package ckconfig

import (
	"fmt"
	"strings"

	commandkit "go.maximumdirect.net/commandkit"
	configkit "go.maximumdirect.net/configkit"
)

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 placeReportValue(root any, path []string, value any) (any, error) {
	if len(path) == 0 {
		return value, nil
	}
	current, ok := root.(map[string]any)
	if !ok {
		return nil, fmt.Errorf("report path ancestor is not a mapping")
	}
	for index, token := range path {
		if index == len(path)-1 {
			current[token] = value
			return root, nil
		}
		child, exists := current[token]
		if !exists {
			next := make(map[string]any)
			current[token] = next
			current = next
			continue
		}
		next, ok := child.(map[string]any)
		if !ok {
			return nil, fmt.Errorf("report path ancestor %q is not a mapping", token)
		}
		current = next
	}
	return root, nil
}

type diffReport struct {
	Additions any      `json:"additions"`
	Changes   any      `json:"changes"`
	Deletions []string `json:"deletions"`
}

func renderDiffReport(differences []configkit.Difference) (any, error) {
	report := diffReport{Additions: map[string]any{}, Changes: map[string]any{}, Deletions: []string{}}
	for _, difference := range differences {
		path := difference.Path.String()
		if difference.Left.Present && !difference.Right.Present {
			report.Deletions = append(report.Deletions, path)
			continue
		}
		if !difference.Right.Present {
			continue
		}
		tokens, err := decodeJSONPointer(path)
		if err != nil {
			return nil, fmt.Errorf("difference at %q: %w", path, err)
		}
		value, err := commandkit.DecodeYAMLReport(strings.NewReader(difference.Right.YAML))
		if err != nil {
			return nil, fmt.Errorf("difference at %q: %w", path, err)
		}
		if difference.Left.Present {
			report.Changes, err = placeReportValue(report.Changes, tokens, value)
		} else {
			report.Additions, err = placeReportValue(report.Additions, tokens, value)
		}
		if err != nil {
			return nil, fmt.Errorf("difference at %q: %w", path, err)
		}
	}
	return report, nil
}