package parser

import (
	"encoding/csv"
	"fmt"
	"io"
	"os"
	"sort"
	"strings"
	"time"

	"git.petrovv.com/nikola/money/internal/config"
	"git.petrovv.com/nikola/money/internal/model"
)

func init() {
	Register("revolut", func(acc *config.Account) (Parser, error) {
		return &revolutParser{digits: acc.Digits(), currency: strings.ToUpper(acc.Currency)}, nil
	})
}

// revolutParser reads a Revolut account-statement CSV export.
//
// Revolut books the fee alongside the transaction rather than as its own line,
// and reports pending transactions that have no balance yet. Both are handled
// the way the original extraction script did: the fee is folded into the
// amount, and anything not COMPLETED is skipped.
//
// A single export can mix currencies. Since an account here has one currency,
// rows in others are skipped and reported; importing them is a matter of
// giving that currency its own account folder.
type revolutParser struct {
	digits   int
	currency string
	warnings []string
}

// Columns the parser needs; a missing one is a hard error rather than a
// silently empty field.
var revolutColumns = []string{
	"Type", "Completed Date", "Description", "Amount", "Fee", "Currency", "State", "Balance",
}

func (p *revolutParser) Warnings() []string { return p.warnings }

func (p *revolutParser) Parse(path string, acc *config.Account) ([]RawTxn, error) {
	p.warnings = nil

	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	r := csv.NewReader(f)
	r.FieldsPerRecord = -1
	r.LazyQuotes = true

	header, err := r.Read()
	if err == io.EOF {
		return nil, fmt.Errorf("%s is empty", path)
	}
	if err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	index, err := revolutHeaderIndex(header)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}

	type record struct {
		fields []string
		line   int
	}
	var records []record
	for line := 2; ; line++ {
		rec, err := r.Read()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, fmt.Errorf("%s: row %d: %w", path, line, err)
		}
		if isBlank(rec) {
			continue
		}
		records = append(records, record{fields: rec, line: line})
	}

	// The export is not in date order, and the running balance only makes
	// sense along one. A stable sort also keeps identical rows in file order,
	// which the importer's deduplication relies on.
	sort.SliceStable(records, func(i, j int) bool {
		return index.get(records[i].fields, "Completed Date") < index.get(records[j].fields, "Completed Date")
	})

	var (
		txns       []RawTxn
		skipped    = map[string]int{}
		currencies = map[string]int{}
	)
	for _, rec := range records {
		get := func(name string) string { return index.get(rec.fields, name) }

		if state := get("State"); state != "COMPLETED" || get("Balance") == "" {
			if state == "" {
				state = "unfinished"
			}
			skipped[state]++
			continue
		}
		if currency := strings.ToUpper(get("Currency")); currency != p.currency {
			currencies[currency]++
			continue
		}

		txn, err := p.row(get, p.digits)
		if err != nil {
			return nil, fmt.Errorf("%s: row %d: %w", path, rec.line, err)
		}
		txns = append(txns, txn)
	}

	for _, state := range sortedKeys(skipped) {
		p.warnings = append(p.warnings, fmt.Sprintf("skipped %d %s transactions", skipped[state], state))
	}
	for _, currency := range sortedKeys(currencies) {
		p.warnings = append(p.warnings, fmt.Sprintf(
			"skipped %d rows in %s; give that currency its own account folder to import them",
			currencies[currency], currency))
	}
	return txns, nil
}

func (p *revolutParser) row(get func(string) string, digits int) (RawTxn, error) {
	d, err := time.Parse("2006-01-02", firstN(get("Completed Date"), 10))
	if err != nil {
		return RawTxn{}, fmt.Errorf("completed date %q: %w", get("Completed Date"), err)
	}

	amount, err := ParseAmount(get("Amount"), ".", "", digits)
	if err != nil {
		return RawTxn{}, fmt.Errorf("amount: %w", err)
	}
	var fee int64
	if raw := strings.TrimSpace(get("Fee")); raw != "" {
		if fee, err = ParseAmount(raw, ".", "", digits); err != nil {
			return RawTxn{}, fmt.Errorf("fee: %w", err)
		}
	}
	balance, err := ParseAmount(get("Balance"), ".", "", digits)
	if err != nil {
		return RawTxn{}, fmt.Errorf("balance: %w", err)
	}

	// The fee is deducted along with the amount rather than booked separately,
	// so it has to be folded in for the balance chain to hold.
	desc := strings.TrimSpace(get("Description"))
	if fee != 0 {
		desc += fmt.Sprintf(" (fee %s)", model.FormatMinor(fee, digits))
	}

	return RawTxn{
		Date:         d.Format("2006-01-02"),
		Description:  desc,
		AmountMinor:  amount - fee,
		Type:         strings.TrimSpace(get("Type")),
		BalanceMinor: &balance,
	}, nil
}

// isBlank reports whether a CSV record holds nothing but whitespace, which is
// what a trailing newline in the export reads as.
func isBlank(rec []string) bool {
	for _, f := range rec {
		if strings.TrimSpace(f) != "" {
			return false
		}
	}
	return true
}

// headerIndex maps a Revolut column name to its position.
type headerIndex map[string]int

func (h headerIndex) get(fields []string, name string) string {
	i, ok := h[name]
	if !ok || i >= len(fields) {
		return ""
	}
	return strings.TrimSpace(fields[i])
}

func revolutHeaderIndex(header []string) (headerIndex, error) {
	index := headerIndex{}
	for i, name := range header {
		index[strings.TrimSpace(name)] = i
	}
	var missing []string
	for _, name := range revolutColumns {
		if _, ok := index[name]; !ok {
			missing = append(missing, name)
		}
	}
	if len(missing) > 0 {
		return nil, fmt.Errorf("statement is missing the %s column(s); header was %v",
			strings.Join(missing, ", "), header)
	}
	return index, nil
}

func sortedKeys(m map[string]int) []string {
	out := make([]string, 0, len(m))
	for k := range m {
		out = append(out, k)
	}
	sort.Strings(out)
	return out
}

func firstN(s string, n int) string {
	if len(s) < n {
		return s
	}
	return s[:n]
}