id: Id,
name: []const u8,
owner: *Build,
makeFn: MakeFn,

dependencies: std.ArrayList(*Step),
/// This field is empty during execution of the user's build script, and
/// then populated during dependency loop checking in the build runner.
dependants: std.ArrayListUnmanaged(*Step),
/// Collects the set of files that retrigger this step to run.
///
/// This is used by the build system's implementation of `--watch` but it can
/// also be potentially useful for IDEs to know what effects editing a
/// particular file has.
///
/// Populated within `make`. Implementation may choose to clear and repopulate,
/// retain previous value, or update.
inputs: Inputs,

state: State,
/// Set this field to declare an upper bound on the amount of bytes of memory it will
/// take to run the step. Zero means no limit.
///
/// The idea to annotate steps that might use a high amount of RAM with an
/// upper bound. For example, perhaps a particular set of unit tests require 4
/// GiB of RAM, and those tests will be run under 4 different build
/// configurations at once. This would potentially require 16 GiB of memory on
/// the system if all 4 steps executed simultaneously, which could easily be
/// greater than what is actually available, potentially causing the system to
/// crash when using `zig build` at the default concurrency level.
///
/// This field causes the build runner to do two things:
/// 1. ulimit child processes, so that they will fail if it would exceed this
/// memory limit. This serves to enforce that this upper bound value is
/// correct.
/// 2. Ensure that the set of concurrent steps at any given time have a total
/// max_rss value that does not exceed the `max_total_rss` value of the build
/// runner. This value is configurable on the command line, and defaults to the
/// total system memory available.
max_rss: usize,

result_error_msgs: std.ArrayListUnmanaged([]const u8),
result_error_bundle: std.zig.ErrorBundle,
result_stderr: []const u8,
result_cached: bool,
result_duration_ns: ?u64,
/// 0 means unavailable or not reported.
result_peak_rss: usize,
test_results: TestResults,

/// The return address associated with creation of this step that can be useful
/// to print along with debugging messages.
debug_stack_trace: []usize,

pub const TestResults = struct {
    fail_count: u32 = 0,
    skip_count: u32 = 0,
    leak_count: u32 = 0,
    log_err_count: u32 = 0,
    test_count: u32 = 0,

    pub fn isSuccess(tr: TestResults) bool {
        return tr.fail_count == 0 and tr.leak_count == 0 and tr.log_err_count == 0;
    }

    pub fn passCount(tr: TestResults) u32 {
        return tr.test_count - tr.fail_count - tr.skip_count;
    }
};

pub const MakeOptions = struct {
    progress_node: std.Progress.Node,
    thread_pool: *std.Thread.Pool,
    watch: bool,
};

pub const MakeFn = *const fn (step: *Step, options: MakeOptions) anyerror!void;

pub const State = enum {
    precheck_unstarted,
    precheck_started,
    /// This is also used to indicate "dirty" steps that have been modified
    /// after a previous build completed, in which case, the step may or may
    /// not have been completed before. Either way, one or more of its direct
    /// file system inputs have been modified, meaning that the step needs to
    /// be re-evaluated.
    precheck_done,
    running,
    dependency_failure,
    success,
    failure,
    /// This state indicates that the step did not complete, however, it also did not fail,
    /// and it is safe to continue executing its dependencies.
    skipped,
    /// This step was skipped because it specified a max_rss that exceeded the runner's maximum.
    /// It is not safe to run its dependencies.
    skipped_oom,
};

pub const Id = enum {
    top_level,
    compile,
    install_artifact,
    install_file,
    install_dir,
    remove_dir,
    fail,
    fmt,
    translate_c,
    write_file,
    update_source_files,
    run,
    check_file,
    check_object,
    config_header,
    objcopy,
    options,
    custom,

    pub fn Type(comptime id: Id) type {
        return switch (id) {
            .top_level => Build.TopLevelStep,
            .compile => Compile,
            .install_artifact => InstallArtifact,
            .install_file => InstallFile,
            .install_dir => InstallDir,
            .remove_dir => RemoveDir,
            .fail => Fail,
            .fmt => Fmt,
            .translate_c => TranslateC,
            .write_file => WriteFile,
            .update_source_files => UpdateSourceFiles,
            .run => Run,
            .check_file => CheckFile,
            .check_object => CheckObject,
            .config_header => ConfigHeader,
            .objcopy => ObjCopy,
            .options => Options,
            .custom => @compileError("no type available for custom step"),
        };
    }
};

pub const CheckFile = @import("Step/CheckFile.zig");
pub const CheckObject = @import("Step/CheckObject.zig");
pub const ConfigHeader = @import("Step/ConfigHeader.zig");
pub const Fail = @import("Step/Fail.zig");
pub const Fmt = @import("Step/Fmt.zig");
pub const InstallArtifact = @import("Step/InstallArtifact.zig");
pub const InstallDir = @import("Step/InstallDir.zig");
pub const InstallFile = @import("Step/InstallFile.zig");
pub const ObjCopy = @import("Step/ObjCopy.zig");
pub const Compile = @import("Step/Compile.zig");
pub const Options = @import("Step/Options.zig");
pub const RemoveDir = @import("Step/RemoveDir.zig");
pub const Run = @import("Step/Run.zig");
pub const TranslateC = @import("Step/TranslateC.zig");
pub const WriteFile = @import("Step/WriteFile.zig");
pub const UpdateSourceFiles = @import("Step/UpdateSourceFiles.zig");

pub const Inputs = struct {
    table: Table,

    pub const init: Inputs = .{
        .table = .{},
    };

    pub const Table = std.ArrayHashMapUnmanaged(Build.Cache.Path, Files, Build.Cache.Path.TableAdapter, false);
    /// The special file name "." means any changes inside the directory.
    pub const Files = std.ArrayListUnmanaged([]const u8);

    pub fn populated(inputs: *Inputs) bool {
        return inputs.table.count() != 0;
    }

    pub fn clear(inputs: *Inputs, gpa: Allocator) void {
        for (inputs.table.values()) |*files| files.deinit(gpa);
        inputs.table.clearRetainingCapacity();
    }
};

pub const StepOptions = struct {
    id: Id,
    name: []const u8,
    owner: *Build,
    makeFn: MakeFn = makeNoOp,
    first_ret_addr: ?usize = null,
    max_rss: usize = 0,
};

pub fn init(options: StepOptions) Step {
    const arena = options.owner.allocator;

    return .{
        .id = options.id,
        .name = arena.dupe(u8, options.name) catch @panic("OOM"),
        .owner = options.owner,
        .makeFn = options.makeFn,
        .dependencies = std.ArrayList(*Step).init(arena),
        .dependants = .{},
        .inputs = Inputs.init,
        .state = .precheck_unstarted,
        .max_rss = options.max_rss,
        .debug_stack_trace = blk: {
            const addresses = arena.alloc(usize, options.owner.debug_stack_frames_count) catch @panic("OOM");
            @memset(addresses, 0);
            const first_ret_addr = options.first_ret_addr orelse @returnAddress();
            var stack_trace = std.builtin.StackTrace{
                .instruction_addresses = addresses,
                .index = 0,
            };
            std.debug.captureStackTrace(first_ret_addr, &stack_trace);
            break :blk addresses;
        },
        .result_error_msgs = .{},
        .result_error_bundle = std.zig.ErrorBundle.empty,
        .result_stderr = "",
        .result_cached = false,
        .result_duration_ns = null,
        .result_peak_rss = 0,
        .test_results = .{},
    };
}

/// If the Step's `make` function reports `error.MakeFailed`, it indicates they
/// have already reported the error. Otherwise, we add a simple error report
/// here.
pub fn make(s: *Step, options: MakeOptions) error{ MakeFailed, MakeSkipped }!void {
    const arena = s.owner.allocator;

    s.makeFn(s, options) catch |err| switch (err) {
        error.MakeFailed => return error.MakeFailed,
        error.MakeSkipped => return error.MakeSkipped,
        else => {
            s.result_error_msgs.append(arena, @errorName(err)) catch @panic("OOM");
            return error.MakeFailed;
        },
    };

    if (!s.test_results.isSuccess()) {
        return error.MakeFailed;
    }

    if (s.max_rss != 0 and s.result_peak_rss > s.max_rss) {
        const msg = std.fmt.allocPrint(arena, "memory usage peaked at {d} bytes, exceeding the declared upper bound of {d}", .{
            s.result_peak_rss, s.max_rss,
        }) catch @panic("OOM");
        s.result_error_msgs.append(arena, msg) catch @panic("OOM");
        return error.MakeFailed;
    }
}

pub fn dependOn(step: *Step, other: *Step) void {
    step.dependencies.append(other) catch @panic("OOM");
}

pub fn getStackTrace(s: *Step) ?std.builtin.StackTrace {
    var len: usize = 0;
    while (len < s.debug_stack_trace.len and s.debug_stack_trace[len] != 0) {
        len += 1;
    }

    return if (len == 0) null else .{
        .instruction_addresses = s.debug_stack_trace,
        .index = len,
    };
}

fn makeNoOp(step: *Step, options: MakeOptions) anyerror!void {
    _ = options;

    var all_cached = true;

    for (step.dependencies.items) |dep| {
        all_cached = all_cached and dep.result_cached;
    }

    step.result_cached = all_cached;
}

pub fn cast(step: *Step, comptime T: type) ?*T {
    if (step.id == T.base_id) {
        return @fieldParentPtr("step", step);
    }
    return null;
}

/// For debugging purposes, prints identifying information about this Step.
pub fn dump(step: *Step, file: std.fs.File) void {
    const w = file.writer();
    const tty_config = std.io.tty.detectConfig(file);
    const debug_info = std.debug.getSelfDebugInfo() catch |err| {
        w.print("Unable to dump stack trace: Unable to open debug info: {s}\n", .{
            @errorName(err),
        }) catch {};
        return;
    };
    const ally = debug_info.allocator;
    if (step.getStackTrace()) |stack_trace| {
        w.print("name: '{s}'. creation stack trace:\n", .{step.name}) catch {};
        std.debug.writeStackTrace(stack_trace, w, ally, debug_info, tty_config) catch |err| {
            w.print("Unable to dump stack trace: {s}\n", .{@errorName(err)}) catch {};
            return;
        };
    } else {
        const field = "debug_stack_frames_count";
        comptime assert(@hasField(Build, field));
        tty_config.setColor(w, .yellow) catch {};
        w.print("name: '{s}'. no stack trace collected for this step, see std.Build." ++ field ++ "\n", .{step.name}) catch {};
        tty_config.setColor(w, .reset) catch {};
    }
}

const Step = @This();
const std = @import("../std.zig");
const Build = std.Build;
const Allocator = std.mem.Allocator;
const assert = std.debug.assert;
const builtin = @import("builtin");
const Cache = Build.Cache;
const Path = Cache.Path;

pub fn evalChildProcess(s: *Step, argv: []const []const u8) ![]u8 {
    const run_result = try captureChildProcess(s, std.Progress.Node.none, argv);
    try handleChildProcessTerm(s, run_result.term, null, argv);
    return run_result.stdout;
}

pub fn captureChildProcess(
    s: *Step,
    progress_node: std.Progress.Node,
    argv: []const []const u8,
) !std.process.Child.RunResult {
    const arena = s.owner.allocator;

    try handleChildProcUnsupported(s, null, argv);
    try handleVerbose(s.owner, null, argv);

    const result = std.process.Child.run(.{
        .allocator = arena,
        .argv = argv,
        .progress_node = progress_node,
    }) catch |err| return s.fail("unable to spawn {s}: {s}", .{ argv[0], @errorName(err) });

    if (result.stderr.len > 0) {
        try s.result_error_msgs.append(arena, result.stderr);
    }

    return result;
}

pub fn fail(step: *Step, comptime fmt: []const u8, args: anytype) error{ OutOfMemory, MakeFailed } {
    try step.addError(fmt, args);
    return error.MakeFailed;
}

pub fn addError(step: *Step, comptime fmt: []const u8, args: anytype) error{OutOfMemory}!void {
    const arena = step.owner.allocator;
    const msg = try std.fmt.allocPrint(arena, fmt, args);
    try step.result_error_msgs.append(arena, msg);
}

pub const ZigProcess = struct {
    child: std.process.Child,
    poller: std.io.Poller(StreamEnum),
    progress_ipc_fd: if (std.Progress.have_ipc) ?std.posix.fd_t else void,

    pub const StreamEnum = enum { stdout, stderr };
};

/// Assumes that argv contains `--listen=-` and that the process being spawned
/// is the zig compiler - the same version that compiled the build runner.
pub fn evalZigProcess(
    s: *Step,
    argv: []const []const u8,
    prog_node: std.Progress.Node,
    watch: bool,
) !?Path {
    if (s.getZigProcess()) |zp| update: {
        assert(watch);
        if (std.Progress.have_ipc) if (zp.progress_ipc_fd) |fd| prog_node.setIpcFd(fd);
        const result = zigProcessUpdate(s, zp, watch) catch |err| switch (err) {
            error.BrokenPipe => {
                // Process restart required.
                const term = zp.child.wait() catch |e| {
                    return s.fail("unable to wait for {s}: {s}", .{ argv[0], @errorName(e) });
                };
                _ = term;
                s.clearZigProcess();
                break :update;
            },
            else => |e| return e,
        };

        if (s.result_error_bundle.errorMessageCount() > 0)
            return s.fail("{d} compilation errors", .{s.result_error_bundle.errorMessageCount()});

        if (s.result_error_msgs.items.len > 0 and result == null) {
            // Crash detected.
            const term = zp.child.wait() catch |e| {
                return s.fail("unable to wait for {s}: {s}", .{ argv[0], @errorName(e) });
            };
            s.result_peak_rss = zp.child.resource_usage_statistics.getMaxRss() orelse 0;
            s.clearZigProcess();
            try handleChildProcessTerm(s, term, null, argv);
            return error.MakeFailed;
        }

        return result;
    }
    assert(argv.len != 0);
    const b = s.owner;
    const arena = b.allocator;
    const gpa = arena;

    try handleChildProcUnsupported(s, null, argv);
    try handleVerbose(s.owner, null, argv);

    var child = std.process.Child.init(argv, arena);
    child.env_map = &b.graph.env_map;
    child.stdin_behavior = .Pipe;
    child.stdout_behavior = .Pipe;
    child.stderr_behavior = .Pipe;
    child.request_resource_usage_statistics = true;
    child.progress_node = prog_node;

    child.spawn() catch |err| return s.fail("unable to spawn {s}: {s}", .{
        argv[0], @errorName(err),
    });

    const zp = try gpa.create(ZigProcess);
    zp.* = .{
        .child = child,
        .poller = std.io.poll(gpa, ZigProcess.StreamEnum, .{
            .stdout = child.stdout.?,
            .stderr = child.stderr.?,
        }),
        .progress_ipc_fd = if (std.Progress.have_ipc) child.progress_node.getIpcFd() else {},
    };
    if (watch) s.setZigProcess(zp);
    defer if (!watch) zp.poller.deinit();

    const result = try zigProcessUpdate(s, zp, watch);

    if (!watch) {
        // Send EOF to stdin.
        zp.child.stdin.?.close();
        zp.child.stdin = null;

        const term = zp.child.wait() catch |err| {
            return s.fail("unable to wait for {s}: {s}", .{ argv[0], @errorName(err) });
        };
        s.result_peak_rss = zp.child.resource_usage_statistics.getMaxRss() orelse 0;

        // Special handling for Compile step that is expecting compile errors.
        if (s.cast(Compile)) |compile| switch (term) {
            .Exited => {
                // Note that the exit code may be 0 in this case due to the
                // compiler server protocol.
                if (compile.expect_errors != null) {
                    return error.NeedCompileErrorCheck;
                }
            },
            else => {},
        };

        try handleChildProcessTerm(s, term, null, argv);
    }

    // This is intentionally printed for failure on the first build but not for
    // subsequent rebuilds.
    if (s.result_error_bundle.errorMessageCount() > 0) {
        return s.fail("the following command failed with {d} compilation errors:\n{s}", .{
            s.result_error_bundle.errorMessageCount(),
            try allocPrintCmd(arena, null, argv),
        });
    }

    return result;
}

fn zigProcessUpdate(s: *Step, zp: *ZigProcess, watch: bool) !?Path {
    const b = s.owner;
    const arena = b.allocator;

    var timer = try std.time.Timer.start();

    try sendMessage(zp.child.stdin.?, .update);
    if (!watch) try sendMessage(zp.child.stdin.?, .exit);

    const Header = std.zig.Server.Message.Header;
    var result: ?Path = null;

    const stdout = zp.poller.fifo(.stdout);

    poll: while (true) {
        while (stdout.readableLength() < @sizeOf(Header)) {
            if (!(try zp.poller.poll())) break :poll;
        }
        const header = stdout.reader().readStruct(Header) catch unreachable;
        while (stdout.readableLength() < header.bytes_len) {
            if (!(try zp.poller.poll())) break :poll;
        }
        const body = stdout.readableSliceOfLen(header.bytes_len);

        switch (header.tag) {
            .zig_version => {
                if (!std.mem.eql(u8, builtin.zig_version_string, body)) {
                    return s.fail(
                        "zig version mismatch build runner vs compiler: '{s}' vs '{s}'",
                        .{ builtin.zig_version_string, body },
                    );
                }
            },
            .error_bundle => {
                const EbHdr = std.zig.Server.Message.ErrorBundle;
                const eb_hdr = @as(*align(1) const EbHdr, @ptrCast(body));
                const extra_bytes =
                    body[@sizeOf(EbHdr)..][0 .. @sizeOf(u32) * eb_hdr.extra_len];
                const string_bytes =
                    body[@sizeOf(EbHdr) + extra_bytes.len ..][0..eb_hdr.string_bytes_len];
                // TODO: use @ptrCast when the compiler supports it
                const unaligned_extra = std.mem.bytesAsSlice(u32, extra_bytes);
                const extra_array = try arena.alloc(u32, unaligned_extra.len);
                @memcpy(extra_array, unaligned_extra);
                s.result_error_bundle = .{
                    .string_bytes = try arena.dupe(u8, string_bytes),
                    .extra = extra_array,
                };
                if (watch) {
                    // This message indicates the end of the update.
                    stdout.discard(body.len);
                    break;
                }
            },
            .emit_digest => {
                const EmitDigest = std.zig.Server.Message.EmitDigest;
                const emit_digest = @as(*align(1) const EmitDigest, @ptrCast(body));
                s.result_cached = emit_digest.flags.cache_hit;
                const digest = body[@sizeOf(EmitDigest)..][0..Cache.bin_digest_len];
                result = .{
                    .root_dir = b.cache_root,
                    .sub_path = try arena.dupe(u8, "o" ++ std.fs.path.sep_str ++ Cache.binToHex(digest.*)),
                };
            },
            .file_system_inputs => {
                s.clearWatchInputs();
                var it = std.mem.splitScalar(u8, body, 0);
                while (it.next()) |prefixed_path| {
                    const prefix_index: std.zig.Server.Message.PathPrefix = @enumFromInt(prefixed_path[0] - 1);
                    const sub_path = try arena.dupe(u8, prefixed_path[1..]);
                    const sub_path_dirname = std.fs.path.dirname(sub_path) orelse "";
                    switch (prefix_index) {
                        .cwd => {
                            const path: Build.Cache.Path = .{
                                .root_dir = Build.Cache.Directory.cwd(),
                                .sub_path = sub_path_dirname,
                            };
                            try addWatchInputFromPath(s, path, std.fs.path.basename(sub_path));
                        },
                        .zig_lib => zl: {
                            if (s.cast(Step.Compile)) |compile| {
                                if (compile.zig_lib_dir) |zig_lib_dir| {
                                    const lp = try zig_lib_dir.join(arena, sub_path);
                                    try addWatchInput(s, lp);
                                    break :zl;
                                }
                            }
                            const path: Build.Cache.Path = .{
                                .root_dir = s.owner.graph.zig_lib_directory,
                                .sub_path = sub_path_dirname,
                            };
                            try addWatchInputFromPath(s, path, std.fs.path.basename(sub_path));
                        },
                        .local_cache => {
                            const path: Build.Cache.Path = .{
                                .root_dir = b.cache_root,
                                .sub_path = sub_path_dirname,
                            };
                            try addWatchInputFromPath(s, path, std.fs.path.basename(sub_path));
                        },
                        .global_cache => {
                            const path: Build.Cache.Path = .{
                                .root_dir = s.owner.graph.global_cache_root,
                                .sub_path = sub_path_dirname,
                            };
                            try addWatchInputFromPath(s, path, std.fs.path.basename(sub_path));
                        },
                    }
                }
            },
            else => {}, // ignore other messages
        }

        stdout.discard(body.len);
    }

    s.result_duration_ns = timer.read();

    const stderr = zp.poller.fifo(.stderr);
    if (stderr.readableLength() > 0) {
        try s.result_error_msgs.append(arena, try stderr.toOwnedSlice());
    }

    return result;
}

pub fn getZigProcess(s: *Step) ?*ZigProcess {
    return switch (s.id) {
        .compile => s.cast(Compile).?.zig_process,
        else => null,
    };
}

fn setZigProcess(s: *Step, zp: *ZigProcess) void {
    switch (s.id) {
        .compile => s.cast(Compile).?.zig_process = zp,
        else => unreachable,
    }
}

fn clearZigProcess(s: *Step) void {
    const gpa = s.owner.allocator;
    switch (s.id) {
        .compile => {
            const compile = s.cast(Compile).?;
            if (compile.zig_process) |zp| {
                gpa.destroy(zp);
                compile.zig_process = null;
            }
        },
        else => unreachable,
    }
}

fn sendMessage(file: std.fs.File, tag: std.zig.Client.Message.Tag) !void {
    const header: std.zig.Client.Message.Header = .{
        .tag = tag,
        .bytes_len = 0,
    };
    try file.writeAll(std.mem.asBytes(&header));
}

pub fn handleVerbose(
    b: *Build,
    opt_cwd: ?[]const u8,
    argv: []const []const u8,
) error{OutOfMemory}!void {
    return handleVerbose2(b, opt_cwd, null, argv);
}

pub fn handleVerbose2(
    b: *Build,
    opt_cwd: ?[]const u8,
    opt_env: ?*const std.process.EnvMap,
    argv: []const []const u8,
) error{OutOfMemory}!void {
    if (b.verbose) {
        // Intention of verbose is to print all sub-process command lines to
        // stderr before spawning them.
        const text = try allocPrintCmd2(b.allocator, opt_cwd, opt_env, argv);
        std.debug.print("{s}\n", .{text});
    }
}

pub inline fn handleChildProcUnsupported(
    s: *Step,
    opt_cwd: ?[]const u8,
    argv: []const []const u8,
) error{ OutOfMemory, MakeFailed }!void {
    if (!std.process.can_spawn) {
        return s.fail(
            "unable to execute the following command: host cannot spawn child processes\n{s}",
            .{try allocPrintCmd(s.owner.allocator, opt_cwd, argv)},
        );
    }
}

pub fn handleChildProcessTerm(
    s: *Step,
    term: std.process.Child.Term,
    opt_cwd: ?[]const u8,
    argv: []const []const u8,
) error{ MakeFailed, OutOfMemory }!void {
    const arena = s.owner.allocator;
    switch (term) {
        .Exited => |code| {
            if (code != 0) {
                return s.fail(
                    "the following command exited with error code {d}:\n{s}",
                    .{ code, try allocPrintCmd(arena, opt_cwd, argv) },
                );
            }
        },
        .Signal, .Stopped, .Unknown => {
            return s.fail(
                "the following command terminated unexpectedly:\n{s}",
                .{try allocPrintCmd(arena, opt_cwd, argv)},
            );
        },
    }
}

pub fn allocPrintCmd(
    arena: Allocator,
    opt_cwd: ?[]const u8,
    argv: []const []const u8,
) Allocator.Error![]u8 {
    return allocPrintCmd2(arena, opt_cwd, null, argv);
}

pub fn allocPrintCmd2(
    arena: Allocator,
    opt_cwd: ?[]const u8,
    opt_env: ?*const std.process.EnvMap,
    argv: []const []const u8,
) Allocator.Error![]u8 {
    var buf: std.ArrayListUnmanaged(u8) = .{};
    if (opt_cwd) |cwd| try buf.writer(arena).print("cd {s} && ", .{cwd});
    if (opt_env) |env| {
        const process_env_map = std.process.getEnvMap(arena) catch std.process.EnvMap.init(arena);
        var it = env.iterator();
        while (it.next()) |entry| {
            const key = entry.key_ptr.*;
            const value = entry.value_ptr.*;
            if (process_env_map.get(key)) |process_value| {
                if (std.mem.eql(u8, value, process_value)) continue;
            }
            try buf.writer(arena).print("{s}={s} ", .{ key, value });
        }
    }
    for (argv) |arg| {
        try buf.writer(arena).print("{s} ", .{arg});
    }
    return buf.toOwnedSlice(arena);
}

/// Prefer `cacheHitAndWatch` unless you already added watch inputs
/// separately from using the cache system.
pub fn cacheHit(s: *Step, man: *Build.Cache.Manifest) !bool {
    s.result_cached = man.hit() catch |err| return failWithCacheError(s, man, err);
    return s.result_cached;
}

/// Clears previous watch inputs, if any, and then populates watch inputs from
/// the full set of files picked up by the cache manifest.
///
/// Must be accompanied with `writeManifestAndWatch`.
pub fn cacheHitAndWatch(s: *Step, man: *Build.Cache.Manifest) !bool {
    const is_hit = man.hit() catch |err| return failWithCacheError(s, man, err);
    s.result_cached = is_hit;
    // The above call to hit() populates the manifest with files, so in case of
    // a hit, we need to populate watch inputs.
    if (is_hit) try setWatchInputsFromManifest(s, man);
    return is_hit;
}

fn failWithCacheError(s: *Step, man: *const Build.Cache.Manifest, err: anyerror) anyerror {
    const i = man.failed_file_index orelse return err;
    const pp = man.files.keys()[i].prefixed_path;
    const prefix = man.cache.prefixes()[pp.prefix].path orelse "";
    return s.fail("{s}: {s}/{s}", .{ @errorName(err), prefix, pp.sub_path });
}

/// Prefer `writeManifestAndWatch` unless you already added watch inputs
/// separately from using the cache system.
pub fn writeManifest(s: *Step, man: *Build.Cache.Manifest) !void {
    if (s.test_results.isSuccess()) {
        man.writeManifest() catch |err| {
            try s.addError("unable to write cache manifest: {s}", .{@errorName(err)});
        };
    }
}

/// Clears previous watch inputs, if any, and then populates watch inputs from
/// the full set of files picked up by the cache manifest.
///
/// Must be accompanied with `cacheHitAndWatch`.
pub fn writeManifestAndWatch(s: *Step, man: *Build.Cache.Manifest) !void {
    try writeManifest(s, man);
    try setWatchInputsFromManifest(s, man);
}

fn setWatchInputsFromManifest(s: *Step, man: *Build.Cache.Manifest) !void {
    const arena = s.owner.allocator;
    const prefixes = man.cache.prefixes();
    clearWatchInputs(s);
    for (man.files.keys()) |file| {
        // The file path data is freed when the cache manifest is cleaned up at the end of `make`.
        const sub_path = try arena.dupe(u8, file.prefixed_path.sub_path);
        try addWatchInputFromPath(s, .{
            .root_dir = prefixes[file.prefixed_path.prefix],
            .sub_path = std.fs.path.dirname(sub_path) orelse "",
        }, std.fs.path.basename(sub_path));
    }
}

/// For steps that have a single input that never changes when re-running `make`.
pub fn singleUnchangingWatchInput(step: *Step, lazy_path: Build.LazyPath) Allocator.Error!void {
    if (!step.inputs.populated()) try step.addWatchInput(lazy_path);
}

pub fn clearWatchInputs(step: *Step) void {
    const gpa = step.owner.allocator;
    step.inputs.clear(gpa);
}

/// Places a *file* dependency on the path.
pub fn addWatchInput(step: *Step, lazy_file: Build.LazyPath) Allocator.Error!void {
    switch (lazy_file) {
        .src_path => |src_path| try addWatchInputFromBuilder(step, src_path.owner, src_path.sub_path),
        .dependency => |d| try addWatchInputFromBuilder(step, d.dependency.builder, d.sub_path),
        .cwd_relative => |path_string| {
            try addWatchInputFromPath(step, .{
                .root_dir = .{
                    .path = null,
                    .handle = std.fs.cwd(),
                },
                .sub_path = std.fs.path.dirname(path_string) orelse "",
            }, std.fs.path.basename(path_string));
        },
        // Nothing to watch because this dependency edge is modeled instead via `dependants`.
        .generated => {},
    }
}

/// Any changes inside the directory will trigger invalidation.
///
/// See also `addDirectoryWatchInputFromPath` which takes a `Build.Cache.Path` instead.
///
/// Paths derived from this directory should also be manually added via
/// `addDirectoryWatchInputFromPath` if and only if this function returns
/// `true`.
pub fn addDirectoryWatchInput(step: *Step, lazy_directory: Build.LazyPath) Allocator.Error!bool {
    switch (lazy_directory) {
        .src_path => |src_path| try addDirectoryWatchInputFromBuilder(step, src_path.owner, src_path.sub_path),
        .dependency => |d| try addDirectoryWatchInputFromBuilder(step, d.dependency.builder, d.sub_path),
        .cwd_relative => |path_string| {
            try addDirectoryWatchInputFromPath(step, .{
                .root_dir = .{
                    .path = null,
                    .handle = std.fs.cwd(),
                },
                .sub_path = path_string,
            });
        },
        // Nothing to watch because this dependency edge is modeled instead via `dependants`.
        .generated => return false,
    }
    return true;
}

/// Any changes inside the directory will trigger invalidation.
///
/// See also `addDirectoryWatchInput` which takes a `Build.LazyPath` instead.
///
/// This function should only be called when it has been verified that the
/// dependency on `path` is not already accounted for by a `Step` dependency.
/// In other words, before calling this function, first check that the
/// `Build.LazyPath` which this `path` is derived from is not `generated`.
pub fn addDirectoryWatchInputFromPath(step: *Step, path: Build.Cache.Path) !void {
    return addWatchInputFromPath(step, path, ".");
}

fn addWatchInputFromBuilder(step: *Step, builder: *Build, sub_path: []const u8) !void {
    return addWatchInputFromPath(step, .{
        .root_dir = builder.build_root,
        .sub_path = std.fs.path.dirname(sub_path) orelse "",
    }, std.fs.path.basename(sub_path));
}

fn addDirectoryWatchInputFromBuilder(step: *Step, builder: *Build, sub_path: []const u8) !void {
    return addDirectoryWatchInputFromPath(step, .{
        .root_dir = builder.build_root,
        .sub_path = sub_path,
    });
}

fn addWatchInputFromPath(step: *Step, path: Build.Cache.Path, basename: []const u8) !void {
    const gpa = step.owner.allocator;
    const gop = try step.inputs.table.getOrPut(gpa, path);
    if (!gop.found_existing) gop.value_ptr.* = .{};
    try gop.value_ptr.append(gpa, basename);
}

fn reset(step: *Step, gpa: Allocator) void {
    assert(step.state == .precheck_done);

    step.result_error_msgs.clearRetainingCapacity();
    step.result_stderr = "";
    step.result_cached = false;
    step.result_duration_ns = null;
    step.result_peak_rss = 0;
    step.test_results = .{};

    step.result_error_bundle.deinit(gpa);
    step.result_error_bundle = std.zig.ErrorBundle.empty;
}

/// Implementation detail of file watching. Prepares the step for being re-evaluated.
pub fn recursiveReset(step: *Step, gpa: Allocator) void {
    assert(step.state != .precheck_done);
    step.state = .precheck_done;
    step.reset(gpa);
    for (step.dependants.items) |dep| {
        if (dep.state == .precheck_done) continue;
        dep.recursiveReset(gpa);
    }
}

test {
    _ = CheckFile;
    _ = CheckObject;
    _ = Fail;
    _ = Fmt;
    _ = InstallArtifact;
    _ = InstallDir;
    _ = InstallFile;
    _ = ObjCopy;
    _ = Compile;
    _ = Options;
    _ = RemoveDir;
    _ = Run;
    _ = TranslateC;
    _ = WriteFile;
    _ = UpdateSourceFiles;
}
