// Package mapping turns Multicase vouchers into Accounted journal entries:
// it translates account numbers, posts customer and supplier subledger lines
// to their control accounts and splits VAT out of gross amounts.
package mapping

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"math"
	"os"
	"sort"
	"strings"

	"simenandre.no/accounted-multicase-intgr/internal/accounted"
	"simenandre.no/accounted-multicase-intgr/internal/multicase"
)

// VATCode describes one Multicase MVA code. Multicase posts revenue and cost
// lines gross with the code attached and leaves the VAT split to the
// receiving ledger, so a code with a rate needs an account to post VAT to.
type VATCode struct {
	Rate    float64 `json:"rate"`
	Account string  `json:"account"`
	TaxCode string  `json:"tax_code"`
}

type Config struct {
	Accounts              map[string]string  `json:"accounts"`
	CustomerLedgerAccount string             `json:"customer_ledger_account"`
	SupplierLedgerAccount string             `json:"supplier_ledger_account"`
	VATCodes              map[string]VATCode `json:"vat_codes"`
	// RoundingAccount takes the öre left over when converted amounts no
	// longer balance, up to MaxRoundingOre.
	RoundingAccount string `json:"rounding_account"`
	MaxRoundingOre  int64  `json:"max_rounding_ore"`
	SourceType      string `json:"source_type"`
	// VoucherSeries maps a Multicase BilagsArt to an Accounted series letter;
	// "*" is the fallback. Empty means the company's default series.
	VoucherSeries map[string]string `json:"voucher_series"`
	// Dimensions maps "department" (AvdNr.) and "project" (Prosjekt) to SIE
	// dimension numbers. Unset means the columns are not sent.
	Dimensions map[string]string `json:"dimensions"`
}

func LoadConfig(path string) (*Config, error) {
	b, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var c Config
	dec := json.NewDecoder(bytes.NewReader(b))
	dec.DisallowUnknownFields()
	if err := dec.Decode(&c); err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	if c.SourceType == "" {
		c.SourceType = "manual"
	}
	if c.MaxRoundingOre == 0 {
		c.MaxRoundingOre = 100
	}
	for code, v := range c.VATCodes {
		if v.Rate < 0 || v.Rate >= 100 {
			return nil, fmt.Errorf("vat_codes.%s: rate must be a percentage between 0 and 100", code)
		}
		if v.Rate > 0 && v.Account == "" {
			return nil, fmt.Errorf("vat_codes.%s: a code with a rate needs an account", code)
		}
	}
	return &c, nil
}

// UnmappedError lists the Multicase accounts and VAT codes a voucher uses
// that the config does not cover.
type UnmappedError struct {
	Accounts []string
	VATCodes []string
}

func (e *UnmappedError) Error() string {
	var parts []string
	if len(e.Accounts) > 0 {
		parts = append(parts, "unmapped accounts "+strings.Join(e.Accounts, ", "))
	}
	if len(e.VATCodes) > 0 {
		parts = append(parts, "unknown VAT codes "+strings.Join(e.VATCodes, ", "))
	}
	return strings.Join(parts, "; ")
}

// ErrEmpty means every line of the voucher has a zero amount, as Multicase
// writes for the customer line of a nullfaktura.
var ErrEmpty = errors.New("voucher has no non-zero lines")

// Build maps one voucher. The fiscal period is left for the caller to set.
func (c *Config) Build(v multicase.Voucher) (accounted.JournalEntry, error) {
	unmapped := &UnmappedError{}
	var lines []accounted.Line
	add := func(account string, amount int64, desc, taxCode string, dims map[string]string) {
		l := accounted.Line{Account: account, Description: desc, TaxCode: taxCode, Dimensions: dims}
		if amount > 0 {
			l.Debit = accounted.Money(amount)
		} else {
			l.Credit = accounted.Money(-amount)
		}
		lines = append(lines, l)
	}

	for _, l := range v.Lines {
		if l.Amount == 0 {
			continue
		}
		account, err := c.account(l)
		if err != nil {
			unmapped.Accounts = appendUnique(unmapped.Accounts, l.Account)
			continue
		}
		desc := lineDescription(l)
		dims := c.dimensions(l)
		if l.VATCode == "" {
			add(account, l.Amount, desc, "", dims)
			continue
		}
		vat, ok := c.VATCodes[l.VATCode]
		if !ok {
			unmapped.VATCodes = appendUnique(unmapped.VATCodes, l.VATCode)
			continue
		}
		if vat.Rate == 0 {
			add(account, l.Amount, desc, vat.TaxCode, dims)
			continue
		}
		net := SplitNet(l.Amount, vat.Rate)
		add(account, net, desc, vat.TaxCode, dims)
		if l.Amount-net != 0 {
			add(vat.Account, l.Amount-net, desc, vat.TaxCode, nil)
		}
	}
	if len(unmapped.Accounts) > 0 || len(unmapped.VATCodes) > 0 {
		return accounted.JournalEntry{}, unmapped
	}
	if len(lines) == 0 {
		return accounted.JournalEntry{}, ErrEmpty
	}

	var sum int64
	for _, l := range lines {
		sum += int64(l.Debit) - int64(l.Credit)
	}
	if sum != 0 {
		if c.RoundingAccount == "" || abs(sum) > c.MaxRoundingOre {
			return accounted.JournalEntry{}, fmt.Errorf("does not balance: off by %s", accounted.Money(sum))
		}
		add(c.RoundingAccount, -sum, "Öresavrundning", "", nil)
	}

	return accounted.JournalEntry{
		EntryDate:     v.Date.Format("2006-01-02"),
		Description:   truncate(strings.TrimSuffix(v.Label()+": "+v.Text, ": "), 200),
		SourceType:    c.SourceType,
		SourceID:      v.Key(),
		VoucherSeries: c.series(v.Type),
		Notes:         fmt.Sprintf("Multicase %s, BilagsArt %s, Bilagsnummer %s, BuntHodeID %s", v.Company, v.Type, v.Number, v.Batch),
		Lines:         lines,
	}, nil
}

func (c *Config) account(l multicase.Line) (string, error) {
	switch {
	case l.CustomerNo != "" && l.Account == l.CustomerNo && c.CustomerLedgerAccount != "":
		return c.CustomerLedgerAccount, nil
	case l.SupplierNo != "" && l.Account == l.SupplierNo && c.SupplierLedgerAccount != "":
		return c.SupplierLedgerAccount, nil
	}
	if a, ok := c.Accounts[l.Account]; ok && a != "" {
		return a, nil
	}
	return "", fmt.Errorf("unmapped account %s", l.Account)
}

func (c *Config) dimensions(l multicase.Line) map[string]string {
	dims := map[string]string{}
	if n := c.Dimensions["department"]; n != "" && l.Department != "" {
		dims[n] = l.Department
	}
	if n := c.Dimensions["project"]; n != "" && l.Project != "" {
		dims[n] = l.Project
	}
	if len(dims) == 0 {
		return nil
	}
	return dims
}

func (c *Config) series(voucherType string) string {
	if s, ok := c.VoucherSeries[voucherType]; ok {
		return s
	}
	return c.VoucherSeries["*"]
}

// SplitNet returns the net part of a gross amount in öre at the given VAT
// rate, rounded half away from zero. VAT is gross minus net, so the two
// always add back up to the gross amount.
func SplitNet(gross int64, rate float64) int64 {
	bp := int64(math.Round(rate * 100))
	num, den := gross*10000, 10000+bp
	if num >= 0 {
		return (2*num + den) / (2 * den)
	}
	return (2*num - den) / (2 * den)
}

func lineDescription(l multicase.Line) string {
	desc := l.Reference
	if l.Currency != "" && l.CurrencyAmt != "" && l.CurrencyAmt != "0" {
		desc = strings.TrimSpace(fmt.Sprintf("%s (%s %s)", desc, l.Currency, l.CurrencyAmt))
	}
	return truncate(desc, 200)
}

func appendUnique(list []string, s string) []string {
	for _, x := range list {
		if x == s {
			return list
		}
	}
	list = append(list, s)
	sort.Strings(list)
	return list
}

func truncate(s string, n int) string {
	r := []rune(s)
	if len(r) <= n {
		return s
	}
	return string(r[:n])
}

func abs(n int64) int64 {
	if n < 0 {
		return -n
	}
	return n
}