return fmt.Errorf("config: %s is required", key)
			}
		}
		return nil
	}
	if err := walk(result, options.Prefix, 0); err != nil {
		return zero, err
	}
	return result.Interface().(T), nil
}

func emptyRequired(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.String:
		return v.Len() == 0
	case reflect.Pointer, reflect.Map, reflect.Slice:
		return v.IsNil()
	}
	return false
}

func parseEnvValue(v reflect.Value, raw string) (err error) {
	defer func() {
		if recover() != nil {
			err = errors.New("invalid custom value")
		}
	}()
	if v.Kind() == reflect.Pointer {
		v.Set(reflect.New(v.Type().Elem()))
		return parseEnvValue(v.Elem(), raw)
	}
	if v.CanAddr() && v.Addr().Type().Implements(textType) {
		return v.Addr().Interface().(encoding.TextUnmarshaler).UnmarshalText([]byte(raw))
	}
	if v.Type() == durationType {
		d, err := time.ParseDuration(raw)
		if err == nil {
			v.SetInt(int64(d))
		}
		return err
	}
	switch v.Kind() {
	case reflect.String:
		v.SetString(raw)
		return nil
	case reflect.Bool:
		x, err := strconv.ParseBool(raw)
		if err == nil {
			v.SetBool(x)
		}
		return err
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		x, err := strconv.ParseInt(raw, 10, v.Type().Bits())
		if err == nil {
			v.SetInt(x)
		}
		return err
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		x, err := strconv.ParseUint(raw, 10, v.Type().Bits())
		if err == nil {
			v.SetUint(x)
		}
		return err
	case reflect.Float32, reflect.Float64:
		x, err := strconv.ParseFloat(raw, v.Type().Bits())
		if err == nil && (math.IsNaN(x) || math.IsInf(x, 0)) {
			return errors.New("nonfinite value")
		}
		if err == nil {
			v.SetFloat(x)
		}
		return err
	case reflect.Slice, reflect.Map:
		var value any
		if err := apierrors.DecodeCatalogJSON([]byte(raw), &value); err != nil {
			return err
		}
		value, err := durations(value, v.Type())
		if err != nil {
			return err
		}
		encoded, err := json.Marshal(value)
		if err != nil {
			return err
		}
		decoder := json.NewDecoder(bytes.NewReader(encoded))
		decoder.DisallowUnknownFields()
		return decoder.Decode(v.Addr().Interface())
	}
	return errors.New("unsupported environment value")
}

func envName(s string) string {
	r := []rune(s)
	var out strings.Builder
	for i, c := range r {
		if i > 0 && unicode.IsUpper(c) && (unicode.IsLower(r[i-1]) || unicode.IsDigit(r[i-1]) || (i+1 < len(r) && unicode.IsLower(r[i+1]))) {
			out.WriteByte('_')
		}
		if c == '-' {
			c = '_'
		}
		out.WriteRune(unicode.ToUpper(c))
	}
	return out.String()
}