package changelog

import (
	"bytes"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"
)

const header = "# Changelog\n\nAll notable changes to this project will be documented in this file.\n\n"

// Entry holds the data for one changelog entry.
type Entry struct {
	Version     string
	Date        time.Time
	Description string
}

// Append prepends a new version entry to the CHANGELOG.md at the given path.
// If the file doesn't exist it will be created with a standard header.
func Append(path string, entry Entry) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return fmt.Errorf("creating dir for %s: %w", path, err)
	}

	existing, err := os.ReadFile(path)
	if err != nil && !os.IsNotExist(err) {
		return fmt.Errorf("reading %s: %w", path, err)
	}

	section := formatSection(entry)

	var buf bytes.Buffer
	if len(existing) == 0 {
		buf.WriteString(header)
		buf.WriteString(section)
	} else {
		content := string(existing)
		// Insert after the header block if present, otherwise prepend.
		insertIdx := findInsertPoint(content)
		buf.WriteString(content[:insertIdx])
		buf.WriteString(section)
		buf.WriteString(content[insertIdx:])
	}

	return os.WriteFile(path, buf.Bytes(), 0644)
}

func formatSection(e Entry) string {
	date := e.Date.Format("2006-01-02")
	var buf bytes.Buffer
	fmt.Fprintf(&buf, "## [%s] - %s\n\n", e.Version, date)
	if strings.TrimSpace(e.Description) != "" {
		buf.WriteString(strings.TrimSpace(e.Description))
		buf.WriteString("\n")
	}
	buf.WriteString("\n")
	return buf.String()
}

// findInsertPoint returns the index in content where new version sections should be inserted.
// It skips over the header (title + unreleased section if any), placing new entries at the top
// of the version list.
func findInsertPoint(content string) int {
	lines := strings.Split(content, "\n")
	for i, line := range lines {
		// Find first ## [version] line (not ## [Unreleased])
		if strings.HasPrefix(line, "## [") && !strings.HasPrefix(line, "## [Unreleased]") {
			// Return position of this line
			return positionOfLine(content, i)
		}
	}
	// No existing version sections: append at end of file
	return len(content)
}

func positionOfLine(content string, lineNum int) int {
	pos := 0
	for i, ch := range content {
		if lineNum == 0 {
			return i
		}
		if ch == '\n' {
			lineNum--
		}
		_ = pos
	}
	return len(content)
}