//! OpenAI Chat Completions streaming provider.
//!
//! Wire format reference: https://platform.openai.com/docs/api-reference/chat/streaming
//!
//! Responsibilities:
//!   - Convert `Conversation` → request JSON (delegated to openai_chat_json.zig)
//!   - POST to `{base_url}/chat/completions` with `stream: true`
//!   - Read the chunked body, feed bytes through SSEParser
//!   - Parse each event payload, drive the block boundary state machine,
//!     and emit Receiver callbacks
//!   - Assemble the final Message and emit onMessageComplete

const std = @import("std");
const Allocator = std.mem.Allocator;
const Io = std.Io;
const http = std.http;
const Uri = std.Uri;

const conversation = @import("conversation.zig");
const provider_mod = @import("provider.zig");
const sse_mod = @import("sse.zig");
const json_mod = @import("openai_chat_json.zig");
const config_mod = @import("config.zig");

/// Active streaming block type tracked by the state machine. Mirrors the
/// `ContentBlock` union variants but adds `.none` for "no block open yet".
const ActiveBlock = enum { none, text, thinking };

pub const OpenAIChatProvider = struct {
    allocator: Allocator,
    io: Io,
    config: config_mod.OpenAIChatConfig,
    http_client: http.Client,

    pub fn init(allocator: Allocator, io: Io, cfg: config_mod.OpenAIChatConfig) OpenAIChatProvider {
        return .{
            .allocator = allocator,
            .io = io,
            .config = cfg,
            .http_client = .{ .allocator = allocator, .io = io },
        };
    }

    pub fn deinit(self: *OpenAIChatProvider) void {
        self.http_client.deinit();
    }

    /// Return a `Provider` interface bound to this concrete provider.
    pub fn provider(self: *OpenAIChatProvider) provider_mod.Provider {
        return .{ .ptr = self, .vtable = &vtable };
    }

    const vtable: provider_mod.ProviderVTable = .{
        .streamStep = vtableStreamStep,
        .deinit = vtableDeinit,
    };

    fn vtableStreamStep(
        ptr: *anyopaque,
        conv: *conversation.Conversation,
        receiver: *provider_mod.Receiver,
    ) anyerror!void {
        const self: *OpenAIChatProvider = @ptrCast(@alignCast(ptr));
        return self.streamStep(conv, receiver);
    }

    /// Called via the `Provider` interface. Tears down the impl AND frees
    /// its heap allocation, since `Provider.init` is the one that allocated
    /// it. Direct stack-allocated users (tests, embedders) call `deinit`
    /// themselves and never hit this path.
    fn vtableDeinit(ptr: *anyopaque) void {
        const self: *OpenAIChatProvider = @ptrCast(@alignCast(ptr));
        const allocator = self.allocator;
        self.deinit();
        allocator.destroy(self);
    }

    pub fn streamStep(
        self: *OpenAIChatProvider,
        conv: *conversation.Conversation,
        receiver: *provider_mod.Receiver,
    ) !void {
        // Outer wrapper guarantees `onError` is called exactly once if
        // anything fails — whether the receiver, the HTTP transport, or the
        // SSE/JSON parsers. The inner `streamStepInner` does the real work.
        self.streamStepInner(conv, receiver) catch |err| {
            receiver.onError(err);
            return err;
        };
    }

    fn streamStepInner(
        self: *OpenAIChatProvider,
        conv: *conversation.Conversation,
        receiver: *provider_mod.Receiver,
    ) !void {
        // Build URL: "{base_url}/chat/completions"
        const url = try std.fmt.allocPrint(
            self.allocator,
            "{s}/chat/completions",
            .{self.config.base_url},
        );
        defer self.allocator.free(url);

        const uri = try Uri.parse(url);

        // Build the request body.
        const body = try json_mod.serializeRequest(self.allocator, &self.config, conv);
        defer self.allocator.free(body);

        // Auth header
        const auth_value = try std.fmt.allocPrint(
            self.allocator,
            "Bearer {s}",
            .{self.config.api_key},
        );
        defer self.allocator.free(auth_value);

        const extra_headers = [_]http.Header{
            .{ .name = "content-type", .value = "application/json" },
            .{ .name = "accept", .value = "text/event-stream" },
            .{ .name = "authorization", .value = auth_value },
        };

        // Open the request. We can't use `fetch()` because it buffers the
        // response; we want to stream the body as it arrives.
        var req = try self.http_client.request(.POST, uri, .{
            .extra_headers = &extra_headers,
            // Disable compression: gzip buffers small SSE frames, defeating
            // the streaming property we paid for `stream: true` to get.
            .headers = .{ .accept_encoding = .{ .override = "identity" } },
            .keep_alive = false,
            .redirect_behavior = .not_allowed,
        });
        defer req.deinit();

        req.transfer_encoding = .{ .content_length = body.len };

        var send_buf: [4096]u8 = undefined;
        var bw = try req.sendBodyUnflushed(&send_buf);
        try bw.writer.writeAll(body);
        try bw.end();
        try req.connection.?.flush();

        // Receive response headers.
        var redirect_buf: [1024]u8 = undefined;
        var response = try req.receiveHead(&redirect_buf);

        if (@intFromEnum(response.head.status) >= 400) {
            // Drain body for diagnostics.
            var transfer_buf: [4096]u8 = undefined;
            const body_reader = response.reader(&transfer_buf);
            var err_buf: std.ArrayList(u8) = .empty;
            defer err_buf.deinit(self.allocator);
            var tmp: [1024]u8 = undefined;
            while (true) {
                const n = body_reader.readSliceShort(&tmp) catch break;
                if (n == 0) break;
                try err_buf.appendSlice(self.allocator, tmp[0..n]);
                if (err_buf.items.len > 16 * 1024) break;
            }
            std.log.err("openai_chat HTTP {d}: {s}", .{
                @intFromEnum(response.head.status),
                err_buf.items,
            });
            return error.HttpError;
        }

        // Stream the body through the SSE parser and event handler.
        var transfer_buf: [4096]u8 = undefined;
        const body_reader = response.reader(&transfer_buf);

        var parser = sse_mod.SSEParser.init(self.allocator);
        defer parser.deinit();

        var state: StreamState = .init(self.allocator);
        defer state.deinit();

        // Use `readVec` so we return to the event loop as soon as *any*
        // bytes arrive, rather than waiting for the buffer to fill.
        // `readSliceShort` blocks until EOF or full, which defeats streaming.
        var chunk: [4096]u8 = undefined;
        var vecs: [1][]u8 = .{&chunk};
        while (true) {
            const n = body_reader.readVec(&vecs) catch |err| switch (err) {
                error.EndOfStream => break,
                else => return err,
            };
            if (n == 0) break;

            const events = try parser.feed(chunk[0..n]);
            defer parser.freeEvents(events);

            for (events) |ev_payload| {
                if (std.mem.eql(u8, ev_payload, "[DONE]")) {
                    try state.finalize(receiver, conv);
                    return;
                }
                try handleEvent(self.allocator, ev_payload, &state, receiver);
                if (state.end_of_stream) {
                    try state.finalize(receiver, conv);
                    return;
                }
            }
        }

        // Stream ended without [DONE] or finish_reason. Finalize anyway.
        try state.finalize(receiver, conv);
    }
};

/// State maintained across the streaming response: which block is currently
/// being assembled, accumulated content, and the assistant message being
/// built up for the final `onMessageComplete` callback.
const StreamState = struct {
    allocator: Allocator,
    started: bool = false,
    /// Set when the wire stream signals end-of-turn (finish_reason or [DONE]).
    /// Tells the outer read loop to stop pulling more events.
    end_of_stream: bool = false,
    /// Set once `finalize` has run, to make it idempotent.
    finalized: bool = false,
    active: ActiveBlock = .none,
    block_index: usize = 0,

    /// Buffer for the currently-streaming block's content.
    /// Owned by this state until the block is completed, at which point
    /// ownership transfers to the assembled Message.
    current_buf: conversation.TextualBlock = .empty,

    /// Assembled blocks for the final message.
    blocks: std.ArrayList(conversation.ContentBlock) = .empty,

    fn init(allocator: Allocator) StreamState {
        return .{ .allocator = allocator };
    }

    fn deinit(self: *StreamState) void {
        self.current_buf.deinit(self.allocator);
        for (self.blocks.items) |*b| b.deinit(self.allocator);
        self.blocks.deinit(self.allocator);
    }

    /// Close the active block (if any) and emit onBlockComplete.
    /// Ownership of `current_buf` transfers into the appended block.
    fn closeActive(self: *StreamState, receiver: *provider_mod.Receiver) !void {
        if (self.active == .none) return;

        const block: conversation.ContentBlock = switch (self.active) {
            .text => .{ .Text = self.current_buf },
            .thinking => .{ .Thinking = .{ .text = self.current_buf } },
            .none => unreachable,
        };
        // The buffer ownership has moved into `block`; replace with empty.
        self.current_buf = .empty;

        try self.blocks.append(self.allocator, block);
        try receiver.onBlockComplete(self.block_index, self.blocks.items[self.blocks.items.len - 1]);

        self.active = .none;
    }

    /// Open a new block of the given type, possibly closing a prior block.
    fn openBlock(
        self: *StreamState,
        new_active: ActiveBlock,
        receiver: *provider_mod.Receiver,
    ) !void {
        if (self.active == new_active) return;
        if (self.active != .none) {
            try self.closeActive(receiver);
            self.block_index += 1;
        }
        self.active = new_active;
        const block_type: provider_mod.ContentBlockType = switch (new_active) {
            .text => .Text,
            .thinking => .Thinking,
            .none => unreachable,
        };
        try receiver.onBlockStart(block_type, self.block_index, null);
    }

    fn appendDelta(
        self: *StreamState,
        receiver: *provider_mod.Receiver,
        delta: []const u8,
    ) !void {
        try self.current_buf.appendSlice(self.allocator, delta);
        try receiver.onContentDelta(self.block_index, delta);
    }

    /// End the stream: close any open block and emit onMessageComplete.
    /// Ownership of the assembled blocks transfers into the conversation.
    fn finalize(
        self: *StreamState,
        receiver: *provider_mod.Receiver,
        conv: *conversation.Conversation,
    ) !void {
        if (self.finalized) return;
        self.finalized = true;

        try self.closeActive(receiver);

        // Move blocks into a fresh conversation message.
        const moved_blocks = try self.blocks.toOwnedSlice(self.allocator);
        defer self.allocator.free(moved_blocks);

        try conv.addAssistantMessage(moved_blocks);

        // The conversation now owns the block buffers. Build a Message view
        // for the callback (borrowed from the conversation).
        const msg = conv.messages.items[conv.messages.items.len - 1];
        try receiver.onMessageComplete(msg);
    }
};

fn handleEvent(
    allocator: Allocator,
    payload: []const u8,
    state: *StreamState,
    receiver: *provider_mod.Receiver,
) !void {
    var parsed = try json_mod.parseStreamEvent(allocator, payload);
    defer parsed.deinit();
    const d = parsed.delta;

    if (!state.started and d.role != null) {
        state.started = true;
        try receiver.onMessageStart(.assistant);
    }

    if (d.reasoning_content) |rc| {
        if (!state.started) {
            state.started = true;
            try receiver.onMessageStart(.assistant);
        }
        try state.openBlock(.thinking, receiver);
        try state.appendDelta(receiver, rc);
    }

    if (d.content) |c| {
        if (!state.started) {
            state.started = true;
            try receiver.onMessageStart(.assistant);
        }
        try state.openBlock(.text, receiver);
        try state.appendDelta(receiver, c);
    }

    if (d.finish_reason) |_| {
        state.end_of_stream = true;
    }
}

// -----------------------------------------------------------------------------
// Tests
// -----------------------------------------------------------------------------

const testing = std.testing;

/// A no-op Receiver that drops every callback. Useful when the test cares
/// about post-stream conversation state rather than callback observability.
const NoopReceiver = struct {
    fn make() provider_mod.Receiver {
        return .{ .ptr = @constCast(@ptrCast(&dummy)), .vtable = &vt };
    }
    var dummy: u8 = 0;
    const vt: provider_mod.ReceiverVTable = .{
        .onMessageStart = noopMsgStart,
        .onBlockStart = noopBlockStart,
        .onContentDelta = noopDelta,
        .onBlockComplete = noopBlockComplete,
        .onMessageComplete = noopMsgComplete,
        .onError = noopErr,
    };
    fn noopMsgStart(_: *anyopaque, _: conversation.MessageRole) anyerror!void {}
    fn noopBlockStart(_: *anyopaque, _: provider_mod.ContentBlockType, _: usize, _: ?provider_mod.BlockMeta) anyerror!void {}
    fn noopDelta(_: *anyopaque, _: usize, _: []const u8) anyerror!void {}
    fn noopBlockComplete(_: *anyopaque, _: usize, _: conversation.ContentBlock) anyerror!void {}
    fn noopMsgComplete(_: *anyopaque, _: conversation.Message) anyerror!void {}
    fn noopErr(_: *anyopaque, _: anyerror) void {}
};

/// Feed a sequence of SSE event payloads through the state machine as if
/// they had been delivered by the wire, finalizing into `conv`.
fn runStreamedTurn(
    allocator: Allocator,
    conv: *conversation.Conversation,
    receiver: *provider_mod.Receiver,
    events: []const []const u8,
) !void {
    var state: StreamState = .init(allocator);
    defer state.deinit();

    for (events) |payload| {
        if (std.mem.eql(u8, payload, "[DONE]")) break;
        try handleEvent(allocator, payload, &state, receiver);
        if (state.end_of_stream) break;
    }
    try state.finalize(receiver, conv);
}

test "two streamed turns persist assistant replies in the conversation" {
    // Regression test for the bug where `finish_reason` arrived before
    // `[DONE]` and `finalize` early-returned without appending the assistant
    // message, so follow-up turns were sent without prior responses.

    const allocator = testing.allocator;

    var conv = conversation.Conversation.init(allocator);
    defer conv.deinit();

    try conv.addSystemMessage("You are a helpful assistant.");
    try conv.addUserMessage("hello!");

    var recv = NoopReceiver.make();

    const turn1 = [_][]const u8{
        \\{"choices":[{"delta":{"role":"assistant"}}]}
        ,
        \\{"choices":[{"delta":{"content":"Hello! "}}]}
        ,
        \\{"choices":[{"delta":{"content":"How can I help you today?"}}]}
        ,
        \\{"choices":[{"delta":{},"finish_reason":"stop"}]}
        ,
        "[DONE]",
    };
    try runStreamedTurn(allocator, &conv, &recv, &turn1);

    try testing.expectEqual(@as(usize, 3), conv.messages.items.len);
    try testing.expectEqual(conversation.MessageRole.assistant, conv.messages.items[2].role);
    try testing.expectEqualStrings(
        "Hello! How can I help you today?",
        conv.messages.items[2].content.items[0].Text.items,
    );

    // Second user turn: the assistant must still see its prior response.
    try conv.addUserMessage("how did you respond to my greeting just now?");

    const turn2 = [_][]const u8{
        \\{"choices":[{"delta":{"role":"assistant"}}]}
        ,
        \\{"choices":[{"delta":{"content":"I replied: \"Hello! How can I help you today?\""}}]}
        ,
        \\{"choices":[{"delta":{},"finish_reason":"stop"}]}
        ,
        "[DONE]",
    };
    try runStreamedTurn(allocator, &conv, &recv, &turn2);

    // System + user + assistant + user + assistant = 5 messages.
    try testing.expectEqual(@as(usize, 5), conv.messages.items.len);
    try testing.expectEqual(conversation.MessageRole.assistant, conv.messages.items[4].role);
    try testing.expectEqualStrings(
        "I replied: \"Hello! How can I help you today?\"",
        conv.messages.items[4].content.items[0].Text.items,
    );
}