package pdfcert

import (
	"fmt"
	"strings"
	"time"
)

// Input for a simple one-page Verae registration certificate PDF.
type Input struct {
	DocumentName string
	SHA256       string
	ReceiptID    string
	CreatedAt    string
	Merkle       string
	VerifyURL    string
	Channel      string
}

// Build returns a minimal PDF (no external deps) describing the stamp.
func Build(in Input) ([]byte, error) {
	created := in.CreatedAt
	if created == "" {
		created = time.Now().UTC().Format(time.RFC3339)
	}
	local := ""
	if t, err := time.Parse(time.RFC3339, created); err == nil {
		local = t.Local().Format(time.RFC1123)
	}
	verify := in.VerifyURL
	if verify == "" && in.SHA256 != "" {
		verify = fmt.Sprintf("https://mcp.verae.ai/register.html?sha256=%s&receipt=%s", in.SHA256, in.ReceiptID)
	}

	lines := []string{
		"Verae Timestamp Registration",
		"",
		"Document: " + sanitize(in.DocumentName),
		"SHA-256: " + in.SHA256,
		"Receipt ID: " + in.ReceiptID,
		"UTC: " + created,
	}
	if local != "" {
		lines = append(lines, "Local: "+local)
	}
	if in.Merkle != "" {
		lines = append(lines, "Merkle/block: "+in.Merkle)
	}
	if in.Channel != "" {
		lines = append(lines, "Channel: "+in.Channel)
	}
	lines = append(lines, "Verify: "+verify)
	lines = append(lines, "", "Digests only — document bytes were not uploaded.")

	return buildTextPDF(lines)
}

func sanitize(s string) string {
	s = strings.ReplaceAll(s, "(", "\\(")
	s = strings.ReplaceAll(s, ")", "\\)")
	s = strings.ReplaceAll(s, "\\", "\\\\")
	return s
}

// Minimal PDF 1.4 with Helvetica text lines.
func buildTextPDF(lines []string) ([]byte, error) {
	var content strings.Builder
	content.WriteString("BT /F1 12 Tf 50 750 Td 14 TL\n")
	for i, line := range lines {
		if i > 0 {
			content.WriteString("T*\n")
		}
		content.WriteString("(")
		content.WriteString(sanitize(line))
		content.WriteString(") Tj\n")
	}
	content.WriteString("ET\n")
	stream := content.String()

	objects := []string{
		"<< /Type /Catalog /Pages 2 0 R >>",
		"<< /Type /Pages /Kids [3 0 R] /Count 1 >>",
		"<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R /Resources << /Font << /F1 5 0 R >> >> >>",
		fmt.Sprintf("<< /Length %d >>\nstream\n%s\nendstream", len(stream), stream),
		"<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>",
	}

	var body strings.Builder
	body.WriteString("%PDF-1.4\n")
	offsets := make([]int, len(objects)+1)
	for i, obj := range objects {
		offsets[i+1] = body.Len()
		fmt.Fprintf(&body, "%d 0 obj\n%s\nendobj\n", i+1, obj)
	}
	xrefStart := body.Len()
	fmt.Fprintf(&body, "xref\n0 %d\n", len(objects)+1)
	body.WriteString("0000000000 65535 f \n")
	for i := 1; i <= len(objects); i++ {
		fmt.Fprintf(&body, "%010d 00000 n \n", offsets[i])
	}
	fmt.Fprintf(&body, "trailer << /Size %d /Root 1 0 R >>\nstartxref\n%d\n%%%%EOF\n", len(objects)+1, xrefStart)
	return []byte(body.String()), nil
}