package wing

import (
	"context"
	"fmt"
	"regexp"
	"slices"
	"strings"
	"unicode"

	xnorm "golang.org/x/text/unicode/norm"
)

// Router places a note with rules only, no model (hyperguild#208):
//
//  1. A subfolder or frontmatter `wing:` is the owner's deliberate choice. It
//     must name a plain, EXISTING wing (a typo never creates one) and then it
//     places, whatever the wing is: extraction runs on the local model, and it
//     is the owner who dropped the note there. Two signals that disagree park.
//  2. With no signal, a note whose text matches ClientBlock is parked; any
//     other note goes to Default, the one wing that is created on first use.
//
// Wings are read from disk on every decision, so a wing created while the
// server runs is usable at once.
type Router struct {
	BrainDir    string
	Default     string
	ClientBlock *regexp.Regexp // nil = no content check
}

// Resolve implements Placer. It never returns an error: anything it cannot
// decide safely is an Exception with the reason spelled out.
func (r Router) Resolve(_ context.Context, n Note) Decision {
	for _, raw := range []string{n.Dir, n.FrontmatterWing} {
		if !isASCII(raw) {
			// strings.ToLower folds some non-ASCII runes to ASCII (the Kelvin
			// sign becomes "k"), which would let a lookalike name a real wing.
			return exception("", 0, fmt.Sprintf("wing name %q contains non-ASCII characters", strings.TrimSpace(raw)))
		}
	}
	dir, fm := norm(n.Dir), norm(n.FrontmatterWing)
	switch {
	case dir != "" && fm != "" && dir != fm:
		return exception("", 0, fmt.Sprintf("subfolder %q and frontmatter wing %q disagree", dir, fm))
	case dir != "":
		return r.explicit(dir, "subfolder")
	case fm != "":
		return r.explicit(fm, "frontmatter")
	}
	if r.ClientBlock != nil && matchesClient(r.ClientBlock, n.Text) {
		return exception("", 0, "text matches the client-content marker; not defaulted into a shared wing")
	}
	if r.Default == "" || !usableWingName(r.Default) {
		return exception("", 0, "no valid default wing configured")
	}
	return Decision{Wing: r.Default, Source: Default, Confidence: 1, Reason: "no wing named; default wing"}
}

func (r Router) explicit(w, via string) Decision {
	if !usableWingName(w) {
		return exception("", 0, fmt.Sprintf("%s names %q, which is not a usable wing name", via, w))
	}
	if w == r.Default {
		return Decision{Wing: w, Source: Explicit, Confidence: 1, Reason: "named by " + via}
	}
	known, err := KnownWings(r.BrainDir)
	if err != nil {
		return exception(w, 0, fmt.Sprintf("cannot list wings: %v", err))
	}
	if !slices.Contains(known, w) {
		return exception(w, 0, fmt.Sprintf("%s names %q: not an existing wing (new wings are never created from a folder name)", via, strings.TrimSpace(w)))
	}
	return Decision{Wing: w, Source: Explicit, Confidence: 1, Reason: "named by " + via}
}

// NewRouter builds the production Router. defaultWing must be a plain wing
// name; clientPattern, when set, is a case-insensitive regexp over note text
// whose match parks an unsignalled note instead of filing it. Both fail at
// startup rather than later: a client filter that silently does nothing is
// worse than none.
func NewRouter(brainDir, defaultWing, clientPattern string) (Router, error) {
	if defaultWing == "" || !usableWingName(defaultWing) {
		return Router{}, fmt.Errorf("default wing %q is not a usable wing name", defaultWing)
	}
	r := Router{BrainDir: brainDir, Default: defaultWing}
	if clientPattern != "" {
		re, err := regexp.Compile("(?i)" + clientPattern)
		if err != nil {
			return Router{}, fmt.Errorf("client-content pattern: %w", err)
		}
		r.ClientBlock = re
	}
	return r, nil
}

// reservedWingNames are directory names under wiki/ that are not wings: the
// retired layout and the root. A note must never be proposed into one.
var reservedWingNames = map[string]bool{"sources": true, "concepts": true, "entities": true, "wiki": true}

// usableWingName: a plain name that is not reserved.
func usableWingName(w string) bool { return plainWingName(w) && !reservedWingNames[w] }

func isASCII(s string) bool {
	for _, r := range s {
		if r > unicode.MaxASCII {
			return false
		}
	}
	return true
}

// matchesClient applies the client-content pattern to the text and to two
// folded forms of it, so the cheap evasions do not defeat it: NFKC (fullwidth
// letters), format characters removed (zero-width joiners, soft hyphens), every
// dash made "-", and whitespace removed ("acme-\nclient", "acme - client"). It catches what it names and nothing else; it is a filter,
// not a classifier.
func matchesClient(re *regexp.Regexp, text string) bool {
	folded := xnorm.NFKC.String(text)
	var b strings.Builder
	for _, r := range folded {
		switch {
		case unicode.Is(unicode.Cf, r):
			continue
		case unicode.Is(unicode.Pd, r) || r == '\u2212':
			b.WriteRune('-')
		default:
			b.WriteRune(r)
		}
	}
	clean := b.String()
	squeezed := strings.Join(strings.Fields(clean), "")
	return re.MatchString(text) || re.MatchString(squeezed)
}