using System.Collections.Concurrent;
using System.IO;
using System.Text.Json.Nodes;
using System.Threading.Channels;
using Microsoft.Extensions.Hosting;

namespace InvoiceParser.Src;

// The single owner of the parse queue. Replaces the old triple-worker setup
// (pending_process.asp, parse_queue.ps1, browser-side parse) so there is no
// claim/steal logic left - one in-process worker drains one channel.
public class ParseWorker : BackgroundService
{
    private readonly AppConfig _cfg;
    private readonly PendingStore _store;
    private readonly LlmClient _llm;
    private readonly EventBroadcaster _events;
    private readonly Channel<string> _queue = Channel.CreateUnbounded<string>();
    private readonly ConcurrentDictionary<string, byte> _inQueue = new();

    public ParseWorker(AppConfig cfg, PendingStore store, LlmClient llm, EventBroadcaster events)
    {
        _cfg = cfg;
        _store = store;
        _llm = llm;
        _events = events;
    }

    public bool Enqueue(string id)
    {
        if (!_inQueue.TryAdd(id, 0)) return false;   // already waiting
        _queue.Writer.TryWrite(id);
        return true;
    }

    // Enqueues queued + errored items (the "Process queued now" button).
    // Returns how many were newly queued and how many were already waiting, so
    // a caller that gets 0 can tell "nothing to do" from "already in flight".
    public (int Queued, int AlreadyWaiting) EnqueueEligible(bool includeErrors)
    {
        int queued = 0, waiting = 0;
        foreach (var item in _store.List())
        {
            var stage = _store.Stage(item.Id);
            bool eligible = stage is "queued" or "queue" || (includeErrors && stage == "error");
            if (!eligible) continue;
            if (Enqueue(item.Id)) queued++;
            else waiting++;
        }
        return (queued, waiting);
    }

    protected override async Task ExecuteAsync(CancellationToken ct)
    {
        // Recover from a previous crash: anything stranded at "processing"
        // goes back to "queued", then all queued work is picked up.
        int recovered = 0;
        foreach (var item in _store.List())
        {
            if (_store.Stage(item.Id) != "processing") continue;
            _store.SetStage(item.Id, "queued");
            recovered++;
        }
        Log.Info("worker", $"started - {recovered} draft(s) reset from processing to queued");

        // The drain loop must outlive any single failure. An exception escaping
        // it leaves the channel with no reader: uploads still queue, _inQueue
        // still dedupes them, and every invoice sits at "queued" forever with no
        // error shown - the host keeps serving because nothing awaits its
        // shutdown request, so the dead queue is completely silent.
        while (!ct.IsCancellationRequested)
        {
            try
            {
                await DrainAsync(ct);
                return;   // channel completed - only happens on shutdown
            }
            catch (OperationCanceledException) when (ct.IsCancellationRequested)
            {
                return;
            }
            catch (Exception ex)
            {
                // Anything stranded in the dedupe set was never read; forget it
                // so the next sweep can queue it again.
                Log.Error("worker", "drain loop crashed - restarting it", ex);
                _inQueue.Clear();
                RequeueUnfinished();
            }
        }
    }

    private async Task DrainAsync(CancellationToken ct)
    {
        EnqueueEligible(includeErrors: false);

        await foreach (var id in _queue.Reader.ReadAllAsync(ct))
        {
            _inQueue.TryRemove(id, out _);
            var started = System.Diagnostics.Stopwatch.StartNew();
            try
            {
                await ProcessOne(id, ct);
                Log.Info("worker", $"{id} finished in {started.ElapsedMilliseconds}ms (stage {_store.Stage(id)})");
            }
            catch (OperationCanceledException) when (ct.IsCancellationRequested) { throw; }
            catch (Exception ex)
            {
                // Includes a request timeout, which arrives as a cancellation
                // that has nothing to do with our token.
                Log.Error("worker", $"{id} failed after {started.ElapsedMilliseconds}ms", ex);
                _store.SetError(id, ex.Message);
                _events.StageChanged(id, "error", ex.Message);
            }
        }
    }

    private void RequeueUnfinished()
    {
        foreach (var item in _store.List())
        {
            if (_store.Stage(item.Id) == "processing") _store.SetStage(item.Id, "queued");
        }
    }

    private async Task ProcessOne(string id, CancellationToken ct)
    {
        var state = _store.ReadState(id);
        if (state == null) return;

        _store.SetStage(id, "processing");
        _events.StageChanged(id, "processing");
        state = _store.ReadState(id)!;
        Log.Info("worker", $"{id} processing \"{state["source"]?["file_name"]}\"");

        // Fresh uploads arrive unprepped so the upload request returns at once;
        // drafts from the old app carry no flag and are already prepped.
        if (state["prepped"] is JsonValue pv && pv.TryGetValue<bool>(out var prepped) && !prepped)
        {
            Prep(id);
            state = _store.ReadState(id)!;
        }

        string parseMode = state["parse_mode"]?.ToString() ?? "";
        string ocrText = state["ocr"]?["text"]?.ToString() ?? "";

        // One request per page, never the whole invoice at once. Asked for a
        // 6-page invoice in a single call the model returned finishReason=STOP
        // after 124 items and silently omitted a whole page of line items - not
        // a token-limit truncation, just fidelity loss over a long output. Page
        // sized requests are small enough to come back complete.
        string backend;
        List<string> responses;
        try
        {
            if (parseMode == "text" && !string.IsNullOrWhiteSpace(ocrText))
            {
                backend = _llm.TextBackend;
                var chunks = SplitTextByPage(ocrText, state);
                var pageImages = _cfg.Llm.AttachPageImages ? PageImagesByNumber(id) : null;
                Log.Info("worker", $"{id} text route ({backend}) - {chunks.Count} page(s), " +
                                   $"{ocrText.Length} chars, images={(pageImages?.Count ?? 0)}");
                responses = await RunPagesAsync(chunks, (c, t) =>
                {
                    (string, string)? img = null;
                    if (pageImages != null && pageImages.TryGetValue(c.Page, out var found)) img = found;
                    return _llm.ParseTextAsync(c.Text, img, t);
                }, ct);
            }
            else
            {
                backend = _llm.ImageBackend;
                var images = CollectImages(id, state);
                Log.Info("worker", $"{id} image route ({backend}) - {images.Count} page image(s)");
                responses = await RunPagesAsync(images,
                    (img, t) => _llm.ParseImagesAsync(new List<(string, string)> { img }, t), ct);
            }
        }
        catch (OperationCanceledException) when (ct.IsCancellationRequested) { throw; }
        catch (Exception ex)
        {
            Log.Error("worker", $"{id} LLM call failed", ex);
            _store.SetError(id, ex.Message);
            _events.StageChanged(id, "error", ex.Message);
            return;
        }

        // The LLM call can take minutes; if the user opened the draft and started
        // editing meanwhile, do not clobber their state.
        if (_store.Stage(id) != "processing")
        {
            Log.Warn("worker", $"{id} left processing while the LLM ran (now {_store.Stage(id)}) - result discarded");
            return;
        }

        JsonNode api;
        JsonNode parsed;
        try
        {
            (api, parsed) = MergeResponses(responses);
        }
        catch (Exception ex)
        {
            Log.Error("worker", $"{id} response merge failed", ex);
            _store.SetError(id, ex.Message);
            _events.StageChanged(id, "error", ex.Message);
            return;
        }

        var newState = BuildParsedState(state, api, parsed, backend);
        _store.WriteState(id, newState);
        _events.StageChanged(id, "parsed");
        Log.Info("worker", $"{id} parsed: invoice={parsed["invoice_number"]} date={parsed["invoice_date"]} " +
                           $"total={parsed["total"]} lines={(parsed["line_items"] as JsonArray)?.Count ?? 0}");
    }

    // Server-side prep (was browser-side in the old app): text-layer extraction
    // for digital PDFs, page rendering for scanned ones.
    private void Prep(string id)
    {
        var src = _store.FindSource(id, out var ext)
            ?? throw new Exception("source file missing after upload");

        string parseMode = "image";
        string ocrText = "", ocrMethod = "";
        JsonNode? lineMap = null;

        if (ext.Equals("pdf", StringComparison.OrdinalIgnoreCase))
        {
            var lines = PdfPrep.ExtractLines(src);
            var text = string.Join("\n", lines.Select(l => l.Text));
            if (text.Trim().Length > 50)
            {
                parseMode = "text";
                ocrText = text;
                ocrMethod = "via PDF text layer";
                lineMap = PdfPrep.LineMapToJson(lines);
            }
            else
            {
                PdfPrep.RenderAllPages(src, Path.Combine(_store.Dir(id), "pages"), _cfg.Llm.MaxImageDim);
            }
        }

        var state = _store.ReadState(id) ?? throw new Exception("state.json missing");
        state["parse_mode"] = parseMode;
        state["ocr"] = new JsonObject
        {
            ["text"] = ocrText,
            ["method"] = ocrMethod,
            ["line_map"] = lineMap
        };
        state["prepped"] = true;
        state["updated_at"] = Util.NowIso();
        _store.WriteState(id, state);
        Log.Info("prep", $"{id} ext={ext} mode={parseMode} textChars={ocrText.Length} method=\"{ocrMethod}\"");
    }

    // Runs the per-page calls with bounded concurrency, preserving page order.
    // Sequentially six pages took longer than the (lossy) single call, because
    // each request pays its own model overhead; overlapping them removes that.
    private async Task<List<string>> RunPagesAsync<T>(
        IReadOnlyList<T> pages, Func<T, CancellationToken, Task<string>> call, CancellationToken ct)
    {
        int limit = Math.Max(1, _cfg.Llm.MaxParallelPages);
        var results = new string[pages.Count];
        using var gate = new SemaphoreSlim(limit);

        var tasks = pages.Select(async (page, i) =>
        {
            await gate.WaitAsync(ct);
            try { results[i] = await call(page, ct); }
            finally { gate.Release(); }
        }).ToList();

        await Task.WhenAll(tasks);
        return results.ToList();
    }

    // Splits the OCR text into one numbered chunk per PDF page, using the
    // per-line page numbers in ocr.line_map. Line numbers stay GLOBAL across
    // chunks so the source_line values the model returns still line up with
    // line_map and the preview markers. Falls back to a single chunk when a
    // draft has no line_map (older drafts, or image sources).
    private static List<(int Page, string Text)> SplitTextByPage(string ocrText, JsonNode state)
    {
        var numbered = LlmClient.NumberLines(ocrText).Replace("\r\n", "\n").Split('\n');

        var map = state["ocr"]?["line_map"] as JsonArray;
        if (map == null || map.Count != numbered.Length)
            return new List<(int, string)> { (1, string.Join("\n", numbered)) };

        var chunks = new List<(int, string)>();
        var current = new List<string>();
        int currentPage = -1;
        for (int i = 0; i < numbered.Length; i++)
        {
            int page = int.TryParse(map[i]?["page"]?.ToString(), out var p) ? p : 1;
            if (page != currentPage && current.Count > 0)
            {
                chunks.Add((currentPage, string.Join("\n", current)));
                current.Clear();
            }
            currentPage = page;
            current.Add(numbered[i]);
        }
        if (current.Count > 0) chunks.Add((currentPage, string.Join("\n", current)));
        return chunks;
    }

    // Rendered pages keyed by page number for hybrid text+image parsing. A
    // text-layer PDF is not rendered during prep, so render it here; the
    // preview endpoint reuses the same files afterwards.
    private Dictionary<int, (string Mime, string B64)> PageImagesByNumber(string id)
    {
        var result = new Dictionary<int, (string, string)>();
        var src = _store.FindSource(id, out var ext);
        if (src == null || !ext.Equals("pdf", StringComparison.OrdinalIgnoreCase)) return result;

        var pagesDir = Path.Combine(_store.Dir(id), "pages");
        var files = Directory.Exists(pagesDir) ? Directory.GetFiles(pagesDir, "page_*.jpg") : Array.Empty<string>();
        if (files.Length == 0)
        {
            PdfPrep.RenderAllPages(src, pagesDir, _cfg.Llm.MaxImageDim);
            files = Directory.GetFiles(pagesDir, "page_*.jpg");
        }
        foreach (var f in files)
            result[PageNumber(f)] = ("image/jpeg", Convert.ToBase64String(File.ReadAllBytes(f)));
        return result;
    }

    // Folds the per-page responses into one invoice. Header fields are taken
    // from the first page that supplies them (they live on page 1 of a real
    // invoice); line items are concatenated in page order.
    private static (JsonNode Api, JsonNode Parsed) MergeResponses(List<string> responses)
    {
        var rawArray = new JsonArray();
        var merged = new JsonObject();
        var items = new JsonArray();
        bool grandTotalFound = false;

        for (int pageNo = 0; pageNo < responses.Count; pageNo++)
        {
            var raw = responses[pageNo];
            JsonNode api;
            JsonNode parsed;
            try
            {
                api = JsonNode.Parse(raw)!;
                parsed = LlmClient.ExtractInvoiceData(api);
            }
            catch (Exception ex)
            {
                // A page that cannot be read is a failed parse, not a shorter
                // invoice: skipping it would drop its line items with no trace.
                throw new Exception($"Page {pageNo + 1} of {responses.Count}: {ex.Message}");
            }

            rawArray.Add(api.DeepClone());

            // Identity fields come from the first page that has them.
            foreach (var field in new[] { "vendor_name", "invoice_number", "invoice_date" })
            {
                if (merged[field] != null) continue;
                var v = parsed[field];
                if (v != null && v.GetValueKind() != System.Text.Json.JsonValueKind.Null &&
                    v.ToString() != "")
                    merged[field] = v.DeepClone();
            }

            // The total: a page whose label names the invoice grand total wins
            // outright. Otherwise the last page with a total is kept, which is
            // wrong when a recap/allowance page follows the totals page - the
            // prompt asks for null there, the label check is the backstop.
            var t = parsed["total"];
            if (t != null && t.GetValueKind() != System.Text.Json.JsonValueKind.Null && t.ToString() != "")
            {
                var label = parsed["total_label"]?.ToString() ?? "";
                bool grand = IsGrandTotalLabel(label);
                if (!grandTotalFound || grand)
                {
                    merged["total"] = t.DeepClone();
                    merged["total_label"] = label;
                    merged["total_page"] = pageNo + 1;
                    grandTotalFound = grandTotalFound || grand;
                }
            }

            if (parsed["line_items"] is JsonArray li)
                foreach (var it in li)
                    if (it != null) items.Add(it.DeepClone());
        }

        if (rawArray.Count == 0) throw new Exception("LLM returned no usable response");

        merged["line_items"] = items;
        return (rawArray, merged);
    }

    private static bool IsGrandTotalLabel(string label) =>
        System.Text.RegularExpressions.Regex.IsMatch(label,
            @"invoice\s*total|total\s*due|amount\s*due|balance\s*due|net\s*total|grand\s*total|net\s*invoice|total\s*invoice",
            System.Text.RegularExpressions.RegexOptions.IgnoreCase);

    // Vision inputs, in order of preference: rendered pages on disk,
    // legacy base64 page_images inside state.json, or the raw source image.
    // A scanned PDF with no rendered pages gets rendered right here.
    private List<(string Mime, string B64)> CollectImages(string id, JsonNode state)
    {
        var pagesDir = Path.Combine(_store.Dir(id), "pages");
        if (Directory.Exists(pagesDir))
        {
            var files = Directory.GetFiles(pagesDir, "page_*.jpg")
                .OrderBy(PageNumber).ToList();
            if (files.Count > 0)
                return files.Select(f => ("image/jpeg", Convert.ToBase64String(File.ReadAllBytes(f)))).ToList();
        }

        if (state["page_images"] is JsonArray legacy && legacy.Count > 0)
            return legacy.Select(n => ("image/jpeg", n?.ToString() ?? "")).ToList();

        var src = _store.FindSource(id, out var ext);
        if (src == null)
            throw new Exception("No page images and no source file for this invoice");

        if (ext.Equals("pdf", StringComparison.OrdinalIgnoreCase))
        {
            PdfPrep.RenderAllPages(src, pagesDir, _cfg.Llm.MaxImageDim);
            var files = Directory.GetFiles(pagesDir, "page_*.jpg").OrderBy(PageNumber).ToList();
            if (files.Count == 0) throw new Exception("PDF produced no page images");
            return files.Select(f => ("image/jpeg", Convert.ToBase64String(File.ReadAllBytes(f)))).ToList();
        }

        return new List<(string, string)>
        {
            (Util.MimeFromExt(ext), Convert.ToBase64String(File.ReadAllBytes(src)))
        };
    }

    private static int PageNumber(string path)
    {
        var name = Path.GetFileNameWithoutExtension(path);
        return int.TryParse(name.Replace("page_", ""), out var n) ? n : 0;
    }

    // Port of jsBuildParsedState from pending_process.asp - same output shape
    // the review screen expects.
    private static JsonNode BuildParsedState(JsonNode state, JsonNode api, JsonNode parsed, string backend)
    {
        var now = Util.NowIso();
        state["stage"] = "parsed";
        state["error"] = "";
        state["updated_at"] = now;
        state["header"] = new JsonObject
        {
            ["vendor_id"] = "",
            ["vendor_name"] = parsed["vendor_name"]?.ToString() ?? "",
            ["vendor_hint"] = parsed["vendor_name"]?.ToString() ?? "",
            ["invoice_number"] = parsed["invoice_number"]?.ToString() ?? "",
            ["invoice_date"] = parsed["invoice_date"]?.ToString() ?? "",
            ["po_number"] = "",
            ["invoice_total"] = parsed["total"]?.ToString() ?? "",
            ["total_label"] = parsed["total_label"]?.ToString() ?? "",
            ["total_page"] = parsed["total_page"]?.ToString() ?? ""
        };

        var items = new JsonArray();
        if (parsed["line_items"] is JsonArray src)
        {
            foreach (var itNode in src)
            {
                var it = itNode as JsonObject ?? new JsonObject();
                items.Add(new JsonObject
                {
                    ["upc"] = it["upc"]?.ToString() ?? "",
                    ["desc"] = it["description"]?.ToString() ?? "",
                    ["cert"] = it["product_code"]?.ToString() ?? "",
                    ["qty"] = it["quantity"]?.ToString() ?? "",
                    ["cost"] = it["unit_price"]?.ToString() ?? "",
                    ["total"] = it["total"]?.ToString() ?? "",
                    ["pack"] = it["pack"]?.ToString() ?? "",
                    ["size"] = it["size"]?.ToString() ?? "",
                    ["uom"] = string.IsNullOrEmpty(it["unit_of_measure"]?.ToString()) ? "EA" : it["unit_of_measure"]!.ToString(),
                    ["source_line"] = it["source_line"]?.ToString() ?? "",
                    ["status"] = "unmatched",
                    ["parsed_desc"] = it["description"]?.ToString() ?? "",
                    ["matched_desc"] = "",
                    ["match_badge_text"] = "",
                    ["match_badge_class"] = "",
                    ["po_status"] = ""
                });
            }
        }
        state["line_items"] = items;
        state["page_images"] = new JsonArray();   // drop legacy heavy images now that parsing is done
        state["llm"] = new JsonObject
        {
            ["backend"] = backend,
            ["at"] = now,
            ["raw_response"] = api.DeepClone(),
            ["parsed"] = parsed.DeepClone()
        };
        return state;
    }
}