// Package rules applies the glob rules from rules.toml to transactions,
// deciding their tag. The most specific rule that fits wins, so a general rule
// and the narrower one carving an exception out of it can be written in either
// order. It is the only thing that decides a tag, so rule_tag is derived state
// and Retag can rewrite it wholesale at any time.
package rules

import (
	"sort"

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

// Engine evaluates rules most specific first; the first match wins.
type Engine struct {
	// rules is file order, which is what Rules and Usage report and what the
	// rules screen numbers and deletes by. Evaluation does not use it.
	rules []config.Rule
	// order indexes into rules, most specific first. Specificity decides
	// precedence rather than position, so "*NIKOLA*" claims what it names
	// wherever it sits relative to the "*NIK*" that would otherwise swallow it.
	order []int
}

// New builds an engine from the parsed rules file, working out once which rule
// is tried before which.
func New(r *config.Rules) *Engine {
	e := &Engine{rules: r.Rule, order: make([]int, len(r.Rule))}
	for i := range e.order {
		e.order[i] = i
	}
	// Stable, so rules of equal specificity keep file order between them and
	// the earlier one still wins.
	sort.SliceStable(e.order, func(a, b int) bool {
		return moreSpecific(e.rules[e.order[a]], e.rules[e.order[b]])
	})
	return e
}

// moreSpecific reports whether a is tried before b.
//
// The literal characters a rule spells out are the evidence: they are what it
// commits to, and what it hands to a '*' is what it gives up. So "*NIKOLA*"
// beats "*NIK*", and a bare "*" sits last of all — the catch-all can be written
// anywhere in the file and still catch only what nothing else wanted.
//
// Two rules spelling out the same amount are separated by what else they pin
// down: fewer '*' first, since "NIKOLA" also fixes both ends where "*NIKOLA*"
// does not, and then an account-scoped rule over one that applies everywhere.
// Rules that tie on all three are left to file order by the stable sort.
func moreSpecific(a, b config.Rule) bool {
	if x, y := literals(a), literals(b); x != y {
		return x > y
	}
	if x, y := stars(a), stars(b); x != y {
		return x < y
	}
	return a.Account != "" && b.Account == ""
}

// literals counts the characters a rule pins down exactly, across every pattern
// it sets: a rule with both match and type has to satisfy both, so both count.
// '?' is not one of them — it fixes a length, not a character.
func literals(r config.Rule) int {
	n := 0
	for _, c := range r.Match + r.Type {
		if c != '*' && c != '?' {
			n++
		}
	}
	return n
}

// stars counts the unbounded wildcards, the only ones that let a pattern match
// a run of any length.
func stars(r config.Rule) int {
	n := 0
	for _, c := range r.Match + r.Type {
		if c == '*' {
			n++
		}
	}
	return n
}

// Rules returns the rules in file order, as loaded from rules.toml. That is the
// order the rules screen shows and deletes by; it is no longer the order they
// are tried in.
func (e *Engine) Rules() []config.Rule { return e.rules }

// Match returns the first rule matching a transaction on the given account, or
// nil if none does.
func (e *Engine) Match(accountSlug string, t model.Transaction) *config.Rule {
	if i := e.MatchIndex(accountSlug, t); i >= 0 {
		return &e.rules[i]
	}
	return nil
}

// MatchIndex returns the file position of the rule claiming a transaction, or
// -1 if none does. Every pattern a rule sets must match: a rule with both
// match and type is an "and", not an "or".
//
// Rules are tried most specific first, so the answer is the narrowest rule that
// fits and not merely the topmost. The returned index is still the position in
// rules.toml, because that is what the caller can point the user at — and it is
// what reveals a rule that is fully shadowed by a more specific one and so
// never applies to anything.
func (e *Engine) MatchIndex(accountSlug string, t model.Transaction) int {
	var (
		description = model.NormalizeDescription(t.Description)
		kind        = model.NormalizeDescription(t.Type)
	)
	for _, i := range e.order {
		r := &e.rules[i]
		if r.Account != "" && r.Account != accountSlug {
			continue
		}
		if r.Match != "" && !glob.Match(r.Match, description) {
			continue
		}
		if r.Type != "" && !glob.Match(r.Type, kind) {
			continue
		}
		return i
	}
	return -1
}

// Usage counts how many transactions each rule actually claims, indexed by file
// position. A rule with a count of zero is dead: either nothing matches it, or
// a more specific rule takes everything it would have caught.
func (e *Engine) Usage(txns []model.Transaction) []int {
	counts := make([]int, len(e.rules))
	for _, t := range txns {
		if i := e.MatchIndex(t.AccountSlug, t); i >= 0 {
			counts[i]++
		}
	}
	return counts
}

// ApplyTxn returns the tag for a transaction, empty if no rule matches.
func (e *Engine) ApplyTxn(accountSlug string, t model.Transaction) string {
	if r := e.Match(accountSlug, t); r != nil {
		return r.Tag
	}
	return ""
}

// Apply is the description-only shorthand, for callers that have nothing else.
func (e *Engine) Apply(accountSlug, description string) string {
	return e.ApplyTxn(accountSlug, model.Transaction{Description: description})
}

// Retag recomputes rule verdicts for every transaction in the index and
// writes them back. It returns how many rows changed.
func (e *Engine) Retag(db *store.DB) (int, error) {
	txns, err := db.Transactions(store.Filter{})
	if err != nil {
		return 0, err
	}
	var changed []store.RuleAssignment
	for _, t := range txns {
		tag := e.ApplyTxn(t.AccountSlug, t)
		if tag == t.RuleTag {
			continue
		}
		changed = append(changed, store.RuleAssignment{ID: t.ID, Tag: tag})
	}
	if len(changed) == 0 {
		return 0, nil
	}
	if err := db.ApplyRuleResults(changed); err != nil {
		return 0, err
	}
	return len(changed), nil
}