From 5edc8a43602296e30b61fbaaa09b1b27024b4190 Mon Sep 17 00:00:00 2001
From: Arun Isaac
Date: Sat, 18 Jun 2022 15:25:35 +0530
Subject: Implement reverse lookup of dictionary.

* thogai.el (thogai-reverse-dictionary): New variable.
(thogai-load-dictionaries): Initialize reverse dictionary.
(thogai-filter-map, thogai-reverse-lookup): New functions.
---
 thogai.el | 39 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 2 deletions(-)

diff --git a/thogai.el b/thogai.el
index d9511ad..daa9372 100644
--- a/thogai.el
+++ b/thogai.el
@@ -81,6 +81,15 @@ not present in `thogai-dictionary-files'.  For example,
 This variable is loaded from files listed in
 `thogai-dictionary-files' on running `thogai-load-dictionaries'.")
 
+(defvar thogai-reverse-dictionary
+  (make-hash-table :test 'equal)
+  "Reverse steno dictionary mapping translations to strokes.
+
+This variable is an exact inverse of the `thogai-dictionary' hash
+table.  It is built when files listed in
+`thogai-dictionary-files' are loaded using
+`thogai-load-dictionaries'.")
+
 (defvar thogai-stroke-ring-size
   1000
   "Size of `thogai-stroke-ring'.
@@ -125,13 +134,18 @@ and the variable is reset to nil.")
 (defun thogai-load-dictionaries ()
   "Load dictionary files in THOGAI-DICTIONARY-FILES."
   (interactive)
-  ;; Reset hash table.
+  ;; Reset forward and reverse dictionaries.
   (clrhash thogai-dictionary)
+  (clrhash thogai-reverse-dictionary)
   (let ((thogai-dictionary-longest-key 0))
     (dolist (dictionary-file thogai-dictionary-files)
       (maphash (lambda (key value)
-                 ;; Load key, value pair into hash table.
+                 ;; Load keys and values into forward and reverse
+                 ;; dictionaries.
                  (puthash key value thogai-dictionary)
+                 (puthash value
+                          (cons key (gethash value thogai-reverse-dictionary))
+                          thogai-reverse-dictionary)
                  ;; Update length of longest key.
                  (setq thogai-dictionary-longest-key
                        (max thogai-dictionary-longest-key
@@ -156,6 +170,27 @@ higher precedence.  If STROKE is in neither, nil is returned."
   (or (map-elt thogai-user-dictionary-alist stroke)
       (map-elt thogai-dictionary stroke)))
 
+(defun thogai-filter-map (function sequence)
+  "Map FUNCTION over SEQUENCE and return only non-nil results.
+
+Map FUNCTION over SEQUENCE using `seq-map', but return a list of
+only the non-nil results."
+  (seq-filter 'identity
+              (seq-map function sequence)))
+
+(defun thogai-reverse-lookup (translation)
+  "Lookup TRANSLATION in reverse dictionary.
+
+A list of all strokes producing TRANSLATION is returned.
+TRANSLATION is looked up in `thogai-user-dictionary-alist' and
+`thogai-reverse-dictionary'."
+  (append
+   (thogai-filter-map (pcase-lambda (`(,key . ,value))
+                        (and (equal value translation)
+                             key))
+                      thogai-user-dictionary-alist)
+   (gethash translation thogai-reverse-dictionary)))
+
 (defun thogai-longest-match (state)
   "Return the longest sublist of STATE that maps to a stroke.
 
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