/*
Copyright © 2026 Craig Williams <craig@avsdev.uk>

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package config

import (
	"errors"
	"fmt"
	"reflect"
	"strconv"
	"strings"
	"time"
)

// Configuration sections are plain structs described with struct tags:
//
//	cfg:"key"        the field's configuration key. Fields without it are
//	                 not part of the configuration.
//	default:"value"  the field's default, parsed the same way as a string
//	                 passed to SetValue (e.g. "10m", "true", "42").
//
// A tagged field whose type is a struct (other than time.Time) is a nested
// section, addressed as "section.key". An embedded struct has its tagged
// fields promoted into the enclosing section, just as Go promotes fields.
//
// time.Duration fields report reflect.String from TypeOf and accept either a
// time.Duration or a duration string such as "90s" in SetValue.
//
// A section can check values before they are stored by implementing
// validator on its pointer type.
const (
	keyTag     = "cfg"
	defaultTag = "default"
)

var (
	errUnknownKey = errors.New("unknown configuration key")
	durationType  = reflect.TypeOf(time.Duration(0))
	timeType      = reflect.TypeOf(time.Time{})
)

// validator is implemented by sections that need to reject some values.
// key is the field's own key within the section; value has already been
// converted to the field's type.
type validator interface {
	validate(key string, value any) error
}

type field struct {
	key        string
	index      []int // path for reflect.Value.FieldByIndex
	typ        reflect.Type
	section    bool
	def        string
	hasDefault bool
}

// fieldsOf lists the configuration fields of a section type in declaration
// order, with the fields of embedded structs promoted into it.
func fieldsOf(t reflect.Type) []field {
	var fields []field
	for i := 0; i < t.NumField(); i++ {
		sf := t.Field(i)
		if sf.Anonymous && sf.Type.Kind() == reflect.Struct {
			for _, f := range fieldsOf(sf.Type) {
				f.index = append([]int{i}, f.index...)
				fields = append(fields, f)
			}
			continue
		}
		key, ok := sf.Tag.Lookup(keyTag)
		if !ok || key == "" || key == "-" || !sf.IsExported() {
			continue
		}
		def, hasDefault := sf.Tag.Lookup(defaultTag)
		fields = append(fields, field{
			key:        key,
			index:      []int{i},
			typ:        sf.Type,
			section:    sf.Type.Kind() == reflect.Struct && sf.Type != timeType,
			def:        def,
			hasDefault: hasDefault,
		})
	}
	return fields
}

// structValue dereferences v down to the section struct it holds.
func structValue(v any) reflect.Value {
	rv := reflect.ValueOf(v)
	for rv.Kind() == reflect.Pointer {
		rv = rv.Elem()
	}
	if rv.Kind() != reflect.Struct {
		panic(fmt.Sprintf("config: %T is not a configuration section", v))
	}
	return rv
}

// lookup resolves a dotted key to the leaf field and the section holding it.
func lookup(section reflect.Value, key string) (reflect.Value, field, error) {
	head, rest, nested := strings.Cut(key, ".")
	for _, f := range fieldsOf(section.Type()) {
		if f.key != head {
			continue
		}
		switch {
		case f.section && nested:
			return lookup(section.FieldByIndex(f.index), rest)
		case !f.section && !nested:
			return section, f, nil
		}
		// Key matched but the shape didn't: a section with no sub-key, or a
		// value with one.
		return reflect.Value{}, field{}, errUnknownKey
	}
	return reflect.Value{}, field{}, errUnknownKey
}

func joinKey(prefix, key string) string {
	if prefix == "" {
		return key
	}
	return prefix + "." + key
}

// keysOf returns the keys directly in a section.
func keysOf(v any) []string {
	fields := fieldsOf(structValue(v).Type())
	keys := make([]string, 0, len(fields))
	for _, f := range fields {
		keys = append(keys, f.key)
	}
	return keys
}

// mapOf converts a section to a map, with nested sections as nested maps.
func mapOf(v any) map[string]any {
	return mapOfValue(structValue(v))
}

func mapOfValue(section reflect.Value) map[string]any {
	m := map[string]any{}
	for _, f := range fieldsOf(section.Type()) {
		fv := section.FieldByIndex(f.index)
		if f.section {
			m[f.key] = mapOfValue(fv)
		} else {
			m[f.key] = fv.Interface()
		}
	}
	return m
}

// flattenOf converts a section to a flat map of dotted keys, each prefixed
// with prefix (if non-empty).
func flattenOf(prefix string, v any) map[string]any {
	m := map[string]any{}
	flattenInto(m, prefix, structValue(v))
	return m
}

func flattenInto(m map[string]any, prefix string, section reflect.Value) {
	for _, f := range fieldsOf(section.Type()) {
		fv := section.FieldByIndex(f.index)
		key := joinKey(prefix, f.key)
		if f.section {
			flattenInto(m, key, fv)
		} else {
			m[key] = fv.Interface()
		}
	}
}

// kindOf is the kind reported by TypeOf, i.e. how the value is read from
// Viper. Durations are read as strings and parsed by SetValue.
func kindOf(t reflect.Type) reflect.Kind {
	if t == durationType {
		return reflect.String
	}
	return t.Kind()
}

func typeOf(v any, key string) (reflect.Kind, error) {
	_, f, err := lookup(structValue(v), key)
	if err != nil {
		return reflect.Invalid, err
	}
	return kindOf(f.typ), nil
}

func valueOf(v any, key string) (any, error) {
	section, f, err := lookup(structValue(v), key)
	if err != nil {
		return nil, err
	}
	return section.FieldByIndex(f.index).Interface(), nil
}

// setValue sets a dotted key on the section that ptr points to.
func setValue(ptr any, key string, value any) error {
	section, f, err := lookup(structValue(ptr), key)
	if err != nil {
		return err
	}
	return setField(section, f, value)
}

func setField(section reflect.Value, f field, value any) error {
	newValue, err := convert(value, f.typ)
	if err != nil {
		return err
	}
	if v, ok := section.Addr().Interface().(validator); ok {
		if err := v.validate(f.key, newValue.Interface()); err != nil {
			return err
		}
	}
	section.FieldByIndex(f.index).Set(newValue)
	return nil
}

// convert turns value into a value of type t. Values already of the right
// type are used as-is; strings are parsed.
func convert(value any, t reflect.Type) (reflect.Value, error) {
	if value != nil {
		rv := reflect.ValueOf(value)
		if rv.Type().AssignableTo(t) {
			return rv, nil
		}
		if s, ok := value.(string); ok {
			return parseString(s, t)
		}
	}
	return reflect.Value{}, fmt.Errorf("unsupported type %T", value)
}

func parseString(s string, t reflect.Type) (reflect.Value, error) {
	v := reflect.New(t).Elem()
	if t == durationType {
		d, err := time.ParseDuration(s)
		if err != nil {
			return reflect.Value{}, fmt.Errorf("unable to convert to duration: %w", err)
		}
		v.SetInt(int64(d))
		return v, nil
	}

	var err error
	switch t.Kind() {
	case reflect.String:
		v.SetString(s)
	case reflect.Bool:
		var b bool
		b, err = strconv.ParseBool(s)
		v.SetBool(b)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		var n int64
		n, err = strconv.ParseInt(s, 10, t.Bits())
		v.SetInt(n)
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		var n uint64
		n, err = strconv.ParseUint(s, 10, t.Bits())
		v.SetUint(n)
	case reflect.Float32, reflect.Float64:
		var n float64
		n, err = strconv.ParseFloat(s, t.Bits())
		v.SetFloat(n)
	default:
		return reflect.Value{}, fmt.Errorf("unsupported type string for %s", t)
	}
	if err != nil {
		return reflect.Value{}, fmt.Errorf("unable to convert %q to %s: %w", s, t, err)
	}
	return v, nil
}

// applyDefaults sets every field that has a default tag, recursing into
// nested sections. ptr must point to a section struct.
func applyDefaults(ptr any) error {
	return applyDefaultsTo("", structValue(ptr))
}

func applyDefaultsTo(prefix string, section reflect.Value) error {
	for _, f := range fieldsOf(section.Type()) {
		key := joinKey(prefix, f.key)
		if f.section {
			if err := applyDefaultsTo(key, section.FieldByIndex(f.index)); err != nil {
				return err
			}
			continue
		}
		if !f.hasDefault {
			continue
		}
		if err := setField(section, f, f.def); err != nil {
			return fmt.Errorf("invalid default for %s: %w", key, err)
		}
	}
	return nil
}