// Package brain provides the wing/hall path taxonomy used by the brain
// wiki layout. A note's canonical location is
// brain/wiki/<wing>/<hall>/<slug>.md, where Wing is a free-form topic
// domain and Hall is one of a closed vocabulary of memory types.
package brain

import (
	"fmt"
	"path/filepath"
	"strings"
)

// ValidHalls is the closed vocabulary of hall names. A hall captures the
// memory type of a note within any wing.
var ValidHalls = map[string]bool{
	"facts":      true,
	"decisions":  true,
	"failures":   true,
	"hypotheses": true,
	"sources":    true,
}

// IsValidHall reports whether h is in the closed Hall vocabulary.
func IsValidHall(h string) bool {
	return ValidHalls[h]
}

// NotePath resolves the canonical filesystem path for a note given a
// wing, hall, and slug. Returns an error if hall is not in ValidHalls
// or if wing/slug sanitise to empty strings.
//
// The returned path is brain/wiki/<wing>/<hall>/<slug>.md with all
// segments sanitised: lowercased, alphanumerics and hyphens only.
func NotePath(brainDir, wing, hall, slug string) (string, error) {
	if !IsValidHall(hall) {
		return "", fmt.Errorf("invalid hall %q: must be one of facts/decisions/failures/hypotheses/sources", hall)
	}
	w := Sanitise(wing)
	if w == "" {
		return "", fmt.Errorf("invalid wing %q: must contain at least one alphanumeric character", wing)
	}
	s := Sanitise(strings.TrimSuffix(slug, ".md"))
	if s == "" {
		return "", fmt.Errorf("invalid slug %q: must contain at least one alphanumeric character", slug)
	}
	return filepath.Join(brainDir, "wiki", w, hall, s+".md"), nil
}

// Sanitise lowercases s and keeps only [a-z0-9-], collapsing any other
// character (including path separators) to a hyphen. Leading/trailing
// hyphens and runs of hyphens are collapsed.
func Sanitise(s string) string {
	s = strings.ToLower(strings.TrimSpace(s))
	var b strings.Builder
	prevHyphen := true
	for _, r := range s {
		switch {
		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
			b.WriteRune(r)
			prevHyphen = false
		case r == '-' || r == '_' || r == ' ' || r == '/' || r == '\\' || r == '.':
			if !prevHyphen {
				b.WriteByte('-')
				prevHyphen = true
			}
		}
	}
	out := b.String()
	return strings.Trim(out, "-")
}