// Package csvimport parses a guest-list CSV into structured rows, with
// tolerant header detection (Excel, Numbers, Google Sheets variants) and
// per-row validation. Streaming-friendly so a 5,000-row import doesn't
// load the entire file into a slice before we know if column 1 is junk.
package csvimport

import (
	"bufio"
	"encoding/csv"
	"errors"
	"fmt"
	"io"
	"net/mail"
	"regexp"
	"strconv"
	"strings"

	"golang.org/x/text/encoding/unicode"
	"golang.org/x/text/transform"
)

// Row is a single validated guest. Empty Email / Phone are allowed (a
// phone-only or name-only guest is valid per the plan).
type Row struct {
	Name     string
	Email    string
	Phone    string
	PlusOnes int
}

// RowError flags one row with the human-readable reason it can't be
// imported. The line number is 1-based and matches the source CSV
// (header counts as line 1, first data row is line 2) so the frontend
// can highlight the offending row.
type RowError struct {
	Row    int    `json:"row"`
	Reason string `json:"reason"`
}

// Result is the outcome of one parse pass.
type Result struct {
	Rows       []Row      `json:"rows,omitempty"`
	Errors     []RowError `json:"errors,omitempty"`
	TotalCount int        `json:"total_count"` // total data rows seen (excluding header)
}

// Options tune limits + behaviour.
type Options struct {
	MaxRows int // hard cap; rows beyond MaxRows return an error instead of being silently dropped
}

const DefaultMaxRows = 5000

// Strict E.164: optional leading +, then a non-zero leading digit (country
// codes never start with 0), followed by 6–14 more digits — total 7–15
// significant digits. Spaces / dashes / parens are tolerated by stripping
// before validation, but local-format numbers like "0244…" or "07700…"
// are rejected here so the host fixes them at upload time rather than at
// WhatsApp-send time.
var phoneRe = regexp.MustCompile(`^\+?[1-9][0-9]{6,14}$`)

// Parse reads a CSV from r and returns the parsed result. Encoding is
// auto-detected: UTF-8 with or without BOM, plus UTF-16 LE/BE BOMs
// (commonly produced by Mac Numbers exports).
func Parse(r io.Reader, opt Options) (*Result, error) {
	max := opt.MaxRows
	if max <= 0 {
		max = DefaultMaxRows
	}

	rd, err := decodingReader(r)
	if err != nil {
		return nil, err
	}

	csvr := csv.NewReader(rd)
	csvr.FieldsPerRecord = -1 // tolerate ragged rows; we re-validate column count ourselves
	csvr.TrimLeadingSpace = true

	header, err := csvr.Read()
	if err != nil {
		if errors.Is(err, io.EOF) {
			return nil, errors.New("csv is empty")
		}
		return nil, fmt.Errorf("read header: %w", err)
	}
	cols, err := detectColumns(header)
	if err != nil {
		return nil, err
	}

	out := &Result{Rows: make([]Row, 0, 64)}
	lineNo := 1 // header was line 1
	for {
		rec, err := csvr.Read()
		if err == io.EOF {
			break
		}
		lineNo++
		if err != nil {
			out.Errors = append(out.Errors, RowError{Row: lineNo, Reason: fmt.Sprintf("malformed csv: %v", err)})
			continue
		}
		out.TotalCount++
		if out.TotalCount > max {
			return nil, fmt.Errorf("import exceeds maximum of %d rows", max)
		}

		// Skip fully-empty rows silently — these appear at the end of
		// Excel exports a lot.
		if rowEmpty(rec) {
			out.TotalCount-- // don't count it
			continue
		}

		row, rerr := buildRow(rec, cols)
		if rerr != "" {
			out.Errors = append(out.Errors, RowError{Row: lineNo, Reason: rerr})
			continue
		}
		out.Rows = append(out.Rows, row)
	}
	return out, nil
}

func rowEmpty(rec []string) bool {
	for _, v := range rec {
		if strings.TrimSpace(v) != "" {
			return false
		}
	}
	return true
}

// decodingReader strips a UTF-8 BOM and decodes UTF-16 LE/BE when their
// BOM is present, returning a UTF-8 reader. Other byte orders fall through
// as raw UTF-8.
func decodingReader(r io.Reader) (*bufio.Reader, error) {
	br := bufio.NewReader(r)
	bom, err := br.Peek(3)
	if err != nil && !errors.Is(err, io.EOF) {
		return nil, err
	}
	switch {
	case len(bom) >= 3 && bom[0] == 0xEF && bom[1] == 0xBB && bom[2] == 0xBF:
		_, _ = br.Discard(3)
		return br, nil
	case len(bom) >= 2 && bom[0] == 0xFF && bom[1] == 0xFE:
		_, _ = br.Discard(2)
		dec := unicode.UTF16(unicode.LittleEndian, unicode.IgnoreBOM).NewDecoder()
		return bufio.NewReader(transform.NewReader(br, dec)), nil
	case len(bom) >= 2 && bom[0] == 0xFE && bom[1] == 0xFF:
		_, _ = br.Discard(2)
		dec := unicode.UTF16(unicode.BigEndian, unicode.IgnoreBOM).NewDecoder()
		return bufio.NewReader(transform.NewReader(br, dec)), nil
	}
	return br, nil
}

// columnSet records which column index each known field lives in. -1 means
// the column was not supplied; only Name is mandatory.
type columnSet struct {
	name, email, phone, plusOnes int
}

func detectColumns(header []string) (columnSet, error) {
	cs := columnSet{name: -1, email: -1, phone: -1, plusOnes: -1}
	for i, raw := range header {
		key := normaliseHeader(raw)
		switch key {
		case "name", "guestname", "fullname":
			cs.name = i
		case "email", "emailaddress", "e-mail":
			cs.email = i
		case "phone", "telephone", "mobile", "phonenumber":
			cs.phone = i
		case "plusones", "plus1", "plus-one", "plus-ones", "+1", "guests", "additionalguests":
			cs.plusOnes = i
		}
	}
	if cs.name < 0 {
		return cs, fmt.Errorf("required column 'name' not found in header: %v", header)
	}
	return cs, nil
}

func normaliseHeader(s string) string {
	s = strings.ToLower(strings.TrimSpace(s))
	// Drop spaces + underscores. Keep `+`, `-` so "+1" / "plus-one" still
	// match exactly.
	return strings.NewReplacer(" ", "", "_", "").Replace(s)
}

func buildRow(rec []string, cs columnSet) (Row, string) {
	get := func(i int) string {
		if i < 0 || i >= len(rec) {
			return ""
		}
		return strings.TrimSpace(rec[i])
	}
	row := Row{
		Name:  get(cs.name),
		Email: strings.ToLower(get(cs.email)),
		Phone: get(cs.phone),
	}
	if row.Name == "" {
		return row, "name is required"
	}

	if row.Email != "" {
		if _, err := mail.ParseAddress(row.Email); err != nil {
			return row, "invalid email"
		}
	}
	if row.Phone != "" {
		stripped := stripPhone(row.Phone)
		if !phoneRe.MatchString(stripped) {
			return row, "phone must be in international format with country code (e.g. +447700900123) — local numbers starting with 0 won't work for SMS or WhatsApp"
		}
		// Normalise: ensure stored form always starts with "+".
		if !strings.HasPrefix(stripped, "+") {
			stripped = "+" + stripped
		}
		row.Phone = stripped
	}
	if raw := get(cs.plusOnes); raw != "" {
		n, err := strconv.Atoi(raw)
		if err != nil || n < 0 {
			return row, "plus_ones must be a non-negative integer"
		}
		row.PlusOnes = n
	}
	return row, ""
}

var phoneStripper = strings.NewReplacer(" ", "", "-", "", "(", "", ")", "", " ", "")

func stripPhone(s string) string {
	return phoneStripper.Replace(s)
}

// TemplateCSV is the sample file served at /events/{id}/guests/import/template.
// Phone numbers MUST include the country code (e.g. +44 for UK, +233 for
// Ghana). Local-format numbers like "0244..." or "07700..." will be
// rejected at upload — the sample below shows the expected shape.
const TemplateCSV = "name,email,phone,plus_ones\n" +
	"Alex Doe,alex@example.com,+447700900123,1\n" +
	"Sam Patel,sam@example.com,,0\n" +
	"Jordan Lee,,+15551234567,2\n" +
	"Mira Patel,mira@example.com,+233244123456,0\n"