;;; symex-data.el --- An evil way to edit Lisp symbolic expressions as trees -*- lexical-binding: t -*-

;; URL: https://github.com/countvajhula/symex.el

;; This program is "part of the world," in the sense described at
;; http://drym.org.  From your perspective, this is no different than
;; MIT or BSD or other such "liberal" licenses that you may be
;; familiar with, that is to say, you are free to do whatever you like
;; with this program.  It is much more than BSD or MIT, however, in
;; that it isn't a license at all but an idea about the world and how
;; economic systems could be set up so that everyone wins.  Learn more
;; at drym.org.
;;
;; This work transcends traditional legal and economic systems, but
;; for the purposes of any such systems within which you may need to
;; operate:
;;
;; This is free and unencumbered software released into the public domain.
;; The authors relinquish any copyright claims on this work.
;;

;;; Commentary:
;;
;; Data abstractions for symex mode.  Defines the linguistic primitives
;; of the symex DSL: moves, maneuvers, precautions, protocols,
;; circuits, and detours.
;;

;;; Code:

(require 'cl-lib)

;;;;;;;;;;;;;;;;;;;;;;;;;
;;; DATA ABSTRACTIONS ;;;
;;;;;;;;;;;;;;;;;;;;;;;;;

(defun symex-make-move (x y)
  "Construct a tree 'move'.

A move represents the number of steps to be taken along the X or
forward-backward axis, and the Y or in-out axis."
  (list 'move x y))

(defun symex--move-x (move)
  "X (horizontal) component of MOVE."
  (nth 1 move))

(defun symex--move-y (move)
  "Y (vertical) component of MOVE."
  (nth 2 move))

(defun symex-move-p (obj)
  "Check if OBJ specifies a move."
  (condition-case nil
      (equal 'move
             (nth 0 obj))
    (error nil)))

(defconst symex--move-zero (symex-make-move 0 0))

(defun symex--are-moves-equal-p (m1 m2)
  "Check if two moves M1 and M2 are identical."
  (equal m1 m2))

(defun symex--add-moves (moves)
  "Add MOVES together as vectors.

This sum indicates height and distance along the branches of the tree."
  (if moves
      (let ((current (car moves))
            (remaining (cdr moves)))
        (let ((result (symex--add-moves remaining)))
          (symex-make-move (+ (symex--move-x current)
                              (symex--move-x result))
                           (+ (symex--move-y current)
                              (symex--move-y result)))))
    symex--move-zero))

(defun symex--move-length (move)
  "Compute the length of the MOVE.

This is most naturally meaningful when the move is entirely along one axis,
but a result will be returned even if the move is across multiple axes,
as standard linear vector magnitude computation is used."
  (let ((x (symex--move-x move))
        (y (symex--move-y move)))
    (if (not (= x 0))
        x
      y)))

(cl-defun symex-make-precaution (traversal &key pre-condition post-condition)
  "A specification to check conditions before/after execution of a TRAVERSAL.

PRE-CONDITION is a boolean function executed before the traversal.  The
traversal is not executed unless this returns true.
POST-CONDITION is a boolean function executed after the traversal.  The
executed traversal is reversed if this returns false."
  (let ((pre-condition (or pre-condition (lambda () t)))
        (post-condition (or post-condition (lambda () t))))
    (list 'precaution
          traversal
          pre-condition
          post-condition)))

(defun symex--precaution-traversal (precaution)
  "The traversal component of the PRECAUTION.

This is the traversal that is to be executed 'with precautions'."
  (nth 1 precaution))

(defun symex--precaution-pre-condition (precaution)
  "Pre-condition of PRECAUTION."
  (nth 2 precaution))

(defun symex--precaution-post-condition (precaution)
  "Post-condition of PRECAUTION."
  (nth 3 precaution))

(defun symex-precaution-p (obj)
  "Check if OBJ specifies a precaution."
  (condition-case nil
      (equal 'precaution
             (nth 0 obj))
    (error nil)))

(defun symex-make-circuit (traversal &optional times)
  "A specification to repeat a TRAVERSAL TIMES times.

If TIMES is nil, repeat indefinitely until the traversal fails."
  (list 'circuit
        traversal
        times))

(defun symex--circuit-traversal (circuit)
  "Get the traversal component of the CIRCUIT.

This is the traversal that is intended to be looped."
  (nth 1 circuit))

(defun symex--circuit-times (circuit)
  "Get the times component of the CIRCUIT.

This is the number of times the traversal should be repeated."
  (nth 2 circuit))

(defun symex-circuit-p (obj)
  "Check if OBJ specifies a circuit."
  (condition-case nil
      (equal 'circuit
             (nth 0 obj))
    (error nil)))

(defun symex-make-maneuver (&rest phases)
  "Construct a maneuver from the given PHASES."
  (list 'maneuver
        phases))

(defun symex--maneuver-phases (maneuver)
  "Get the phases of a MANEUVER.

Each phase could be any traversal."
  (nth 1 maneuver))

(defun symex-maneuver-p (obj)
  "Check if OBJ specifies a maneuver."
  (condition-case nil
      (equal 'maneuver
             (nth 0 obj))
    (error nil)))

(defun symex-make-detour (reorientation traversal)
  "Construct a detour.

A detour consists of two components -- a TRAVERSAL that we wish to execute, and
a REORIENTATION which is a transformation we want to apply prior to attempting
the traversal.  Both the reorientation as well as the traversal could be any
type of traversal, for instance a detour or a maneuver.

The reorientation is applied repeatedly and the traversal is re-attempted each
time, until it succeeds.  If the reorientation itself fails, then the detour
fails as well."
  (list 'detour
        reorientation
        traversal))

(defun symex--detour-reorientation (detour)
  "Get the reorientation component of the DETOUR."
  (nth 1 detour))

(defun symex--detour-traversal (detour)
  "Get the traversal component of the DETOUR."
  (nth 2 detour))

(defun symex-detour-p (obj)
  "Check if OBJ specifies a detour."
  (condition-case nil
      (equal 'detour
             (nth 0 obj))
    (error nil)))

(defun symex-make-protocol (&rest options)
  "Construct a protocol abstraction for the given OPTIONS.

An option could be either a maneuver, or a protocol itself."
  (list 'protocol
        options))

(defun symex--protocol-options (protocol)
  "Get the set of options that are part of the PROTOCOL."
  (nth 1 protocol))

(defun symex-protocol-p (obj)
  "Check if OBJ specifies a protocol."
  (condition-case nil
      (equal 'protocol
             (nth 0 obj))
    (error nil)))

(defun symex-make-decision (condition consequent alternative)
  "A specification to choose between two traversals.

If CONDITION is true, then the CONSEQUENT traversal is executed,
otherwise the ALTERNATIVE traversal is executed.

This is analogous to an `if` statement in common languages."
  (list 'decision
        condition
        consequent
        alternative))

(defun symex--decision-condition (decision)
  "Get the condition component of the DECISION.

This is the condition upon which the decision
to choose one or the other traversal is based."
  (nth 1 decision))

(defun symex--decision-consequent (decision)
  "Get the consequent component of the DECISION.

This is the traversal that will be chosen if the condition is true."
  (nth 2 decision))

(defun symex--decision-alternative (decision)
  "Get the alternative component of the DECISION.

This is the traversal that will be chosen if the condition is false."
  (nth 3 decision))

(defun symex-decision-p (obj)
  "Check if OBJ specifies a decision."
  (condition-case nil
      (equal 'decision
             (nth 0 obj))
    (error nil)))

(defun symex-traversal-p (obj)
  "Check if OBJ specifies a traversal."
  (or (symex-move-p obj)
      (symex-maneuver-p obj)
      (symex-circuit-p obj)
      (symex-detour-p obj)
      (symex-precaution-p obj)
      (symex-protocol-p obj)
      (symex-decision-p obj)))


(provide 'symex-data)
;;; symex-data.el ends here