;;; cycle.el --- Simple module for working with cycles -*- lexical-binding: t -*-
;; Author: William Carroll <wpcarro@gmail.com>
;; Version: 0.0.1
;; Package-Requires: ((emacs "24.3"))
;;; Commentary:
;; Something like this may already exist, but I'm having trouble finding it, and
;; I think writing my own is a nice exercise for learning more Elisp.
;;; Code:
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;; Dependencies
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(require 'dash)
(require 'maybe)
(require 'struct)
(require 'cl-lib)
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;; Wish list
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;; - TODO: Provide immutable variant.
;; - TODO: Replace mutable consumption with immutable variant.
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;; Library
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;; `current-index' tracks the current index
;; `xs' is the original list
(cl-defstruct cycle current-index previous-index xs)
(defun cycle-from-list (xs)
"Create a cycle from a list of `XS'."
(if (= 0 (length xs))
(make-cycle :current-index nil
:previous-index nil
:xs xs)
(make-cycle :current-index 0
:previous-index nil
:xs xs)))
(defun cycle-new (&rest xs)
"Create a cycle with XS as the values."
(cycle-from-list xs))
(defun cycle-to-list (xs)
"Return the list representation of a cycle, XS."
(cycle-xs xs))
(defun cycle-previous-focus (cycle)
"Return the previously focused entry in CYCLE."
(let ((i (cycle-previous-index cycle)))
(if (maybe-some? i)
(nth i (cycle-xs cycle))
nil)))
(defun cycle-focus-previous! (xs)
"Jump to the item in XS that was most recently focused; return the cycle.
This will error when previous-index is nil. This function mutates the
underlying struct."
(let ((i (cycle-previous-index xs)))
(if (maybe-some? i)
(progn
(cycle-jump! i xs)
(cycle-current xs))
(error "Cannot focus the previous element since cycle-previous-index is nil"))))
(defun cycle-next! (xs)
"Return the next value in `XS' and update `current-index'."
(let* ((current-index (cycle-current-index xs))
(next-index (cycle--next-index-> 0 (cycle-count xs) current-index)))
(struct-set! cycle previous-index current-index xs)
(struct-set! cycle current-index next-index xs)
(nth next-index (cycle-xs xs))))
(defun cycle-prev! (xs)
"Return the previous value in `XS' and update `current-index'."
(let* ((current-index (cycle-current-index xs))
(next-index (cycle--next-index<- 0 (cycle-count xs) current-index)))
(struct-set! cycle previous-index current-index xs)
(struct-set! cycle current-index next-index xs)
(nth next-index (cycle-xs xs))))
(defun cycle-current (cycle)
"Return the current value in `CYCLE'."
(nth (cycle-current-index cycle) (cycle-xs cycle)))
(defun cycle-count (cycle)
"Return the length of `xs' in `CYCLE'."
(length (cycle-xs cycle)))
(defun cycle-jump! (i xs)
"Jump to the I index of XS."
(let ((current-index (cycle-current-index xs))
(next-index (mod i (cycle-count xs))))
(struct-set! cycle previous-index current-index xs)
(struct-set! cycle current-index next-index xs))
xs)
(defun cycle-focus! (p cycle)
"Focus the element in CYCLE for which predicate, P, is t."
(let ((i (->> cycle
cycle-xs
(-find-index p))))
(if i
(cycle-jump! i cycle)
(error "No element in cycle matches predicate"))))
(defun cycle-focus-item! (x xs)
"Focus item, X, in cycle XS.
ITEM is the first item in XS that t for `equal'."
(cycle-focus! (lambda (y) (equal x y)) xs))
(defun cycle-append! (x xs)
"Add X to the left of the focused element in XS.
If there is no currently focused item, add X to the beginning of XS."
(if (cycle-empty? xs)
(progn
(struct-set! cycle xs (list x) xs)
(struct-set! cycle current-index 0 xs)
(struct-set! cycle previous-index nil xs))
(let ((curr-i (cycle-current-index xs))
(prev-i (cycle-previous-index xs)))
(if curr-i
(progn
(struct-set! cycle xs (-insert-at curr-i x (cycle-xs xs)) xs)
(when (and prev-i (>= prev-i curr-i))
(struct-set! cycle previous-index (1+ prev-i) xs))
(when curr-i (struct-set! cycle current-index (1+ curr-i) xs)))
(progn
(struct-set! cycle xs (cons x (cycle-xs xs)) xs)
(when prev-i (struct-set! cycle previous-index (1+ prev-i) xs))))
xs)))
(defun cycle-remove! (x xs)
"Attempt to remove X from XS.
X is found using `equal'.
If X is the currently focused value, after it's deleted, current-index will be
nil. If X is the previously value, after it's deleted, previous-index will be
nil."
(let ((curr-i (cycle-current-index xs))
(prev-i (cycle-previous-index xs))
(rm-i (-elem-index x (cycle-xs xs))))
(struct-set! cycle xs (-remove-at rm-i (cycle-xs xs)) xs)
(when prev-i
(when (> prev-i rm-i) (struct-set! cycle previous-index (1- prev-i) xs))
(when (= prev-i rm-i) (struct-set! cycle previous-index nil xs)))
(when curr-i
(when (> curr-i rm-i) (struct-set! cycle current-index (1- curr-i) xs))
(when (= curr-i rm-i) (struct-set! cycle current-index nil xs)))
xs))
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;; Predicates
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(defun cycle-contains? (x xs)
"Return t if cycle, XS, has member X."
(maybe-some? (-contains? (cycle-xs xs) x)))
(defun cycle-empty? (xs)
"Return t if cycle XS has no elements."
(= 0 (length (cycle-xs xs))))
(defun cycle-focused? (xs)
"Return t if cycle XS has a non-nil value for current-index."
(maybe-some? (cycle-current-index xs)))
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;; Helper Functions
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(defun cycle--next-index<- (lo hi x)
"Return the next index in a cycle when moving downwards.
- `LO' is the lower bound.
- `HI' is the upper bound.
- `X' is the current index."
(if (< (- x 1) lo)
(- hi 1)
(- x 1)))
(defun cycle--next-index-> (lo hi x)
"Return the next index in a cycle when moving upwards.
- `LO' is the lower bound.
- `HI' is the upper bound.
- `X' is the current index."
(if (>= (+ 1 x) hi)
lo
(+ 1 x)))
(provide 'cycle)
;;; cycle.el ends here
