Support functions for navigating buffer caches

I've wanted an MRU/LRU sort of my "source code buffers" in Emacs. This commit
support three ways for working with a cache of source code buffers.

So first, what's a source code buffer? Well it isn't a buffer like *Messages*;
we can call these "Emacs-generated" buffers for convenience. Other problematic
buffers are buffers like `magit-status` and `dired-mode` and `erc` buffers.

I added some predicates for querying buffers for their major modes.

Supporting three KBDs for quickly accessing these functions:
1. <SPC><SPC> Toggle previous buffer
2. <SPC>b     Use ivy to fuzzily search source code buffers
3. C-{f,b}    Cycle {forwards,backwards} through the source code buffer cache.

configs/shared/.emacs.d/wpc/buffer.el

@@ -9,6 +9,13 @@
 ;; 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:
 
@@ -16,20 +23,66 @@
 ;; Dependencies
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+(require 'prelude)
 (require 'maybe)
+(require 'set)
+(require 'cycle)
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Library
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-(defun buffer/exists? (name)
+(defconst buffer/enable-tests? t
-  "Return t if buffer, NAME, exists."
+  "When t, run the test suite.")
-  (maybe/some? (buffer/find name)))
+
+(defconst buffer/install-kbds? t
+  "When t, install the keybindings defined herein.")
+
+(defconst buffer/source-code-blacklist
+  (set/new 'dired-mode 'erc-mode 'magit-status-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))
@@ -47,5 +100,87 @@ Return a reference to that buffer."
   "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)
+  "Cycle forwards or backwards 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
+          (setf (source-code-cycle-cycle buffer/source-code-cycle-state)
+                (cycle/from-list (buffer/source-code-buffers)))
+          (let ((cycle (source-code-cycle-cycle
+                        buffer/source-code-cycle-state)))
+            (funcall cycle-fn cycle)
+            (switch-to-buffer (cycle/current cycle)))
+          (setf (source-code-cycle-last-called buffer/source-code-cycle-state)
+                (ts-now)))
+      (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)))
+
+;; TODO: Replace `evil-leader/set-key' with `general-def'.
+(when buffer/install-kbds?
+  (general-define-key
+   :states '(normal)
+   "C-f" #'buffer/cycle-next
+   "C-b" #'buffer/cycle-prev)
+  (evil-leader/set-key
+   "b" #'buffer/ivy-source-code
+   "<SPC>" #'buffer/show-previous)
+  )
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; 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

configs/shared/.emacs.d/wpc/packages/wpc-keybindings.el

@@ -110,7 +110,6 @@
     "f"   #'wpc/find-file
     "n"   #'flycheck-next-error
     "N"   #'smerge-next
-    "b"   #'ivy-switch-buffer
     "W"   #'balance-windows
     "gs"  #'magit-status
     "E"   #'refine