//! Debug-build log redirection.
//!
//! Debug builds emit a *lot* of `std.log.debug` traffic — most usefully the
//! raw JSON of every provider API request and response. Writing that to stderr
//! makes the interactive TUI unusable (every line scribbles over the frame).
//!
//! This module installs a custom `std.Options.logFn` (wired up via
//! `std_options` in `main.zig`). In **Debug** builds it routes the entire log
//! stream to a per-session file under
//!
//!     ($PANTO_HOME | $XDG_DATA_HOME/panto | ~/.local/share/panto)/debug/<session-id>.log
//!
//! and writes *nothing* to the terminal, leaving the TUI pristine. In any
//! other build mode it delegates to `std.log.defaultLog` (stderr, level-gated)
//! so release behavior is unchanged.
//!
//! The file is opened with `O_TRUNC`, so each session starts fresh; the file
//! holds exactly one session's logs and never grows across runs. (A *new*
//! session id per invocation already gives a fresh file; truncation guards the
//! resume case where the same id is reused.)
//!
//! The log function may be called from any thread (the `Io.Threaded` worker
//! pool as well as the main loop), so all writes are serialized behind a
//! mutex. Writes go straight to the fd via `std.c.write` — no `Io` handle is
//! needed (and none is available at a `logFn` call site), matching the raw-fd
//! approach already used by `tui_terminal.zig`.

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

const panto_home = @import("panto_home.zig");

/// True only in Debug builds; gates the file-redirect path at comptime.
const enabled = builtin.mode == .Debug;

/// A tiny atomic spinlock serializing writes. `logFn` can be called from any
/// thread but has no `Io` handle, so `Io.Mutex` (which needs one) is out;
/// contention is rare and each critical section is a single bounded write.
var log_lock = std.atomic.Value(bool).init(false);

fn lockLog() void {
    while (log_lock.cmpxchgWeak(false, true, .acquire, .monotonic) != null) {
        std.atomic.spinLoopHint();
    }
}

fn unlockLog() void {
    log_lock.store(false, .release);
}
/// The open log file descriptor, or -1 before `init` (or if init failed).
var log_fd: std.c.fd_t = -1;
/// Monotonic per-line counter. Wall-clock time needs an `Io` handle (not
/// available at a `logFn` call site), so lines are stamped with a sequence
/// number instead — enough to order events within the session.
var seq: u64 = 0;
/// Buffer backing the per-call `Writer`. Guarded by `log_mutex`.
var writer_buf: [16 * 1024]u8 = undefined;

/// Open the per-session debug log file and arm `logFn` to write to it. No-op
/// in non-Debug builds. Best-effort: any failure leaves `log_fd == -1`, and
/// `logFn` silently drops messages (debug logging must never break startup).
///
/// Safe to call once, after the session id is known. `environ_map` is used to
/// resolve `$PANTO_HOME`; `io` is used only to create the `debug/` directory.
pub fn init(
    allocator: Allocator,
    io: Io,
    environ_map: *const std.process.Environ.Map,
    session_id: []const u8,
) void {
    if (!enabled) return;

    var layout = panto_home.resolve(allocator, environ_map) catch return;
    defer layout.deinit();

    const debug_dir = std.fs.path.join(allocator, &.{ layout.home, "debug" }) catch return;
    defer allocator.free(debug_dir);

    Io.Dir.cwd().createDirPath(io, debug_dir) catch |err| switch (err) {
        error.PathAlreadyExists => {},
        else => return,
    };

    const file_name = std.fmt.allocPrint(allocator, "{s}.log", .{session_id}) catch return;
    defer allocator.free(file_name);
    const path = std.fs.path.joinZ(allocator, &.{ debug_dir, file_name }) catch return;
    defer allocator.free(path);

    const flags = std.c.O{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true };
    const fd = std.c.open(path.ptr, flags, @as(std.c.mode_t, 0o600));
    if (fd < 0) return;

    lockLog();
    defer unlockLog();
    log_fd = fd;
    seq = 0;
}

/// Custom `std.Options.logFn`. Debug builds capture everything to the session
/// file (terminal stays clean); other builds use the stderr default.
pub fn logFn(
    comptime level: std.log.Level,
    comptime scope: @EnumLiteral(),
    comptime format: []const u8,
    args: anytype,
) void {
    if (!enabled) {
        std.log.defaultLog(level, scope, format, args);
        return;
    }

    lockLog();
    defer unlockLog();

    const fd = log_fd;
    // Before the file is open (early bootstrap), drop the message rather than
    // letting it reach the terminal — keeping the TUI pristine is the point.
    if (fd < 0) return;

    var fw: FdWriter = .{
        .fd = fd,
        .interface = .{ .vtable = &fd_vtable, .buffer = &writer_buf },
    };
    const w = &fw.interface;

    seq += 1;
    w.print("#{d:>6} {s}", .{ seq, level.asText() }) catch {};
    if (scope != .default) w.print("({t})", .{scope}) catch {};
    w.writeAll(": ") catch {};
    w.print(format, args) catch {};
    w.writeAll("\n") catch {};
    w.flush() catch {};
}

/// A minimal `std.Io.Writer` whose sink is a raw file descriptor. Writes are
/// best-effort (logging must never fault): short writes are looped, `EINTR` is
/// retried, and any other error silently drops the remaining bytes.
const FdWriter = struct {
    fd: std.c.fd_t,
    interface: std.Io.Writer,
};

const fd_vtable: std.Io.Writer.VTable = .{ .drain = drainFd };

fn drainFd(w: *std.Io.Writer, data: []const []const u8, splat: usize) std.Io.Writer.Error!usize {
    const self: *FdWriter = @alignCast(@fieldParentPtr("interface", w));
    const fd = self.fd;

    // 1. Flush any buffered bytes first (they were logically written already).
    if (w.end != 0) {
        writeAllFd(fd, w.buffer[0..w.end]);
        w.end = 0;
    }

    // 2. Write each data slice in order; the last is repeated `splat` times.
    if (data.len == 0) return 0;
    var consumed: usize = 0;
    for (data[0 .. data.len - 1]) |slice| {
        writeAllFd(fd, slice);
        consumed += slice.len;
    }
    const last = data[data.len - 1];
    var i: usize = 0;
    while (i < splat) : (i += 1) {
        writeAllFd(fd, last);
        consumed += last.len;
    }
    return consumed;
}

fn writeAllFd(fd: std.c.fd_t, bytes: []const u8) void {
    var off: usize = 0;
    while (off < bytes.len) {
        const n = std.c.write(fd, bytes.ptr + off, bytes.len - off);
        if (n < 0) {
            if (std.posix.errno(n) == .INTR) continue;
            return; // best-effort: drop the rest
        }
        if (n == 0) return;
        off += @intCast(n);
    }
}

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

const testing = std.testing;

test "FdWriter: buffered + splat writes reach the fd intact" {
    const path = "/tmp/panto_debug_log_drain_test.txt";
    const wr_flags = std.c.O{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true };
    const fd = std.c.open(path, wr_flags, @as(std.c.mode_t, 0o600));
    try testing.expect(fd >= 0);

    {
        // A deliberately tiny buffer forces several drains mid-message,
        // exercising the buffer-flush path and the splat repetition.
        var buf: [8]u8 = undefined;
        var fw: FdWriter = .{
            .fd = fd,
            .interface = .{ .vtable = &fd_vtable, .buffer = &buf },
        };
        const w = &fw.interface;
        try w.print("hello {d} world {s}", .{ 42, "xyz" });
        try w.splatBytesAll("ab", 3); // splat: "ababab"
        try w.flush();
    }
    _ = std.c.close(fd);

    const rfd = std.c.open(path, std.c.O{ .ACCMODE = .RDONLY }, @as(std.c.mode_t, 0));
    try testing.expect(rfd >= 0);
    defer _ = std.c.close(rfd);
    var rbuf: [128]u8 = undefined;
    const n = std.c.read(rfd, &rbuf, rbuf.len);
    try testing.expect(n > 0);
    try testing.expectEqualStrings("hello 42 world xyzababab", rbuf[0..@intCast(n)]);
}