const builtin = @import("builtin");
const Os = builtin.Os;
const debug = @import("../debug.zig");
const assert = debug.assert;
const mem = @import("../mem.zig");
const fmt = @import("../fmt/index.zig");
const Allocator = mem.Allocator;
const os = @import("index.zig");
const math = @import("../math.zig");
const posix = os.posix;
const c = @import("../c/index.zig");
const cstr = @import("../cstr.zig");

pub const sep_windows = '\\';
pub const sep_posix = '/';
pub const sep = if (is_windows) sep_windows else sep_posix;

pub const delimiter_windows = ';';
pub const delimiter_posix = ':';
pub const delimiter = if (is_windows) delimiter_windows else delimiter_posix;

const is_windows = builtin.os == builtin.Os.windows;

/// Naively combines a series of paths with the native path seperator.
/// Allocates memory for the result, which must be freed by the caller.
pub fn join(allocator: &Allocator, paths: ...) -> %[]u8 {
    if (is_windows) {
        return joinWindows(allocator, paths);
    } else {
        return joinPosix(allocator, paths);
    }
}

pub fn joinWindows(allocator: &Allocator, paths: ...) -> %[]u8 {
    return mem.join(allocator, sep_windows, paths);
}

pub fn joinPosix(allocator: &Allocator, paths: ...) -> %[]u8 {
    return mem.join(allocator, sep_posix, paths);
}

test "os.path.join" {
    assert(mem.eql(u8, %%joinWindows(&debug.global_allocator, "c:\\a\\b", "c"), "c:\\a\\b\\c"));
    assert(mem.eql(u8, %%joinWindows(&debug.global_allocator, "c:\\a\\b\\", "c"), "c:\\a\\b\\c"));

    assert(mem.eql(u8, %%joinWindows(&debug.global_allocator, "c:\\", "a", "b\\", "c"), "c:\\a\\b\\c"));
    assert(mem.eql(u8, %%joinWindows(&debug.global_allocator, "c:\\a\\", "b\\", "c"), "c:\\a\\b\\c"));

    assert(mem.eql(u8, %%joinWindows(&debug.global_allocator,
        "c:\\home\\andy\\dev\\zig\\build\\lib\\zig\\std", "io.zig"),
        "c:\\home\\andy\\dev\\zig\\build\\lib\\zig\\std\\io.zig"));

    assert(mem.eql(u8, %%joinPosix(&debug.global_allocator, "/a/b", "c"), "/a/b/c"));
    assert(mem.eql(u8, %%joinPosix(&debug.global_allocator, "/a/b/", "c"), "/a/b/c"));

    assert(mem.eql(u8, %%joinPosix(&debug.global_allocator, "/", "a", "b/", "c"), "/a/b/c"));
    assert(mem.eql(u8, %%joinPosix(&debug.global_allocator, "/a/", "b/", "c"), "/a/b/c"));

    assert(mem.eql(u8, %%joinPosix(&debug.global_allocator, "/home/andy/dev/zig/build/lib/zig/std", "io.zig"),
        "/home/andy/dev/zig/build/lib/zig/std/io.zig"));
}

pub fn isAbsolute(path: []const u8) -> bool {
    if (is_windows) {
        return isAbsoluteWindows(path);
    } else {
        return isAbsolutePosix(path);
    }
}

pub fn isAbsoluteWindows(path: []const u8) -> bool {
    if (path[0] == '/')
        return true;

    if (path[0] == '\\') {
        return true;
    }
    if (path.len < 3) {
        return false;
    }
    if (path[1] == ':') {
        if (path[2] == '/')
            return true;
        if (path[2] == '\\')
            return true;
    }
    return false;
}

pub fn isAbsolutePosix(path: []const u8) -> bool {
    return path[0] == sep_posix;
}

test "os.path.isAbsoluteWindows" {
    testIsAbsoluteWindows("/", true);
    testIsAbsoluteWindows("//", true);
    testIsAbsoluteWindows("//server", true);
    testIsAbsoluteWindows("//server/file", true);
    testIsAbsoluteWindows("\\\\server\\file", true);
    testIsAbsoluteWindows("\\\\server", true);
    testIsAbsoluteWindows("\\\\", true);
    testIsAbsoluteWindows("c", false);
    testIsAbsoluteWindows("c:", false);
    testIsAbsoluteWindows("c:\\", true);
    testIsAbsoluteWindows("c:/", true);
    testIsAbsoluteWindows("c://", true);
    testIsAbsoluteWindows("C:/Users/", true);
    testIsAbsoluteWindows("C:\\Users\\", true);
    testIsAbsoluteWindows("C:cwd/another", false);
    testIsAbsoluteWindows("C:cwd\\another", false);
    testIsAbsoluteWindows("directory/directory", false);
    testIsAbsoluteWindows("directory\\directory", false);
}

test "os.path.isAbsolutePosix" {
    testIsAbsolutePosix("/home/foo", true);
    testIsAbsolutePosix("/home/foo/..", true);
    testIsAbsolutePosix("bar/", false);
    testIsAbsolutePosix("./baz", false);
}

fn testIsAbsoluteWindows(path: []const u8, expected_result: bool) {
    assert(isAbsoluteWindows(path) == expected_result);
}

fn testIsAbsolutePosix(path: []const u8, expected_result: bool) {
    assert(isAbsolutePosix(path) == expected_result);
}

pub fn drive(path: []const u8) -> ?[]const u8 {
    if (path.len < 2)
        return null;
    if (path[1] != ':')
        return null;
    return path[0..2];
}

pub fn networkShare(path: []const u8) -> ?[]const u8 {
    if (path.len < "//a/b".len)
        return null;

    // TODO when I combined these together with `inline for` the compiler crashed
    {
        const this_sep = '/';
        const two_sep = []u8{this_sep, this_sep};
        if (mem.startsWith(u8, path, two_sep)) {
            if (path[2] == this_sep)
                return null;

            var it = mem.split(path, []u8{this_sep});
            _ = (it.next() ?? return null);
            _ = (it.next() ?? return null);
            return path[0..it.index];
        }
    }
    {
        const this_sep = '\\';
        const two_sep = []u8{this_sep, this_sep};
        if (mem.startsWith(u8, path, two_sep)) {
            if (path[2] == this_sep)
                return null;

            var it = mem.split(path, []u8{this_sep});
            _ = (it.next() ?? return null);
            _ = (it.next() ?? return null);
            return path[0..it.index];
        }
    }
    return null;
}

test "os.path.networkShare" {
    assert(mem.eql(u8, ??networkShare("//a/b"), "//a/b"));
    assert(mem.eql(u8, ??networkShare("\\\\a\\b"), "\\\\a\\b"));

    assert(networkShare("\\\\a\\") == null);
}

pub fn root(path: []const u8) -> []const u8 {
    if (is_windows) {
        return rootWindows(path);
    } else {
        return rootPosix(path);
    }
}

pub fn rootWindows(path: []const u8) -> []const u8 {
    return drive(path) ?? (networkShare(path) ?? []u8{});
}

pub fn rootPosix(path: []const u8) -> []const u8 {
    if (path.len == 0 or path[0] != '/')
        return []u8{};

    return path[0..1];
}

pub fn drivesEqual(drive1: []const u8, drive2: []const u8) -> bool {
    assert(drive1.len == 2);
    assert(drive2.len == 2);
    assert(drive1[1] == ':');
    assert(drive2[1] == ':');
    return asciiUpper(drive1[0]) == asciiUpper(drive2[0]);
}

fn asciiUpper(byte: u8) -> u8 {
    return switch (byte) {
        'a' ... 'z' => 'A' + (byte - 'a'),
        else => byte,
    };
}

/// Converts the command line arguments into a slice and calls `resolveSlice`.
pub fn resolve(allocator: &Allocator, args: ...) -> %[]u8 {
    var paths: [args.len][]const u8 = undefined;
    comptime var arg_i = 0;
    inline while (arg_i < args.len) : (arg_i += 1) {
        paths[arg_i] = args[arg_i];
    }
    return resolveSlice(allocator, paths);
}

/// On Windows, this calls `resolveWindows` and on POSIX it calls `resolvePosix`.
pub fn resolveSlice(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
    if (is_windows) {
        return resolveWindows(allocator, paths);
    } else {
        return resolvePosix(allocator, paths);
    }
}

/// This function is like a series of `cd` statements executed one after another.
/// It resolves "." and "..".
/// The result does not have a trailing path separator.
/// If all paths are relative it uses the current working directory as a starting point.
/// Each drive has its own current working directory.
/// Path separators are canonicalized to '\\' and drives are canonicalized to capital letters.
pub fn resolveWindows(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
    if (paths.len == 0) {
        assert(is_windows); // resolveWindows called on non windows can't use getCwd
        return os.getCwd(allocator);
    }

    // determine which drive we want to result with
    var result_drive_upcase: ?u8 = null;
    var have_abs = false;
    var first_index: usize = 0;
    var max_size: usize = 0;
    for (paths) |p, i| {
        const is_abs = isAbsoluteWindows(p);
        if (is_abs) {
            have_abs = true;
            first_index = i;
            max_size = 0;
        }
        if (drive(p)) |d| {
            result_drive_upcase = asciiUpper(d[0]);
        } else if (networkShare(p)) |_| {
            result_drive_upcase = null;
        }
        max_size += p.len + 1;
    }


    // if we will result with a drive, loop again to determine
    // which is the first time the drive is absolutely specified, if any
    // and count up the max bytes for paths related to this drive
    if (result_drive_upcase) |res_dr| {
        have_abs = false;
        first_index = 0;
        max_size = "_:".len;
        var correct_drive = false;

        for (paths) |p, i| {
            if (drive(p)) |dr| {
                correct_drive = asciiUpper(dr[0]) == res_dr;
            } else if (networkShare(p)) |_| {
                continue;
            }
            if (!correct_drive) {
                continue;
            }
            const is_abs = isAbsoluteWindows(p);
            if (is_abs) {
                first_index = i;
                max_size = "_:".len;
                have_abs = true;
            }
            max_size += p.len + 1;
        }
    }

    var drive_buf = "_:";
    var result: []u8 = undefined;
    var result_index: usize = 0;
    var root_slice: []const u8 = undefined;

    if (have_abs) {
        result = %return allocator.alloc(u8, max_size);

        if (result_drive_upcase) |res_dr| {
            drive_buf[0] = res_dr;
            root_slice = drive_buf[0..];

            mem.copy(u8, result, root_slice);
            result_index += root_slice.len;
        } else {
            // We know it looks like //a/b or \\a\b because of earlier code
            var it = mem.split(paths[first_index], "/\\");
            const server_name = ??it.next();
            const other_name = ??it.next();

            result[result_index] = '\\';
            result_index += 1;
            result[result_index] = '\\';
            result_index += 1;
            mem.copy(u8, result[result_index..], server_name);
            result_index += server_name.len;
            result[result_index] = '\\';
            result_index += 1;
            mem.copy(u8, result[result_index..], other_name);
            result_index += other_name.len;
            
            root_slice = result[0..result_index];
        }
    } else {
        assert(is_windows); // resolveWindows called on non windows can't use getCwd
        // TODO get cwd for result_drive if applicable
        const cwd = %return os.getCwd(allocator);
        defer allocator.free(cwd);
        result = %return allocator.alloc(u8, max_size + cwd.len + 1);
        mem.copy(u8, result, cwd);
        result_index += cwd.len;

        root_slice = rootWindows(result[0..result_index]);
    }
    %defer allocator.free(result);

    var correct_drive = true;
    for (paths[first_index..]) |p, i| {
        if (result_drive_upcase) |res_dr| {
            if (drive(p)) |dr| {
                correct_drive = asciiUpper(dr[0]) == res_dr;
            } else if (networkShare(p)) |_| {
                continue;
            }
            if (!correct_drive) {
                continue;
            }
        }
        var it = mem.split(p[rootWindows(p).len..], "/\\");
        while (it.next()) |component| {
            if (mem.eql(u8, component, ".")) {
                continue;
            } else if (mem.eql(u8, component, "..")) {
                while (true) {
                    if (result_index == 0 or result_index == root_slice.len)
                        break;
                    result_index -= 1;
                    if (result[result_index] == '\\' or result[result_index] == '/')
                        break;
                }
            } else {
                result[result_index] = sep_windows;
                result_index += 1;
                mem.copy(u8, result[result_index..], component);
                result_index += component.len;
            }
        }
    }

    if (result_index == root_slice.len) {
        result[result_index] = '\\';
        result_index += 1;
    }

    return result[0..result_index];
}

/// This function is like a series of `cd` statements executed one after another.
/// It resolves "." and "..".
/// The result does not have a trailing path separator.
/// If all paths are relative it uses the current working directory as a starting point.
pub fn resolvePosix(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
    if (paths.len == 0) {
        assert(!is_windows); // resolvePosix called on windows can't use getCwd
        return os.getCwd(allocator);
    }

    var first_index: usize = 0;
    var have_abs = false;
    var max_size: usize = 0;
    for (paths) |p, i| {
        if (isAbsolutePosix(p)) {
            first_index = i;
            have_abs = true;
            max_size = 0;
        }
        max_size += p.len + 1;
    }

    var result: []u8 = undefined;
    var result_index: usize = 0;

    if (have_abs) {
        result = %return allocator.alloc(u8, max_size);
    } else {
        assert(!is_windows); // resolvePosix called on windows can't use getCwd
        const cwd = %return os.getCwd(allocator);
        defer allocator.free(cwd);
        result = %return allocator.alloc(u8, max_size + cwd.len + 1);
        mem.copy(u8, result, cwd);
        result_index += cwd.len;
    }
    %defer allocator.free(result);

    for (paths[first_index..]) |p, i| {
        var it = mem.split(p, "/");
        while (it.next()) |component| {
            if (mem.eql(u8, component, ".")) {
                continue;
            } else if (mem.eql(u8, component, "..")) {
                while (true) {
                    if (result_index == 0)
                        break;
                    result_index -= 1;
                    if (result[result_index] == '/')
                        break;
                }
            } else {
                result[result_index] = '/';
                result_index += 1;
                mem.copy(u8, result[result_index..], component);
                result_index += component.len;
            }
        }
    }

    if (result_index == 0) {
        result[0] = '/';
        result_index += 1;
    }

    return result[0..result_index];
}

test "os.path.resolve" {
    const cwd = %%os.getCwd(&debug.global_allocator);
    if (is_windows) {
        assert(mem.eql(u8, testResolveWindows([][]const u8{"."}), cwd));
    } else {
        assert(mem.eql(u8, testResolvePosix([][]const u8{"a/b/c/", "../../.."}), cwd));
        assert(mem.eql(u8, testResolvePosix([][]const u8{"."}), cwd));
    }
}

test "os.path.resolveWindows" {
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/blah\\blah", "d:/games", "c:../a"}), "C:\\blah\\a"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/blah\\blah", "d:/games", "C:../a"}), "C:\\blah\\a"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/ignore", "d:\\a/b\\c/d", "\\e.exe"}), "D:\\e.exe"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/ignore", "c:/some/file"}), "C:\\some\\file"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"d:/ignore", "d:some/dir//"}), "D:\\ignore\\some\\dir"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"//server/share", "..", "relative\\"}), "\\\\server\\share\\relative"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/", "//"}), "C:\\"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/", "//dir"}), "C:\\dir"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/", "//server/share"}), "\\\\server\\share\\"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/", "//server//share"}), "\\\\server\\share\\"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"c:/", "///some//dir"}), "C:\\some\\dir"));
    assert(mem.eql(u8, testResolveWindows([][]const u8{"C:\\foo\\tmp.3\\", "..\\tmp.3\\cycles\\root.js"}),
        "C:\\foo\\tmp.3\\cycles\\root.js"));
}

test "os.path.resolvePosix" {
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/a/b", "c"}), "/a/b/c"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/a/b", "c", "//d", "e///"}), "/d/e"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/a/b/c", "..", "../"}), "/a"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/", "..", ".."}), "/"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/a/b/c/"}), "/a/b/c"));

    assert(mem.eql(u8, testResolvePosix([][]const u8{"/var/lib", "../", "file/"}), "/var/file"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/var/lib", "/../", "file/"}), "/file"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/some/dir", ".", "/absolute/"}), "/absolute"));
    assert(mem.eql(u8, testResolvePosix([][]const u8{"/foo/tmp.3/", "../tmp.3/cycles/root.js"}), "/foo/tmp.3/cycles/root.js"));
}

fn testResolveWindows(paths: []const []const u8) -> []u8 {
    return %%resolveWindows(&debug.global_allocator, paths);
}

fn testResolvePosix(paths: []const []const u8) -> []u8 {
    return %%resolvePosix(&debug.global_allocator, paths);
}

pub fn dirname(path: []const u8) -> []const u8 {
    if (is_windows) {
        return dirnameWindows(path);
    } else {
        return dirnamePosix(path);
    }
}

pub fn dirnameWindows(path: []const u8) -> []const u8 {
    if (path.len == 0)
        return path[0..0];

    const root_slice = rootWindows(path);
    if (path.len == root_slice.len)
        return path;

    const have_root_slash = path.len > root_slice.len and (path[root_slice.len] == '/' or path[root_slice.len] == '\\');

    var end_index: usize = path.len - 1;

    while ((path[end_index] == '/' or path[end_index] == '\\') and end_index > root_slice.len) {
        if (end_index == 0)
            return path[0..0];
        end_index -= 1;
    }

    while (path[end_index] != '/' and path[end_index] != '\\' and end_index > root_slice.len) {
        if (end_index == 0)
            return path[0..0];
        end_index -= 1;
    }

    if (have_root_slash and end_index == root_slice.len) {
        end_index += 1;
    }

    return path[0..end_index];
}

pub fn dirnamePosix(path: []const u8) -> []const u8 {
    if (path.len == 0)
        return path[0..0];

    var end_index: usize = path.len - 1;
    while (path[end_index] == '/') {
        if (end_index == 0)
            return path[0..1];
        end_index -= 1;
    }

    while (path[end_index] != '/') {
        if (end_index == 0)
            return path[0..0];
        end_index -= 1;
    }

    if (end_index == 0 and path[end_index] == '/')
        return path[0..1];

    return path[0..end_index];
}

test "os.path.dirnamePosix" {
    testDirnamePosix("/a/b/c", "/a/b");
    testDirnamePosix("/a/b/c///", "/a/b");
    testDirnamePosix("/a", "/");
    testDirnamePosix("/", "/");
    testDirnamePosix("////", "/");
    testDirnamePosix("", "");
    testDirnamePosix("a", "");
    testDirnamePosix("a/", "");
    testDirnamePosix("a//", "");
}

test "os.path.dirnameWindows" {
    testDirnameWindows("c:\\", "c:\\");
    testDirnameWindows("c:\\foo", "c:\\");
    testDirnameWindows("c:\\foo\\", "c:\\");
    testDirnameWindows("c:\\foo\\bar", "c:\\foo");
    testDirnameWindows("c:\\foo\\bar\\", "c:\\foo");
    testDirnameWindows("c:\\foo\\bar\\baz", "c:\\foo\\bar");
    testDirnameWindows("\\", "\\");
    testDirnameWindows("\\foo", "\\");
    testDirnameWindows("\\foo\\", "\\");
    testDirnameWindows("\\foo\\bar", "\\foo");
    testDirnameWindows("\\foo\\bar\\", "\\foo");
    testDirnameWindows("\\foo\\bar\\baz", "\\foo\\bar");
    testDirnameWindows("c:", "c:");
    testDirnameWindows("c:foo", "c:");
    testDirnameWindows("c:foo\\", "c:");
    testDirnameWindows("c:foo\\bar", "c:foo");
    testDirnameWindows("c:foo\\bar\\", "c:foo");
    testDirnameWindows("c:foo\\bar\\baz", "c:foo\\bar");
    testDirnameWindows("file:stream", "");
    testDirnameWindows("dir\\file:stream", "dir");
    testDirnameWindows("\\\\unc\\share", "\\\\unc\\share");
    testDirnameWindows("\\\\unc\\share\\foo", "\\\\unc\\share\\");
    testDirnameWindows("\\\\unc\\share\\foo\\", "\\\\unc\\share\\");
    testDirnameWindows("\\\\unc\\share\\foo\\bar", "\\\\unc\\share\\foo");
    testDirnameWindows("\\\\unc\\share\\foo\\bar\\", "\\\\unc\\share\\foo");
    testDirnameWindows("\\\\unc\\share\\foo\\bar\\baz", "\\\\unc\\share\\foo\\bar");
    testDirnameWindows("/a/b/", "/a");
    testDirnameWindows("/a/b", "/a");
    testDirnameWindows("/a", "/");
    testDirnameWindows("", "");
    testDirnameWindows("/", "/");
    testDirnameWindows("////", "/");
    testDirnameWindows("foo", "");
}

fn testDirnamePosix(input: []const u8, expected_output: []const u8) {
    assert(mem.eql(u8, dirnamePosix(input), expected_output));
}

fn testDirnameWindows(input: []const u8, expected_output: []const u8) {
    assert(mem.eql(u8, dirnameWindows(input), expected_output));
}

pub fn basename(path: []const u8) -> []const u8 {
    if (builtin.os == builtin.Os.windows) {
        @compileError("TODO implement os.path.basename for windows");
    }
    if (path.len == 0)
        return []u8{};

    var end_index: usize = path.len - 1;
    while (path[end_index] == '/') {
        if (end_index == 0)
            return []u8{};
        end_index -= 1;
    }
    var start_index: usize = end_index;
    end_index += 1;
    while (path[start_index] != '/') {
        if (start_index == 0)
            return path[0..end_index];
        start_index -= 1;
    }

    return path[start_index + 1..end_index];
}

test "os.path.basename" {
    testBasename("", "");
    testBasename("/", "");
    testBasename("/dir/basename.ext", "basename.ext");
    testBasename("/basename.ext", "basename.ext");
    testBasename("basename.ext", "basename.ext");
    testBasename("basename.ext/", "basename.ext");
    testBasename("basename.ext//", "basename.ext");
    testBasename("/aaa/bbb", "bbb");
    testBasename("/aaa/", "aaa");
    testBasename("/aaa/b", "b");
    testBasename("/a/b", "b");
    testBasename("//a", "a");
}
fn testBasename(input: []const u8, expected_output: []const u8) {
    assert(mem.eql(u8, basename(input), expected_output));
}

/// Returns the relative path from ::from to ::to. If ::from and ::to each
/// resolve to the same path (after calling ::resolve on each), a zero-length
/// string is returned.
pub fn relative(allocator: &Allocator, from: []const u8, to: []const u8) -> %[]u8 {
    if (is_windows) {
        return windowsRelative(allocator, from, to);
    } else {
        return posixRelative(allocator, from, to);
    }
}

fn windowsRelative(allocator: &Allocator, from: []const u8, to: []const u8) -> %[]u8 {
    @compileError("TODO implement this");
}

fn posixRelative(allocator: &Allocator, from: []const u8, to: []const u8) -> %[]u8 {
    const resolved_from = %return resolve(allocator, from);
    defer allocator.free(resolved_from);

    const resolved_to = %return resolve(allocator, to);
    defer allocator.free(resolved_to);

    var from_it = mem.split(resolved_from, "/");
    var to_it = mem.split(resolved_to, "/");
    while (true) {
        const from_component = from_it.next() ?? return mem.dupe(allocator, u8, to_it.rest());
        const to_rest = to_it.rest();
        if (to_it.next()) |to_component| {
            if (mem.eql(u8, from_component, to_component))
                continue;
        }
        var up_count: usize = 1;
        while (from_it.next()) |_| {
            up_count += 1;
        }
        const up_index_end = up_count * "../".len;
        const result = %return allocator.alloc(u8, up_index_end + to_rest.len);
        %defer allocator.free(result);

        var result_index: usize = 0;
        while (result_index < up_index_end) {
            result[result_index] = '.';
            result_index += 1;
            result[result_index] = '.';
            result_index += 1;
            result[result_index] = '/';
            result_index += 1;
        }
        if (to_rest.len == 0) {
            // shave off the trailing slash
            return result[0..result_index - 1];
        }

        mem.copy(u8, result[result_index..], to_rest);
        return result;
    }

    return []u8{};
}

test "os.path.relative" {
    if (is_windows) {
        testRelative("c:/blah\\blah", "d:/games", "d:\\games");
        testRelative("c:/aaaa/bbbb", "c:/aaaa", "..");
        testRelative("c:/aaaa/bbbb", "c:/cccc", "..\\..\\cccc");
        testRelative("c:/aaaa/bbbb", "c:/aaaa/bbbb", "");
        testRelative("c:/aaaa/bbbb", "c:/aaaa/cccc", "..\\cccc");
        testRelative("c:/aaaa/", "c:/aaaa/cccc", "cccc");
        testRelative("c:/", "c:\\aaaa\\bbbb", "aaaa\\bbbb");
        testRelative("c:/aaaa/bbbb", "d:\\", "d:\\");
        testRelative("c:/AaAa/bbbb", "c:/aaaa/bbbb", "");
        testRelative("c:/aaaaa/", "c:/aaaa/cccc", "..\\aaaa\\cccc");
        testRelative("C:\\foo\\bar\\baz\\quux", "C:\\", "..\\..\\..\\..");
        testRelative("C:\\foo\\test", "C:\\foo\\test\\bar\\package.json", "bar\\package.json");
        testRelative("C:\\foo\\bar\\baz-quux", "C:\\foo\\bar\\baz", "..\\baz");
        testRelative("C:\\foo\\bar\\baz", "C:\\foo\\bar\\baz-quux", "..\\baz-quux");
        testRelative("\\\\foo\\bar", "\\\\foo\\bar\\baz", "baz");
        testRelative("\\\\foo\\bar\\baz", "\\\\foo\\bar", "..");
        testRelative("\\\\foo\\bar\\baz-quux", "\\\\foo\\bar\\baz", "..\\baz");
        testRelative("\\\\foo\\bar\\baz", "\\\\foo\\bar\\baz-quux", "..\\baz-quux");
        testRelative("C:\\baz-quux", "C:\\baz", "..\\baz");
        testRelative("C:\\baz", "C:\\baz-quux", "..\\baz-quux");
        testRelative("\\\\foo\\baz-quux", "\\\\foo\\baz", "..\\baz");
        testRelative("\\\\foo\\baz", "\\\\foo\\baz-quux", "..\\baz-quux");
        testRelative("C:\\baz", "\\\\foo\\bar\\baz", "\\\\foo\\bar\\baz");
        testRelative("\\\\foo\\bar\\baz", "C:\\baz", "C:\\baz")
    } else {
        testRelative("/var/lib", "/var", "..");
        testRelative("/var/lib", "/bin", "../../bin");
        testRelative("/var/lib", "/var/lib", "");
        testRelative("/var/lib", "/var/apache", "../apache");
        testRelative("/var/", "/var/lib", "lib");
        testRelative("/", "/var/lib", "var/lib");
        testRelative("/foo/test", "/foo/test/bar/package.json", "bar/package.json");
        testRelative("/Users/a/web/b/test/mails", "/Users/a/web/b", "../..");
        testRelative("/foo/bar/baz-quux", "/foo/bar/baz", "../baz");
        testRelative("/foo/bar/baz", "/foo/bar/baz-quux", "../baz-quux");
        testRelative("/baz-quux", "/baz", "../baz");
        testRelative("/baz", "/baz-quux", "../baz-quux");
    }
}
fn testRelative(from: []const u8, to: []const u8, expected_output: []const u8) {
    const result = %%relative(&debug.global_allocator, from, to);
    assert(mem.eql(u8, result, expected_output));
}

error AccessDenied;
error FileNotFound;
error NotSupported;
error NotDir;
error NameTooLong;
error SymLinkLoop;
error InputOutput;
error Unexpected;
/// Return the canonicalized absolute pathname.
/// Expands all symbolic links and resolves references to `.`, `..`, and
/// extra `/` characters in ::pathname.
/// Caller must deallocate result.
pub fn real(allocator: &Allocator, pathname: []const u8) -> %[]u8 {
    switch (builtin.os) {
        Os.windows => @compileError("TODO implement os.path.real for windows"),
        Os.darwin, Os.macosx, Os.ios => {
            // TODO instead of calling the libc function here, port the implementation
            // to Zig, and then remove the NameTooLong error possibility.
            const pathname_buf = %return allocator.alloc(u8, pathname.len + 1);
            defer allocator.free(pathname_buf);

            const result_buf = %return allocator.alloc(u8, posix.PATH_MAX);
            %defer allocator.free(result_buf);

            mem.copy(u8, pathname_buf, pathname);
            pathname_buf[pathname.len] = 0;

            const err = posix.getErrno(posix.realpath(pathname_buf.ptr, result_buf.ptr));
            if (err > 0) {
                return switch (err) {
                    posix.EINVAL => unreachable,
                    posix.EBADF => unreachable,
                    posix.EFAULT => unreachable,
                    posix.EACCES => error.AccessDenied,
                    posix.ENOENT => error.FileNotFound,
                    posix.ENOTSUP => error.NotSupported,
                    posix.ENOTDIR => error.NotDir,
                    posix.ENAMETOOLONG => error.NameTooLong,
                    posix.ELOOP => error.SymLinkLoop,
                    posix.EIO => error.InputOutput,
                    else => error.Unexpected,
                };
            }
            return cstr.toSlice(result_buf.ptr);
        },
        Os.linux => {
            const fd = %return os.posixOpen(pathname, posix.O_PATH|posix.O_NONBLOCK|posix.O_CLOEXEC, 0, allocator);
            defer os.posixClose(fd);

            var buf: ["/proc/self/fd/-2147483648".len]u8 = undefined;
            const proc_path = fmt.bufPrint(buf[0..], "/proc/self/fd/{}", fd);

            return os.readLink(allocator, proc_path);
        },
        else => @compileError("TODO implement os.path.real for " ++ @enumTagName(builtin.os)),
    }
}