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+;;; bytes.el --- Working with byte values -*- lexical-binding: t -*-
+;; Author: William Carroll <wpcarro@gmail.com>
+
+;;; Commentary:
+;; Functions to help with human-readable representations of byte values.
+;;
+;; Usage:
+;; See the test cases for example usage.  Or better yet, I should use a type of
+;; structured documentation that would allow me to expose a view into the test
+;; suite here.  Is this currently possible in Elisp?
+;;
+;; API:
+;; - serialize :: Integer -> String
+;;
+;; Wish list:
+;; - Rounding: e.g. (bytes (* 1024 1.7)) => "2KB"
+
+;;; Code:
+
+;; TODO: Support -ibabyte variants like Gibibyte (GiB).
+
+;; Ranges:
+;;  B: [   0,  1e3)
+;; KB: [ 1e3,  1e6)
+;; MB: [ 1e6,  1e6)
+;; GB: [ 1e9, 1e12)
+;; TB: [1e12, 1e15)
+;; PB: [1e15, 1e18)
+;;
+;; Note: I'm currently not support exabytes because that causes the integer to
+;;  overflow.  I imagine a larger integer type may exist, but for now, I'll
+;;  treat this as a YAGNI.
+
+(require 'prelude)
+(require 'tuple)
+(require 'math)
+(require 'number)
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Constants
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defconst bytes/kb (math/exp 2 10)
+  "Number of bytes in a kilobyte.")
+
+(defconst bytes/mb (math/exp 2 20)
+  "Number of bytes in a megabytes.")
+
+(defconst bytes/gb (math/exp 2 30)
+  "Number of bytes in a gigabyte.")
+
+(defconst bytes/tb (math/exp 2 40)
+  "Number of bytes in a terabyte.")
+
+(defconst bytes/pb (math/exp 2 50)
+  "Number of bytes in a petabyte.")
+
+(defconst bytes/eb (math/exp 2 60)
+  "Number of bytes in an exabyte.")
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Functions
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(defun bytes/classify (x)
+  "Return unit that closest fits byte count, X."
+  (prelude/assert (number/whole? x))
+  (cond
+   ((and (>= x 0)        (< x bytes/kb))     'byte)
+   ((and (>= x bytes/kb) (< x bytes/mb)) 'kilobyte)
+   ((and (>= x bytes/mb) (< x bytes/gb)) 'megabyte)
+   ((and (>= x bytes/gb) (< x bytes/tb)) 'gigabyte)
+   ((and (>= x bytes/tb) (< x bytes/pb)) 'terabyte)
+   ((and (>= x bytes/pb) (< x bytes/eb)) 'petabyte)))
+
+(defun bytes/to-string (x)
+  "Convert integer X into a human-readable string."
+  (let ((base-and-unit
+         (pcase (bytes/classify x)
+           ('byte     (tuple/from        1 "B"))
+           ('kilobyte (tuple/from bytes/kb "KB"))
+           ('megabyte (tuple/from bytes/mb "MB"))
+           ('gigabyte (tuple/from bytes/gb "GB"))
+           ('terabyte (tuple/from bytes/tb "TB"))
+           ('petabyte (tuple/from bytes/pb "PB")))))
+    (string/format "%d%s"
+                   (round x (tuple/first base-and-unit))
+                   (tuple/second base-and-unit))))
+
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Tests
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(progn
+  (prelude/assert
+   (equal "1000B" (bytes/to-string 1000)))
+  (prelude/assert
+   (equal "2KB" (bytes/to-string (* 2 bytes/kb))))
+  (prelude/assert
+   (equal "17MB" (bytes/to-string (* 17 bytes/mb))))
+  (prelude/assert
+   (equal "419GB" (bytes/to-string (* 419 bytes/gb))))
+  (prelude/assert
+   (equal "999TB" (bytes/to-string (* 999 bytes/tb))))
+  (prelude/assert
+   (equal "2PB" (bytes/to-string (* 2 bytes/pb)))))
+
+(provide 'bytes)
+;;; bytes.el ends here