Alter pushd to add more bash-like behaviour

The following behaviour is added:
- an empty pushd exchanges the top two directories in the stack;
- pushd +<n> rotates the stack so that the n-th directory (counting from the left of the list shown by dirs, starting with zero) is at the top;
- pushd -<n> rotates the stack so that the nth directory (counting from the right of the list shown by dirs, starting with zero) is at the top.

share/functions/pushd.fish

@@ -1,16 +1,75 @@
 
 
-function pushd --description "Push directory to stack"
+function pushd --description 'Push directory to stack'
 	if count $argv >/dev/null
+		# check for --help
 		switch $argv[1]
 			case -h --h --he --hel --help
 				__fish_print_help pushd
 				return 0
 		end
+
+		# emulate bash by checking if argument of form +n or -n
+		set rot_l (echo $argv[1] | sed -n '/^+\([0-9]\)$/  s//\1/g p')
+		set rot_r (echo $argv[1] | sed -n '/^-\([0-9]\)$/  s//\1/g p')
+	end
+
+	# emulate bash: an empty pushd should switch the top of dirs
+	if test (count $argv) -eq 0
+		# check that the stack isn't empty
+		if test (count $dirstack) -eq 0
+			echo "pushd: no other directory"
+			return 1
+		end
+
+		# get the top two values of the dirs stack ... the first is pwd
+		set -l top_dir (command pwd)
+		set -l next_dir $dirstack[1]
+
+		# alter the top of dirstack and move to directory
+		set -g dirstack[1] $top_dir
+		cd $next_dir
+		return
+	end
+
+	# emulate bash: check for rotations
+	if test -n "$rot_l" -o -n "$rot_r"
+		# grab the current stack
+		set -l stack (command pwd) $dirstack
+
+		# translate a right rotation to a left rotation
+		if test -n "$rot_r"
+			# check the rotation in range
+			if test $rot_r -gt (math (count $stack) - 1)
+				echo "pushd: -$rot_r: directory stack index out of range"
+				return 1
+			end
+
+			set rot_l (math (count $stack) - 1 - $rot_r)
+		end
+
+		# check the rotation in range
+		if test $rot_l -gt (math (count $stack) - 1)
+			echo "pushd: +$rot_l: directory stack index out of range"
+			return 1
+		else
+			# rotate stack unless rot_l is 0
+			if test $rot_l -gt 0
+				set stack $stack[(math $rot_l + 1)..(count $stack)] $stack[1..$rot_l]
+			end
+
+			# now reconstruct dirstack and change directory
+			set -g dirstack $stack[2..(count $stack)]
+			cd $stack[1]
+		end
+
+		# print the new stack
+		dirs
+		return
 	end
 
 	# Comment to avoid set completions
 	set -g dirstack (command pwd) $dirstack
 	cd $argv[1]
-end
 
+end