// Package report aggregates transactions for the summary views.
package report

import (
	"fmt"
	"sort"
	"strings"

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

// TagTotal is one row of a spending breakdown.
type TagTotal struct {
	Tag      string
	Currency string
	In       int64 // sum of inflows, positive
	Out      int64 // sum of outflows, positive
	Count    int
	Digits   int
}

// Net is inflow minus outflow.
func (t TagTotal) Net() int64 { return t.In - t.Out }

// ByTag groups transactions by currency and effective tag. Untagged rows are
// collected under "(untagged)" so they stay visible instead of vanishing.
const Untagged = "(untagged)"

// Order is how the rows of a breakdown are arranged within a currency.
//
// Currency is always the outer key and no order can change that: the tool holds
// no exchange rates, so two currencies interleaved by amount would invite a
// comparison between numbers that cannot be compared.
type Order int

const (
	// OrderOut is the default, and the question a spending report is usually
	// opened to answer: where did the money go.
	OrderOut   Order = iota // largest outflow first
	OrderIn                 // largest inflow first
	OrderNet                // lowest net first, so the biggest losses lead
	OrderCount              // most transactions first
	OrderTag                // tag name, A→Z

	numOrders // how many there are, for cycling; not an order itself
)

// Next is the order after o, wrapping round. There are too many for a toggle,
// so a view offering the choice cycles through them.
func (o Order) Next() Order { return (o + 1) % numOrders }

// String is the short name the `--sort` flag takes, and ParseOrder accepts.
func (o Order) String() string {
	switch o {
	case OrderIn:
		return "in"
	case OrderNet:
		return "net"
	case OrderCount:
		return "count"
	case OrderTag:
		return "tag"
	}
	return "out"
}

// Label says what the order actually does, for a view with room to say it.
func (o Order) Label() string {
	switch o {
	case OrderIn:
		return "largest in first"
	case OrderNet:
		return "lowest net first"
	case OrderCount:
		return "most transactions first"
	case OrderTag:
		return "tag A→Z"
	}
	return "largest out first"
}

// Column is the heading the order sorts on, so a view can mark the column the
// rows are arranged by without keeping its own copy of this mapping.
func (o Order) Column() string {
	switch o {
	case OrderIn:
		return "In"
	case OrderNet:
		return "Net"
	case OrderCount:
		return "N"
	case OrderTag:
		return "Tag"
	}
	return "Out"
}

// Ascending reports which way the order runs. Only the tag reads naturally
// upwards; every amount and count is asked for biggest-first.
func (o Order) Ascending() bool { return o == OrderTag }

// Orders is every order, in the cycle Next follows.
func Orders() []Order {
	out := make([]Order, 0, numOrders)
	for o := Order(0); o < numOrders; o++ {
		out = append(out, o)
	}
	return out
}

// ParseOrder turns a short name back into an order, naming the alternatives
// when it cannot, since a misspelt sort would otherwise silently report in an
// order the caller did not ask for.
func ParseOrder(s string) (Order, error) {
	for _, o := range Orders() {
		if o.String() == s {
			return o, nil
		}
	}
	var names []string
	for _, o := range Orders() {
		names = append(names, o.String())
	}
	return OrderOut, fmt.Errorf("unknown sort %q: want one of %s", s, strings.Join(names, ", "))
}

// ByTag returns totals grouped by currency and arranged within each by order.
//
// Matched transfer legs are left out entirely. Both legs of a transfer are
// dropped together, so a report over everything is unchanged in total by money
// the user moved between their own accounts; a report scoped to one account or
// one month may see only one leg, and dropping it is still right, since the
// money was not spent. What was left out is available from Excluded.
//
// A pair whose definition sets a tolerance_pct is dropped the same way, fee and
// all — so with such a definition in play the report *is* short by the fee, and
// callers are expected to show Excluded alongside it rather than leave that
// difference unexplained.
func ByTag(txns []model.Transaction, order Order) []TagTotal {
	type key struct{ currency, tag string }
	acc := map[key]*TagTotal{}

	for _, t := range txns {
		if t.IsTransferLeg() {
			continue
		}
		tag := t.RuleTag
		if tag == "" {
			tag = Untagged
		}
		k := key{t.Currency, tag}
		row, ok := acc[k]
		if !ok {
			row = &TagTotal{Tag: tag, Currency: t.Currency, Digits: t.MinorDigits}
			acc[k] = row
		}
		if t.AmountMinor < 0 {
			row.Out += -t.AmountMinor
		} else {
			row.In += t.AmountMinor
		}
		row.Count++
	}

	out := make([]TagTotal, 0, len(acc))
	for _, row := range acc {
		out = append(out, *row)
	}
	sort.Slice(out, func(i, j int) bool {
		a, b := out[i], out[j]
		if a.Currency != b.Currency {
			return a.Currency < b.Currency
		}
		switch order {
		case OrderIn:
			if a.In != b.In {
				return a.In > b.In
			}
		case OrderNet:
			if a.Net() != b.Net() {
				return a.Net() < b.Net()
			}
		case OrderCount:
			if a.Count != b.Count {
				return a.Count > b.Count
			}
		case OrderTag:
			// The tag is the whole order here, and also the tie-break below.
		default:
			if a.Out != b.Out {
				return a.Out > b.Out
			}
		}
		// Every order falls back to the tag, so rows that tie on the chosen
		// column keep a fixed position instead of shuffling between reloads.
		return a.Tag < b.Tag
	})
	return out
}

// CurrencyTotal is the bottom line for one currency.
type CurrencyTotal struct {
	Currency string
	In       int64
	Out      int64
	Digits   int
}

// Net is inflow minus outflow.
func (c CurrencyTotal) Net() int64 { return c.In - c.Out }

// Totals sums the per-tag rows per currency. Currencies are never combined,
// because the tool holds no exchange rates.
func Totals(rows []TagTotal) []CurrencyTotal {
	acc := map[string]*CurrencyTotal{}
	for _, r := range rows {
		c, ok := acc[r.Currency]
		if !ok {
			c = &CurrencyTotal{Currency: r.Currency, Digits: r.Digits}
			acc[r.Currency] = c
		}
		c.In += r.In
		c.Out += r.Out
	}
	out := make([]CurrencyTotal, 0, len(acc))
	for _, c := range acc {
		out = append(out, *c)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Currency < out[j].Currency })
	return out
}

// TransferTotal is what ByTag left out for one currency.
//
// It counts legs rather than pairs on purpose: a report scoped to one account
// or month usually holds only one side of a movement, and claiming a whole
// transfer was excluded when only half of it was in view would be a lie. The
// same goes for the two directions, which is why they are separate totals: an
// exchange between currencies has its legs in different columns entirely, so
// one currency can see only what left and the other only what arrived.
type TransferTotal struct {
	Currency string
	Out      int64 // sum of the leaving legs in view, positive
	In       int64 // sum of the arriving legs in view
	Legs     int
	Digits   int
	// Fee is what those movements cost: the amount that left less the amount
	// that arrived, for pairs whose legs are *both* in view. A definition with a
	// tolerance_pct pairs legs that disagree, and since the pair leaves the
	// report entirely, this is the only place that difference is still money
	// rather than nothing. Zero when no definition allows a mismatch.
	Fee int64
	// Pairs counts the movements Fee was computed from — complete pairs in this
	// currency, not legs. A report scoped to one account or month usually holds
	// one side of a movement, and half a pair cannot say what the other half
	// received, so it contributes to Legs and Out/In but not here.
	Pairs int
}

// Excluded summarises the transfer legs ByTag dropped, so a report can account
// for the difference between its total and the account balances.
func Excluded(txns []model.Transaction) []TransferTotal {
	// Both legs of a pair carry the same TransferID, so the ones whose partner
	// is also in view can be found without going back to the index.
	byID := map[int64][]model.Transaction{}
	for _, t := range txns {
		if t.IsTransferLeg() {
			byID[*t.TransferID] = append(byID[*t.TransferID], t)
		}
	}

	acc := map[string]*TransferTotal{}
	row := func(t model.Transaction) *TransferTotal {
		r, ok := acc[t.Currency]
		if !ok {
			r = &TransferTotal{Currency: t.Currency, Digits: t.MinorDigits}
			acc[t.Currency] = r
		}
		return r
	}

	for _, t := range txns {
		if !t.IsTransferLeg() {
			continue
		}
		r := row(t)
		r.Legs++
		if t.AmountMinor < 0 {
			r.Out += -t.AmountMinor
		} else {
			r.In += t.AmountMinor
		}
	}

	for _, legs := range byID {
		// An exchange has its legs in two currencies, and there is no rate here
		// to subtract one from the other, so it has no fee to report.
		if len(legs) != 2 || legs[0].Currency != legs[1].Currency {
			continue
		}
		r := row(legs[0])
		r.Pairs++
		r.Fee -= legs[0].AmountMinor + legs[1].AmountMinor
	}

	out := make([]TransferTotal, 0, len(acc))
	for _, r := range acc {
		out = append(out, *r)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Currency < out[j].Currency })
	return out
}