const builtin = @import("builtin");
const std = @import("std");
const Build = std.Build;

const Coverage = @This();

/// Whether the host platform supports kcov-based coverage.
/// Only x86_64 and aarch64 Linux are supported (kcov binary availability).
/// On unsupported platforms, the coverage step will fail at runtime with
/// a clear error from the kcov download or execution step.
// pub const supported = builtin.os.tag == .linux and
//     (builtin.cpu.arch == .x86_64 or builtin.cpu.arch == .aarch64);

/// Initialize coverage infrastructure. Creates the "coverage" build step,
/// registers build options (-Dcoverage-threshold, -Dcoverage-dir),
/// and sets up the kcov download step. The kcov binary is downloaded into the
/// zig cache on first use and reused thereafter.
///
/// Use `zig build coverage --verbose` to see per-file coverage breakdown.
///
/// Call `addModule()` on the returned value to add the test module to the
/// coverage run.
///
/// Because addModule creates a new test executable from the root module provided,
/// if there are any linking steps being done to your test executable, those
/// must also be done to the test_exe returned by addModule.
pub fn init(b: *Build) Coverage {
    // Add options
    const coverage_threshold = b.option(u7, "coverage-threshold", "Minimum coverage percentage required") orelse 0;
    const coverage_dir = b.option([]const u8, "coverage-dir", "Coverage output directory") orelse
        b.pathJoin(&.{ b.build_root.path orelse ".", "coverage" });
    const coverage_step = b.step("coverage", "Generate test coverage report");

    // Set up kcov download.
    // We can't download directly because we are sandboxed during build, but
    // we can create a helper program and run it. First we need the destination
    // directory, keyed by architecture.
    const arch_name = switch (builtin.cpu.arch) {
        .x86_64 => "x86_64",
        .aarch64 => "aarch64",
        else => @tagName(builtin.cpu.arch),
    };

    const Algo = std.crypto.hash.sha2.Sha256;
    var hasher = Algo.init(.{});
    hasher.update("kcov-");
    hasher.update(arch_name);
    var cache_hash: [Algo.digest_length]u8 = undefined;
    hasher.final(&cache_hash);

    const cache_dir = b.pathJoin(&.{
        b.cache_root.path.?,
        "o",
        b.fmt("{s}", .{std.fmt.bytesToHex(cache_hash, .lower)}),
    });

    const kcov_path = b.pathJoin(&.{ cache_dir, b.fmt("kcov-{s}", .{arch_name}) });

    // Create the download helper executable
    const download_exe = b.addExecutable(.{
        .name = "download-kcov",
        .root_module = b.createModule(.{
            .root_source_file = b.path("build/download_kcov.zig"),
            .target = b.resolveTargetQuery(.{}),
        }),
    });

    const run_download = b.addRunArtifact(download_exe);
    run_download.addArg(kcov_path);
    run_download.addArg(arch_name);

    return .{
        .b = b,
        .coverage_step = coverage_step,
        .coverage_dir = coverage_dir,
        .coverage_threshold = coverage_threshold,
        .kcov_path = kcov_path,
        .run_download = run_download,
    };
}

/// Add a test module to the coverage run. Runs kcov on the test binary,
/// then reads the coverage JSON and prints a summary (with per-file
/// breakdown if --verbose). Fails if below -Dcoverage-threshold.
///
/// Returns the test executable so the caller can add any extra linking steps.
pub fn addModule(self: *Coverage, root_module: *Build.Module, name: []const u8) *Build.Step.Compile {
    const b = self.b;

    // Set up kcov run: filter to src/ only, use custom CSS for HTML report
    const run_coverage = b.addSystemCommand(&.{self.kcov_path});
    const include_path = b.pathJoin(&.{ b.build_root.path.?, "src" });
    run_coverage.addArgs(&.{ "--include-path", include_path });
    const css_file = b.pathJoin(&.{ b.build_root.path.?, "build", "bcov.css" });
    run_coverage.addArg(b.fmt("--configure=css-file={s}", .{css_file}));
    run_coverage.addArg(self.coverage_dir);

    // Create a test executable for this module.
    // We need to set use_llvm because the self-hosted backend
    // does not emit the DWARF data that kcov needs.
    const test_exe = b.addTest(.{
        .name = name,
        .root_module = root_module,
        .use_llvm = true,
    });
    run_coverage.addArtifactArg(test_exe);
    run_coverage.step.dependOn(&test_exe.step);
    run_coverage.step.dependOn(&self.run_download.step);

    // Wire up the threshold check step after kcov completes
    const check = b.allocator.create(Coverage) catch @panic("OOM");
    check.* = .{
        .b = b,
        .coverage_step = undefined,
        .coverage_dir = undefined,
        .coverage_threshold = undefined,
        .kcov_path = undefined,
        .run_download = undefined,
        .step = Build.Step.init(.{
            .id = .custom,
            .name = "check coverage",
            .owner = b,
            .makeFn = make,
        }),
        .json_path = b.fmt("{s}/{s}/coverage.json", .{ self.coverage_dir, name }),
        .threshold = self.coverage_threshold,
    };
    check.step.dependOn(&run_coverage.step);
    self.coverage_step.dependOn(&check.step);

    return test_exe;
}

// ── Coverage struct fields ──────────────────────────────────

// Fields used by init() to configure the shared coverage infrastructure
b: *Build,
coverage_step: *Build.Step,
coverage_dir: []const u8,
coverage_threshold: u7,
kcov_path: []const u8,
run_download: *Build.Step.Run,

// Fields used by make() for the threshold check (set by addModule)
step: Build.Step = undefined,
json_path: []const u8 = "",
threshold: u7 = 0,

// This must be kept in step with kcov per-binary coverage.json format
const CoverageReport = struct {
    files: []const CoverageFile,
};

const CoverageFile = struct {
    file: []const u8,
    covered_lines: usize,
    total_lines: usize,
};

const File = struct {
    file: []const u8,
    percent_covered: f64,
    covered_lines: usize,
    total_lines: usize,

    pub fn coverageLessThanDesc(_: void, lhs: File, rhs: File) bool {
        return lhs.percent_covered > rhs.percent_covered;
    }
};

/// Build step make function: reads kcov JSON output, prints a summary
/// (with per-file breakdown if verbose), and fails if below threshold.
fn make(step: *Build.Step, options: Build.Step.MakeOptions) !void {
    _ = options;
    const check: *Coverage = @fieldParentPtr("step", step);
    const allocator = step.owner.allocator;

    const file = std.fs.cwd().openFile(check.json_path, .{}) catch |err| {
        return step.fail("Failed to open coverage report {s}: {}", .{ check.json_path, err });
    };
    defer file.close();

    const content = try file.readToEndAlloc(allocator, 10 * 1024 * 1024);
    defer allocator.free(content);

    const json = std.json.parseFromSlice(CoverageReport, allocator, content, .{
        .ignore_unknown_fields = true,
    }) catch |err| {
        return step.fail("Failed to parse coverage JSON: {}", .{err});
    };
    defer json.deinit();

    var total_covered: usize = 0;
    var total_lines: usize = 0;

    var file_list = std.ArrayList(File).empty;
    defer file_list.deinit(allocator);

    for (json.value.files) |f| {
        const pct: f64 = if (f.total_lines > 0)
            @as(f64, @floatFromInt(f.covered_lines)) / @as(f64, @floatFromInt(f.total_lines)) * 100.0
        else
            0;
        try file_list.append(allocator, .{
            .file = f.file,
            .covered_lines = f.covered_lines,
            .total_lines = f.total_lines,
            .percent_covered = pct,
        });
        total_covered += f.covered_lines;
        total_lines += f.total_lines;
    }

    std.mem.sort(File, file_list.items, {}, File.coverageLessThanDesc);

    var stdout_buffer: [1024]u8 = undefined;
    var stdout_writer = std.fs.File.stdout().writer(&stdout_buffer);
    const stdout = &stdout_writer.interface;
    if (step.owner.verbose) {
        for (file_list.items) |f| {
            try stdout.print(
                "{d: >5.1}% {d: >5}/{d: <5}:{s}\n",
                .{ f.percent_covered, f.covered_lines, f.total_lines, f.file },
            );
        }
    }

    const total_pct: f64 = if (total_lines > 0)
        @as(f64, @floatFromInt(total_covered)) / @as(f64, @floatFromInt(total_lines)) * 100.0
    else
        0;
    try stdout.print(
        "Total test coverage: {d:.2}% ({d}/{d})\n",
        .{ total_pct, total_covered, total_lines },
    );
    try stdout.flush();

    if (@as(u7, @intFromFloat(@floor(total_pct))) < check.threshold)
        return step.fail("Coverage {d:.2}% is below threshold {d}%", .{ total_pct, check.threshold });
}