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

	// FilterChunk is how many bytes of a note the content filter folds and
	// matches at a time (0 = 1 MiB). Folding allocates several times its input,
	// so a 64 MiB note is checked in pieces (with an overlap, so a marker on a
	// boundary is still found) instead of all at once.
	FilterChunk int
}

// 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, r.FilterChunk) {
		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
}

const (
	defaultFilterChunk = 1 << 20
	filterOverlap      = 4096
)

// matchesClient applies the client-content pattern to the text in bounded
// chunks, each overlapping the next, so memory stays proportional to the chunk
// and a marker on a chunk edge is still found.
func matchesClient(re *regexp.Regexp, text string, chunk int) bool {
	if chunk <= 0 {
		chunk = defaultFilterChunk
	}
	overlap := filterOverlap
	if overlap >= chunk {
		overlap = chunk / 2
	}
	for start := 0; start < len(text); {
		end := start + chunk
		if end >= len(text) {
			end = len(text)
		}
		if matchesFolded(re, text[start:end]) {
			return true
		}
		if end == len(text) {
			return false
		}
		// A cut inside a rune is harmless: the overlap carries every marker
		// shorter than it whole into the next chunk.
		start = end - overlap
	}
	return false
}

// matchesFolded matches the pattern against the chunk as it is and against
// folded forms, so the cheap evasions do not defeat it. Folding: NFKD (fullwidth
// letters, accents become base letter + mark), then marks, format characters and
// invisible fillers dropped, Cyrillic/Greek lookalikes mapped to Latin, every
// dash made "-". Views: whitespace collapsed to one space (so "acme corp" and
// \s work), whitespace removed (so "acme-\nclient" works), and the same with a
// soft hyphen read as a dash. It catches what it names and nothing else:
// leetspeak, punctuation splits (a.c.m.e), encodings and the many homoglyphs
// not in the small table below are not caught. It is a filter, not a classifier.
func matchesFolded(re *regexp.Regexp, chunk string) bool {
	if re.MatchString(chunk) {
		return true
	}
	decomposed := xnorm.NFKD.String(chunk)
	return re.MatchString(fold(decomposed, false, true)) ||
		re.MatchString(fold(decomposed, false, false)) ||
		re.MatchString(fold(decomposed, true, false))
}

// fold builds one view. softAsDash reads U+00AD as "-" instead of dropping it;
// keepSpace collapses whitespace to a single space instead of removing it.
func fold(s string, softAsDash, keepSpace bool) string {
	var b strings.Builder
	b.Grow(len(s))
	space := false
	for _, r := range s {
		switch {
		case r == '\u00ad' && softAsDash:
			b.WriteRune('-')
			space = false
			continue
		case unicode.Is(unicode.Mn, r) || unicode.Is(unicode.Me, r) || unicode.Is(unicode.Mc, r) ||
			unicode.Is(unicode.Cf, r) || invisibleFiller[r]:
			continue
		case unicode.IsSpace(r) || unicode.Is(unicode.Zs, r):
			if keepSpace && !space {
				b.WriteByte(' ')
				space = true
			}
			continue
		}
		space = false
		switch {
		case unicode.Is(unicode.Pd, r) || r == '\u2212':
			b.WriteRune('-')
		default:
			if l, ok := latinLookalike[r]; ok {
				r = l
			}
			b.WriteRune(r)
		}
	}
	return b.String()
}

// invisibleFiller are characters that render as nothing but are not format
// characters: Hangul fillers and the blank Braille pattern.
var invisibleFiller = map[rune]bool{0x115F: true, 0x1160: true, 0x3164: true, 0xFFA0: true, 0x2800: true}

// latinLookalike maps the commonest Cyrillic and Greek letters that read as
// Latin ones. Deliberately small; see matchesFolded.
var latinLookalike = map[rune]rune{
	'\u0430': 'a', '\u0435': 'e', '\u043e': 'o', '\u0440': 'p', '\u0441': 'c', '\u0445': 'x', '\u0443': 'y',
	'\u0456': 'i', '\u0458': 'j', '\u0455': 's', '\u04bb': 'h', '\u0501': 'd', '\u043a': 'k', '\u043d': 'h', '\u0442': 't',
	'\u0410': 'A', '\u0412': 'B', '\u0415': 'E', '\u041a': 'K', '\u041c': 'M', '\u041d': 'H', '\u041e': 'O', '\u0420': 'P', '\u0421': 'C', '\u0422': 'T', '\u0425': 'X',
	'\u03bf': 'o', '\u03bd': 'v', '\u03c1': 'p', '\u0391': 'A', '\u0392': 'B', '\u0395': 'E', '\u039f': 'O', '\u03a1': 'P',
}