package ckconfig

import (
	"bytes"
	"fmt"
	"strings"

	configkit "gitea.maximumdirect.net/eric/ConfigKit"
	"go.yaml.in/yaml/v4"
)

func decodeSafeSnapshot(snapshot string) (*yaml.Node, error) {
	var document yaml.Node
	decoder := yaml.NewDecoder(strings.NewReader(snapshot))
	if err := decoder.Decode(&document); err != nil {
		return nil, fmt.Errorf("decode safe YAML snapshot: %w", err)
	}
	if document.Kind != yaml.DocumentNode || len(document.Content) == 0 {
		return nil, fmt.Errorf("decode safe YAML snapshot: document has no value")
	}
	return document.Content[0], nil
}

func decodeJSONPointer(pointer string) ([]string, error) {
	if pointer == "" {
		return nil, nil
	}
	if pointer[0] != '/' {
		return nil, fmt.Errorf("decode JSON pointer %q: path must start with /", pointer)
	}

	parts := strings.Split(pointer[1:], "/")
	for index, part := range parts {
		var decoded strings.Builder
		decoded.Grow(len(part))
		for offset := 0; offset < len(part); offset++ {
			if part[offset] != '~' {
				decoded.WriteByte(part[offset])
				continue
			}
			if offset+1 >= len(part) {
				return nil, fmt.Errorf("decode JSON pointer %q: incomplete escape in token %d", pointer, index)
			}
			offset++
			switch part[offset] {
			case '0':
				decoded.WriteByte('~')
			case '1':
				decoded.WriteByte('/')
			default:
				return nil, fmt.Errorf("decode JSON pointer %q: invalid escape in token %d", pointer, index)
			}
		}
		parts[index] = decoded.String()
	}
	return parts, nil
}

func placeYAMLValue(category *yaml.Node, path []string, value *yaml.Node) (*yaml.Node, error) {
	if value == nil {
		return nil, fmt.Errorf("place YAML value: value node is nil")
	}
	if len(path) == 0 {
		return value, nil
	}
	if category == nil || category.Kind != yaml.MappingNode {
		return nil, fmt.Errorf("place YAML value at %q: category ancestor is not a mapping", pointerFromTokens(path[:1]))
	}

	current := category
	for index, token := range path {
		keyIndex := mappingKeyIndex(current, token)
		if index == len(path)-1 {
			if keyIndex >= 0 {
				current.Content[keyIndex+1] = value
			} else {
				current.Content = append(current.Content, stringKey(token), value)
			}
			return category, nil
		}

		if keyIndex < 0 {
			child := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
			current.Content = append(current.Content, stringKey(token), child)
			current = child
			continue
		}

		child := current.Content[keyIndex+1]
		if child.Kind != yaml.MappingNode {
			return nil, fmt.Errorf("place YAML value at %q: ancestor %q is not a mapping", pointerFromTokens(path), pointerFromTokens(path[:index+1]))
		}
		current = child
	}
	return category, nil
}

func mappingKeyIndex(mapping *yaml.Node, key string) int {
	for index := 0; index+1 < len(mapping.Content); index += 2 {
		candidate := mapping.Content[index]
		if candidate.Kind == yaml.ScalarNode && candidate.Tag == "!!str" && candidate.Value == key {
			return index
		}
	}
	return -1
}

func stringKey(value string) *yaml.Node {
	return &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: value}
}

func pointerFromTokens(tokens []string) string {
	if len(tokens) == 0 {
		return ""
	}
	var pointer strings.Builder
	for _, token := range tokens {
		pointer.WriteByte('/')
		pointer.WriteString(strings.ReplaceAll(strings.ReplaceAll(token, "~", "~0"), "/", "~1"))
	}
	return pointer.String()
}

func renderDiffReport(differences []configkit.Difference) ([]byte, error) {
	additions := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
	changes := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
	deletions := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}

	for _, difference := range differences {
		path := difference.Path.String()
		switch {
		case !difference.Left.Present && difference.Right.Present:
			tokens, err := decodeJSONPointer(path)
			if err != nil {
				return nil, fmt.Errorf("render addition at %q: %w", path, err)
			}
			value, err := decodeSafeSnapshot(difference.Right.YAML)
			if err != nil {
				return nil, fmt.Errorf("render addition at %q: %w", path, err)
			}
			additions, err = placeYAMLValue(additions, tokens, value)
			if err != nil {
				return nil, fmt.Errorf("render addition at %q: %w", path, err)
			}
		case difference.Left.Present && difference.Right.Present:
			tokens, err := decodeJSONPointer(path)
			if err != nil {
				return nil, fmt.Errorf("render change at %q: %w", path, err)
			}
			value, err := decodeSafeSnapshot(difference.Right.YAML)
			if err != nil {
				return nil, fmt.Errorf("render change at %q: %w", path, err)
			}
			changes, err = placeYAMLValue(changes, tokens, value)
			if err != nil {
				return nil, fmt.Errorf("render change at %q: %w", path, err)
			}
		case difference.Left.Present && !difference.Right.Present:
			deletions.Content = append(deletions.Content, stringKey(path))
		}
	}

	root := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map", Content: []*yaml.Node{
		stringKey("additions"), additions,
		stringKey("changes"), changes,
		stringKey("deletions"), deletions,
	}}
	document := &yaml.Node{Kind: yaml.DocumentNode, Content: []*yaml.Node{root}}
	var output bytes.Buffer
	if err := encodeYAML(&output, document); err != nil {
		return nil, fmt.Errorf("encode configuration diff report: %w", err)
	}
	return output.Bytes(), nil
}