# Copyright 2017-2021 Free Software Foundation, Inc.

# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# This file implements some simple data structures in Tcl.

# A namespace/commands to support a stack.
#
# To create a stack, call ::Stack::new, recording the returned object ID
# for future calls to manipulate the stack object.
#
# Example:
#
# set sid [::Stack::new]
# stack push $sid a
# stack push $sid b
# stack empty $sid;  # returns false
# stack pop $sid;    # returns "b"
# stack pop $sid;    # returns "a"
# stack pop $sid;    # errors with "stack is empty"
# stack delete $sid1

namespace eval ::Stack {
    # A counter used to create object IDs
    variable num_ 0

    # An array holding all object lists, indexed by object ID.
    variable data_

    # Create a new stack object, returning its object ID.
    proc new {} {
	variable num_
	variable data_

	set oid [incr num_]
	set data_($oid) [list]
	return $oid
    }

    # Delete the given stack ID.
    proc delete {oid} {
	variable data_

	error_if $oid
	unset data_($oid)
    }

    # Returns whether the given stack is empty.
    proc empty {oid} {
	variable data_

	error_if $oid
	return [expr {[llength $data_($oid)] == 0}]
    }

    # Push ELEM onto the stack given by OID.
    proc push {oid elem} {
	variable data_

	error_if $oid
	lappend data_($oid) $elem
    }

    # Return and pop the top element on OID.  It is an error to pop
    # an empty stack.
    proc pop {oid} {
	variable data_

	error_if $oid
	if {[llength $data_($oid)] == 0} {
	    ::error "stack is empty"
	}
	set elem [lindex $data_($oid) end]
	set data_($oid) [lreplace $data_($oid) end end]
	return $elem
    }

    # Returns the depth of a given ID.
    proc length {oid} {
	variable data_

	error_if $oid
	return [llength $data_($oid)]
    }

    # Error handler for invalid object IDs.
    proc error_if {oid} {
	variable data_

	if {![info exists data_($oid)]} {
	    ::error "object ID $oid does not exist"
	}
    }

    # Export procs to be used.
    namespace export empty push pop new delete length error_if

    # Create an ensemble command to use instead of requiring users
    # to type namespace proc names.
    namespace ensemble create -command ::stack
}

# A namespace/commands to support a queue.
#
# To create a queue, call ::Queue::new, recording the returned queue ID
# for future calls to manipulate the queue object.
#
# Example:
#
# set qid [::Queue::new]
# queue push $qid a
# queue push $qid b
# queue empty $qid;  # returns false
# queue pop $qid;    # returns "a"
# queue pop $qid;    # returns "b"
# queue pop $qid;    # errors with "queue is empty"
# queue delete $qid

namespace eval ::Queue {

    # Remove and return the oldest element in the queue given by OID.
    # It is an error to pop an empty queue.
    proc pop {oid} {
	variable ::Stack::data_

	error_if $oid
	if {[llength $data_($oid)] == 0} {
	    error "queue is empty"
	}
	set elem [lindex $data_($oid) 0]
	set data_($oid) [lreplace $data_($oid) 0 0]
	return $elem
    }

    # "Unpush" ELEM back to the head of the queue given by QID.
    proc unpush {oid elem} {
	variable ::Stack::data_

	error_if $oid
	set data_($oid) [linsert $data_($oid) 0 $elem]
    }

    # Re-use some common routines from the Stack implementation.
    namespace import ::Stack::create ::Stack::new ::Stack::empty \
	::Stack::delete ::Stack::push ::Stack::length ::Stack::error_if

    # Export procs to be used.
    namespace export new empty push pop new delete length error_if unpush

    # Create an ensemble command to use instead of requiring users
    # to type namespace proc names.
    namespace ensemble create -command ::queue
}