// Package scanner implements the core fuzzy-diff engine of NaviWatcher.
//
// It compares a user's local albums (from Navidrome) against an artist's
// external discography (from MusicBrainz) and reports the releases that are
// present externally but have no sufficiently similar local album.
package scanner

import (
	"github.com/lithammer/fuzzysearch/fuzzy"
	"naviwatcher/internal/normalize"
)

// DefaultThreshold is the fallback similarity threshold used when a caller
// passes threshold == 0. It matches config.Scanner.FuzzyThreshold default.
const DefaultThreshold = 0.85

// resolveThreshold returns the provided threshold, or DefaultThreshold when
// the caller passes zero (unset). This keeps the engine usable when config
// defaults are not threaded through explicitly.
func resolveThreshold(threshold float64) float64 {
	if threshold == 0 {
		return DefaultThreshold
	}
	return threshold
}

// Similarity returns a normalized similarity score in the range [0.0, 1.0]
// between two strings. The strings are normalized first (lowercased,
// bracketed/parenthesized content and years stripped, special characters
// removed), then compared with a Levenshtein-distance-based ratio.
//
// A score of 1.0 means the normalized strings are identical; 0.0 means they
// share nothing. Empty strings (after normalization) always score 0.0.
func Similarity(a, b string) float64 {
	na := normalize.NormalizeString(a)
	nb := normalize.NormalizeString(b)

	// Two empty inputs are not considered a match.
	if na == "" && nb == "" {
		return 0.0
	}
	// One empty, one non-empty: no similarity.
	if na == "" || nb == "" {
		return 0.0
	}

	dist := fuzzy.LevenshteinDistance(na, nb)
	maxLen := len(na)
	if len(nb) > maxLen {
		maxLen = len(nb)
	}

	// 1.0 - normalized distance → higher is more similar.
	score := 1.0 - float64(dist)/float64(maxLen)
	if score < 0.0 {
		return 0.0
	}
	return score
}

// IsMatch reports whether a and b are similar enough to be considered the
// same release, given the provided threshold in [0.0, 1.0]. A threshold of 0
// (unset) falls back to DefaultThreshold, so this primitive honors the same
// zero-means-default contract as FindMissingReleases/ScanArtist/ScanAll rather
// than treating 0 as "always match".
func IsMatch(a, b string, threshold float64) bool {
	return Similarity(a, b) >= resolveThreshold(threshold)
}