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+;;; buffer.el --- Working with buffers -*- lexical-binding: t -*-
+
+;; Author: William Carroll <wpcarro@gmail.com>
+;; Version: 0.0.1
+;; URL: https://git.wpcarro.dev/wpcarro/briefcase
+;; Package-Requires: ((emacs "24.3"))
+
+;;; Commentary:
+;; Utilities for CRUDing buffers in Emacs.
+;;
+;; Many of these functions may seem unnecessary especially when you consider
+;; there implementations.  In general I believe that Elisp suffers from a
+;; library disorganization problem.  Providing simple wrapper functions that
+;; rename functions or reorder parameters is worth the effort in my opinion if
+;; it improves discoverability (via intuition) and improve composability.
+;;
+;; I support three ways for switching between what I'm calling "source code
+;; buffers":
+;; 1. Toggling previous: <SPC><SPC>
+;; 2. Using `ivy-read': <SPC>b
+;; TODO: These obscure evil KBDs.  Maybe a hydra definition would be best?
+;; 3. Cycling (forwards/backwards): C-f, C-b
+
+;;; Code:
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Dependencies
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(require 'prelude)
+(require 'maybe)
+(require 'set)
+(require 'cycle)
+(require 'struct)
+(require 'ts)
+(require 'general)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Library
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defconst buffer-enable-tests? t
+  "When t, run the test suite.")
+
+(defconst buffer-install-kbds? t
+  "When t, install the keybindings defined herein.")
+
+(defconst buffer-source-code-blacklist
+  (set-new 'dired-mode
+           'erc-mode
+           'vterm-mode
+           'magit-status-mode
+           'magit-process-mode
+           'magit-log-mode
+           'magit-diff-mode
+           'org-mode
+           'fundamental-mode)
+  "A blacklist of major-modes to ignore for listing source code buffers.")
+
+(defconst buffer-source-code-timeout 2
+  "Number of seconds to wait before invalidating the cycle.")
+
+(cl-defstruct source-code-cycle cycle last-called)
+
+(defun buffer-emacs-generated? (name)
+  "Return t if buffer, NAME, is an Emacs-generated buffer.
+Some buffers are Emacs-generated but are surrounded by whitespace."
+  (let ((trimmed (s-trim name)))
+    (and (s-starts-with? "*" trimmed))))
+
+(defun buffer-find (buffer-or-name)
+  "Find a buffer by its BUFFER-OR-NAME."
+  (get-buffer buffer-or-name))
+
+(defun buffer-major-mode (name)
+  "Return the active `major-mode' in buffer, NAME."
+  (with-current-buffer (buffer-find name)
+    major-mode))
+
+(defun buffer-source-code-buffers ()
+  "Return a list of source code buffers.
+This will ignore Emacs-generated buffers, like *Messages*.  It will also ignore
+  any buffer whose major mode is defined in `buffer-source-code-blacklist'."
+  (->> (buffer-list)
+       (list-map #'buffer-name)
+       (list-reject #'buffer-emacs-generated?)
+       (list-reject (lambda (name)
+                      (set-contains? (buffer-major-mode name)
+                                     buffer-source-code-blacklist)))))
+
+(defvar buffer-source-code-cycle-state
+  (make-source-code-cycle
+   :cycle (cycle-from-list (buffer-source-code-buffers))
+   :last-called (ts-now))
+  "State used to manage cycling between source code buffers.")
+
+(defun buffer-exists? (name)
+  "Return t if buffer, NAME, exists."
+  (maybe-some? (buffer-find name)))
+
+(defun buffer-new (name)
+  "Return a newly created buffer NAME."
+  (generate-new-buffer name))
+
+(defun buffer-find-or-create (name)
+  "Find or create buffer, NAME.
+Return a reference to that buffer."
+  (let ((x (buffer-find name)))
+    (if (maybe-some? x)
+        x
+      (buffer-new name))))
+
+;; TODO: Should this consume: `display-buffer' or `switch-to-buffer'?
+(defun buffer-show (buffer-or-name)
+  "Display the BUFFER-OR-NAME, which is either a buffer reference or its name."
+  (display-buffer buffer-or-name))
+
+;; TODO: Move this and `buffer-cycle-prev' into a separate module that
+;; encapsulates all of this behavior.
+
+(defun buffer-cycle (cycle-fn)
+  "Using CYCLE-FN, move through `buffer-source-code-buffers'."
+  (let ((last-called (source-code-cycle-last-called
+                      buffer-source-code-cycle-state))
+        (cycle (source-code-cycle-cycle
+                buffer-source-code-cycle-state)))
+    (if (> (ts-diff (ts-now) last-called)
+           buffer-source-code-timeout)
+        (progn
+          (struct-set! source-code-cycle
+                       cycle
+                       (cycle-from-list (buffer-source-code-buffers))
+                       buffer-source-code-cycle-state)
+          (let ((cycle (source-code-cycle-cycle
+                        buffer-source-code-cycle-state)))
+            (funcall cycle-fn cycle)
+            (switch-to-buffer (cycle-current cycle)))
+          (struct-set! source-code-cycle
+                       last-called
+                       (ts-now)
+                       buffer-source-code-cycle-state))
+      (progn
+        (funcall cycle-fn cycle)
+        (switch-to-buffer (cycle-current cycle))))))
+
+(defun buffer-cycle-next ()
+  "Cycle forward through the `buffer-source-code-buffers'."
+  (interactive)
+  (buffer-cycle #'cycle-next))
+
+(defun buffer-cycle-prev ()
+  "Cycle backward through the `buffer-source-code-buffers'."
+  (interactive)
+  (buffer-cycle #'cycle-prev))
+
+(defun buffer-ivy-source-code ()
+  "Use `ivy-read' to choose among all open source code buffers."
+  (interactive)
+  (ivy-read "Source code buffer: "
+            (-drop 1 (buffer-source-code-buffers))
+            :sort nil
+            :action #'switch-to-buffer))
+
+(defun buffer-show-previous ()
+  "Call `switch-to-buffer' on the previously visited buffer.
+This function ignores Emacs-generated buffers, i.e. the ones that look like
+  this: *Buffer*.  It also ignores buffers that are `dired-mode' or `erc-mode'.
+  This blacklist can easily be changed."
+  (interactive)
+  (let* ((xs (buffer-source-code-buffers))
+         (candidate (list-get 1 xs)))
+    (prelude-assert (maybe-some? candidate))
+    (switch-to-buffer candidate)))
+
+(when buffer-install-kbds?
+  (general-define-key
+   :states '(normal)
+   "C-f" #'buffer-cycle-next
+   "C-b" #'buffer-cycle-prev)
+  (general-define-key
+   :prefix "<SPC>"
+   :states '(normal)
+   "b" #'buffer-ivy-source-code
+   "<SPC>" #'buffer-show-previous
+   "k" #'kill-buffer))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(when buffer-enable-tests?
+  (prelude-assert
+   (list-all? #'buffer-emacs-generated?
+              '("*scratch*"
+                "*Messages*"
+                "*shell*"
+                "*Shell Command Output*"
+                "*Occur*"
+                "*Warnings*"
+                "*Help*"
+                "*Completions*"
+                "*Apropos*"
+                "*info*"))))
+
+(provide 'buffer)
+;;; buffer.el ends here