using w4c_workflows.Models;

namespace w4c_workflows.Services;

/// <summary>
/// Precomputes the execution graph from a parsed YAML definition so validation
/// and compilation share one deterministic interpretation:
///   - <see cref="HeadId"/> is the entry of the success chain (the top-level
///     task with no incoming `next` that is not purely a compensation target).
///   - <see cref="Chain"/> is the linear happy path root → head → … → terminal.
///   - <see cref="CompensationOnly"/> marks tasks that are only ever reached as
///     an `onError` target (they have no `next` of their own).
/// </summary>
internal sealed class WorkflowGraph
{
    public required IReadOnlyList<TaskDefinition> Tasks { get; init; }
    public required IReadOnlyDictionary<string, TaskDefinition> ById { get; init; }
    public required IReadOnlyDictionary<string, int> IncomingNext { get; init; }
    public required IReadOnlySet<string> OnErrorTargets { get; init; }
    public required IReadOnlySet<string> CompensationOnly { get; init; }
    public string? HeadId { get; init; }
    public required IReadOnlyList<string> Chain { get; init; }

    public static WorkflowGraph Compute(WorkflowDefinition def)
    {
        var tasks = def.Tasks ?? new List<TaskDefinition>();
        var byId = new Dictionary<string, TaskDefinition>(StringComparer.Ordinal);

        foreach (var task in tasks)
            if (!string.IsNullOrEmpty(task.Id))
                byId[task.Id] = task;

        var incomingNext = new Dictionary<string, int>(StringComparer.Ordinal);
        var onErrorTargets = new HashSet<string>(StringComparer.Ordinal);

        foreach (var task in tasks)
        {
            if (!string.IsNullOrEmpty(task.Next) && byId.ContainsKey(task.Next))
                incomingNext[task.Next] = incomingNext.GetValueOrDefault(task.Next) + 1;
            if (!string.IsNullOrEmpty(task.OnError) && byId.ContainsKey(task.OnError))
                onErrorTargets.Add(task.OnError);
        }

        // A compensation-only task is an onError target that does not continue
        // the success chain (no `next`). It lives outside the happy path.
        var compensationOnly = new HashSet<string>(StringComparer.Ordinal);
        foreach (var task in tasks)
        {
            if (task.Id == null) continue;
            if (onErrorTargets.Contains(task.Id) && string.IsNullOrEmpty(task.Next))
                compensationOnly.Add(task.Id);
        }

        // Head: top-level (parent == root), no incoming `next`, not compensation-only.
        string? headId = null;
        foreach (var task in tasks)
        {
            if (task.Id == null) continue;
            var parent = string.IsNullOrEmpty(task.Parent) ? "root" : task.Parent;
            if (!string.Equals(parent, "root", StringComparison.Ordinal)) continue;
            if (incomingNext.GetValueOrDefault(task.Id) > 0) continue;
            if (compensationOnly.Contains(task.Id)) continue;
            headId = task.Id;
            break;
        }

        // Walk the success chain (each node has at most one `next` after validation).
        var chain = new List<string>();
        var visited = new HashSet<string>(StringComparer.Ordinal);
        var cursor = headId;
        while (cursor != null && visited.Add(cursor) && byId.TryGetValue(cursor, out var task))
        {
            chain.Add(cursor);
            cursor = string.IsNullOrEmpty(task.Next) ? null : task.Next;
        }

        return new WorkflowGraph
        {
            Tasks = tasks,
            ById = byId,
            IncomingNext = incomingNext,
            OnErrorTargets = onErrorTargets,
            CompensationOnly = compensationOnly,
            HeadId = headId,
            Chain = chain,
        };
    }
}