using Microsoft.EntityFrameworkCore;
using Novelly.Api.Common;
using Novelly.Api.Common.Validation;
using Novelly.Api.Data;

namespace Novelly.Api.Characters;

/// <summary>
/// A character's arc: a flat, ordered list of the changes they go through. Same shape as
/// a chapter's beats, and for the same reason — an arc is a sequence, not a tree.
/// </summary>
/// <remarks>
/// Arcs are only really worth keeping for main characters, but nothing here refuses one
/// on a supporting character. Demoting someone should not delete work, and a character
/// who turns out to matter gets promoted after the arc is already sketched.
/// </remarks>
public class CharacterArcService(
    INovelDbContext db,
    ILogger<CharacterArcService> logger,
    IModelValidator<CreateArcStageRequest> createValidator,
    IModelValidator<UpdateArcStageRequest> updateValidator,
    IModelValidator<ReorderArcStagesRequest> reorderValidator)
{
    public async Task<IReadOnlyList<ArcStageDto>> ListAsync(Guid characterId, CancellationToken ct = default)
    {
        Guard.Default(characterId, nameof(characterId));

        logger.LogInformation("Listing arc stages for character {CharacterId}", characterId);

        var stages = await Query()
            .Where(s => s.CharacterId == characterId)
            .OrderBy(s => s.SortOrder)
            .ToListAsync(ct);

        return [.. stages.Select(s => s.ToDto())];
    }

    /// <summary>Null when no arc stage has this id — a lookup miss is expected, not exceptional.</summary>
    public async Task<ArcStageDto?> GetAsync(Guid id, CancellationToken ct = default)
    {
        Guard.Default(id, nameof(id));

        logger.LogInformation("Getting arc stage {ArcStageId}", id);
        return (await FindAsync(id, ct))?.ToDto();
    }

    public async Task<ArcStageDto> CreateAsync(
        Guid characterId, CreateArcStageRequest request, CancellationToken ct = default)
    {
        Guard.Default(characterId, nameof(characterId));
        Guard.Null(request, nameof(request));
        createValidator.Validate(request).ThrowIfInvalid();

        logger.LogInformation("Creating arc stage {Title} for character {CharacterId}", request.Title, characterId);

        var character = await db.Characters.FirstOrDefaultAsync(c => c.Id == characterId, ct);
        if (character is null)
        {
            logger.LogWarning("Rejected arc stage creation: character {CharacterId} not found", characterId);
            throw new NotFoundException(nameof(Character), characterId);
        }

        await EnsureChapterIsInSameProjectAsync(character, request.ChapterId, ct);

        var stage = new CharacterArcStage
        {
            CharacterId = characterId,
            Title = request.Title,
            SortOrder = request.SortOrder ?? await NextSortOrderAsync(characterId, ct),
            Description = request.Description,
            ChapterId = request.ChapterId
        };

        db.CharacterArcStages.Add(stage);
        await db.SaveChangesAsync(ct);

        // Just created it — the reload is only to pick up includes, not to check existence.
        return (await FindAsync(stage.Id, ct))!.ToDto();
    }

    public async Task<ArcStageDto?> UpdateAsync(
        Guid id, UpdateArcStageRequest request, CancellationToken ct = default)
    {
        Guard.Default(id, nameof(id));
        Guard.Null(request, nameof(request));
        updateValidator.Validate(request).ThrowIfInvalid();

        logger.LogInformation("Updating arc stage {ArcStageId}", id);

        var stage = await FindAsync(id, ct);
        if (stage is null)
        {
            return null;
        }

        var character = await db.Characters.FirstOrDefaultAsync(c => c.Id == stage.CharacterId, ct);
        if (character is null)
        {
            // The stage's own character should always exist via the FK — this is an
            // invariant failing, not a caller mistake, so it stays exceptional.
            logger.LogError("Arc stage {ArcStageId} references character {CharacterId} which does not exist", id, stage.CharacterId);
            throw new NotFoundException(nameof(Character), stage.CharacterId);
        }

        await EnsureChapterIsInSameProjectAsync(character, request.ChapterId, ct);

        stage.Title = Patch.Apply(stage.Title, request.Title) ?? stage.Title;
        stage.SortOrder = request.SortOrder ?? stage.SortOrder;
        stage.Description = Patch.Apply(stage.Description, request.Description);
        stage.ChapterId = request.ChapterId ?? stage.ChapterId;
        stage.UpdatedAt = DateTimeOffset.UtcNow;

        await db.SaveChangesAsync(ct);
        return (await FindAsync(id, ct))!.ToDto();
    }

    /// <summary>True if an arc stage was deleted; false if no stage had this id.</summary>
    public async Task<bool> DeleteAsync(Guid id, CancellationToken ct = default)
    {
        Guard.Default(id, nameof(id));

        logger.LogInformation("Deleting arc stage {ArcStageId}", id);

        var stage = await FindAsync(id, ct);
        if (stage is null)
        {
            return false;
        }

        db.CharacterArcStages.Remove(stage);
        await db.SaveChangesAsync(ct);
        return true;
    }

    /// <summary>
    /// Renumbers a character's arc to match the order given. Stages left out keep their
    /// relative position after the ones listed, exactly as beat reordering works.
    /// </summary>
    public async Task<IReadOnlyList<ArcStageDto>> ReorderAsync(
        Guid characterId, ReorderArcStagesRequest request, CancellationToken ct = default)
    {
        Guard.Default(characterId, nameof(characterId));
        Guard.Null(request, nameof(request));
        reorderValidator.Validate(request).ThrowIfInvalid();

        logger.LogInformation("Reordering {Count} arc stages for character {CharacterId}", request.StageIds.Count, characterId);

        var stages = await db.CharacterArcStages
            .Where(s => s.CharacterId == characterId)
            .ToListAsync(ct);

        var missing = request.StageIds.Where(id => stages.All(s => s.Id != id)).ToList();
        if (missing.Count > 0)
        {
            logger.LogWarning("Reorder for character {CharacterId} referenced missing arc stage {ArcStageId}", characterId, missing[0]);
            throw new NotFoundException(nameof(CharacterArcStage), missing[0]);
        }

        var order = 1;
        foreach (var id in request.StageIds)
        {
            stages.Single(s => s.Id == id).SortOrder = order++;
        }

        foreach (var stage in stages.Where(s => !request.StageIds.Contains(s.Id)).OrderBy(s => s.SortOrder))
        {
            stage.SortOrder = order++;
        }

        await db.SaveChangesAsync(ct);
        return await ListAsync(characterId, ct);
    }

    private async Task EnsureChapterIsInSameProjectAsync(
        Character character, Guid? chapterId, CancellationToken ct)
    {
        if (chapterId is not { } id)
        {
            return;
        }

        logger.LogDebug("Checking chapter {ChapterId} belongs to project {ProjectId}", id, character.ProjectId);

        var belongs = await db.Chapters.AnyAsync(c => c.Id == id && c.ProjectId == character.ProjectId, ct);

        if (!belongs)
        {
            logger.LogWarning("Rejected arc stage: chapter {ChapterId} does not belong to project {ProjectId}", id, character.ProjectId);
            throw new InvalidOperationException(
                "An arc stage can only point at a chapter in the same project as its character.");
        }
    }

    private async Task<int> NextSortOrderAsync(Guid characterId, CancellationToken ct)
    {
        logger.LogDebug("Computing next sort order for character {CharacterId}", characterId);

        var max = await db.CharacterArcStages
            .Where(s => s.CharacterId == characterId)
            .MaxAsync(s => (int?)s.SortOrder, ct);

        return (max ?? 0) + 1;
    }

    private IQueryable<CharacterArcStage> Query() => db.CharacterArcStages.Include(s => s.Chapter);

    private async Task<CharacterArcStage?> FindAsync(Guid id, CancellationToken ct)
    {
        logger.LogDebug("Finding arc stage {ArcStageId}", id);

        var stage = await Query().FirstOrDefaultAsync(s => s.Id == id, ct);
        if (stage is null)
        {
            logger.LogInformation("CharacterArcStage {ArcStageId} not found", id);
        }
        else
        {
            logger.LogDebug("Found arc stage {ArcStageId}", id);
        }

        return stage;
    }
}