AmiReel/Services/SourceCropFilters.cs

using System.Globalization;
using System.Text.RegularExpressions;
using AmiReel.Models;

namespace AmiReel.Services;

/// <summary>Shared FFmpeg crop filters used by both Video and Shorts pipelines.</summary>
internal static class SourceCropFilters
{
    internal const string CenterFourByThreeFilter =
        "crop='min(iw,ih*4/3)':'min(ih,iw*3/4)':'(iw-ow)/2':'(ih-oh)/2'";

    private static readonly Regex CropSpecRegex = new(
        @"crop=(\d+):(\d+):(\d+):(\d+)",
        RegexOptions.CultureInvariant);

    internal static string BuildManualCropFilter(int left, int right, int top, int bottom) =>
        $"crop=iw-{left}-{right}:ih-{top}-{bottom}:{left}:{top}";

    internal static bool HasManualCrop(int left, int right, int top, int bottom) =>
        left > 0 || right > 0 || top > 0 || bottom > 0;

    internal static string Resolve(
        ShortSourceCropMode mode,
        int left,
        int right,
        int top,
        int bottom,
        string? autoDetected)
    {
        if (HasManualCrop(left, right, top, bottom))
            return BuildManualCropFilter(left, right, top, bottom);

        return mode switch
        {
            ShortSourceCropMode.None => "",
            ShortSourceCropMode.Auto when !string.IsNullOrWhiteSpace(autoDetected) => autoDetected,
            ShortSourceCropMode.Auto => CenterFourByThreeFilter,
            _ => CenterFourByThreeFilter,
        };
    }

    internal static string Prefix(string cropFilter, string nextFilter) =>
        string.IsNullOrWhiteSpace(cropFilter) ? nextFilter : $"{cropFilter},{nextFilter}";

    internal static List<string> BuildHorizontalCropDetectArguments(string inputFile, string start, double durationSeconds) =>
    [
        "-hide_banner",
        "-ss",
        string.IsNullOrWhiteSpace(start) ? "00:00:00" : start,
        "-t",
        durationSeconds.ToString("0.###", CultureInfo.InvariantCulture),
        "-i",
        inputFile,
        "-an",
        "-vf",
        "crop=iw:trunc(ih*0.70/2)*2:0:trunc(ih*0.15/2)*2,cropdetect=limit=28:round=2:reset=0",
        "-f",
        "null",
        "-",
    ];

    internal static string? SelectMostCommonCropSpec(string ffmpegOutput)
    {
        Dictionary<string, int> counts = new(StringComparer.Ordinal);
        foreach (Match match in CropSpecRegex.Matches(ffmpegOutput))
        {
            string spec = match.Value;
            counts[spec] = counts.GetValueOrDefault(spec) + 1;
        }

        return counts.Count == 0
            ? null
            : counts.OrderByDescending(pair => pair.Value).ThenBy(pair => pair.Key, StringComparer.Ordinal).First().Key;
    }

    internal static bool TryParseCropSpec(string spec, out int width, out int height, out int x, out int y)
    {
        Match match = CropSpecRegex.Match(spec);
        if (!match.Success)
        {
            width = height = x = y = 0;
            return false;
        }

        width = int.Parse(match.Groups[1].Value, CultureInfo.InvariantCulture);
        height = int.Parse(match.Groups[2].Value, CultureInfo.InvariantCulture);
        x = int.Parse(match.Groups[3].Value, CultureInfo.InvariantCulture);
        y = int.Parse(match.Groups[4].Value, CultureInfo.InvariantCulture);
        return true;
    }

    internal static bool TryBuildAutoHorizontalCrop(int detectedWidth, int detectedX, out int width, out int x)
    {
        int inset = (int)(detectedWidth * 0.005 / 2) * 2;
        width = ((detectedWidth - (2 * inset)) / 2) * 2;
        x = ((detectedX + inset) / 2) * 2;
        if (width < 16)
            return false;
        return true;
    }

    internal static string? TryBuildAutoHorizontalFilter(string ffmpegOutput)
    {
        string? spec = SelectMostCommonCropSpec(ffmpegOutput);
        if (spec is null || !TryParseCropSpec(spec, out int width, out _, out int x, out _))
            return null;
        if (!TryBuildAutoHorizontalCrop(width, x, out int cropWidth, out int cropX))
            return null;

        return $"crop={cropWidth}:ih:{cropX}:0";
    }
}