const std = @import("std");

const config_mod = @import("config.zig");
const conversation = @import("conversation.zig");
const tool_registry_mod = @import("tool_registry.zig");
const session_mod = @import("session.zig");
pub const ToolRegistry = tool_registry_mod.ToolRegistry;
pub const Usage = session_mod.Usage;

pub const ContentBlockType = enum {
    Text,
    Thinking,
    ToolUse,
    ToolResult,
};

/// Heuristic detector for provider context-overflow rejections, applied to
/// an HTTP 400 error response body. Both OpenAI-compatible and Anthropic
/// APIs reject oversized requests on the input side with HTTP 400 and a
/// recognizable message; matching it lets the agent compact and retry
/// instead of surfacing a hard error.
///
/// Markers (case-sensitive substrings, as the wire emits them):
///   - OpenAI:    `context_length_exceeded`, `maximum context length`
///   - Anthropic: `prompt is too long`
pub fn isContextOverflowBody(body: []const u8) bool {
    const markers = [_][]const u8{
        "context_length_exceeded",
        "maximum context length",
        "prompt is too long",
        "context window",
    };
    for (markers) |m| {
        if (std.mem.indexOf(u8, body, m) != null) return true;
    }
    return false;
}

/// Distinct provider/API failure classes the agent needs in order to decide
/// between retrying, compacting, and hard-failing. The transport/stream layer
/// maps HTTP status codes and connection failures onto these so the agent's
/// retry policy can switch on a stable, provider-agnostic name rather than a
/// broad `error.HttpError`.
///
/// Zig errors cannot carry payloads, so status code, `Retry-After`, and the
/// provider's diagnostic message ride alongside via `ProviderDiagnostic` (an
/// out-parameter the provider fills before returning the error).
pub const ProviderError = error{
    /// HTTP 429. Retryable; honor `Retry-After` when present.
    ProviderRateLimited,
    /// HTTP 503 or a server explicitly signalling unavailability. Retryable.
    ProviderUnavailable,
    /// HTTP 500/502/504. Retryable.
    ProviderServerError,
    /// Connection reset, DNS/connect failure, TLS failure, request timeout
    /// (HTTP 408), or other transport-level failure before a response. Also
    /// covers retryable conflict/not-ready statuses (409, 425). Retryable.
    ProviderTransport,
    /// The provider stream ended or was malformed before a complete
    /// assistant message was committed. Retryable (subject to policy).
    ProviderStreamMalformed,
    /// HTTP 401/403. Not retryable — a credentials/permissions problem.
    ProviderAuthFailed,
    /// HTTP 400 (other than context overflow). Not retryable — the request
    /// itself is malformed.
    ProviderBadRequest,
    /// HTTP 404 shaped as an unknown-model error. Not retryable.
    ProviderModelNotFound,
    /// The input context exceeds the model's window (HTTP 400 + a recognized
    /// context marker). Handled by one-shot compaction, not ordinary retry.
    ContextOverflow,
};

/// Side-channel for the payload a `ProviderError` cannot carry. The provider
/// fills the relevant fields immediately before returning a classified error;
/// the agent reads them to drive backoff and retry notifications. Reset to
/// `.{}` before each provider attempt.
pub const ProviderDiagnostic = struct {
    /// The HTTP status code, when the failure carried one.
    status_code: ?u16 = null,
    /// Parsed `Retry-After` delay in milliseconds, when the provider sent it.
    retry_after_ms: ?u64 = null,
    /// The provider's diagnostic message (borrowed/owned per caller; the
    /// agent treats it as borrowed for the lifetime of the failed attempt).
    message: ?[]const u8 = null,

    pub fn reset(self: *ProviderDiagnostic) void {
        self.* = .{};
    }
};

/// True for the provider errors the agent's retry policy may retry. Context
/// overflow is deliberately excluded — it has a separate one-shot compaction
/// path. Auth, bad-request, and model-not-found are terminal.
pub fn isRetryableProviderError(err: anyerror) bool {
    return switch (err) {
        error.ProviderRateLimited,
        error.ProviderUnavailable,
        error.ProviderServerError,
        error.ProviderTransport,
        error.ProviderStreamMalformed,
        => true,
        else => false,
    };
}

/// Map an HTTP status code from a provider response onto a `ProviderError`.
/// The `body` is inspected only for the 400 case, to separate context
/// overflow (compact-and-retry) from an ordinary bad request (hard-fail).
///
/// Caller is responsible for stashing the status code (and any `Retry-After`)
/// into a `ProviderDiagnostic`; this function only chooses the error name.
pub fn classifyHttpStatus(status: u16, body: []const u8) ProviderError {
    return switch (status) {
        400 => if (isContextOverflowBody(body)) error.ContextOverflow else error.ProviderBadRequest,
        401, 403 => error.ProviderAuthFailed,
        404 => error.ProviderModelNotFound,
        408 => error.ProviderTransport,
        409, 425 => error.ProviderTransport,
        429 => error.ProviderRateLimited,
        503 => error.ProviderUnavailable,
        500, 502, 504 => error.ProviderServerError,
        else => if (status >= 500) error.ProviderServerError else error.ProviderBadRequest,
    };
}

/// Parse an HTTP `Retry-After` header value into milliseconds. Supports the
/// delta-seconds form (`"120"`); the HTTP-date form is not parsed and returns
/// null (the agent then falls back to its computed backoff). Returns null for
/// empty or unparseable values.
pub fn parseRetryAfterMs(value: []const u8) ?u64 {
    const trimmed = std.mem.trim(u8, value, " \t\r\n");
    if (trimmed.len == 0) return null;
    const secs = std.fmt.parseInt(u64, trimmed, 10) catch return null;
    return secs *| std.time.ms_per_s;
}

/// Find a `Retry-After` header (case-insensitive) in an HTTP response head
/// and parse it into milliseconds. `head` is a `std.http.Client.Response.Head`
/// (taken as `anytype` to avoid importing the http types here). Returns null
/// when absent or unparseable.
pub fn retryAfterFromHead(head: anytype) ?u64 {
    var it = head.iterateHeaders();
    while (it.next()) |h| {
        if (std.ascii.eqlIgnoreCase(h.name, "retry-after")) {
            return parseRetryAfterMs(h.value);
        }
    }
    return null;
}

/// Details handed to the receiver before the agent sleeps for a provider
/// retry. Purely a UI/runtime event — never persisted to the session.
pub const ProviderRetryInfo = struct {
    /// The attempt that just failed, 1-based (1 = the initial attempt).
    attempt: usize,
    /// Total attempts the policy will make, including the first.
    max_attempts: usize,
    /// How long the agent will sleep before the next attempt, in ms.
    delay_ms: u64,
    /// The classified error that triggered the retry.
    err: anyerror,
    /// HTTP status code, when known.
    status_code: ?u16 = null,
    /// Provider-sent `Retry-After`, when present, in ms.
    retry_after_ms: ?u64 = null,
    /// Provider diagnostic message, when known.
    message: ?[]const u8 = null,
    /// True when the retry is a context-overflow compaction attempt rather
    /// than an ordinary backoff retry. Compaction retries report
    /// `delay_ms == 0`.
    compaction: bool = false,
};

/// Vtable for receiving streaming events from a Provider.
///
/// The lifecycle callbacks (`onMessageStart` ... `onMessageComplete`) return
/// `anyerror!void`. Returning an error aborts the in-flight turn: the Provider
/// stops streaming, calls `onError(err)`, and propagates `err` out of
/// `streamStep`. No partial assistant message is appended to the conversation.
///
/// Tool-use identity (`id`, `name`) is delivered via `onToolDetails`, fired
/// once per ToolUse block at the earliest moment both fields are known. For
/// Anthropic this is immediately after `onBlockStart`, before any deltas. For
/// OpenAI Chat Completions this may be partway through the arg-deltas, since
/// the wire protocol can split `id` and `name` across multiple streaming
/// chunks. The only guarantees are: it fires strictly after the block's
/// `onBlockStart`, strictly before its `onBlockComplete`, and at most once
/// per ToolUse block. It never fires for non-ToolUse blocks. If a tool_use
/// block is dropped because identity never fully arrived, `onToolDetails`
/// (and `onBlockComplete`) never fire for it.
///
/// `onError` is the receiver's cleanup hook. It fires exactly once per failed
/// turn, whether the error originated in the receiver itself (a write failure)
/// or in the Provider (HTTP/parse/stream failure). It is the last callback the
/// receiver will see for that turn. `onError` itself cannot fail; receivers
/// must swallow secondary failures during cleanup.
///
/// `onMessageComplete`'s `usage` argument carries the wire-reported token
/// counts for the just-finished assistant turn. Providers fire this exactly
/// once per successful turn. `usage` is `null` only when the wire genuinely
/// did not deliver any usage information — chiefly OpenAI-compatible proxies
/// (OpenRouter, vLLM, some self-hosted backends) that ignore
/// `stream_options.include_usage`. Receivers that compute cost should record
/// the null case explicitly ("unknown") rather than treating it as zero.
pub const ReceiverVTable = struct {
    onMessageStart: *const fn (*anyopaque, conversation.MessageRole) anyerror!void,
    onBlockStart: *const fn (*anyopaque, ContentBlockType, usize) anyerror!void,
    onToolDetails: *const fn (*anyopaque, usize, []const u8, []const u8) anyerror!void,
    onContentDelta: *const fn (*anyopaque, usize, []const u8) anyerror!void,
    onBlockComplete: *const fn (*anyopaque, usize, conversation.ContentBlock) anyerror!void,
    onMessageComplete: *const fn (*anyopaque, conversation.Message, ?Usage) anyerror!void,
    onError: *const fn (*anyopaque, anyerror) void,
    /// Fired by the agent after a provider attempt fails and before it
    /// sleeps for the next attempt. Purely informational: receivers surface
    /// it (e.g. a dim CLI status line) but cannot abort or alter the retry.
    /// Never fires for tool-call failures (those become tool results) nor
    /// for terminal provider errors. May be a no-op.
    onProviderRetry: *const fn (*anyopaque, ProviderRetryInfo) void,
};

pub const Receiver = struct {
    ptr: *anyopaque,
    vtable: *const ReceiverVTable,

    pub fn onMessageStart(self: Receiver, role: conversation.MessageRole) !void {
        try self.vtable.onMessageStart(self.ptr, role);
    }

    pub fn onBlockStart(self: Receiver, block_type: ContentBlockType, index: usize) !void {
        try self.vtable.onBlockStart(self.ptr, block_type, index);
    }

    pub fn onToolDetails(self: Receiver, block_index: usize, id: []const u8, name: []const u8) !void {
        try self.vtable.onToolDetails(self.ptr, block_index, id, name);
    }

    pub fn onContentDelta(self: Receiver, block_index: usize, delta: []const u8) !void {
        try self.vtable.onContentDelta(self.ptr, block_index, delta);
    }

    pub fn onBlockComplete(self: Receiver, block_index: usize, block: conversation.ContentBlock) !void {
        try self.vtable.onBlockComplete(self.ptr, block_index, block);
    }

    pub fn onMessageComplete(self: Receiver, message: conversation.Message, usage: ?Usage) !void {
        try self.vtable.onMessageComplete(self.ptr, message, usage);
    }

    pub fn onError(self: Receiver, err: anyerror) void {
        self.vtable.onError(self.ptr, err);
    }

    pub fn onProviderRetry(self: Receiver, info: ProviderRetryInfo) void {
        self.vtable.onProviderRetry(self.ptr, info);
    }
};

/// Drive one streaming provider turn against the active config snapshot.
///
/// This is the single dispatch point: it switches on `cfg.provider`'s
/// `APIStyle` tag, builds a transient per-request object bound to the
/// process-global HTTP client, and runs it. There is no persistent
/// provider object — every turn re-reads `cfg`, so swapping the agent's
/// `*const Config` between turns changes provider, model, base_url, and
/// the visible tool set with no transport teardown.
///
/// The tool registry is taken from `cfg.registry`; the serializers receive
/// it directly.
pub fn streamStep(
    allocator: std.mem.Allocator,
    io: std.Io,
    cfg: *const config_mod.Config,
    conv: *conversation.Conversation,
    receiver: *Receiver,
    diag: ?*ProviderDiagnostic,
) anyerror!void {
    // Imported lazily to break the circular module graph:
    // provider.zig <- provider_openai_chat.zig <- provider.zig.
    const provider_openai_chat = @import("provider_openai_chat.zig");
    const provider_anthropic_messages = @import("provider_anthropic_messages.zig");
    const client = config_mod.httpClient();
    switch (cfg.provider) {
        .openai_chat => |*c| {
            var req: provider_openai_chat.OpenAIChatRequest = .{
                .allocator = allocator,
                .io = io,
                .config = c,
                .http_client = client,
                .diag = diag,
            };
            return req.streamStep(conv, cfg.registry, receiver);
        },
        .anthropic_messages => |*c| {
            var req: provider_anthropic_messages.AnthropicMessagesRequest = .{
                .allocator = allocator,
                .io = io,
                .config = c,
                .http_client = client,
                .diag = diag,
            };
            return req.streamStep(conv, cfg.registry, receiver);
        },
    }
}

/// The shape of `streamStep`, exposed as a function-pointer type so the
/// agent can carry an injectable seam (real dispatch in production, a stub
/// in tests) without resurrecting a per-provider vtable.
pub const StreamFn = *const fn (
    allocator: std.mem.Allocator,
    io: std.Io,
    cfg: *const config_mod.Config,
    conv: *conversation.Conversation,
    receiver: *Receiver,
    diag: ?*ProviderDiagnostic,
) anyerror!void;

test "isContextOverflowBody - matches known markers, rejects others" {
    const t2 = std.testing;
    try t2.expect(isContextOverflowBody("{\"error\":{\"code\":\"context_length_exceeded\"}}"));
    try t2.expect(isContextOverflowBody("This model's maximum context length is 8192 tokens"));
    try t2.expect(isContextOverflowBody("prompt is too long: 250000 tokens > 200000 maximum"));
    try t2.expect(!isContextOverflowBody("{\"error\":{\"code\":\"invalid_api_key\"}}"));
    try t2.expect(!isContextOverflowBody("rate limit exceeded"));
}

test "classifyHttpStatus - maps statuses to provider errors" {
    const t2 = std.testing;
    try t2.expectEqual(error.ContextOverflow, classifyHttpStatus(400, "prompt is too long"));
    try t2.expectEqual(error.ProviderBadRequest, classifyHttpStatus(400, "bad json"));
    try t2.expectEqual(error.ProviderAuthFailed, classifyHttpStatus(401, ""));
    try t2.expectEqual(error.ProviderAuthFailed, classifyHttpStatus(403, ""));
    try t2.expectEqual(error.ProviderModelNotFound, classifyHttpStatus(404, ""));
    try t2.expectEqual(error.ProviderTransport, classifyHttpStatus(408, ""));
    try t2.expectEqual(error.ProviderTransport, classifyHttpStatus(409, ""));
    try t2.expectEqual(error.ProviderTransport, classifyHttpStatus(425, ""));
    try t2.expectEqual(error.ProviderRateLimited, classifyHttpStatus(429, ""));
    try t2.expectEqual(error.ProviderServerError, classifyHttpStatus(500, ""));
    try t2.expectEqual(error.ProviderServerError, classifyHttpStatus(502, ""));
    try t2.expectEqual(error.ProviderUnavailable, classifyHttpStatus(503, ""));
    try t2.expectEqual(error.ProviderServerError, classifyHttpStatus(504, ""));
    try t2.expectEqual(error.ProviderServerError, classifyHttpStatus(599, ""));
    try t2.expectEqual(error.ProviderBadRequest, classifyHttpStatus(418, ""));
}

test "isRetryableProviderError - retryable vs terminal" {
    const t2 = std.testing;
    try t2.expect(isRetryableProviderError(error.ProviderRateLimited));
    try t2.expect(isRetryableProviderError(error.ProviderUnavailable));
    try t2.expect(isRetryableProviderError(error.ProviderServerError));
    try t2.expect(isRetryableProviderError(error.ProviderTransport));
    try t2.expect(isRetryableProviderError(error.ProviderStreamMalformed));
    try t2.expect(!isRetryableProviderError(error.ProviderAuthFailed));
    try t2.expect(!isRetryableProviderError(error.ProviderBadRequest));
    try t2.expect(!isRetryableProviderError(error.ProviderModelNotFound));
    try t2.expect(!isRetryableProviderError(error.ContextOverflow));
    try t2.expect(!isRetryableProviderError(error.Canceled));
}

test "parseRetryAfterMs - delta-seconds and rejects" {
    const t2 = std.testing;
    try t2.expectEqual(@as(?u64, 120_000), parseRetryAfterMs("120"));
    try t2.expectEqual(@as(?u64, 0), parseRetryAfterMs("0"));
    try t2.expectEqual(@as(?u64, 2_000), parseRetryAfterMs("  2 "));
    try t2.expectEqual(@as(?u64, null), parseRetryAfterMs(""));
    // HTTP-date form is not parsed.
    try t2.expectEqual(@as(?u64, null), parseRetryAfterMs("Wed, 21 Oct 2015 07:28:00 GMT"));
}