package releaseprep

import (
	"fmt"
	"os"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"
)

const (
	defaultVersionFile = "internal/releaseprep/version/version.go"
	defaultVersionExpr = `const String = "([^"]+)"`
	defaultChangelog   = "changelog.md"
)

type Options struct {
	VersionFile    string
	VersionPattern string
	Changelog      string
}

type semver struct {
	major int
	minor int
	patch int
}

type resolvedOptions struct {
	VersionFile string
	VersionExpr *regexp.Regexp
	Changelog   string
}

func Prepare(rootDir, version, releaseDate string, options Options) error {
	normalizedVersion := strings.TrimPrefix(strings.TrimSpace(version), "v")
	if normalizedVersion == "" {
		return fmt.Errorf("version must not be empty")
	}

	releaseDate = strings.TrimSpace(releaseDate)
	if releaseDate == "" {
		return fmt.Errorf("release date must not be empty")
	}

	resolved, err := resolveOptions(options)
	if err != nil {
		return err
	}

	if err := updateVersionFile(rootDir, normalizedVersion, resolved); err != nil {
		return err
	}

	if err := updateChangelog(rootDir, normalizedVersion, releaseDate, resolved.Changelog); err != nil {
		return err
	}

	return nil
}

func RecommendedTag(rootDir string, options Options) (string, error) {
	resolved, err := resolveOptions(options)
	if err != nil {
		return "", err
	}

	currentVersion, err := readCurrentVersion(rootDir, resolved)
	if err != nil {
		return "", err
	}

	unreleasedBody, err := readUnreleasedBody(rootDir, resolved.Changelog)
	if err != nil {
		return "", err
	}

	parsed, err := parseSemver(currentVersion)
	if err != nil {
		return "", err
	}

	switch {
	case strings.Contains(unreleasedBody, "### Breaking"), sectionHasEntries(unreleasedBody, "Removed"):
		parsed.major++
		parsed.minor = 0
		parsed.patch = 0
	case sectionHasEntries(unreleasedBody, "Added"):
		parsed.minor++
		parsed.patch = 0
	default:
		parsed.patch++
	}

	return fmt.Sprintf("v%d.%d.%d", parsed.major, parsed.minor, parsed.patch), nil
}

func sectionHasEntries(unreleasedBody, sectionName string) bool {
	heading := "### " + sectionName
	sectionStart := strings.Index(unreleasedBody, heading)
	if sectionStart == -1 {
		return false
	}

	afterHeading := unreleasedBody[sectionStart+len(heading):]
	if strings.HasPrefix(afterHeading, "\r") {
		afterHeading = afterHeading[1:]
	}
	if strings.HasPrefix(afterHeading, "\n") {
		afterHeading = afterHeading[1:]
	}

	nextHeading := strings.Index(afterHeading, "\n### ")
	sectionBody := afterHeading
	if nextHeading != -1 {
		sectionBody = afterHeading[:nextHeading]
	}

	return strings.TrimSpace(sectionBody) != ""
}

func resolveOptions(options Options) (resolvedOptions, error) {
	versionFile := strings.TrimSpace(options.VersionFile)
	if versionFile == "" {
		versionFile = defaultVersionFile
	}

	pattern := strings.TrimSpace(options.VersionPattern)
	if pattern == "" {
		pattern = defaultVersionExpr
	}
	versionExpr, err := regexp.Compile(pattern)
	if err != nil {
		return resolvedOptions{}, fmt.Errorf("compile version pattern: %w", err)
	}
	if versionExpr.NumSubexp() != 1 {
		return resolvedOptions{}, fmt.Errorf("version pattern must contain exactly one capture group")
	}

	changelog := strings.TrimSpace(options.Changelog)
	if changelog == "" {
		changelog = defaultChangelog
	}

	return resolvedOptions{VersionFile: versionFile, VersionExpr: versionExpr, Changelog: changelog}, nil
}

func updateVersionFile(rootDir, version string, options resolvedOptions) error {
	path := filepath.Join(rootDir, options.VersionFile)
	contents, err := os.ReadFile(path)
	if err != nil {
		return fmt.Errorf("read version file: %w", err)
	}

	match := options.VersionExpr.FindStringSubmatch(string(contents))
	if len(match) < 2 {
		return fmt.Errorf("version value not found in %s", path)
	}

	replacement := strings.Replace(match[0], match[1], version, 1)
	updated := strings.Replace(string(contents), match[0], replacement, 1)
	if updated == string(contents) {
		return nil
	}

	if err := os.WriteFile(path, []byte(updated), 0o644); err != nil {
		return fmt.Errorf("write version file: %w", err)
	}

	return nil
}

func updateChangelog(rootDir, version, releaseDate, changelogPath string) error {
	unreleasedBody, text, afterHeader, nextSectionStart, path, err := readChangelogState(rootDir, changelogPath)
	if err != nil {
		return err
	}

	if strings.TrimSpace(unreleasedBody) == "" {
		return fmt.Errorf("unreleased section is empty")
	}

	newSection := fmt.Sprintf("## [%s] - %s\n", version, releaseDate)
	newSection += "\n" + unreleasedBody
	if !strings.HasSuffix(newSection, "\n") {
		newSection += "\n"
	}

	updated := text[:afterHeader] + "\n" + newSection + text[nextSectionStart:]
	if err := os.WriteFile(path, []byte(updated), 0o644); err != nil {
		return fmt.Errorf("write changelog: %w", err)
	}

	return nil
}

func readCurrentVersion(rootDir string, options resolvedOptions) (string, error) {
	path := filepath.Join(rootDir, options.VersionFile)
	contents, err := os.ReadFile(path)
	if err != nil {
		return "", fmt.Errorf("read version file: %w", err)
	}

	match := options.VersionExpr.FindStringSubmatch(string(contents))
	if len(match) < 2 {
		return "", fmt.Errorf("version value not found in %s", path)
	}

	return strings.TrimSpace(match[1]), nil
}

func readUnreleasedBody(rootDir, changelogPath string) (string, error) {
	unreleasedBody, _, _, _, _, err := readChangelogState(rootDir, changelogPath)
	if err != nil {
		return "", err
	}

	if strings.TrimSpace(unreleasedBody) == "" {
		return "", fmt.Errorf("unreleased section is empty")
	}

	return unreleasedBody, nil
}

func readChangelogState(rootDir, changelogPath string) (string, string, int, int, string, error) {
	path := filepath.Join(rootDir, changelogPath)
	contents, err := os.ReadFile(path)
	if err != nil {
		return "", "", 0, 0, "", fmt.Errorf("read changelog: %w", err)
	}

	text := string(contents)
	unreleasedHeader := "## [Unreleased]\n"
	start := strings.Index(text, unreleasedHeader)
	if start == -1 {
		return "", "", 0, 0, "", fmt.Errorf("unreleased section not found in changelog")
	}

	afterHeader := start + len(unreleasedHeader)
	nextSectionRelative := strings.Index(text[afterHeader:], "\n## [")
	if nextSectionRelative == -1 {
		nextSectionRelative = len(text[afterHeader:])
	}
	nextSectionStart := afterHeader + nextSectionRelative
	unreleasedBody := strings.TrimLeft(text[afterHeader:nextSectionStart], "\n")

	return unreleasedBody, text, afterHeader, nextSectionStart, path, nil
}

func parseSemver(version string) (semver, error) {
	parts := strings.Split(strings.TrimSpace(version), ".")
	if len(parts) != 3 {
		return semver{}, fmt.Errorf("version %q is not semantic major.minor.patch", version)
	}

	major, err := strconv.Atoi(parts[0])
	if err != nil {
		return semver{}, fmt.Errorf("parse major version: %w", err)
	}
	minor, err := strconv.Atoi(parts[1])
	if err != nil {
		return semver{}, fmt.Errorf("parse minor version: %w", err)
	}
	patch, err := strconv.Atoi(parts[2])
	if err != nil {
		return semver{}, fmt.Errorf("parse patch version: %w", err)
	}

	return semver{major: major, minor: minor, patch: patch}, nil
}