stemify/ios/Stemify/EQ/ApoConfig.swift

import Foundation

/// Equalizer APO's config text, the format AutoEQ publishes headphone corrections in
/// (ParametricEQ.txt):
///
///   Preamp: -6.2 dB
///   Filter 1: ON LSC Fc 105 Hz Gain 6.0 dB Q 0.70
///   Filter 2: ON PK Fc 2300 Hz Gain -3.1 dB Q 1.41
///
/// Also understood: "Channel: L" / "R" / "all" lines, BW Oct instead of Q, and the older
/// type names (LS, HS, LP, HP, PEQ ...). Other commands (GraphicEQ, Delay, Copy) are skipped.
enum ApoConfig {
    struct Result { var filters: [EqFilter]; var preamp: Float?; var skipped: Int }

    private static let types: [String: FilterType] = [
        "PK": .Peak, "PEQ": .Peak, "MODAL": .Peak,
        "LS": .LowShelf, "LSC": .LowShelf, "LSQ": .LowShelf,
        "HS": .HighShelf, "HSC": .HighShelf, "HSQ": .HighShelf,
        "LP": .LowPass, "LPQ": .LowPass,
        "HP": .HighPass, "HPQ": .HighPass,
        "BP": .BandPass, "NO": .Notch, "AP": .AllPass,
    ]

    /// Nil when the text has no preamp or filter lines at all.
    static func parse(_ text: String) -> Result? {
        var filters: [EqFilter] = []
        var preamp: Float?
        var channel = EqChannel.Both
        var skipped = 0
        for raw in text.components(separatedBy: .newlines) {
            let line = (raw.split(separator: "#", maxSplits: 1, omittingEmptySubsequences: false).first.map(String.init) ?? "")
                .trimmingCharacters(in: .whitespaces)
            if line.isEmpty { continue }
            guard let colon = line.firstIndex(of: ":") else { continue }
            let cmd = line[..<colon].trimmingCharacters(in: .whitespaces).lowercased()
            let args = line[line.index(after: colon)...].split(whereSeparator: { $0 == " " || $0 == "\t" }).map(String.init)
            if cmd == "preamp" {
                guard let v = args.first.flatMap(Float.init) else { continue }
                preamp = (preamp ?? 0) + v
            } else if cmd == "channel" {
                let set = Set(args.map { $0.uppercased() })
                channel = set == ["L"] ? .Left : set == ["R"] ? .Right : .Both
            } else if cmd.hasPrefix("filter") {
                if let f = parseFilter(args, channel) { filters.append(f) } else { skipped += 1 }
            } else {
                skipped += 1
            }
        }
        if filters.isEmpty && preamp == nil { return nil }
        return Result(filters: filters, preamp: preamp, skipped: skipped)
    }

    private static func parseFilter(_ args: [String], _ channel: EqChannel) -> EqFilter? {
        func at(_ i: Int) -> String? { i < args.count ? args[i] : nil }
        var i = 0
        var on = true
        switch at(0)?.uppercased() {
        case "ON": i += 1
        case "OFF": on = false; i += 1
        default: break
        }
        guard let t = at(i)?.uppercased(), let type = types[t] else { return nil }
        i += 1
        var freq: Float?
        var gain: Float = 0
        var q: Float?
        var slope = 12
        while i < args.count {
            let key = args[i].lowercased()
            let num = at(i + 1).flatMap(Float.init)
            if key == "fc", let num { freq = num; i += 2 }
            else if key == "gain", let num { gain = num; i += 2 }
            else if key == "q", let num { q = num; i += 2 }
            else if key == "bw" && at(i + 1)?.lowercased() == "oct" {
                if let bw = at(i + 2).flatMap(Double.init) { q = Float(pow(2, bw).squareRoot() / (pow(2, bw) - 1)) }
                i += 3
            } else if key.hasSuffix("db"), let s = Float(key.dropLast(2)) {
                // "LSC 12dB Fc ...": a shelf or pass slope in dB per octave.
                slope = Int(s); i += 1
            } else {
                i += 1
            }
        }
        guard let fc = freq else { return nil }
        let defaultQ: Float = type == .Peak ? EQ_Q : 0.707
        return EqFilter(id: EqController.newFilterId(), type: type, freq: fc, gain: type.hasGain ? gain : 0, q: q ?? defaultQ,
                        slope: type.hasSlope ? min(max(slope, 6), 48) : 12, channel: channel, on: on)
    }

    /// The profile as an Equalizer APO config: preamp, graphic bands as peaks, then the filters.
    static func format(_ p: EqProfile) -> String {
        func n(_ v: Float) -> String {
            var s = String(format: "%.2f", locale: Locale(identifier: "en_US_POSIX"), v)
            while s.hasSuffix("0") { s.removeLast() }
            if s.hasSuffix(".") { s.removeLast() }
            return s
        }
        var out = "# \(p.name), from Stemify\n"
        out += "Preamp: \(n(p.preamp)) dB\n"
        var channel = EqChannel.Both
        var k = 1
        for f in EqFilter.graphic(p.gains) + p.filters {
            if f.channel != channel {
                channel = f.channel
                out += "Channel: " + (channel == .Left ? "L" : channel == .Right ? "R" : "all") + "\n"
            }
            let head = "\(f.on ? "ON" : "OFF") \(f.type.apo) Fc \(n(f.freq)) Hz"
            if f.type.hasGain {
                out += "Filter \(k): \(head) Gain \(n(f.gain)) dB Q \(n(f.q))\n"; k += 1
            } else if !f.type.hasSlope || f.slope == 12 {
                out += "Filter \(k): \(head) Q \(n(f.q))\n"; k += 1
            } else if f.slope < 12 {
                out += "# 6 dB/oct \(f.type.label.lowercased()): APO has no first-order filter, so this is a gentle 12 dB/oct one\n"
                out += "Filter \(k): \(head) Q 0.5\n"; k += 1
            } else {
                // Steeper slopes are Butterworth: one APO filter per second-order section.
                let order = f.slope / 6
                for j in 1...(order / 2) {
                    let q = Float(1 / (2 * cos(Double(2 * j - 1) * .pi / Double(2 * order))))
                    out += "Filter \(k): \(head) Q \(n(q))\n"; k += 1
                }
            }
        }
        return out
    }
}