package config

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

	"gopkg.in/yaml.v3"
)

// yamlMergeTag is the tag YAML gives a merge key (<<), which adds the keys
// of the map it names to the map it sits in.
const yamlMergeTag = "!!merge"

// EnvSection is the top-level key of a map of environment variables that
// smartconfig sets while it loads the config. It is not a vaultik setting,
// so its keys are not checked.
const EnvSection = "env"

var (
	errUnknownKey           = errors.New("unknown config key")
	errAnchorContainsItself = errors.New("anchor contains itself")
)

// nodeAndType is a node of a parsed config and the setting type it is
// checked against.
type nodeAndType struct {
	node *yaml.Node
	typ  reflect.Type
}

// SettingType returns the type of the setting that the dotted key names in
// Config, following the fields' yaml tags, as for s3.bucket or
// snapshots.home.exclude.0. A snapshot name or a list index may be any
// word. It returns an unknown key error when no setting has that key.
func SettingType(keys []string) (reflect.Type, error) {
	typ := reflect.TypeFor[Config]()

	for _, key := range keys {
		switch {
		case typ.Kind() == reflect.Map || typ.Kind() == reflect.Slice:
			typ = typ.Elem()
		case typ.Kind() == reflect.Struct:
			field, ok := yamlField(typ, key)
			if !ok {
				return nil, fmt.Errorf("%w %q", errUnknownKey, strings.Join(keys, "."))
			}

			typ = field.Type
		default:
			return nil, fmt.Errorf("%w %q", errUnknownKey, strings.Join(keys, "."))
		}
	}

	return typ, nil
}

// yamlField returns the field of struct type typ whose yaml tag names key.
func yamlField(typ reflect.Type, key string) (reflect.StructField, bool) {
	for field := range typ.Fields() {
		name, _, _ := strings.Cut(field.Tag.Get("yaml"), ",")
		if name == key && name != "-" {
			return field, true
		}
	}

	return reflect.StructField{}, false
}

// checkKeys returns an error naming the first key in node, a parsed config
// or part of one at the dotted path, that no setting in typ has. The error
// gives the key's line in the file, when it has one. A value of the wrong
// type is left for the YAML decoder to report. following holds the aliases
// the walk is inside. Meeting one of them again means its anchor contains
// itself, which is an error. checked holds each anchored node the walk has
// checked completely, with the type it was checked against. An alias to
// that node is not followed again for that type.
func checkKeys(node *yaml.Node, typ reflect.Type, path string,
	following map[*yaml.Node]bool, checked map[nodeAndType]bool,
) error {
	switch node.Kind {
	case yaml.DocumentNode:
		for _, child := range node.Content {
			err := checkKeys(child, typ, path, following, checked)
			if err != nil {
				return err
			}
		}
	case yaml.AliasNode:
		if following[node] {
			return fmt.Errorf("%w: %q on line %d",
				errAnchorContainsItself, node.Value, node.Line)
		}

		target := nodeAndType{node.Alias, typ}
		if checked[target] {
			return nil
		}

		following[node] = true
		defer delete(following, node)

		err := checkKeys(node.Alias, typ, path, following, checked)
		if err != nil {
			return err
		}

		checked[target] = true
	case yaml.SequenceNode:
		if typ.Kind() != reflect.Slice {
			return nil
		}

		for i, child := range node.Content {
			err := checkKeys(child, typ.Elem(), joinKey(path, strconv.Itoa(i)),
				following, checked)
			if err != nil {
				return err
			}
		}
	case yaml.MappingNode:
		return checkMappingKeys(node, typ, path, following, checked)
	case yaml.ScalarNode:
		// A scalar has no keys.
	}

	return nil
}

// checkFilledKeys is checkKeys for data, the config after smartconfig has
// filled in its ${...} references. A key that a reference brings in is not
// in the file, and the copy that Encode makes has no line numbers.
func checkFilledKeys(data map[string]any) error {
	var filled yaml.Node

	err := filled.Encode(data)
	if err != nil {
		return fmt.Errorf("failed to marshal config data: %w", err)
	}

	return checkKeys(&filled, reflect.TypeFor[Config](), "",
		map[*yaml.Node]bool{}, map[nodeAndType]bool{})
}

// checkMappingKeys is checkKeys for a map.
func checkMappingKeys(node *yaml.Node, typ reflect.Type, path string,
	following map[*yaml.Node]bool, checked map[nodeAndType]bool,
) error {
	for i := 0; i+1 < len(node.Content); i += 2 {
		key, value := node.Content[i], node.Content[i+1]

		if key.Tag == yamlMergeTag {
			// The value is one map or a list of maps.
			merged := []*yaml.Node{value}
			if value.Kind == yaml.SequenceNode {
				merged = value.Content
			}

			for _, m := range merged {
				err := checkKeys(m, typ, path, following, checked)
				if err != nil {
					return err
				}
			}

			continue
		}

		keyPath := joinKey(path, key.Value)
		if keyPath == EnvSection {
			continue
		}

		var valueType reflect.Type

		switch {
		case typ.Kind() == reflect.Map:
			valueType = typ.Elem()
		case typ.Kind() == reflect.Struct:
			field, ok := yamlField(typ, key.Value)
			if !ok && key.Line == 0 {
				// A key that a ${...} reference brought in has no line.
				return fmt.Errorf("%w %q", errUnknownKey, keyPath)
			}

			if !ok {
				return fmt.Errorf("%w %q on line %d",
					errUnknownKey, keyPath, key.Line)
			}

			valueType = field.Type
		default:
			return nil
		}

		err := checkKeys(value, valueType, keyPath, following, checked)
		if err != nil {
			return err
		}
	}

	return nil
}

// joinKey appends key to the dotted path.
func joinKey(path, key string) string {
	if path == "" {
		return key
	}

	return path + "." + key
}