os: rewrite os.walk and os.walk_with_context to use iteration, instead of recursion

vlib/os/os.c.v

@@ -738,6 +738,41 @@ pub fn is_link(path string) bool {
 	}
 }
 
+struct PathKind {
+mut:
+	is_dir  bool
+	is_link bool
+}
+
+fn kind_of_existing_path(path string) PathKind {
+	mut res := PathKind{}
+	$if windows {
+		attr := C.GetFileAttributesW(path.to_wide())
+		if attr != u32(C.INVALID_FILE_ATTRIBUTES) {
+			if (int(attr) & C.FILE_ATTRIBUTE_DIRECTORY) != 0 {
+				res.is_dir = true
+			}
+			if (int(attr) & 0x400) != 0 {
+				res.is_link = true
+			}
+		}
+	} $else {
+		statbuf := C.stat{}
+		// ref: https://code.woboq.org/gcc/include/sys/stat.h.html
+		res_stat := unsafe { C.lstat(&char(path.str), &statbuf) }
+		if res_stat == 0 {
+			kind := (int(statbuf.st_mode) & s_ifmt)
+			if kind == s_ifdir {
+				res.is_dir = true
+			}
+			if kind == s_iflnk {
+				res.is_link = true
+			}
+		}
+	}
+	return res
+}
+
 // chdir changes the current working directory to the new directory in `path`.
 pub fn chdir(path string) ? {
 	ret := $if windows { C._wchdir(path.to_wide()) } $else { C.chdir(&char(path.str)) }

vlib/os/os.v

@@ -569,8 +569,9 @@ fn impl_walk_ext(path string, ext string, mut out []string) {
 }
 
 // walk traverses the given directory `path`.
-// When a file is encountred it will call the
-// callback function `f` with current file as argument.
+// When a file is encountred, it will call the callback `f` with current file as argument.
+// Note: walk can be called even for deeply nested folders,
+// since it does not recurse, but processes them iteratively.
 pub fn walk(path string, f fn (string)) {
 	if path.len == 0 {
 		return
@@ -578,29 +579,36 @@ pub fn walk(path string, f fn (string)) {
 	if !is_dir(path) {
 		return
 	}
-	mut files := ls(path) or { return }
-	mut local_path_separator := path_separator
-	if path.ends_with(path_separator) {
-		local_path_separator = ''
+	mut remaining := []string{cap: 1000}
+	clean_path := path.trim_right(path_separator)
+	$if windows {
+		remaining << clean_path.replace('/', '\\')
+	} $else {
+		remaining << clean_path
 	}
-	for file in files {
-		p := path + local_path_separator + file
-		if is_dir(p) && !is_link(p) {
-			walk(p, f)
-		} else if exists(p) {
-			f(p)
+	for remaining.len > 0 {
+		cpath := remaining.pop()
+		pkind := kind_of_existing_path(cpath)
+		if pkind.is_link || !pkind.is_dir {
+			f(cpath)
+			continue
+		}
+		mut files := ls(cpath) or { continue }
+		for idx := files.len - 1; idx >= 0; idx-- {
+			remaining << cpath + path_separator + files[idx]
 		}
 	}
-	return
 }
 
 // FnWalkContextCB is used to define the callback functions, passed to os.walk_context
 pub type FnWalkContextCB = fn (voidptr, string)
 
 // walk_with_context traverses the given directory `path`.
-// For each encountred file and directory, it will call your `fcb` callback,
+// For each encountred file *and* directory, it will call your `fcb` callback,
 // passing it the arbitrary `context` in its first parameter,
 // and the path to the file in its second parameter.
+// Note: walk_with_context can be called even for deeply nested folders,
+// since it does not recurse, but processes them iteratively.
 pub fn walk_with_context(path string, context voidptr, fcb FnWalkContextCB) {
 	if path.len == 0 {
 		return
@@ -608,19 +616,30 @@ pub fn walk_with_context(path string, context voidptr, fcb FnWalkContextCB) {
 	if !is_dir(path) {
 		return
 	}
-	mut files := ls(path) or { return }
-	mut local_path_separator := path_separator
-	if path.ends_with(path_separator) {
-		local_path_separator = ''
+	mut remaining := []string{cap: 1000}
+	clean_path := path.trim_right(path_separator)
+	$if windows {
+		remaining << clean_path.replace('/', '\\')
+	} $else {
+		remaining << clean_path
 	}
-	for file in files {
-		p := path + local_path_separator + file
-		fcb(context, p)
-		if is_dir(p) && !is_link(p) {
-			walk_with_context(p, context, fcb)
+	mut loops := 0
+	for remaining.len > 0 {
+		loops++
+		cpath := remaining.pop()
+		// call `fcb` for everything, but the initial folder:
+		if loops > 1 {
+			fcb(context, cpath)
+		}
+		pkind := kind_of_existing_path(cpath)
+		if pkind.is_link || !pkind.is_dir {
+			continue
+		}
+		mut files := ls(cpath) or { continue }
+		for idx := files.len - 1; idx >= 0; idx-- {
+			remaining << cpath + path_separator + files[idx]
 		}
 	}
-	return
 }
 
 // log will print "os.log: "+`s` ...

vlib/os/os_js.js.v

@@ -29,6 +29,24 @@ pub fn is_link(path string) bool {
 	return res
 }
 
+struct PathKind {
+	is_dir  bool
+	is_link bool
+}
+
+fn kind_of_existing_path(path string) PathKind {
+	is_link := false
+	is_dir := false
+	$if js_node {
+		#is_link.val = $fs.existsSync(path.str) && $fs.lstatSync(path.str).isSymbolicLink()
+		#is_dir.val = $fs.existsSync(path,str) && $fs.lstatSync(path.str).isDirectory()
+	}
+	return PathKind{
+		is_dir: is_dir
+		is_link: is_link
+	}
+}
+
 pub fn exists(path string) bool {
 	res := false
 	$if js_node {