diff --git a/README.md b/README.md
index ac9cff6ec5..8cbf31b3e1 100644
--- a/README.md
+++ b/README.md
@@ -43,15 +43,12 @@ clarity.
  * Cross-compiling is a primary use case.
  * In addition to creating executables, creating a C library is a primary use
    case. You can export an auto-generated .h file.
- * Standard library supports Operating System abstractions for:
-   * `x86_64` `linux`
-   * `x86_64` `macos`
-   * Support for all popular operating systems and architectures is planned.
  * For OS development, Zig supports all architectures that LLVM does. All the
    standard library that does not depend on an OS is available to you in
    freestanding mode.
 
 ### Support Table
+
 Freestanding means that you do not directly interact with the OS
 or you are writing your own OS.
 
diff --git a/std/mem.zig b/std/mem.zig
index e16e947c25..bc25c6e706 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -216,20 +216,28 @@ pub fn dupe(allocator: &Allocator, comptime T: type, m: []const T) -> %[]T {
 
 /// Linear search for the index of a scalar value inside a slice.
 pub fn indexOfScalar(comptime T: type, slice: []const T, value: T) -> ?usize {
-    for (slice) |item, i| {
-        if (item == value) {
+    return indexOfScalarPos(T, slice, 0, value);
+}
+
+pub fn indexOfScalarPos(comptime T: type, slice: []const T, start_index: usize, value: T) -> ?usize {
+    var i: usize = start_index;
+    while (i < slice.len) : (i += 1) {
+        if (slice[i] == value)
             return i;
-        }
     }
     return null;
 }
 
-// TODO boyer-moore algorithm
 pub fn indexOf(comptime T: type, haystack: []const T, needle: []const T) -> ?usize {
+    return indexOfPos(T, haystack, 0, needle);
+}
+
+// TODO boyer-moore algorithm
+pub fn indexOfPos(comptime T: type, haystack: []const T, start_index: usize, needle: []const T) -> ?usize {
     if (needle.len > haystack.len)
         return null;
 
-    var i: usize = 0;
+    var i: usize = start_index;
     const end = haystack.len - needle.len;
     while (i <= end) : (i += 1) {
         if (eql(T, haystack[i .. i + needle.len], needle))
@@ -303,15 +311,15 @@ pub fn eql_slice_u8(a: []const u8, b: []const u8) -> bool {
     return eql(u8, a, b);
 }
 
-/// Returns an iterator that iterates over the slices of ::s that are not
-/// the byte ::c.
-/// split("   abc def    ghi  ")
+/// Returns an iterator that iterates over the slices of `buffer` that are not
+/// any of the bytes in `split_bytes`.
+/// split("   abc def    ghi  ", " ")
 /// Will return slices for "abc", "def", "ghi", null, in that order.
-pub fn split(s: []const u8, c: u8) -> SplitIterator {
+pub fn split(buffer: []const u8, split_bytes: []const u8) -> SplitIterator {
     SplitIterator {
         .index = 0,
-        .s = s,
-        .c = c,
+        .buffer = buffer,
+        .split_bytes = split_bytes,
     }
 }
 
@@ -328,31 +336,32 @@ pub fn startsWith(comptime T: type, haystack: []const T, needle: []const T) -> b
 }
 
 const SplitIterator = struct {
-    s: []const u8,
-    c: u8,
+    buffer: []const u8,
+    split_bytes: []const u8, 
     index: usize,
 
     pub fn next(self: &SplitIterator) -> ?[]const u8 {
         // move to beginning of token
-        while (self.index < self.s.len and self.s[self.index] == self.c) : (self.index += 1) {}
+        while (self.index < self.buffer.len and self.isSplitByte(self.buffer[self.index])) : (self.index += 1) {}
         const start = self.index;
-        if (start == self.s.len) {
+        if (start == self.buffer.len) {
             return null;
         }
 
         // move to end of token
-        while (self.index < self.s.len and self.s[self.index] != self.c) : (self.index += 1) {}
+        while (self.index < self.buffer.len and !self.isSplitByte(self.buffer[self.index])) : (self.index += 1) {}
         const end = self.index;
 
-        return self.s[start..end];
+        return self.buffer[start..end];
     }
 
-    /// Returns a slice of the remaining bytes. Does not affect iterator state.
-    pub fn rest(self: &const SplitIterator) -> []const u8 {
-        // move to beginning of token
-        var index: usize = self.index;
-        while (index < self.s.len and self.s[index] == self.c) : (index += 1) {}
-        return self.s[index..];
+    fn isSplitByte(self: &SplitIterator, byte: u8) -> bool {
+        for (self.split_bytes) |split_byte| {
+            if (byte == split_byte) {
+                return true;
+            }
+        }
+        return false;
     }
 };
 
diff --git a/std/os/index.zig b/std/os/index.zig
index ccd7527975..3065396fa0 100644
--- a/std/os/index.zig
+++ b/std/os/index.zig
@@ -474,18 +474,28 @@ pub const args = struct {
 
 /// Caller must free the returned memory.
 pub fn getCwd(allocator: &Allocator) -> %[]u8 {
-    var buf = %return allocator.alloc(u8, 1024);
-    %defer allocator.free(buf);
-    while (true) {
-        const err = posix.getErrno(posix.getcwd(buf.ptr, buf.len));
-        if (err == posix.ERANGE) {
-            buf = %return allocator.realloc(u8, buf, buf.len * 2);
-            continue;
-        } else if (err > 0) {
-            return error.Unexpected;
-        }
+    switch (builtin.os) {
+        Os.windows => {
+            @panic("implement getCwd for windows");
+            //if (windows.GetCurrentDirectoryA(buf_len(out_cwd), buf_ptr(out_cwd)) == 0) {
+            //    zig_panic("GetCurrentDirectory failed");
+            //}
+        },
+        else => {
+            var buf = %return allocator.alloc(u8, 1024);
+            %defer allocator.free(buf);
+            while (true) {
+                const err = posix.getErrno(posix.getcwd(buf.ptr, buf.len));
+                if (err == posix.ERANGE) {
+                    buf = %return allocator.realloc(u8, buf, buf.len * 2);
+                    continue;
+                } else if (err > 0) {
+                    return error.Unexpected;
+                }
 
-        return cstr.toSlice(buf.ptr);
+                return cstr.toSlice(buf.ptr);
+            }
+        },
     }
 }
 
diff --git a/std/os/path.zig b/std/os/path.zig
index cfed8d7b2d..bce866a5fd 100644
--- a/std/os/path.zig
+++ b/std/os/path.zig
@@ -11,39 +11,223 @@ const posix = os.posix;
 const c = @import("../c/index.zig");
 const cstr = @import("../cstr.zig");
 
-pub const sep = switch (builtin.os) {
-    Os.windows => '\\',
-    else => '/',
-};
-pub const delimiter = switch (builtin.os) {
-    Os.windows => ';',
-    else => ':',
-};
+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 {
-    mem.join(allocator, sep, paths)
+    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, %%join(&debug.global_allocator, "/a/b", "c"), "/a/b/c"));
-    assert(mem.eql(u8, %%join(&debug.global_allocator, "/a/b/", "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, %%join(&debug.global_allocator, "/", "a", "b/", "c"), "/a/b/c"));
-    assert(mem.eql(u8, %%join(&debug.global_allocator, "/a/", "b/", "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, %%join(&debug.global_allocator, "/home/andy/dev/zig/build/lib/zig/std", "io.zig"),
+    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 {
-    switch (builtin.os) {
-        Os.windows => @compileError("Unsupported OS"),
-        else => return path[0] == sep,
+    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;
+
+    {
+        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;
+            const index_host = mem.indexOfScalarPos(u8, path, 3, this_sep) ?? return null;
+            const next_start = index_host + 1;
+            if (next_start >= path.len)
+                return null;
+            const index_root = mem.indexOfScalarPos(u8, path, next_start, this_sep) ?? path.len;
+            return path[0..index_root];
+        }
+    }
+    {
+        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;
+            const index_host = mem.indexOfScalarPos(u8, path, 3, this_sep) ?? return null;
+            const next_start = index_host + 1;
+            if (next_start >= path.len)
+                return null;
+            const index_root = mem.indexOfScalarPos(u8, path, next_start, this_sep) ?? path.len;
+            return path[0..index_root];
+        }
+    }
+    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 preferredSepWindows(path: []const u8) -> u8 {
+    for (path) |byte| {
+        if (byte == '/' or byte == '\\') {
+            return byte;
+        }
+    }
+    return sep_windows;
+}
+
+pub fn preferredSep(path: []const u8) -> u8 {
+    if (is_windows) {
+        return preferredSepWindows(path);
+    } else {
+        return sep_posix;
+    }
+}
+
+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 asciiLower(drive1[0]) == asciiLower(drive2[0]);
+}
+
+fn asciiLower(byte: u8) -> u8 {
+    return switch (byte) {
+        'A' ... 'Z' => 'a' + (byte - 'A'),
+        else => byte,
+    };
+}
+
 /// This function is like a series of `cd` statements executed one after another.
 /// The result does not have a trailing path separator.
 pub fn resolve(allocator: &Allocator, args: ...) -> %[]u8 {
@@ -56,17 +240,130 @@ pub fn resolve(allocator: &Allocator, args: ...) -> %[]u8 {
 }
 
 pub fn resolveSlice(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
-    if (builtin.os == builtin.Os.windows) {
-        @compileError("TODO implement os.path.resolve for windows");
+    if (is_windows) {
+        return resolveWindows(allocator, paths);
+    } else {
+        return resolvePosix(allocator, paths);
     }
-    if (paths.len == 0)
+}
+
+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: ?[]const 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 = d;
+        } else if (is_abs) {
+            result_drive = 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) |res_dr| {
+        have_abs = false;
+        first_index = 0;
+        max_size = 0;
+        var correct_drive = false;
+
+        for (paths) |p, i| {
+            if (drive(p)) |dr| {
+                correct_drive = drivesEqual(dr, res_dr);
+            }
+            if (!correct_drive) {
+                continue;
+            }
+            const is_abs = isAbsoluteWindows(p);
+            if (is_abs) {
+                first_index = i;
+                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);
+        mem.copy(u8, result, paths[first_index]);
+        result_index += paths[first_index].len;
+        first_index += 1;
+    } 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;
+    }
+    %defer allocator.free(result);
+
+    var correct_drive = false;
+    const rootSlice = rootWindows(result[0..result_index]);
+    const preferred_path_sep = preferredSepWindows(result[0..result_index]);
+    for (paths[first_index..]) |p, i| {
+        if (result_drive) |res_dr| {
+            if (drive(p)) |dr| {
+                correct_drive = drivesEqual(dr, res_dr);
+            }
+            if (!correct_drive) {
+                continue;
+            }
+        }
+        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 or result_index == rootSlice.len)
+                        break;
+                    result_index -= 1;
+                    if (result[result_index] == '\\' or result[result_index] == '/')
+                        break;
+                }
+            } else {
+                result[result_index] = preferred_path_sep;
+                result_index += 1;
+                mem.copy(u8, result[result_index..], component);
+                result_index += component.len;
+            }
+        }
+    }
+
+    return result[0..result_index];
+}
+
+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 (isAbsolute(p)) {
+        if (isAbsolutePosix(p)) {
             first_index = i;
             have_abs = true;
             max_size = 0;
@@ -80,6 +377,7 @@ pub fn resolveSlice(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
     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);
@@ -89,7 +387,7 @@ pub fn resolveSlice(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
     %defer allocator.free(result);
 
     for (paths[first_index..]) |p, i| {
-        var it = mem.split(p, '/');
+        var it = mem.split(p, "/");
         while (it.next()) |component| {
             if (mem.eql(u8, component, ".")) {
                 continue;
@@ -119,22 +417,95 @@ pub fn resolveSlice(allocator: &Allocator, paths: []const []const u8) -> %[]u8 {
 }
 
 test "os.path.resolve" {
-    assert(mem.eql(u8, testResolve("/a/b", "c"), "/a/b/c"));
-    assert(mem.eql(u8, testResolve("/a/b", "c", "//d", "e///"), "/d/e"));
-    assert(mem.eql(u8, testResolve("/a/b/c", "..", "../"), "/a"));
-    assert(mem.eql(u8, testResolve("/", "..", ".."), "/"));
-    assert(mem.eql(u8, testResolve("/a/b/c/"), "/a/b/c"));
+    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));
+    }
 }
-fn testResolve(args: ...) -> []u8 {
-    return %%resolve(&debug.global_allocator, args);
+
+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 (builtin.os == builtin.Os.windows) {
-        @compileError("TODO implement os.path.dirname for windows");
+    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 rootSlice = rootWindows(path);
+    if (path.len == rootSlice.len)
+        return path;
+
+    const have_root_slash = path.len > rootSlice.len and (path[rootSlice.len] == '/' or path[rootSlice.len] == '\\');
+
+    var end_index: usize = path.len - 1;
+
+    while ((path[end_index] == '/' or path[end_index] == '\\') and end_index > rootSlice.len) {
+        if (end_index == 0)
+            return path[0..0];
+        end_index -= 1;
+    }
+
+    while (path[end_index] != '/' and path[end_index] != '\\' and end_index > rootSlice.len) {
+        if (end_index == 0)
+            return path[0..0];
+        end_index -= 1;
+    }
+
+    if (have_root_slash and end_index == rootSlice.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)
@@ -154,19 +525,60 @@ pub fn dirname(path: []const u8) -> []const u8 {
     return path[0..end_index];
 }
 
-test "os.path.dirname" {
-    testDirname("/a/b/c", "/a/b");
-    testDirname("/a/b/c///", "/a/b");
-    testDirname("/a", "/");
-    testDirname("/", "/");
-    testDirname("////", "/");
-    testDirname("", "");
-    testDirname("a", "");
-    testDirname("a/", "");
-    testDirname("a//", "");
+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//", "");
 }
-fn testDirname(input: []const u8, expected_output: []const u8) {
-    assert(mem.eql(u8, dirname(input), expected_output));
+
+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 {
@@ -215,9 +627,18 @@ fn testBasename(input: []const u8, expected_output: []const u8) {
 /// 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 (builtin.os == builtin.Os.windows) {
-        @compileError("TODO implement os.path.relative for windows");
+    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);
 
@@ -263,18 +684,45 @@ pub fn relative(allocator: &Allocator, from: []const u8, to: []const u8) -> %[]u
 }
 
 test "os.path.relative" {
-    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");
+    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);